December 11-13th MW/GL Snowstorm

[OmahaSnowFan]

What’s cool is that all the hi res models have consistently been showing that small bullseye on the north side of Omaha (me) and the county right north of me

[Grizzcoat]

Good AFD by DMX--- they too mention a later change than what CAM's are showing. But overall- a good setup for many. Also mention enhanced totals in NE IA for those peeps...

Area Forecast Discussion
National Weather Service Des Moines IA
404 PM CST Fri Dec 11 2020

.DISCUSSION.../Tonight through Friday/
Issued at 404 PM CST Fri Dec 11 2020

Synoptic and Meso Features:

Main focus will be on the deformation zone developing over central
Iowa this afternoon and evening with accumulating precipitation.
Strong short-wave trough has developed negative tilt, with the axis
extending from the Colorado Front Range to the Texas Gulf Coast. The
strong CVA associated with the jet streak with this short-wave is
phasing well with the surface cyclone, and the cyclone has continued
to deepen this afternoon. The surface cyclone center will stay south
of Iowa, but is on a northeast track as the jet stream begins to
round the base. Within the deformation zone, there is quite a bit to
look at. Analyzing frontogenesis fields, both synoptic scale models
and CAMs show a strong ribbon that starts near Atlantic and moves
ESE through the DSM Metro into east-central Iowa. This is largely
concentrated between the H85 and H7 levels, though there are hints
of more forcing around 925mb as well. This will move through between
now and 12z Saturday. After 6z another area of enhanced
frontogenesis develops near CID, but most of this becomes focused
east of the forecast area, but will result in heavier snow for
portions of eastern and northeast Iowa through Saturday Morning.
These same areas are also characterized by moderate to strong Q-
vector convergence, which makes sense with strong jet streak and
associated kinematic reactions. Delving into model soundings, cloud-
ice does not appear to be hard to come by for counties in Iowa
mainly south of U.S. Hwy 20 to around  U.S. Hwy. 34. With ice
introduction, this leaves rain, rain-snow mix, or pure snow the
viable precipitation type for this event. There may be a brief
period of other wintry mixes, but overnight will transition to all
snow. Temperatures will largely drive rain vs. snow this evening.
Temperatures late this afternoon will remain near 35-36F, but should
see some amount of cooling within the precipitation shield heading
into the evening. By 06z, with temperatures between 32-34F and
dewpoints closer to 30-31F, wetbulb temperatures should be near 32F
and thus is when snow is expected to take over. One quick note about
areas along and north of Hwy. 18, there does appear to be some
strong frontogenesis that is starting but cross-section and vertical
sounding analysis show a very dry layer above the surface, thus
northern Iowa will be missing out on most of this precipitation
event.


Snow Amounts and Model Guidance Comparison:

The bottom line up front, CAMs has been much more robust with
respect to total snow then some of the synoptic models. However,
both have the greatest snow accumulations along a line from near Red
Oak, IA (south of Council Bluffs) through Des Moines to
Marshalltown, with decreasing amounts then northwest of this line
and southeast of it. This area is highlighted by the current Winter
Weather Advisory. The differences, is the 12z HREF mean values have
4-6 inches within this band and localized pockets of 8-10 inches. In
the 12z GEFS members, greatest amounts were 2-4 inches, and do not
necessarily have the localized heavy amounts, but this is likely due
to the coarse resolution. In a brief discussion with WPC, it was
noted that several of the GEFS members were warm with respect to
temperatures, and as a result was turning it into liquid rain for an
extended period of time, while CAMs depicted a slightly quicker
change to snow. Given the strong frontogenesis, as well as strong
omega profiles through the DGZ, it is anticipated that there will be
enough forcing to overcome a lot of these challenges presented by
the warmer temperatures in the area of greatest accumulations.
Therefore, will have a snowfall forecast that lies in between the
HREF and GEFS members, but certainly could see some locally higher
amounts where frontogenesis is strong. The warmer temperatures and
melting are the reason to not go nearly as high as the 12z HREF
mean, but the frontogenesis and lift through the DGZ justifies being
higher than GEFS mean, and even the 12z deterministic GFS for that
matter. Other considerations with this will be snowfall rate. 12z
HREF was not overly robust with this, greatest 1 hr snowfall rates
only reach about half an inch per hour. With the high moisture
content, visibility could still drop to around 1 mile as the column
saturates. With wind gusts only forecast up to 25 MPH, blowing snow
is not expected to be a big issue.

[Grizzcoat]

I guess not really concerned as I was about this rain being shown as snow in few hours which is not happening. Never was supposed. The defo band is what counts. Good Luck to all the winter weather weanies!!!

[bud2380]

18z RDPS

 

[East Dubzz]

Nice to see snow again! Here is how it’s going so far..

[clintbeed1993]

I drove to my parents in Aurora, Nebraska to experience some snow since I've been missing it a ton. Glad I did. 4 inches down and probably a couple more to go this evening.

Now i have 2 locations to utilize for chasing snowstorms. Double your odds. Not a bad startegy

[james1976]

Nice write up from DMX. Good luck to you all. My backyard looks to get grazed but im up in St Paul so ill have to pop in to see how its going!

Defo band is setting up nicely.

[Hawkeye]

Bud, I don't expect any accumulation here til after midnight.  It's good that it's changing over to snow in Dubuque.  It has to change there before here.

[hlcater]

Most models put my backyard right on the wrong side of the cutoff. Quite a consensus for that, so it is probably correct. 

Gonna go with 1.5" in downtown IC and 4.5" in Hiawatha.

[bud2380]

I see some slush on my windshield 

[bud2380]

I have to head to anamosa and heard it’s changing to snow there already 

[hlcater]

Mixing altitude is about 1200' AGL along I80.

[Stormhunter87]

Saw some flakes heading over to western Iowa but no full change yet

[gabel23]

View of my back yard. Shocked to say sitting at 4” of snow and hope to pick up another 2-4”! The Nam, euro, ukie and gdps model nailed that for me! 

[Hawkeye]

Holy cow, it's snowing here!

[BMT]

Snowing here in Marion!

[Bellona]

It's changed in Waterloo, but weenie flakes so far. 

[bud2380]

Snowing in spring villle 

[bud2380]

Snowing very hard 

[bud2380]

Down to 32°

[Stormhunter87]

Still just sleet here

[Hawkeye]

It's snowing lightly here, but still 35º, so it's more like white rain.  About 15 miles north it's 33º.

[jcwxguy]

Oax

[OmahaSnowFan]

Still just a mix of crap falling. 33.4 degrees now.

Guess it’s going to have to fall below 32 before it changes over

[Hawkeye]

The snow is picking up a bit here, temp down to 34º.  It's still melting on contact.

[Hawkeye]

Big flakes are pouring down now and it's beginning to stick.  I'm impressed by how quickly the changeover crashed south after sunset.  It appears we'll get a couple hours of snow followed by a lull for a few hours.

[bud2380]

I’m temporarily in Anamosa for the next hour or so and the flakes here are huge. Sticking to the grass. And the gravel roads. 

[bud2380]

These are the biggest snowflakes I think I’ve ever seen. 

[Hawkeye]

The temp here is down to 33º.

First snow accum report of the event.

At 6:00 PM CST, 1 NW Dubuque Regional A [Dubuque Co, IA] OFFICIAL NWS OBS reports SNOW of 1.10 INCH. 6 HOUR TOTAL.

 

 

[hlcater]

Snow in Iowa City. Temp 35.2

[Stormhunter87]

Been inside with some family so haven't been out but definitely have snow on the ground. Air temp and winds have dropped the temperature.

[Stormhunter87]

Man that radar makes it appear like this band is just crawling.

[Grizzcoat]

[bud2380]

23z HRRR with 6” for Iowa city and CR. 9” if you use 10:1 ratio 

[Hawkeye]

Heavier returns have moved back over CR on radar, but so far it's light snow with a bit of sleet mixed in.  There has been no additional accumulation over the last hour.  There is still only a bit of white on the grass.

[East Dubzz]

14 minutes ago, bud2380 said:

23z HRRR with 6” for Iowa city and CR. 9” if you use 10:1 ratio 

Dubuque?

[Grizzcoat]

Change over W side of DSM-- nothing accumulating but done with the cold miserable rain.

Really like where I stt for main event.  Several runs in a row of something similar to this and HRRR is getting into better range.

 

[Grizzcoat]

Hydrometeror classification radar from COD has been spot on tonight.

https://weather.cod.edu/satrad/nexrad/index.php?parms=DMX-HHC-1-12-100-usa-rad

[Grizzcoat]

00Z RAP-- lining up well with HRRR for C.IA

[Bellona]

Starting to whiten up a bit in Waterloo. Snowing at a decent rate.