Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202114

Area Forecast Discussion
National Weather Service Des Moines IA
314 PM CST Tue Feb 20 2018

.SHORT TERM.../Tonight through Wednesday/
Issued at 312 PM CST Tue Feb 20 2018

There will be little of concern into tomorrow outside of minor
precip  chances through the early evening hours. The long wave
pattern will not change much tonight, but the widespread mid/high
level moisture fetch in the SW flow from TX into the Great Lakes
will finally exit by 12z. Much of the precip associated with this
has exited our forecast area however. While right entrance region
divergence SW of the Upper MS Valley jet max is fairly strong and
coupled with kinematic forcing of several weak ripples riding the
flow and H85/H7 frontogenesis, any lift is being offset by 1-3km
cold advection and limited moisture depth. Thus expect only the far
SE corner of the forecast area to be brushed by a brief wintry mix
before ending shortly.

Farther below these mechanisms to the north and west, some flurries
are being generated in shallow mixing, very weak instability and
cold advection. There is very shallow and weak convection barely
showing up on radar, even in VCP 31 long pulse, which is capped just
above 700m where the inversion starts. Thus have included scattered
flurries through 00z until the cold/dry advection dissipates the
stratus sufficiently. Guidance seems to have a decent handle on
temps so have followed a blend for lows tonight and highs Wednesday.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 312 PM CST Tue Feb 20 2018

The next system to impact Iowa will move in beginning Wednesday
evening.  At the surface, high pressure will shift off to the east
northeast bringing winds around to the east and eventually east
southeast.  Aloft, a deep trough will begin to lift out of the
southwest U.S. but ahead of that feature, a compact shortwave will
ride up the flow across Iowa from southwest to northeast.  Forecast
soundings show some quick saturation Wednesday evening with some
theta-e advection lifting across the state during this time.  Light
snow should develop in the early evening and spread from southwest
Iowa northward overnight.  Portions of southwest Iowa...from Des
Moines to Red Oak will see around an inch to an inch and a half of
snow from this with lighter amounts north and east.

During the day Thursday, ptype gets interesting as warm air noses in
aloft while surface temps are near to below freezing...except for
the southeast.  Precip will transition from snow to freezing rain
across the south, spreading northward through the day then in the
afternoon, a brief period of rain will occur across the far south to
east central forecast area before transitioning back to freezing
rain in the evening.  Further adding to the hodge podge of ptypes,
soundings show periods where we lose our ice introduction and have
some low level dry layers which would give us areas of freezing
drizzle.  For now I did not try to split hairs as to when there
would be freezing rain vs freezing drizzle but this level of detail
will be added in the next forecast package or so.  So the ice
accrual in our grids is likely a bit to high but still very
reasonable given the extensive period of freezing precip.

There will be a brief respite from the weather on Friday but Friday
night and Saturday the aforementioned upper trough bodily lifts up
over the Upper Midwest bringing another shot of forcing across Iowa
late Friday night and Saturday.  For now I kept the ptype rain or
snow but there will likely be some sort of wintery mix south.  Right
now there is enough variability in the models with respect to far
much warm air will nose into the south to keep confidence low on
ptype and where that might happen.

Once that system pushes off early Sunday morning quieter weather is
in store for the forecast area for the remainder of the long term


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1202 PM CST Tue Feb 20 2018

A wintry mix of precipitation may affect KOTM early this
afternoon, possibly down to IFR briefly, but otherwise dry
conditions are anticipated. Elsewhere stratus will linger with
widespread MVFR ceilings, but they should improve to VFR NW to SE
toward sunset with medium confidence in VFR into Wed.




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