National Weather Service Raw Text Product

134 
FXUS63 KDMX 200828
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
328 AM CDT Wed Sep 20 2017

.SHORT TERM.../Today through Tonight/
Issued at 328 AM CDT Wed Sep 20 2017

Confidence: Medium to High

Boundary making progress into northwest Iowa early this morning. As 
expected...showers and thunderstorms continue to weaken as they move 
through the area...though there may be enough forcing to keep the 
activity going with lessening chance through 12z to the east. 
Today will feature the boundary sliding into the southeast portion
of the forecast area...then stalling...and retreating north 
overnight tonight. We are beginning the day quite warm over the 
south and afternoon highs should have little trouble recovering 
into the mid 70s over the northwest where cooler air will move 
into the region with mid to upper 80s in the south. Though mixing 
diminishes with lowering H850 wind fields this afternoon/early 
evening...H850 temperatures near the boundary remain in the upper 
teens which should support the warmer temperatures south. Some 
clouds...both mid and high level can be expected this morning and 
early afternoon with partly cloudy conditions later this 
afternoon. Model guidance is in good agreement today with the high
forecast. The Euro/GFS/NAM are in generally good agreement with 
the placement of the boundary from 00z to 12z Thursday 
morning...generally near or just south of the IA MO border during 
this time. With the boundary nearly stalled and return H850 flow 
overnight increasing to 20-25 kts into the boundary...showers and 
thunderstorms are expected over the south. There is some 
uncertainty as to the northerly extent...but for now have included
some slight PoP up to just south of the I80 corridor. Will let 
the day shift evaluate further given some uncertainty and more 
data available later today. Today's Day 1 outlook has the 
southeast in a marginal risk. Though instability is a bit more 
favorable this afternoon southeast than tonight...return flow and 
lift is more favorable tonight. Overall...the risk will remain low
with any development favoring mainly hail/wind. Winds 
aloft...already becoming more southerly will also support mild 
overnight temperatures in the south with cooler readings closer to
the ridge axis in the far north. Clouds south with partly cloudy 
to clear conditions north will also favor more temperature spread 
from north/south tonight.

.LONG TERM.../Thursday through Tuesday/
Issued at 328 AM CDT Wed Sep 20 2017

The long term weather pattern will feature a trough of low pressure 
extending from western Canada into the Northwest US that will become 
amplified as a 130 knot jet enters the base of the trough on 
Thursday. The ridge will also amplify over the eastern US while 
Tropical Storm Jose meanders off the Northeast US Coast. This will 
place Iowa in deep southwesterly flow through the weekend into early 
next week. The trough of low pressure and associated surface cold 
front will approach from the northern Plains ever so slowly this 
weekend into early next week as the pattern holds for Jose and 
current Hurricane Maria to move through the western Atlantic. 

As for the details, a cold front will have moved through the area
Wednesday night stalling near or south of the state before 
quickly lifting back northward over Iowa on Thursday. Within this
warm advection regime, some scattered showers and isolated 
thunderstorms will be possible as this lifts north of the state by
Thursday afternoon. Temperatures will surge upward as winds from 
the south deliver warmer and moister air with highs peaking well 
into the 80s. Have incorporated some of the warmer guidance into 
initial Superblend as 850mb temperatures peak around 21C. Some 
clouds and isolated showers/storms may be able to form in the 
moisture rich environment Thursday night into Friday morning with 
GFS/NAM cross sections show the best likelihood of this over the 
northwest part of the forecast area. 

For much of central Iowa, Friday through the first half of Sunday
will be dry with unseasonably warm conditions. As was done on 
Thursday, high temperatures Friday through Sunday were reached by 
blending in warmer guidance to account for 850mb temperatures 
around 20C with the persistent, deep southwest flow. A caveat was 
that for Sunday highs, they were manually adjusted over the 
western part of the forecast area as clouds will be increasing 
with the approaching cold front. Low temperatures were also 
increased generally using varying amounts of CONSRaw with
Superblend. As stated yesterday morning, for some locations 
record high minimums may be reached or exceeded. This is most 
likely Friday at least at a few locations, but is also possible 
Saturday. The aforementioned cold front may bring a few showers or
thunderstorms to the far northwest part of the forecast area 
Friday night through Saturday night, but overall model consensus 
is that most of the rainfall will be northwest of our forecast 
area. This will gradually change later on Sunday into Sunday night
as the cold front begins to enter the state with more clouds and 
showers and storms arriving. With the slow-moving front, leaned 
toward the GFS/ECMWF that have a similar timing. The CMC was 
discounted as it cuts off a low at 500mb, which is contrast to the
open wave of the other global models. Therefore, forecast has 
showers and thunderstorms in the forecast Monday into a good part 
of Tuesday before drier air filters in across all of the forecast 
area Tuesday night. Much cooler air will also arrive behind the
cold front with highs in the 60s and 70s on Tuesday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1157 PM CDT Tue Sep 19 2017

A few thunderstorms still possible over far northern Iowa tonight
and may pass near vicinity of KMCW. A storm may also be possible
in the KOTM area late in the period. Expect breezy winds to
persist the next several hours then diminish on Wednesday as high
pressure moves into NW IA. Localized MVFR cigs possible late
tonight into early Wednesday morning over the north.

&&

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

$$

SHORT TERM...REV
LONG TERM...Ansorge
AVIATION...Donavon