773 
FXUS63 KDMX 112204
AFDDMX

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. 

Extended: 

Northwest flow will keep conditions cooler. There will be active 
flow north and south of Iowa, but no features of note move through, 
keeping the extended forecast dry.  

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1125 AM CST Fri Dec 11 2020

Poor aviation conditions across central Iowa as winter storm
system moves through the region. Overcast skies will remain in
place for most of the period, with long period of IFR conditions.
Snow will be expected at most terminals, MCW being the only
terminal to miss out on accumulating snowfall. Snowfall rates
overnight may send visibility down to 1 mile. The winds will be
gusty at times up to 25 kts, but are not expected to be a major
issue. Conditions will slowly improve by the middle of Saturday
afternoon. 


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM CST Saturday for IAZ028-
037>039-045>050-057>062-070>075-081>085-092>094.

&&

$$

DISCUSSION...Krull
AVIATION...Krull