Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221101

Area Forecast Discussion
National Weather Service Des Moines IA
601 AM CDT Sun Sep 22 2019

.DISCUSSION.../Today through Saturday/
Issued at 245 AM CDT Sun Sep 22 2019

A line of strong storms that moved through much of the forecast
area overnight has nearly cleared the area to the east at this
time, however, a large area of light to moderate stratiform rain
remains upstream per regional radar imagery, with a few stronger
storms lingering back over Nebraska/Kansas. High-resolution
short-range models consistently propagate this rain east
northeastward through the remainder of this morning, consistent
with recent radar trends, leading to fairly hefty QPF over the
next 8-10 hours or so. However, it would appear that while good
additional rainfall totals are possible during that time frame,
they will be relatively long duration instead of the intense rates
we saw with the earlier convection overnight. Therefore, have
elected not to expand the flash flood watch at this time. The
greatest threat should remain across southern Iowa, while further
north toward the Des Moines and Waterloo metro areas, flash
flooding will be possible but at a lower probability and primarily
focused on urban areas. This threat will continue to be closely
monitored today.

Continued rain/storms and thick cloud cover will keep temperatures
down today, with much of the forecast area likely not climbing
out of the 60s. All in all it will be a cool but humid, gray and
wet day for much of Iowa, especially the southeastern half or so.
As a cool front just to our northwest finally surges eastward
across the region later today and this evening, it will scour out
the rain, clouds, and unseasonably high dewpoints that have
plagued us and bring a brief respite for the early part of the
week. In fact from Sunday night into Tuesday the weather will be
gorgeous for most outdoor interests, with dewpoints easing into
the 50s and high temperatures Monday afternoon in the mid to upper
70s beneath mostly sunny skies.

From Monday night through Tuesday the 500 mb flow overhead will
shift from northwesterly to more zonal, while at the surface
southerly flow will return bringing a rebound of temperatures and
especially dewpoints, with the latter returning to the mid 60s or
so by Tuesday afternoon. An even stronger 500 mb trough and
surface front will then cross the region on Tuesday night,
bringing a brief chance for showers and storms along the frontal
zone, followed by even cooler and drier weather on Wednesday into
Thursday. Depending on the timing of the frontal passage some
storms in our area could be strong to severe around Tuesday
evening, as we will have ample opportunity to destabilize during
the afternoon and the convergence and shear around the frontal
zone appear moderately impressive. After the hydrologic concerns
today, this potential for severe weather late Tuesday will
be the primary focus for potentially hazardous weather in the
next week. The good news is it will be short lived as the front
will surge quickly through, and by Wednesday will see high
temperatures only in the upper 60s to mid 70s across Iowa with
brisk northwesterly breezes. By Thursday morning temperatures will
bottom out in the mid 40s to lower 50s, easily the coolest
readings seen in recent weeks as we have been unseasonably warm
for some time now.

Looking into the extended forecast period, all signs point to a
return of unseasonably warm and humid weather from the end of the
coming week into the beginning of the following week. At 500 mb,
the trough responsible for the midweek cool front will swing
quickly eastward away from the region, while a series of northern
stream shortwaves coming ashore over the Pacific northwest coast
will quickly carve out a large western U.S. trough by Friday. From
then onward, through the foreseeable future, a blocking pattern
will set up with the persistent, large trough dominating the
western states and a large, hot high pressure system over the
southeastern U.S. slowly retrograding westward and dominating the
southern states through at least the early portion of the first
week of October. Within this steady pattern Iowa will reside
beneath persistent southwesterly flow aloft, supporting warm,
humid weather with intermittent chances for rain and storms
particularly as any shortwave impulses move through the
southwesterly flow. It appears summer is not yet quite over, and
may actually extend its grip into the first days of October.


.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 559 AM CDT Sun Sep 22 2019

Widespread showers and scattered thunderstorms will affect the
area through this morning, prompting prevailing MVFR conditions
with localized/transient IFR conditions as included in the
outgoing 12Z TAFs, along with tempo groups at FOD/DSM/OTM where
reduced vsbys/cigs are most likely. Should see gradual improvement
during the afternoon as SHRA/TSRA begins to move out, with VFR
conditions prevailing by later tonight and thereafter. Amendments
are likely this morning into the afternoon to reflect short-term
radar and observational trends.


Flash Flood Watch through this evening for IAZ074-075-083>086-



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