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Displaying AFOS PIL: AFDDMX Received: 2018-01-03 05:40 UTC

FXUS63 KDMX 030540

Area Forecast Discussion
National Weather Service Des Moines IA
1140 PM CST Tue Jan 2 2018

.DISCUSSION.../Tonight through next Tuesday/ 
Issued at 354 PM CST Tue Jan 2 2018

GOES 16 shows the next PV anomaly/clipper moving through North 
Dakota and northern Minnesota southeastward this afternoon. Radar 
representation of this band has diminished since this morning and 
only a few ASOS/AWOS sites have recently reported light snow along 
with a couple of mPING reports. Many of the CAMs have been showing 
this narrow band of snow arriving around 5-6pm over far northern 
Iowa, 8-9pm for Des Moines and Waterloo areas, and closer to 10pm
for Ottumwa. Soundings show saturation within the dendritic 
growth zone (DGZ) as well as beneath the DGZ. While there will be 
light snow or flurries, the omega within the DGZ is near 0 or 
positive. Overall, the atmosphere is rather dry with precipitable 
water values peaking around 0.15". The QPF will be light and snow 
ratios will be in the upper teens/low 20s to one. Lagging behind 
the snow area will be a cold front, which will bring a quick 
reinforcing shot of cold air. Winds behind the front have been 
picking up this afternoon. Looking at upstream observations have 
sustained winds of 20-25 knots with a few gusts around 30 knots. 
Therefore, expect similar winds to develop later tonight behind 
the front with gusts over 30 mph over northern Iowa. This is 
likely to cause at least patchy blowing snow and have this 
mentioned over northern Iowa tonight into early tomorrow morning. 
At this point, it does not look like winds will reach wind 
advisory criteria, but something the evening and overnight shifts 
will need to monitor. There will also be a 6-8 hour window of wind
chills below -20F late tonight into tomorrow morning. However, 
with the recent prolonged period of wind chill headlines, decision
is to hold off on any headlines tonight.

After a not as frigid day today, temperatures will take a step 
backward by about 5 degrees tomorrow with cold air advection 
continuing as the cold front drops farther southward into Missouri, 
Illinois, and Kansas. A 1036mb high pressure will begin to drop 
southeastward over the southern Canadian prairies with an extension 
in the northern Plains. With high pressure fostering mainly clear 
skies, will have temperatures fall back into the teens below zero 
over much of the northern 2/3rds of Iowa with single digits below 
zero elsewhere. While winds will be under 10 knots, the combination 
will still create wind chill values below -20F. As stated above, 
confidence is high that there will be a period of advisory level 
wind chills, but wanted to give everyone a break of a wind chill 
headline. Therefore, either tonight or tomorrow we'll likely need to 
return a wind chill headline to a good portion of central Iowa.

As the high slowly moves eastward, yet another clipper will streak 
southeastward out of Canada toward our area Thursday night. While
Superblend provided some low end PoPs, it did not give any QPF. 
This was a result of timing and placement differences amongst the 
models. Therefore, used a blend of the NAM/GFS/WPC for QPF amounts
with a blend of NAM/GFS for PoPs Thursday night into Friday 
morning. ECMWF is not as strong with its forcing and is farther 
west compared to the GFS, which shows modest Q vector divergence 
in the low and mid-levels. While there looks to be a 1 to perhaps
2 inch accumulating snowfall, confidence is a little below normal
on where the highest totals will be.

The eastern US trough pattern will break down this weekend as the 
surface high moves into the Eastern US. As a shortwave trough moves 
out of the Pacific Northwest across the Rockies, will see the mid-
level north and northwesterly flow of the past week or so ease and 
have origins more of the Pacific than the Arctic. Surface winds will 
become from the south and eventually southwest this weekend, which 
will allow for a warmup. The shortwave trough along with the 
aforementioned surface winds will bring moisture back into the 
state. This will allow for precipitation on Sunday into Sunday 
evening with the best chance over southern Iowa. As temperatures 
climb above freezing into the middle 30s (+1 to perhaps +3C) on 
Sunday, will have to determine where the rain/snow transition line 
will set up. While colder air will follow this system early next 
week, at this point it does not to appears as bitterly cold as what 
has happened over the past week.


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1140 PM CST Tue Jan 2 2018

A swath of stratus clouds with scattered flurries/snow showers and
MVFR ceilings/vsbys will move across the area over the next
several hours. Thereafter, skies will largely clear out and VFR
conditions will prevail for the remainder of the TAF period. Have
taken best shot at timing the lowest conditions overnight and
timing of improvement, but amendments may be necessary.