October 2016 Observations and Model Discussion for the Pacific Northwest

[Phil]

Seems neutral to slightly positive to me...

Based on which features, exactly? The computed PNA values reflect a weak negative PNA in Dec/Jan, followed by a heavily negative PNA in February.

[snow_wizard]

Yeah, when juxtaposed with a -PNA, the -NAO can actually assist in keeping troughing in the west, as opposed to a January 2014 type deal. The prolonged +NAO in recent winters I think has been a contributing factor to the warmth in the western US, given that a +NAO constructively teleconnects with a +PNA, in most cases.

 

Meanwhile, the -NAO stretch from 2008-2013 featured improved winters over the western US as a whole.

 

Very interesting.  I wasn't even aware the NAO was in a negative phase during those favorable winters.

[westcoastexpat]

Hopefully we can get the PNA to go negative like last December. With a -AO it could really flood the PNW with frigid temps. Really impressed with the snowpack in eastern Siberia so far... no doubt the PNW will have access to a solid cold source early.

[snow_wizard]

Looks like the back edge of the frontal boundary is finally swinging through.

[snow_wizard]

Hopefully we can get the PNA to go negative like last December. With a -AO it could really flood the PNW with frigid temps. Really impressed with the snowpack in eastern Siberia so far... no doubt the PNW will have access to a solid cold source early.

 

I feel good about our chances of getting hit hard this winter.  The big question is what part or parts will be the best.

[SilverFallsAndrew]

An ideal winter for me would be a couple periods of extreme blocking with an arctic blast and then lots of cool NW flow. 

[snow_wizard]

An ideal winter for me would be a couple periods of extreme blocking with an arctic blast and then lots of cool NW flow. 

 

For me it would be.

 

1. Arctic front with snow

2. cold and clear for 4 or 5 days

3. reload with snow

4. cold and clear

 

Basically Arctic blasts alternating with cold / dry northerly flow or a ridge right over us with a few days of freezing fog with snow on the ground.

[Phil]

Hopefully we can get the PNA to go negative like last December. With a -AO it could really flood the PNW with frigid temps. Really impressed with the snowpack in eastern Siberia so far... no doubt the PNW will have access to a solid cold source early.

The PNA wasn't negative last December. It was quite positive, actually.

 

Last December was a strong +EPO/+NAM, with that flat equatorward NPAC ridge representing the climatological loading pattern for a +EPO. Definitely not a -PNA.

[snow_wizard]

C-Zone beginning to form around Seattle now.  If this were winter chances are high today's precip would have been snow in this area.  Can't wait for the frost tomorrow night.

[westcoastexpat]

The PNA wasn't negative last December. It was quite positive, actually.

 

Last December was a raging +NAM/+EPO w/ the flat equatorward NPAC ridge..not a -PNA, just the +NAM loading pattern.

 

I think the month may have averaged positive, but I recall a pretty distinct period mid month where the PNA was negative... remember it being one of the only good periods last winter. My old place near Peace Arch had like 4 days of flurries.

[Phil]

Very interesting. I wasn't even aware the NAO was in a negative phase during those favorable winters.

The winters of 2009/10, 2010/11, and 2012/13 were -NAO, while 2008/09 was neutral NAO. The winters of 2011-12, 2014/15, and 2015/16 were +NAO.

[snow_wizard]

I think the month may have averaged positive, but I recall a pretty distinct period mid month where the PNA was negative... remember it being one of the only good periods last winter. My old place near Peace Arch had like 4 days of flurries.

 

I think you're right.  There may have been a week to 10 day period of negative PNA.

[Phil]

I think the month may have averaged positive, but I recall a pretty distinct period mid month where the PNA was negative... remember it being one of the only good periods last winter. My old place near Peace Arch had like 4 days of flurries.

The index went weakly negative for 5-6 days, but really that wasn't a real -PNA, just an extremely contracted, fully coupled polar vortex and corresponding flat height rises over the mid-latitude Pacific.

 

Obviously, a pattern like that won't ever do the trick in the lowlands.

[snow_wizard]

The winters of 2009/10, 2010/11, and 2012/13 were -NAO, while 2008/09 was neutral NAO. The winters of 2011-12, 2014/15, and 2015/16 were +NAO.

 

So much for Westerners who think negative NAO is bad for us.

[Phil]

For reference:

 

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/pna.timeseries.gif

[Phil]

So much for Westerners who think negative NAO is bad for us.

Oh, 1942-43 also had a massive -NAO in January. Second strongest -NAO of any Niña winter, actually.

 

The strongest -NAO winter of all time is 1959-60. Here's what it looked like, temperature wise:

 

http://i724.photobucket.com/albums/ww243/phillywillie/Mobile%20Uploads/9F624EB5-F4A1-4A02-81AF-9894D0543A01_zpsargshgwb.png

[westcoastexpat]

For reference:

 

 

These appear to be monthly averages though, no? The -PNA period in the approximately December 11-25 range isn't going to show up. Anyway, not a big deal, but hopefully we can get more persistent -PNA periods like the one last December in conjunction with a -AO... really need that -AO.

[Phil]

January 1943 @ 500mb. Block party, with -NAO dominance.

 

[stuffradio]

Hopefully this winter isn't an OR focused winter. I still want OR to get snow, but I hope the lowlands of SW BC and WA get snow too. My thinking is there will be many Fraser outflow events. The only thing I hate about those is the wind, because the wind tends to dry my skin and crack it open when it gets so dry and stagnant here. At that point, I then just wish for warm rain.

[Phil]

These appear to be monthly averages though, no? The -PNA period in the approximately December 11-25 range isn't going to show up. Anyway, not a big deal, but hopefully we can get more persistent -PNA periods like the one last December in conjunction with a -AO... really need that -AO.

That's the monthly average, yes. The daily averages are here if you're interested:

 

ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.pna.index.b500101.current.ascii

 

There was a weak -PNA stretch the lasted 5 days. Less than one standard deviation below average..basically a non signal relative to the EPO/NAM.

[westcoastexpat]

I think you're right.  There may have been a week to 10 day period of negative PNA.

 

Hi Jim, this seems to confirm our suspicions:

 

 

Definitely had a -PNA look for that period in December. 

[Phil]

That's not how the PNA is calculated. See the link in my previous post. No real -PNA to speak of during last December..just a behemoth GOA vortex and equatorward Pacific Jet.

[Phil]

Anyway, not a big deal, but hopefully we can get more persistent -PNA periods like the one last December in conjunction with a -AO... really need that -AO.

 

Yeah, I agree that we need some form of high latitude blocking this winter. I don't think it has to be a -AO, but a raging +AO/+EPO like last December won't work, obviously. Perhaps even a +EPO could work under a healthy -NAO/-PNA, neutral AO? Or a deep -EPO/-PNA/+NAO can work too, though that's more fragile I think, pattern wise...kinda hit & run, so to peak.

[stuffradio]

This last December was such a tease. I had so many rain to slush events, but nothing that accumulated... not even the slush!

[westcoastexpat]

That's not how the PNA is calculated. See the link in my previous post. No real -PNA to speak of during last December..just a behemoth GOA vortex and equatorward Pacific Jet.

 

Seems to me that there was indeed a -PNA period in December, as Jim and I had suspected... confirmed by both that link you provided and the resulting temp anomalies! Thanks for your help!

 

 

This last December was such a tease. I had so many rain to slush events, but nothing that accumulated... not even the slush!

 

I was able to get dustings in the mornings, but come late morning it'd melt away.

[stuffradio]

Can someone help me figure out what is needed for forecasting frost? Is it true that the Dewpoint has to be around 32, and the 2m has to be around 32 or below for frost to form?

[snow_wizard]

Hopefully this winter isn't an OR focused winter. I still want OR to get snow, but I hope the lowlands of SW BC and WA get snow too. My thinking is there will be many Fraser outflow events. The only thing I hate about those is the wind, because the wind tends to dry my skin and crack it open when it gets so dry and stagnant here. At that point, I then just wish for warm rain.

 

No way to know for sure, but I will say these things always even out.  Given that our chances are good for more northern areas to do well.

[snow_wizard]

Can someone help me figure out what is needed for forecasting frost? Is it true that the Dewpoint has to be around 32, and the 2m has to be around 32 or below for frost to form?

 

You can get frost with low temps more like 35 since colder air hugs the ground.  Going into a clear night I always like to see dps no higher than 40 or so.  You don't need a 32 dp going into night time because the dp will fall with the temp to some extent.  Then of course you want little if any wind.

[westcoastexpat]

Can someone help me figure out what is needed for forecasting frost? Is it true that the Dewpoint has to be around 32, and the 2m has to be around 32 or below for frost to form?

 

2 ways:

 

1) Temperature is cooled to dew point, dew is formed, and temps eventually reach 32 or lower as the night progresses, causing the dew to freeze.

2) Dew point is 32 or lower, and temperature is cooled to the dew point, creating frost.

 

Remember that radiative cooling on a clear, windless night is often underestimated by models.

 

2m temps can be above freezing... really just need that thin, interfacial layer of air hugging the surface (e.g. grass blades) to get cool enough.

[snow_wizard]

January 1943 @ 500mb. Block party, with -NAO dominance.

 

 

Two excellent Januaries here had strong minus NAO then.  Very interesting!

[Phil]

2 ways:

 

1) Temperature is cooled to dew point, dew is formed, and temps eventually reach 32 or lower as the night progresses, causing the dew to freeze.

2) Dew point is 32 or lower, and temperature is cooled to the dew point, creating frost.

 

Remember that radiative cooling on a clear, windless night is often underestimated by models.

Well, it's the surfaces accreting frost that cool below 32F, not necessarily the air itself.

 

That's why you don't usually see frost on cloudy nights (radiative exchange between the clouds/nucleated aerosols and the surface prevents the surface from cooling below the LTE threshold).

 

This is also why you'll often see frost on open fields but not under trees, thanks to backradiation preventing the ground from radiating efficiently.

 

If it were just the temperature cooling to the dewpoint below 32F, you'd have freezing fog, or just regular fog. There's a difference here. The dewpoint doesn't have to drop below 32F.

[westcoastexpat]

Well, it's the surfaces accreting frost that cool below 32F, not necessarily the air itself.

 

That's why you don't usually see frost on cloudy nights (radiative exchange between the clouds/nucleated aerosols and the surface). This is also why you'll often see frost on open fields but not under trees, thanks to backradiation preventing the ground from radiating efficiently.

 

If it were just the temperature cooling to the dewpoint below 32F, you'd have freezing fog, or just regular fog. There's a difference here.

 

Yes... certainly we can all agree on that... his question was about forecasting the frost, not how frost is created. 

[snow_wizard]

Nice little chilly period on the 0z later in week two.  534 thickness with 850s below zero.  Like seeing that!

[Phil]

Yes... certainly we can all agree on that... his question was about forecasting the frost, not how frost is created.

Well, the answer is no, the dewpoint doesn't have to be below 32F for frost to form. The frost-point is always higher than the dewpoint (stronger molecular bonding requires a higher temperature to complete a full break cycle at/below that threshold relative to surrounding H^2O molecules).

 

This is why aircraft often accumulates ice when flying through thunderheads.

[snow_wizard]

Nice end on the 0z.  Exactly what we want to see in October.

[snow_wizard]

Well, the answer is no, the dewpoint doesn't have to be below 32F for frost to form. The frost-point is always higher than the dewpoint (stronger molecular bonding requires a higher temperature to complete break cycle at/below that threshold).

 

This is why aircraft often accumulates ice when flying through thunderheads.

 

I think you're probably making this much more complicated than the answer he was looking for.

[Phil]

I think you're probably making this much more complicated than the answer he was looking for.

Haha, probably.

 

Here's a basic rundown on the issue if you're interested. Simple, no equations or difficult physics.

 

http://www.theweatherprediction.com/habyhints/347/

[Bryant]

Nice end on the 0z. Exactly what we want to see in October.

Timing seems right for that vortex to break up

[snow_wizard]

SEA is already down into the 40s tonight.  Now we're getting somewhere!

[westcoastexpat]

Well, the answer is no, the dewpoint doesn't have to be below 32F for frost to form. The frost-point is always higher than the dewpoint (stronger molecular bonding requires a higher temperature to complete a full break cycle at/below that threshold relative to surrounding H^2O molecules).

 

This is why aircraft often accumulates ice when flying through thunderheads.

 

Agreed, either the dew point (32 F or below) or frost point needs to be reached at the interfacial layer, but he's not going to find frost point from a model output unless he extrapolates using a skew-T. I'm also pretty sure stuffradio doesn't live on an aircraft, lol!   Always important to know your audience when answering a question!