Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 241135
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
535 AM CST Mon Feb 24 2020

...Aviation Update...

.DISCUSSION.../Today through Sunday/
Issued at 409 AM CST Mon Feb 24 2020

Summary...

Quiet, warm, and pleasant conditions over the weekend are in the
rear view mirror and cooler/colder and wetter conditions will begin
to set in as a pair of waves move through the region and interact.
Light rain will primarily be seen today over southern and southeast
Iowa, before more widespread chances for snowfall as temperatures
cool and forcing increases. Greatest snowfall amounts continue to
push further east and south, roughly along a line from Ottumwa
northeast through the Quad Cities currently, though uncertainty
remains. Coldest temperatures of the week will be ushered in on
the heels of breezy northwest winds, dropping highs Wednesday and
Thursday in the 20s/low 30s with coldest lows Thursday and Friday
morning in the single digits to teens. Temperatures will rebound
through the weekend with the likely return of 40s/low 50s for many
by Sunday and Monday.

Monday through Wednesday...

The upcoming weather makers are currently along the Kansas/Oklahoma
border and dropping into Wyoming respectively, as nicely seen on WV
imagery. General model guidance trends continue to push heaviest
QPF/snowfall to the east as the surface low is now progged to move
NE through SE Missouri/S Illinois/S Indiana. While there is relative
consensus in track, the phasing with the northern stream wave
continues to handled differently by the flagship synoptic models and
their ensemble members as well as within the few hi-res windows
available. The 00z GFS and Euro continue to depict a Fujiwhara type
effect, which pulls/re-develops portions of the surface low further
west, though the GFS does so much further west than the current 00z
Euro, which puts C/N Illinois and N Indiana more in the cross hairs
than E IA. Additionally, the cutoff between light snowfall and
heaviest amounts continues to look as though it will be quite sharp,
especially with dry 850 to 700mb air impinging on the north/west
sides, but lack of confidence in location yields the too broad total
snowfall. Given the continued trends, have dropped heaviest
QPF/snowfall amounts over southeast Iowa further than initial NBM and
WPC guidance, but still worry that current heaviest amounts near
Ottumwa of 4 to nearly 6 inches remain too high.

Elsewhere and prior to the phasing, as the northern stream wave
moves across, light snow remains likely back into central and
northern Iowa. The bulk of the forcing across C/N/W Iowa will be
located above 700 mb, within the DGZ. The problem though is the
aforementioned 850 to 700 mb dry air being brought in from broad
anti-cyclonic flow to the north. It will take time for the column to
saturate and snowfall will at least initially be provided via seeder-
feeder with shallow low level moisture present. Overall do not
expect much in the way of accumulations in these area, yielding
around an inch in C/W IA and less as you move N into more prevalent
dry air and weaker upper level forcing.

All combined, highest impacts (mainly travel) are expected over
southeast portions of the forecast area where the greatest potential
for accumulating snowfall exists. With the eastward trends
continuing, have trimmed off a number of counties from the watch
leaving only far E-C/SE counties. Discussion with DVN yielded the
decision to leave as a watch and allow the day shift one more model
cycle to help determine best path forward.

As the eventually paired systems exit, pressure rises on the back
side will be enough to yield N/NW winds possibly gusting to around
30 kts, but look to remain sub advisory. Blowing snow may be a
concern in areas that receive multiple inches of snow, but have
kept blowing snow out of the forecast given expectation of
strongest winds over areas to receive around an inch or less.
Regardless, those winds will help usher in colder conditions and
a brief return to winter temperatures Wednesday into Friday.

Thursday through Sunday...

Cold conditions will continue into Friday with northwest steering
flow dominating, yielding highs in the 20s to low 30s and lows in
the single digits to teens. While the GFS and Euro depict fast
moving shortwaves at times Thursday and Friday, lack of consensus
and moisture starved nature yield little to no confidence an
worthwhile changes to currently dry forecast. Should some QPF be
squeezed out by any of the passing waves, flurries appear the most
likely outcome.

Upper level ridging, southerly surface winds, and general
temperature moderation return through the weekend. Highs by Sunday
and early next week slated to return to the upper 40s to low 50s for
many.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 530 AM CST Mon Feb 24 2020

VFR will primarily prevail thorugh the TAF period at all sites
besides KOTM. Towards the end of the period and especially just
beyond, MVFR and lower ceilings and chances of -SN creep into
remaining sites. At KOTM, bulk of -RA will remain south today
before -SN spreading north overnight. Additionally KOTM ceilings
will drop into MVFR/IFR ranges between 00z to 06z and beyond.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late tonight through late Tuesday night
for IAZ062-075-085-086-096-097.

&&

$$

DISCUSSION...Curtis
AVIATION...Curtis



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