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Dynamics of convective events in the PNW?

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#1
NorthBurnabyWeather

Posted 04 January 2016 - 12:44 PM

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Hi,

 

I'm a long time weather watcher from British Columbia. For the last few years, I've been a follower of Scott Sistek over at KOMO-TV as well as Cliff Mass' blog. I actually prefer these soruces (along with the forecasts from the NWS Seattle office) over Environment Canada's forecast. Especially in summer, EC has gotten it wrong on convection countless times.

 

Anyway, from reading Cliff's blog, I am familiar with the weather patterns in the PNW in summer and how they limit convection. However, I want to know the dynamics that could trigger thunderstorm activity: west or east of the Cascades; in or out of summer, from Bellingham/the Lower Mainland down to Eugene...how do cold or warm-core convection events get triggered in this part of the continent when the dominant air mass is the North Pacific High, onshore flow dominates, the Cascades and Rockies block moisture, and dewpoints rarely surpass 60F/15C? As for east of the Cascades or the Willemette Valley, what makes warm-core convection so much more frequent than in, say, the Puget Sound or the Lower Mainland?

 

Essentially, what I want to know is something like this, but for thunderstorms.

 

There are a couple of things I know:

 

  • Most convective events west of the Cascades occur in the spring, when the air is coldest aloft (source: Cliff Mass)
  • Warm core convective events west of the Cascades are typically a dry lightning event. Thundervirga is also not uncommon. As such, the biggest danger is not wind, hail, or heavy rain, but actually wildfires. To my count, there has been one Tornado Warning west of the Cascades in my lifetime (the 5/31/97 tornado outbreak), one Severe Thunderstorm Watch north of Portland (on 8/12/14, in SW WA), and one Severe Thunderstorm Warning in Metro Vancouver/Fraser Valley (on 7/25/09, for the western Fraser Valley including Abbotsford). However, there are Red Flag Warnings every summer.
  • Cloud tops aren't as high as east of the Rockies; very rare to see echo tops over 30,000 feet (east of the Cascades is a different story)
  • At least here in the Lower Mainland, thunderstorms almost always come in a general south-north direction, or east-west, whereas fall/winter storms usually travel southwest to northeast.
  • CAPE values max out at 800-1000 J/kg west of the Cascades*. East of the Cascades (example) and in the Willamette Valley, CAPE readings can hit 1500-2000 J/kg, but CAPE potential is more limited because the source of moisture is the North American Monsoon instead of the Gulf of Mexico, which basically acts like a heat pump/sauna for most of North America east of the Continental Divide. CAPE values in the Midwest and Mid-Atlantic can reach 5000-6000 J/kg.

This question has been bugging me for many years. I emailed both Cliff Mass and Scott Sistek for answers, but I never got a reply. I would also love to learn how to analyze the data on Twisteradata.com, like professional storm chasers do, and infer clues as to thunderstorm development. Like, "oh, the 850mb wind direction is different, and the 850mb temperature is 2C, and the CAPE is 850 J/kg with high voricity and water vapour, we will get thunderstorms".

 

* I'm making an educated guess because I haven't been able to find archived surface CAPE reading that are available to the public. UW's GRF-WRF product (which includes CAPE readings) only archives data for 12 days, and the NCDC doesn't have surface CAPE data archived. Though, I think there hasn't been a 1000 J/kg day west of the Cascades since 8/3/99 or maybe even 5/31/97.

 

Edit: 5/31/97 was tornado warning, not STW.


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#2
iFred

Posted 04 January 2016 - 05:31 PM

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Hopefully someone responds. I'd be curious what some here would think what is needed for our severe events.

#3
NorthBurnabyWeather

Posted 04 January 2016 - 10:07 PM

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There isn't enough moisture and instability for severe events, at least until you get over the Cascade crest.

 

Convective events are really all I want to know more about.



#4
BLI snowman

Posted 05 January 2016 - 12:58 AM

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Definitely use Plymouth State's archived tools if you want some data

 

Look for the main sounding locations in the PNW. KSLE (Salem), KUIL (Quillayute), and CYZT (Port Hardy)

 

http://vortex.plymouth.edu/u-make.html

 

You can play around with these tools in various forms if you know what to look for. I'd recommend using SLE, as their parameters are going to be the most indicative of what the populous PNW lowlands see in most given events.

 

The best, basic tools you can use are Lifted Index and CAPE. 

 

As an example, here is Salem's hodogram from a decent westside convective event way back on June 5, 1958

 

Note the LI of -5.8 and CAPE of over 1500 J/KG. About as good of a sounding as you'll ever see around these parts, really. 

 

Attached File  160105082945.gif   12.66KB   0 downloads

 

The best strong convective setups for much of the PNW tend to occur in the mid to late spring (late April to late June). That's when we by far will have the best combination of heating, 500mb dynamics, and moisture.

 

The ideal setup for a strong warm-core convective event is a strong upper level trough, cut-off low, or even a weaker shortwave which can wrap the winds aloft around it and trigger moisture advection from the south or southeast. 

 

This is a pretty textbook PNW convection setup, albeit a little later than normal during the usually drier midsummer period

 

http://mp1.met.psu.e...1983/us0719.php

 

A more recent example

 

http://mp1.met.psu.e...2009/us0605.php

 

Cold core events are a totally different animal and can occur in any month, although in the summer they are quite rare. These events stem of course from strong upper level troughs overhead and actually do produce the majority of our severe weather, most notably spin-up type tornadoes like what we saw in Battle Ground, WA last month that form with very localized pockets of lower level rotation as opposed to deep layer shear and supercells.


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#5
NorthBurnabyWeather

Posted 05 January 2016 - 10:28 AM

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Definitely use Plymouth State's archived tools if you want some data

 

Look for the main sounding locations in the PNW. KSLE (Salem), KUIL (Quillayute), and CYZT (Port Hardy)

 

http://vortex.plymouth.edu/u-make.html

 

You can play around with these tools in various forms if you know what to look for. I'd recommend using SLE, as their parameters are going to be the most indicative of what the populous PNW lowlands see in most given events.

 

The best, basic tools you can use are Lifted Index and CAPE. 

 

As an example, here is Salem's hodogram from a decent westside convective event way back on June 5, 1958

 

Note the LI of -5.8 and CAPE of over 1500 J/KG. About as good of a sounding as you'll ever see around these parts, really. 

 

attachicon.gif160105082945.gif

 

The best strong convective setups for much of the PNW tend to occur in the mid to late spring (late April to late June). That's when we by far will have the best combination of heating, 500mb dynamics, and moisture.

 

The ideal setup for a strong warm-core convective event is a strong upper level trough, cut-off low, or even a weaker shortwave which can wrap the winds aloft around it and trigger moisture advection from the south or southeast. 

 

This is a pretty textbook PNW convection setup, albeit a little later than normal during the usually drier midsummer period

 

http://mp1.met.psu.e...1983/us0719.php

 

A more recent example

 

http://mp1.met.psu.e...2009/us0605.php

 

Cold core events are a totally different animal and can occur in any month, although in the summer they are quite rare. These events stem of course from strong upper level troughs overhead and actually do produce the majority of our severe weather, most notably spin-up type tornadoes like what we saw in Battle Ground, WA last month that form with very localized pockets of lower level rotation as opposed to deep layer shear and supercells.

 

This is great, I can't thank you enough. Though I'll admit I don't know how to read a sounding or a hodo®graph.

 

I tried looking through some sample dates (for which I know thunderstorms occured) out of KSLE but kept getting high LI's and low CAPE; what gives?

 

Examples: 8/3/99, 7/23/00, 7/3/088/14/15



#6
BLI snowman

Posted 05 January 2016 - 12:23 PM

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This is great, I can't thank you enough. Though I'll admit I don't know how to read a sounding or a hodo®graph.

 

I tried looking through some sample dates (for which I know thunderstorms occured) out of KSLE but kept getting high LI's and low CAPE; what gives?

 

Examples: 8/3/99, 7/23/00, 7/3/088/14/15

 

Generally these soundings are just a brief snapshot and don't capture the best dynamics for us, which tend to be pretty fleeting. The July 22, 2000 thunderstorm outbreak for example was triggered by a shortwave moving quickly up the coast. The 00z sounding at SLE on 7/22 was relatively impressive

 

Attached File  160105201730.gif   26.22KB   0 downloads

 

But as you can see, 24 hours later those dynamics had completely shifted north. 

 

Like with snow, it's a very delicate balance for us in getting the right combination of elements for warm-core thunderstorms. There's always going to be a fine line between an encroaching cooler, stable marine layer and the warmer, moist unstable air. These conducive elements generally don't last too long, although in a less progressive pattern an event like August 1999 can happen where areas do see storms several days in a row.


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#7
NorthBurnabyWeather

Posted 12 January 2016 - 12:55 PM

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Generally these soundings are just a brief snapshot and don't capture the best dynamics for us, which tend to be pretty fleeting. The July 22, 2000 thunderstorm outbreak for example was triggered by a shortwave moving quickly up the coast. The 00z sounding at SLE on 7/22 was relatively impressive

 

attachicon.gif160105201730.gif

 

But as you can see, 24 hours later those dynamics had completely shifted north. 

 

Like with snow, it's a very delicate balance for us in getting the right combination of elements for warm-core thunderstorms. There's always going to be a fine line between an encroaching cooler, stable marine layer and the warmer, moist unstable air. These conducive elements generally don't last too long, although in a less progressive pattern an event like August 1999 can happen where areas do see storms several days in a row.

 

After tooling around with the soundings, it seems that they don't represent a wide enough time frame for such fleeting dynamics (it's similar to the snow events you mentioned), so I want to ask you if you know the mesoscale setups in the PNW for some of these epic thunderstorm events:

 

-August 1999

 

-7/25/09 which hammered SW BC but not W WA

 

-July 2012, which hammered W WA but not Vancouver

 

-8/7/12 again which hammered SW BC and not W WA

 

-The thunderstorms in late August and early September 2013, which were the rarest of the rare, warm-core moist thunderstorms.

 

-8/11/14-8/12/14 which triggered the STW in SW WA

 

-8/14/15, which was the last warm core thunderstorm I can think of.



#8
Abbotsford_wx

Posted 12 January 2016 - 05:00 PM

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Out where I am, I've noticed that most of our storms are of the warm-core variety and typically occur between June and September. I rarely see any lightning here with cold-core convection, just heavy rain/hail.

 

Further west toward the coast, it seems like the cold-core variety are more common and typically occur in the spring and fall.



#9
BLI snowman

Posted 13 January 2016 - 12:24 AM

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After tooling around with the soundings, it seems that they don't represent a wide enough time frame for such fleeting dynamics (it's similar to the snow events you mentioned), so I want to ask you if you know the mesoscale setups in the PNW for some of these epic thunderstorm events:

 

-August 1999

 

-7/25/09 which hammered SW BC but not W WA

 

-July 2012, which hammered W WA but not Vancouver

 

-8/7/12 again which hammered SW BC and not W WA

 

-The thunderstorms in late August and early September 2013, which were the rarest of the rare, warm-core moist thunderstorms.

 

-8/11/14-8/12/14 which triggered the STW in SW WA

 

-8/14/15, which was the last warm core thunderstorm I can think of.

 

This link is helpful in recreating some of those upper level setups

 

 http://mp1.met.psu.e.../NARR/2013.html

 

You can play around with the dates there and notice the general similarities in the weather patterns.

 

Generally we'll be on the western side of an upper level ridge and there will be to some degree or another an upper level trough or low pressure area to our west which triggers positive vorticity advection and creates lift. When the air-flow is due southerly or southeasterly (storms can drift off the Cascades) ahead of this trough or vorticity max, we'll see our best dynamics to moisten up and destabilize the atmosphere.

 

September 5-6, 2013 for example was a classic 500mb setup with a compact cut-off low parked off the coast before moving onshore to the south. The difference there was that we managed to tap into a surprisingly deep layer of mid level moisture and had some very impressive precipitable water values for a summertime airmass here. 

 

http://mp1.met.psu.e...2013/us0905.php


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#10
IbrChris

Posted 23 February 2016 - 10:47 PM

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I'll try and answer a few of these questions. For historical CAPE values you can look at the SPC sounding climatology page. It will note the mean, percentiles and maximum CAPE (SBCAPE, MLCAPE and MUCAPE) for each sounding location in the lower 48 states. UIL is probably a decent proxy for Vancouver, BC.

My thoughts on thunder in the PNW (West of the Cascades):

While our convection tends to be focused on the spring months (March through June) this convection is rarely deep enough to produce much lightning. Interestingly the bulk of the lightning activity offshore from cold-core convection are positive strikes rather than the usually more common negative strikes. Positive strikes are generally more powerful than their negative cousins. The reason behind the comparative lack of lightning accompanying cold-core convection can be best answered by noting that low-topped Cb common here rarely attain significant charge separation for frequent lightning which is common with thunder east of the Rockies. Most cold-core convection tops out at 15-20 kft, occasionally 25 kft if the buoyant layer is sufficiently deep, such as a cold ULL in later spring when surface temps are often into the 70s with sunshine. At that elevation (roughly 400-500 mb) the temperature is often well above -40c which is the temperature in which supercooled water vapor instantaneously crystalizes (it doesn't require a suitable particle for an ice crystal nucleus). Thus most cold-core convection is at least partially composed of supercooled droplets even within the upper portions of the updraft, precluding a strong charge separation between the cloud and the surface.

Warm-season/warm-core convection is usually elevated above the boundary layer and often synonymous with surges in monsoonal moisture and elevated instability. Bases are often 10 kft or higher above the surface and tops can reach 30-40 kft (especially for convection over the Cascades). I've personally witnessed a warm-core severe thunderstorm on Aug 10, 2013 drop 1.5" diameter hail near Government Camp, OR (west of the crest). Due to the often dry sub-cloud layer wind is the biggest threat with warm-core convection however if a marine layer is present downdrafts may not penetrate to the surface. When forecasting these events I look for 700 mb theta-e ridges (maxima) extending west of the Cascades and some sort of trigger...often a shortwave rotating northward around a ULL off the S Oregon or N California coast. Warm season thunderstorms tend to be nocturnal in the Willamette Valley, averaging one or two events each summer and occasionally a multi-day event with two or more convective "episodes" can occur. The reason the storms tend to be nocturnal is that they form on higher terrain to the SE and S of the Willamette Valley during the late afternoon and evening hours, tracking northward in the evening maintained by elevated instability (high theta-e). Often these events are comprised of scattered small cells with rather short life-cycles of 30 minutes to an hour, but often highly-electrified with strike frequencies as high as once per second, more often once every 10-20 seconds.


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#11
NorthBurnabyWeather

Posted 26 February 2016 - 11:49 PM

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Thank you for your informative posts. I learned a lot.

 

I would like to see if there is any academic research on this subject. The NWS Portland office wrote a paper on the 1/10/08 tornado in Vancouver, WA that included some climatology of severe convective weather in Portland, and that's as close as I've gotten:

 

 

Two synoptic patterns are locally recognized for producing severe convective weather in Portland's CWA. The first, more common pattern, occurs during the spring and summer convective season, and involves an upper level cutoff low off the northern California coast. The weakly sheared, dry environment supports mostly elevated, single cell storms, at times producing downbursts and hail. The 10 January 2008 tornado synoptic pattern falls in a second regime. This regime is more frequent in fall and winter months, and involves the approach or passage, of an upper level trough. [The region of tornado genesis on 1/10/08] was located on the cyclonic side of a strong polar jet, under mid level cold advection, with an onshore flow of moist Pacific air. This frequently occurs in a postfrontal air mass.

 

Which is informative, but I think the nuances of storms in the Willmette Valley are different from those of the Puget Sound, and those of Bellingham/Vancouver BC.

 

Does anyone have leads as to academic literature about thunderstorm climatology?

 

Our weather is so moderate that it's not worth studying! (Besides windstorms, of course)



#12
Timmy_Supercell

Posted 29 February 2016 - 12:18 PM

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Pretty much if you want a guarantee of a severe event happening you need to be east of the cascades. For widespread text-book events like 06/04/2009 or 05/19/1993 to pan out, multiple things need to come together just perfectly in the valleys (typically only a couple ingredients are present while another huge ingredient might be missing in some events). I've noticed we can lack heavily with ample wind shear, which in high amounts can help longevity and organization of storm cells (or as we've seen in 2009; state-wide storm complexes with embedded rotation). While shear is not 100% needed for severe storms to develop (I've seen severe storms happen in a low shear environment), but it does help an overall event from looking better and lasting longer, and sometimes increases the number of storms seen in a region. Another problem the lowlands have west of the mountains, is it can take several hours to get out of a CAP. 

 

Sometimes moisture really isn't a problem; evidently in much of summer 2014 and 2015 most of the PNW had lots of moisture to pull from Upper Level Low activity frequently occurring. Not quite comparable to anywhere east of the Rockies, but for PNW standards I had noticed large quantities of it (numerous NWS Medford AFD's mentioned PWAT's ranging from 1.25" to 1.50" in July and August in southern Oregon). 

 

On another note, I have wondered myself why a nocturnal storm might exhibit much more erratic lightning characteristics. For example, early in the morning on 06/09/2015 in Klamath Falls, 2 storms (1 near the border of CA, and 1 just to my west) absolutely exploded with frequent lightning from 4am to sunrise. (ranging from 8-12+ flashes per minute). This was the most lightning I had personally witnessed in my life. By far, more lightning than any Severe Thunderstorms have accomplished here in the afternoon/evening hours. So much ionization happened that I began to smell a charged ozone type air, to the point where it was not as easy to breathe it and made me feel nauseous.

 

Though there is an exceptional severe storm that occurred on 08/05/2012 from 7pm-8pm while I was out of town, with a spotter report indicating 3-4 strikes per 10 seconds frequency. I am assuming this is extraordinary for a late afternoon-evening type setup in this area. The storm itself was a 10-15+ year type storm based on what neighbors have said (folks who have spent more than 30 years in Klamath Falls). I spent all of summer 2012 visiting my brother in Beaverton, so I missed a great event. Though for some strange reason, that was the only t'storm to occur in K-Falls while I was away. 


Weather Data for Klamath Falls, OR

------------------------------------------------------------

(Personal Winter Totals since 2010)

'10-'11 = 58.20" (161% of normal)

'11-'12 = 49.00" (136% of normal)

'12-'13 = 16.70" (46% of normal)

'13-'14 = 9.05" (25% of normal)

'14-'15 = 2.85" (8% of normal)

'15-'16 = 54.45" (151% of normal)

'16-'17 = 63.00" (175% of normal)

 

Nov '16: 1.20" (30% of normal)

Dec '16: 11.10" (123% of normal)

Jan '17: 29.50" (246% of normal)

Feb '17: 12.90" (161% of normal)

Mar '17: 5.60" (224% of normal)

Apr '17: 2.70"

 

Nov '15: 4.00" (100% of normal) (Avg: 4.00")

Dec '15: 33.10" (367% of normal) (Avg: 9.00")

Jan '16: 10.75" (90% of normal) (Avg: 12.00")

Feb '16: 3.50" (43% of normal) (Avg: 8.00")

Mar '16: 3.10" (124% of normal) (Avg: 2.50")

Apr '16: T

 

OTHER WEATHER DATA

-------------------------------------------------------------

*Max 1 Day Snowfall: 12.40" (01/03/2017)*

*Max Snow Depth: 21.00" (01/07/2017)*, 18.00" (12/24/2015)

Max High (F): 101 (07/02/2013), 99 (07/02/2015)

Min High (F): 6 (12/08/2013), 7 (01/06/2017), 8 (01/05/2017)

Max Low (F): 63 (07/04/2015)

Min Low (F): -20 (12/08/2013), -19 (01/06/2017), -17 (01/05/2017)

Max Wind Gusts:

58-60 (10/15/2016), 60-65 (10/26/2016) ( 55+ MPH (09/12/2016), 67 MPH (01/19/2016), 65 MPH (02/06/2015), 63 MPH (02/05/2015), 62 MPH (02/17/2016),

56 MPH (02/08/2015), 55 MPH (12/03/2015), 58 MPH (10/25/2014), 55 MPH (12/30/2011), 58 MPH (09/04/2011), 54 MPH (03/13/2011), 58 MPH (02/15/2011), 60+ (02/14/2011)

T'storm Days: 12 (2016), 20 (2015), 21 (2014), 16 (2013), 2 (2012), 12 (2011) - 1980-2015 Avg = 12 Days

Severe T'storms: 2 (01/19/2016), (08/05/2012)

Vicinity Severe T'storms: 9 (dates below)

09/04/2011, 09/12/2011, 08/12/2013, 08/22/2013, 08/04/2014, 08/05/2014, 06/09/2015, 07/05/2015, 07/09/2015

Earliest Warm-Core T'storm: (04/03/2016)

Latest Cold-Core T'storm (06/17/2016)

Latest <32 low (06/18/2014)

Latest "20's" low (06/11/2016) (28 degrees)


#13
IbrChris

Posted 04 March 2016 - 11:50 AM

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Which is informative, but I think the nuances of storms in the Willmette Valley are different from those of the Puget Sound, and those of Bellingham/Vancouver BC.

 

Warm-core "SW Monsoon"-type convection is less common the further north you travel, although storms can and do form over western WA and conceivably even in SW BC/Vancouver Island in these scenarios. I've personally seen lightning detected from OR to BC in some big events. Cold-core convection is more aligned with the 500 mb cold pool, which is more common further north as it takes a more amplified trough or a closed low to get the mid-level cold pool to sag south into Oregon. From this my guess as a meteorologist would be that coastal BC seems more cold-core events which are generally benign and not really noteworthy, and less warm-core elevated convection. Of course thunder is much more common in interior BC during spring and summer months.


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#14
snow_wizard

Posted 13 March 2016 - 07:37 PM

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On another note, I have wondered myself why a nocturnal storm might exhibit much more erratic lightning characteristics. For example, early in the morning on 06/09/2015 in Klamath Falls, 2 storms (1 near the border of CA, and 1 just to my west) absolutely exploded with frequent lightning from 4am to sunrise. (ranging from 8-12+ flashes per minute). This was the most lightning I had personally witnessed in my life. By far, more lightning than any Severe Thunderstorms have accomplished here in the afternoon/evening hours. So much ionization happened that I began to smell a charged ozone type air, to the point where it was not as easy to breathe it and made me feel nauseous.

 

 

 

Lightning or even large electric sparks can produce nitrogen dioxide which turns to nitric acid when it comes into contact with water.  Sounds like that was an extreme case.


Death To Warm Anomalies!
 
winter.jpg

Winter 2016-17 Stats

Total snow = 9.8"
Days Min 32 or below = 61
Days Max 32 or below = 1
Days Max Below 40 = 29
Coldest Min = 16

#15
Timmy_Supercell

Posted 14 March 2016 - 01:29 PM

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Lightning or even large electric sparks can produce nitrogen dioxide which turns to nitric acid when it comes into contact with water.  Sounds like that was an extreme case.

 

It could also be that at higher elevations, this is so much more noticeable in severe lightning events. I don't know anyone else personally who lives at 4000-5000 feet and been through a wide variety of t'storm events over a period of years.

 

If this were in the Willamette Valley, I'd assume I wouldn't have had such an experience. 


Weather Data for Klamath Falls, OR

------------------------------------------------------------

(Personal Winter Totals since 2010)

'10-'11 = 58.20" (161% of normal)

'11-'12 = 49.00" (136% of normal)

'12-'13 = 16.70" (46% of normal)

'13-'14 = 9.05" (25% of normal)

'14-'15 = 2.85" (8% of normal)

'15-'16 = 54.45" (151% of normal)

'16-'17 = 63.00" (175% of normal)

 

Nov '16: 1.20" (30% of normal)

Dec '16: 11.10" (123% of normal)

Jan '17: 29.50" (246% of normal)

Feb '17: 12.90" (161% of normal)

Mar '17: 5.60" (224% of normal)

Apr '17: 2.70"

 

Nov '15: 4.00" (100% of normal) (Avg: 4.00")

Dec '15: 33.10" (367% of normal) (Avg: 9.00")

Jan '16: 10.75" (90% of normal) (Avg: 12.00")

Feb '16: 3.50" (43% of normal) (Avg: 8.00")

Mar '16: 3.10" (124% of normal) (Avg: 2.50")

Apr '16: T

 

OTHER WEATHER DATA

-------------------------------------------------------------

*Max 1 Day Snowfall: 12.40" (01/03/2017)*

*Max Snow Depth: 21.00" (01/07/2017)*, 18.00" (12/24/2015)

Max High (F): 101 (07/02/2013), 99 (07/02/2015)

Min High (F): 6 (12/08/2013), 7 (01/06/2017), 8 (01/05/2017)

Max Low (F): 63 (07/04/2015)

Min Low (F): -20 (12/08/2013), -19 (01/06/2017), -17 (01/05/2017)

Max Wind Gusts:

58-60 (10/15/2016), 60-65 (10/26/2016) ( 55+ MPH (09/12/2016), 67 MPH (01/19/2016), 65 MPH (02/06/2015), 63 MPH (02/05/2015), 62 MPH (02/17/2016),

56 MPH (02/08/2015), 55 MPH (12/03/2015), 58 MPH (10/25/2014), 55 MPH (12/30/2011), 58 MPH (09/04/2011), 54 MPH (03/13/2011), 58 MPH (02/15/2011), 60+ (02/14/2011)

T'storm Days: 12 (2016), 20 (2015), 21 (2014), 16 (2013), 2 (2012), 12 (2011) - 1980-2015 Avg = 12 Days

Severe T'storms: 2 (01/19/2016), (08/05/2012)

Vicinity Severe T'storms: 9 (dates below)

09/04/2011, 09/12/2011, 08/12/2013, 08/22/2013, 08/04/2014, 08/05/2014, 06/09/2015, 07/05/2015, 07/09/2015

Earliest Warm-Core T'storm: (04/03/2016)

Latest Cold-Core T'storm (06/17/2016)

Latest <32 low (06/18/2014)

Latest "20's" low (06/11/2016) (28 degrees)


#16
NorthBurnabyWeather

Posted 26 May 2016 - 10:19 PM

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Can any whizzes help me understand how in the world a sounding works?

 

I am trying to compare soundings between two days. One day was a day in which heavy thunderstorms were observed in southwestern BC (Aug. 6, 1997), and the other is a day more charactaristic of the usual summertime pattern of an onshore ridge of high pressure, clear skies and low dewpoints (Jul. 23, 2015).

 

I'll be using the Qualiayute soundings for reference, because it's the best analogue to Vancouver and the BC south coast that I could find. For the first event, I'm using the 00Z 8/6 and 12Z 8/6 soundings. These correspond to 17:00 8/5 and 05:00 8/6 in the PDT time zone. The storms started firing up mid-morning on the 6th and lasted until the mid-afternoon, so those two particular soundings I believe would be an apt thermodynamic "tale of the tape", as it were. For the second event, I'm using the 00Z 7/23 sounding.

 

I tried reading a "how to read a Skew-T diagram" page from the NWS Storm Prediction Center, but since I was a layperson it didn't didn't help me. They were glossing over the various types of isotherms and dry adibats and whatnot. I don't know what the solid lines represent and what the dashed lines represent, nor the yellow line in the diagram. Which one's the isotherm, and which one's the dry adibat, and which one's the wet bulb temperature profile? When it comes to the wind barbs, if they're sticking out to the northwest does that mean the winds are coming from the north west? Or is there some other counterintuitive orientation? (i.e. if the wind barbs are sticking out of the northwest, that means the winds are coming out of the *southeast*)



#17
NorthBurnabyWeather

Posted 05 August 2016 - 06:28 PM

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I recently started a thread on heat events, inspired by all the talk of heat domes in the central and eastern U.S. as of late. While the following is related to heat domes, I thought it was more appropriate to post in here.

 

A side effect of heat domes is the "ring of fire" phenomenon, in which heavy thunderstorms develop along the periphery of a hot air mass. Examples include the "super derecho" in 2012. 

 

My gut instinct is that geography gets in the way of "ring of fire"-esque events. The Rockies block moisture from the Gulf of Mexico while the Cascades block surface moisture originating from the monsoon down in Arizona.

 

What's more, there's a lot less 28+ degree water in the Gulf of California and the tropical eastern Pacific, compared to the Gulf of Mexico as can be evidenced by NOAA's SST maps.

 

I think that's why thunderstorms in the western U.S. tend to be dry, and we don't get the kind of drenching storms that they get east of the Rockies.

 

You won't get them unless the source of moisture is something like the remnants of an eastern Pacific hurricane (like 7/20/12), or there's a freak progressive event like 8/6/97, 9/5/13, 8/3-5/99, 6/22/93 or 8/12/14.


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#18
Phil

Posted 05 August 2016 - 07:54 PM

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I recently started a thread on heat events, inspired by all the talk of heat domes in the central and eastern U.S. as of late. While the following is related to heat domes, I thought it was more appropriate to post in here.

A side effect of heat domes is the "ring of fire" phenomenon, in which heavy thunderstorms develop along the periphery of a hot air mass. Examples include the "super derecho" in 2012.

My gut instinct is that geography gets in the way of "ring of fire"-esque events. The Rockies block moisture from the Gulf of Mexico while the Cascades block surface moisture originating from the monsoon down in Arizona.

What's more, there's a lot less 28+ degree water in the Gulf of California and the tropical eastern Pacific, compared to the Gulf of Mexico as can be evidenced by NOAA's SST maps.

I think that's why thunderstorms in the western U.S. tend to be dry, and we don't get the kind of drenching storms that they get east of the Rockies.

You won't get them unless the source of moisture is something like the remnants of an eastern Pacific hurricane (like 7/20/12), or there's a freak progressive event like 8/6/97, 9/5/13, 8/3-5/99, 6/22/93 or 8/12/14.


Outside the lower tropospheric moisture loading via the Gulf of Mexico and SW Atlantic, derecho events (and most other severe weather events) that occur here in the summer arise under the EML (elevated mixed layer) along zones of focused streamflow on the periphery of the aforementioned "heat some". The EML is essentially what the acronym stands for. This warm air aloft has much more adiabatic potential and steepens lapse rates substantially.

The Rockies/Intermountain West is the source region for this airmass, in that these higher elevations conduct heat higher into the atmosphere, relatively speaking, before this air is shunted eastward over the Plains, Midwest, and East/Southeast.
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Personal Weather Station, Live Stream on Wunderground: https://www.wundergr...BETHE62#history

Warm season 2017:
Thunderstorm days: 4
Severe days: 3
Hail: 1 (pea sized)
Wind: 2 (62mph, 58mph)
Rain total: 4.54"

#19
NorthBurnabyWeather

Posted 03 January 2017 - 07:22 PM

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Hi all,

 

I hope you had a good holiday.

 

Over the holidays, I've been trying to get a handle on upper air and surface analyses, and see if I can deduce the setup to thunderstorm events. Why did BC's Lower Mainland and Fraser Valley get rocked on August 7, 2012 and May 2, 2016 while Seattle and Portland were untouched? Why did Seattle get storms on August 2, 2014 but Vancouver did not? That kind of thing.

 

Growing up in the Northwest all my life, studying upper air charts via Plymouth State University have allowed me to recognize several "over-arching" setups for warm-core convective events west of the Cascades:

  • Type I: Shortwave trough, no onshore flow through depth, southerly surface flow: These "monsoonal plume" kind of events are a part of summer when you get east of the Cascades, but for them to happen in the interior lowlands of W WA or the Willamette Valley, the onshore flow has to be cut off. This is possible if the North Pacific High is located unusually far south and west (closer to the Hawaiian islands), there is robust monsoonal activity (and a mid-level (about 500 hPa) anticyclone) in the Desert Southwest, and a trough is located in the right position. The main danger is wildfires since there is abundant dry lightning. Virga is not uncommon. Examples: September 2-3, 2009; August 2, 2014
  • Type II: Cut off low off the OR/CA coast, no onshore flow through depth, monsoonal moisture: The second primary lifting mechanism for warm-core convective events is a cut off upper level low (cut off meaning it formed independent of the jet stream) spinning off the Oregon coast, which can tap into warmer water off Baja California or a monsoonal trough over land. Couple that with residual heat from a weakening desert high, and you have a recipe for fireworks. As the cells are pushed over the Cascades, high-based lightning becomes more frequent. Examples: August 9-10, 2013; August 11-13, 2014; August 14, 2015
  • Type III: Cut off low off the OR/CA coast, no onshore flow through depth, mid level Pacific moisture: An uncommon cousin of type 2, if there's abundant mid level moisture (like an atmospheric river of the remnants of a Pacific hurricane), you can get drenching thunderstorms that are more characteristic of the Midwest or East Coast. Examples: September 5, 2013; July 20, 2012
  • Type IV: Upper level low churning over E WA: Extremely rare. If there's a closed cut off low located east of the Cascades and sufficient daytime heating, it can push storms east-to-west, forcing them over the Cascade crest and intensifying them. Examples: July 25, 2009

 

I have attempted to teach myself mesoscale analyses, but I'm having a lot of trouble. I received a lot of great information from the other replies in this thread, but I want to go to the next level, to the quantitative "nitty-gritty". I thought I would start with 8/2/14 since it was the day that that guy recorded himself being struck by lightning on camera. Here's what I have so far, using nullschool.net since I found their dynamic and interactive animations meant it was easier for me to switch heights and times. If you'll indulge me:

 

Thursday, July 31, 11am: An immoving Omega block is over Aluetian Islands. The North Pacific high is unusually south and west, allowing it to tap into the warmer waters of the tropical Pacific, and lead to a bit of an "atmospheric river" effect along the west coast of North America. There's a better shot of the Pacific high at 250 hPa, which also lets you see the vigorous low (~5700dm; try as I might I was unable to find archived barometric pressure map data (using mbar); nor could I find a way to convert geopotential heights into rough barometric pressure) located 950 km off Haida Gwaii. The "river" airflow and the upper level low airflow are converging over the west coast of Vancouver Island.

 

 
Saturday, August 2

 

Unfortunately, I've become a bit stuck. It was straightforward to get a "big-picture" look from the nullschool visualizations, but I'm not completely satisfied. Nullschool's visualizations only go as far back as 2013-11-01 00Z. Upper air charts on Plymouth State University's website, on the other hand, go back 60 years. The NWS has a large historical archive of surface charts so I'm fine there. I guess what I'm trying to say is, how do I know what to look for? There is so much information that you can map via PSWC: just look at the list! My gut instinct tells me that I should probably get schooled in DIY mesoscale analysis, like through theweatherprediction.com, before I start jumping into charts and whatnot. But, at least you can see what I have so far.

 

If you're still reading so far, the TL;DR is as follows: what would I have to know to write something like this, but for 8/3/99 or 9/5/13 in the Northwest, rather than 4/27/11 in Dixie.