Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211808
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1255 PM CDT Thu Sep 21 2017

.SHORT TERM.../Today through Tonight/
Issued at 416 AM CDT Thu Sep 21 2017

The primary concerns will be convective trends early this morning
and possibly later today, and then temperatures as well.
Thunderstorms have continued to train across southeast IA overnight
along a nearly stationary frontal boundary and zone of strong 0-2km
moisture convergence. Moderate to strong 850mb moisture transport
has continued to feed this back building convection, and it has
moved little with the mean wind parallel to the front and
aforementioned moisture transport equal and opposite keeping the
upwind propagation vector nil in a zone of 1.5-1.8 precipitation
water, which is anomalously high for late September. Antecedent soil
moisture conditions are quite dry there, but with doppler radar
estimates now pushing six inches across Wapello Co and more
coming upstream felt a Flash Flood Warning was prudent.

Although they are likely underdeveloped, most convection allowing
models are handling general convective trends ok this morning and
suggest weakening over the next 3-4 hours with the typical diurnal
waning of the low level jet and associated convergence and warm
advection. A recent weak northward progression of the MCS may
indicate the complex is becoming somewhat more elevated and could
support this overall weakening trend as it taps lower effective
instability. The question will be whether redevelopment can occur
this afternoon, either surface based or elevated. GFS and NAM
soundings both depict little inhibition by 21z with moderate to high
instability, but there appears to be little convergence or focus and
surface based convection is difficult to develop on retreating warm
fronts. Confidence in these trends isn`t great, with the potential
to bust in either direction, but have included slight chances
northeast where the advancing boundary and theta-e advection aloft
would be during peak heating. The baroclinic zone should be north of
Iowa into the night leaving Iowa in the warm and humid, but precip
free warm sector. This advancing warm front will allow better mixing
today and temps may reach 90 in spots central and south with muggy
lows in the lower 70s tonight, possibly setting new record high mins
in spots.

.LONG TERM.../Friday through Wednesday/
Issued at 416 AM CDT Thu Sep 21 2017

Very warm weather will end the week and prevail into the weekend
before it starts to wane as a lethargic cold front approaches the
area through the weekend. The slowness of this front is due in
part to Tropical Storm Jose and Hurricane Marie that will be
working their way through the western Atlantic underneath an upper
level ridge over the eastern US. Meanwhile, the cold front and
associated high amplitude trough over the western US will not
quickly push east as the flow becomes parallel to the front.

High temperatures Friday through Sunday were generally adjusted
upward as broad southerly flow continues over the state bringing in
the warmer and moister air. For Friday and Saturday highs, mixed
down temperatures from 850mb and blended with Superblend, which
yielded highs a few degrees either side of 90. Sunday highs were more
a blend of the GFS and Superblend as the clouds begin to arrive over
part of the state ahead of the slow moving cold front. Friday
through Sunday lows were adjusted upward using CONSRaw as winds do
not drop off allowing the boundary layer to remain mixed. Lows in
the upper 60s to low 70s will be near or above record high minimums
at a few locations, especially Friday and perhaps Saturday. As
has been alluded to, it will be breezy and have incorporated
CONSMOS for winds Friday and Saturday. PoPs were trimmed back and
generally removed from our forecast area from Saturday afternoon
through Sunday afternoon. Previous runs of the GFS were moving the
rain into the northwest part of the forecast area by late
Saturday and reaching central Iowa by midday Sunday. Tonight`s 00z
GFS has caught on to what the ECMWF and CMC have shown in the
past of a very slow, very gradually arrival of the rain. Rain and
thunderstorms will start to enter the forecast area more so after
00z Monday reaching central Iowa during the day Monday. The rain
will last through much of the day Tuesday and end from west to
east across central Iowa by Tuesday night. Much cooler and drier
air will arrive on Tuesday behind the front and last through the
end of the period as the upper trough moves overhead.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1255 PM CDT Thu Sep 21 2017

Warm front to quickly lift through northern/northeastern TAF
locations through 00Z. Any MVFR cigs remaining should quickly become
VFR.  Brisk winds will occur through the period from the surface up
through 3 or 4Kft.  For now I took out the wind shear from the TAF`s
because winds will be strong from the surface up and pretty
unidirectional.  Not sure there will be a 20kt difference.  Thi s
will need to be monitored and may be put back in, in later
forecasts, especially KMCW and KALO.

&&

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

&&

$$

SHORT TERM...Small
LONG TERM...Ansorge
AVIATION...FAB



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