Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202342
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
542 PM CST Tue Nov 20 2018

...Aviation Update...

.SHORT TERM.../Tonight through Thanksgiving Day/
Issued at 333 PM CST Tue Nov 20 2018

It will be relatively benign for the start of the holiday travel
period.  Currently, stratus shifting across Iowa marks the leading
edge of warm advection across the state.  Rapid clearing is taking
place behind the stratus and there is a notable change in wind speed
and temps.  Western Iowa is already seeing temps in near 40 early
this afternoon while over the east, temps are still in the mid 20`s
to lower 30`s.  For the rest of the afternoon into the evening, the
warm air will act to melt remaining snow cover and moderate temps
but this will be limited by a cold front expected to pass through
the state overnight.  This will bring a wind shift, colder air and
more stratus for at least northern Iowa.

By 18Z Wednesday the cold front will exit the state to the south as
a Canadian high pressure system slides into the Great Lakes.  Any
clouds from tonight should quickly dissipate in the morning.
Soundings show a decent warm nose aloft and we should m and with ix
to at least 880mb.  This won`t tap much of the warm air aloft but
enough for lower to mid 40s for highs...aided by sunshine in the
afternoon.

.LONG TERM.../Thursday night through Tuesday/
Issued at 333 PM CST Tue Nov 20 2018

Thursday night and Friday an upper trough sweeps through the region
and soundings have really been consistent in some very deep moisture
advecting up ahead of this.  The good news is that temps will be warm
enough for much of the precip to fall as rain.  There is a brief
period early Friday morning across the northeast where thermal
profiles suggest a wintery mix for a couple hours.  This will not
amount to much...if it in fact even occurs.

By Friday evening that trough pushes east and we precip ends, though
cloud cover may remain.  We will still be very warm Friday
night/Saturday.  We will be in weak ridging to zonal flow with
clearing skies.  This may be the warmest day of the Holiday travel
weekend as we may see 50`s.

It will be short lived however as a digging trough is pushing out of
the Rockies with a surface low developing in the central/southern
Plains. By Saturday night moisture and forcing begin to spread into
the northwest and what is somewhat in question is how much cold air
initially comes down with this.  That could mean the difference
between rain, rain/snow or all snow.  For now kept the model blend
solution of starting out with rain then transitioning to snow.

Differences between the Euro and GFS and narrowing and both
solutions now develop a closed upper low around Kansas City by
Sunday morning but the surface low is somewhat in question.  The GFS
is a skosh further north and east with the surface low and with time
deepens the upper trough more which slows the progression of the
surface low.  The Euro is a little quicker and warmer though it does
not appear that it will be warm enough to prevent a changeover to
snow...even across southern Iowa. Confidence is growing in the model
solution but still not confident enough given it is day 5 to mention
anything more than the potential for snow and strong wind.  If the
GFS...which didn`t show anything much 24 hours ago verifies, it
could be a major winter storm for the area. Will definitely bear
watching.

Beyond Sunday...colder air dives back into the region behind this
system and highs drop back into the 20`s and 30`s with lows in the
single digits and teens.

&&

.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 533 PM CST Tue Nov 20 2018

Only concern within the TAF period will be the stratus deck
moving in this evening/overnight across the northern TAF sites,
KMCW/KALO/KFOD. A few guidance members attempt to bring in brief
IFR, but limited confidence at the this time. Winds will shift
clockwise throughout the period, eventually prevailing out of the
east as high pressure passes to the north of the area, but will
remain under 10kts in most cases.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...FAB
LONG TERM...FAB
AVIATION...Curtis


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