Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220844
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
344 AM CDT Fri Sep 22 2017

.SHORT TERM.../Today through Tonight/
Issued at 343 AM CDT Fri Sep 22 2017

Overall, little change in the weather is anticipated through the
period with the primary concern temperature trends as our high
amplitude pattern remains in place. Iowa will stay between the
western CONUS long wave trough and Upper OH Valley high, which may
even nudge a bit closer by tonight. This will keep us thoroughly in
the warm sector with anomalously warm and humid conditions. There
was little decoupling overnight and 07z temps were anywhere from 2
to 26 degrees warmer than this time last night south to north. The
airmass is still quite unstable at the present time with MLCAPEs
2000-3500 j/kg at 07z, but inhibition and the lack of convergence
will keep that from being realized. The southern fringe of strong
warm/theta-e advection currently driving clusters of Dakotas/MN
storms is trying to produce convection this far south with isolated
high based convection in NW Iowa. IR satellite trends suggest the
best potential and vertical development will remain west of our
forecast area, but have added a brief period of isolated convection
for NW.

Have nudged temps up to, or just exceeding the high end of raw and
MOS guidance for highs today with favorable wind aided warming and
similar thinking for lows tonight. Considering where we have started
will likely also see new high min temp records today. With afternoon
highs 90+ and dewpoints hovering close to 70F, peak heat indices
will be well through the 90s possibly approaching 100F in spots.
Thus will issue an SPS to increase awareness of this late season
heat episode. especially considering this evening`s football games.

.LONG TERM.../Saturday through Thursday/
Issued at 343 AM CDT Fri Sep 22 2017

By Saturday morning, the longwave, high amplitude, slightly positive
tilted trough of low pressure will be over the western US while a
ridge of high pressure continues to build over the eastern US. What
remains of current Tropical Storm Lee and Hurricane Maria will also
be working through the western Atlantic, which will slow the
progression of the trough. As has been the forecast the last few
days, the trough will be slow to move eastward and thus so will the
associated cold front, which at times will stall with the flow aloft
being nearly parallel to the boundary. With a breezy southerly flow
ahead of this boundary, it will remain unusually warm and let`s just
say muggy for the latter part of September. High temperatures were
adjusted slightly upward from Superblend for Saturday, which yielded
highs around 90 degrees. Current forecast highs would be near record
values in some cases. Lows Sunday morning were primarily CONSRaw,
which would place them near record high minimum values. However,
concern is that temperatures may drop below records before turning
the calendar to the next date. Beyond record highs, of more
concern is the prolonged, late season heat that is ongoing
presently (it is 78 at our office as I type this) and will
continue really through Sunday. Heat index values on Saturday will
generally be in the low to middle 90s with values in the upper
80s to low 90s on Sunday. The Heat Health System website shows
moist tropical double plus Friday through Sunday at both Waterloo
and Des Moines, which would be consideration for an advisory or
warning, respectively. This is likely due to the late seasonality
of this event. Since the heat index values fall below NWS heat
advisory/warning criteria, will highlight the late season heat
primarily on our webpage and on social media given the number of
outdoor events and activities through the weekend.

Rain chances were once again pushed back until later Sunday entering
the northwest forecast area and eventually spreading southeastward
through the day Monday. The best shot for rainfall will be with the
convergence around the cold front Monday night into Tuesday as it
slowly moves through central Iowa. The severe threat looks low
overall at this time with deep layer shear generally under 30
knots and MUCAPE less than 1000 J/kg with the exception of around
midday Monday where it reaches around or just above 1000 J/kg.
More impressive are the precipitable water values for this time of
year, which average around 1.5 inches through the event. However,
the heavier rain axis looks to set up northwest of central Iowa.
While the CMC holds onto precipitation after 12z Wednesday, have
sided with the drier GFS/ECMWF given the timing of the frontal
passage. Much cooler and drier air will follow behind the cold
front Tuesday and especially Wednesday through the end of the week
as a zone of high pressure builds out from the Rockies onto the
Plains.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1148 PM CDT Thu Sep 21 2017

VFR conditions will continue through the forecast period. The wind
will become gusty again on Friday before diminishing by the
evening. Low level wind shear is possible through 12z.

&&

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

$$

SHORT TERM...Small
LONG TERM...Ansorge
AVIATION...Donavon



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