Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211458

Area Forecast Discussion
National Weather Service Des Moines IA
958 AM CDT Tue Mar 21 2017

Issued at 958 AM CDT Tue Mar 21 2017

Band of snow moving across west central IA into central IA
bringing a brief period of snow to the area this morning. Have
updated to reflect the latest radar trends and increase
pops/coverage area of snow mention. Expect the precip to
transition to rain through mid-day as it moves southward. However
prior to the transition, some light snow accumulations will be
possible with the band with up to a half inch to an inch in areas
where the heaviest snow is occurring likely across far west
central IA. Visibilities are dropping to around a half mile to 3/4
mile in the snow.


.SHORT TERM.../Today through Tonight/
Issued at 324 AM CDT Tue Mar 21 2017

The primary concern will be precip trends today with conditions
immediately upstream unimpressive as of yet. Right entrance region
divergence associated with the northern Plains/Great Lakes jet
segment is aiding mid/upper level lift through NE and SD with other
DPVA contributions from weak short waves embedded in the flow,
including the strongest near the WY/MT border. Thermodynamically
there is weak, high based warm advection which is expected to also
help lower ceilings and increase precip into the morning as it
reaches west central IA, which is supported by many high resolution
model simulated reflectivities. The precip will likely not expand
appreciably into IA until the main kinematic forcing associated with
the WY/MT wave approaches the MO River. This will be nearly
coincident with increasing 1-3km subsidence however as the H85/H7
ridge, anti-cyclonic flow and resultant cold advection strengthens
into IA starting at 18z. With the aforementioned short wave already
into IA by this time, the high res model depiction of quickly
weakening precip makes sense. Thus have higher PoPs west by late
morning, but weakening SE by afternoon. Any QPF amounts look fairly
light regardless.

Any remaining precip should be gone by 00z with fair weather heading
into tonight. Mid/high clouds may linger SW associated with the MO
Valley baroclinic zone as warm advection develops upstream and
begins to return into the next period. There may very well be decent
low temp gradient SW-NE with mostly clear skies and light winds NE
so have dipped mins there to 20 or less.

.LONG TERM.../Wednesday through Monday/
Issued at 324 AM CDT Tue Mar 21 2017

The long term will be an active period with frequent periods of
precipitation. For Wednesday there will be some weak energy
topping an upper ridge and coincident with warm advection taking
place over Western Iowa. With cold air still in place we should
see some light snow across the West in the early morning which
will mix with then change over to rain in the late morning. A
short wave will then top the ridge and and push the precip off to
the Northeast.

A much stronger trough will dig into the Southwest U.S. as a
surface low develops over the Plains. A warm front will extend
from the low into the Upper Midwest or Missouri. There will be a
lot of lift on Thursday and precip should blossom over the state
through the morning. Forecast soundings indicate that ptype may be
mixed or a brief period of very light freezing rain. Any freezing
precip would not last more than a couple hours before changing
over to rain.

This system will be come stacked with the surface low nearly
coincident with the upper low and thus move very slowly...keeping
precip chances going into the weekend. Friday to warm front will
lift across Iowa bringing increased instability and some forcing.
This will be sufficient for thunderstorms from Thursday evening
through Friday evening. Some storms later on Friday could be

The system continues to shift East across Iowa on Saturday and if
you believe the GFS...Western Iowa could see nearly 2 inches of
rain between Friday and Saturday. We will have a short break from
the precip on Sunday but Sunday night through Monday another low
similar to the previous system moves across the Plains bringing
more precip to the region. This system will be more progressive
and thus move through faster.

Another system similar to the past two then arrives for Tuesday
into Wednesday. Temps are much warmer with the GFS and near normal
with the Euro. At this point given the uncertainty of the track of
the lows, I went with persistence for temps.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1151 PM CDT Mon Mar 20 2017

Generally VFR conditions for all locations during the forecast
period. However, a narrow ribbon of MVFR ceilings and
visibilities will be possible from west central into southeast
Iowa with a narrow band of forcing associated with an upper
system and may affect KDSM and KOTM. Winds will remain northerly
for the entire forecast period.





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