National Weather Service Raw Text Product

477 
FXUS63 KDMX 211219
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
719 AM 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 12Z TAFS through 12Z Friday morning/
Issued at 718 AM CDT Thu Sep 21 2017

The primary concerns will be convection and stratus trends this 
morning and then LLWS potential a lesser concern late. Much of the
storms should be VFR so TAFs remain as such due to limited 
confidence in timing MVFR or less occurrences. Farther north 
patchy IFR or MVFR stratus is occurring on the periphery of the 
convection but should be relatively brief and dissipate later this
morning. There is fairly high confidence in VFR conditions 
through the remainder of the period. LLWS may develop in the 
strengthening southerly flow overnight and has been mentioned at 
higher confidence locations /KMCW/.

&&

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

&&

$$

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