Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171056
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
556 AM CDT Tue Sep 17 2019

...Updated for 12z Aviation Discussion...

.DISCUSSION.../Today through Monday/
Issued at 416 AM CDT Tue Sep 17 2019

*/ Highlights...

 -Several rounds of showers and thunderstorms possible over the next
several days.
 -Heaviest rainfall may be in central to northern Iowa, with a non-
zero chance of flooding issues
 -Well above-normal temperatures and humid conditions to continue
through this week

*/ Summary...

Another hot and humid day is in store for central
Iowa. Expect temperatures for today to be similar to where they
were yesterday, topping out well into the 80s. With dewpoints
expected to remain in the low 70s, heat index values will likely
push the low 90s once again. Spotty showers and thunderstorms may
develop in east-central Iowa. Though these storms are not expected
to be long-lasting, a few of these storms may be strong.

For tonight, moderate rainfall is possible primarily across
north- central to northwestern Iowa- where rainfall amounts may be
in the one-quarter to one-half an inch range...with locally
heavier amounts possible.

Another shot of moderate to heavy rainfall is possible Wednesday
night into Thursday morning. The potential rainfall amounts in this
round of precip is worth monitoring closely, as central to northern
Iowa may be impacted. Should the moderate rain forecast for tomorrow
night pan out, the grounds may be well-saturated in advance of this
next round of rainfall... introducing the non-zero chance of
flooding.

Yet another slug of showers and storms are forecast sometime near
Saturday that could bring widespread moderate rainfall totals.
Above- normal temperatures and humid conditions should continue
through Saturday, before a cool-down temporarily arrives for
Sunday.

*/ Technical Discussion...

Today...

With 12z Tue 850mb temps similar to 12z Mon 850mb temps, plus
scattered clouds expected, temperatures should be similar for
Tuesday to where they were yesterday. Introduced spotty showers and
storms to east-central Iowa. Instability will be sufficient for
convection. Weak focusing mechanism presented in low-levels with
slight sfc convergence, 925-950mb frontogenetical forcing, and
850mb moisture flux convergence. This should be sufficient to
punch through cap... trouble is the 0-6km shear profile is not the
most impressive as indicated by a tight hodograph profile.
Ultimately, expecting any storms to be more pulse-like in nature
versus the sustained strong/severe category.

Tonight...

08z Tue water vapor imagery shows a well-defined upper low spinning
near the Washington coastline. Models in good agreement showing this
upper low racing across the Rockies, making it into North Dakota by
12z Wed. In advance of this upper low, a weak impulse is picked up
from western Kansas through New Mexico. This impulse will be
dragged up into NW Iowa overnight. Low-level support is very
impressive as shown by an elongated 50kt LLJ in the NW corner of
the state. 0-6km mixing ratios shatter 8 g/kg as PWAT values reach
1.5 to 1.75 inches. 0-6km cross-section across the northern/NW
portion of Iowa shows RH values around 80 percent, with warm cloud
depths reaching 4km. So expecting mildly decent precip
efficiency.

Tomorrow night...

Another shortwave blasts through the Midwest and into NW Iowa
between 06 and 12z Thursday. Similar to tonight`s setup, decent
moisture available with overall mesoscale features similar to
tonight. Orientation of this precip round is much more extended in
areal coverage, stretching from Mason City through Creston.
Models, especially the 00z Tue ECMWF, may be too far north with
the core of this precip... so have begun to nudge higher QPF
amounts further south/SW. Big thing to watch with this round is
the potential for heavy precip to fall over already saturated
areas. Over the past 7 days, northern to north-central Iowa has
received around 600 percent of their normal precip. If rainfall
tonight exceeds going forecast, ground may not be able to take too
much more rain. So will monitor for flood potential.

Friday through Saturday...

A rather potent-looking upper low is being progged to come onshore
near the Oregon coastline and dip down through the Four corners
Region. Meanwhile, another shortwave is being progged to propagate
eastward across the northern CONUS. These two features more-or-less
Fujiwara over the upper Plains and eject towards Minnesota. The
impact of this would be yet another...potentially stronger... surge
of moisture being advected into Iowa. Long-range models are pushing
PWATs of 2 inches into Iowa. So will have to monitor this for
another moderate to heavy rainfall potential. Behind this, high
pressure should come in for Sunday and cool us down for a day or two.

&&

.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 553 AM CDT Tue Sep 17 2019

High confidence in winds in the 10 to 20 kt range today at all
sites except KOTM. Medium to high confidence in showers and
thunderstorms arriving in NW Iowa towards the end of this TAF
period. KFOD first to be impacted, followed by KMCW and KALO.
Future TAF updates will address this in better detail.

&&

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

$$

DISCUSSION...Kotenberg
AVIATION...Kotenberg


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