Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 262105

Area Forecast Discussion
National Weather Service Des Moines IA
405 PM CDT Wed Apr 26 2017

.SHORT TERM.../Tonight through Thursday/
Issued at 359 PM CDT Wed Apr 26 2017

The weather will turn quite active into the weekend and early next
week, but initially the concerns through tomorrow will be precip
trends southeast this evening and then frost potential southwest
into Thursday morning. The current situation has a deep PV
anomaly/short wave moving through TX with a large area of
kinematically forced lift moving through the MS Valley, including
some right entrance region effects from a small Upper MS Valley jet
segment. Much of the warm/theta-e advection and frontogenetical
forcing is south and east of Iowa, and should remain that way
tonight. The mid level mechanically forced lift may just brush far
southeast sections this evening with only chance PoPs.

Frost is a concern southwest overnight but there are questions with
regard to wind, temps and cloud cover. HiRes window ARW/NNM runs
suggest some clearing southwest half overnight, however the ESRL
HRRR, HRRR and RAP are both more aggressive and the current stratus
shield still extends well into NE. Winds may not drop below 8kts and
mins are even marginal with no lower than 34F expected. With these
inputs in low confidence have opted to not issue a headline for the
time being. The cloud trends would be the main concern and can be
further evaluated this evening if need be. Although IA will be
thoroughly embedded in the broad mean trough, a subtle short wave
ridge through the MO Valley may give us a brief glimpse of sun
Thursday before the forecast turns.

.LONG TERM.../Thursday night through Wednesday/
Issued at 359 PM CDT Wed Apr 26 2017

The overall pattern will have changed little heading into the later
periods with strong Pacific NW jet continuing to feed short waves
into the base of the mean trough. The next wave of forcing is
expected to lift through the Plains and MO Valley Thursday Night
with a strong baroclinic zone lifting south to north into IA. The
phasing of deep moisture and lift is not there all the time, but
should result in at least some precip into Friday. Have also
mentioned some frost north with temps at or below freezing and skies
mostly clear for a good deal of the night.

After a relative lull to start the weekend, the long wave trough
will deepen and amplify leading to strong MO Valley cyclogenesis
into IA. The ECMWF and GFS are in good agreement and fairly
impressive spreading deformation zone precip into NW with strong
thermodynamic forcing just to the south and east into central IA.
Both models depict tremendous theta-e advection and
frontogenetical forcing leading to likely PoPs into Sunday, even
at this time range. WPC Day 5 QPF forecasts suggest widespread
1-2" accumulations may occur through the weekend. Temperatures
will be right on the brink of being cool enough to support snow,
so there continues to be at least a mix wording into early Monday
morning. Timing as of now suggests the mid/upper level core of the
low will be over IA Sunday Night, which would be a more favored
time for snow rather the during nearly early May daytime heating.
Gusty winds will also need to be watched with a strong MSLP
gradient just ahead of and then behind the low. Confidence is
certainly not high, with a noted dry slot also rotating into IA,
but strong forcing and precip rates at night could lead to accums
and overwhelm any melting at night.

Some low end PoPs linger into Tue and Wed but forcing is somewhat
non-descript as another NW flow wave attempts to deepen the
central CONUS mean trough. Temperatures through the extended will
certainly not be warm with cool and wet the primary themes, even
beyond valid forecast period.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1238 PM CDT Wed Apr 26 2017

Large stratus shield extending well into central NE and SD. I expect
some erosion of the stratus through the day but this should fill in
some again tonight so MVFR/IFR conditions should persist over TAF
locations.  After 12Z improving conditions will be common. Surface
flow will be W to NW and remain at 10kts or above.  Even if we
clear, frost is not a big concern due to the wind however if we
clear some and winds drop then locations W of a KFOD to KDSM would
be at risk of some frost.  Confidence is low on both of these things
occurring. Some lgt precip possible this afternoon and late evening
at KOTM as an upper level feature pivots across the area lifting the
precip over MO across SE into Eastern IA.





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