Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201134

Area Forecast Discussion
National Weather Service Des Moines IA
634 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 12Z TAFS through 12Z Thursday morning/
Issued at 633 AM CDT Wed Sep 20 2017

Main concern today will be patchy MVFR areas prior to 15z...then
thunder aft 21z southeast into the night south of the I 80 corridor.
Front moving east now with all previous -tsra diminished. Winds
become more WNW today. As front stalls aft 21z some additional
-tsra possible near KOTM. Aft 00z increasing low level jet may aid
in additional thunder for KOTM mainly possibly as far north as
KDSM. Confidence on coverage limited so VCTS for now. /rev




LONG TERM...Ansorge
AVIATION...REV is the U.S. government's official web portal to all federal, state and local government web resources and services.