Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182333

Area Forecast Discussion
National Weather Service Des Moines IA
633 PM CDT Tue Sep 18 2018

.DISCUSSION.../Tonight through Tuesday/
Issued at 414 PM CDT Tue Sep 18 2018

At 20z Tue, a somewhat broad, stretched more northerly-southerly,
area of anticyclonic flow was centered over eastern Oklahoma.
Meanwhile height falls were noted across the western Plains as low
pressure has begun to try to establish itself off the Rockies.
With this synoptic setup, moisture was being transported from the
Gulf of Mexico through West Texas and into Kansas. As this
anticyclone propagates eastward the next couple of days, low-
level moisture transport will make it into Iowa.

Mesoscale has dominated as an outflow boundary went pushed southward
all the way from northern Iowa into Missouri. Behind this boundary,
clouds kept temperatures down... confining highs to the 60s... where
southern Iowa made it into the low 90s before the boundary arrived
there. The RAP has had the best handle on the progression of this
boundary, so have leaned towards it much of the day.

For this evening into tonight...
PWAT values continue to push 2 inches across most of Iowa through
the next couple of days. A Theta- e ridge has begun to build over
eastern Nebraska, with the ribbon cutting right through western
Iowa. A 30 KT 850mb LLJ pokes its way into NW Iowa after 06z Wed.
In short, severe threat will diminish to nil overnight as MUCAPE
values drop to around 500 J/KG. Main focus is on flooding
potential. Northern/NW Iowa has had generally 1 to 2 inches of
rain the past 48 hours, with some locations closer to Mason City
approaching 3 to 4 inches this morning alone. The GFS is excited
about orienting a band of E-W oriented 925mb frontogenetical
forcing to serve as a focusing mechanism for where reinforcing
precip could set up and dump itself. At this time, think NW Iowa
is the most likely location for best phasing. Again, if this
shifts eastward closer to I-35 and especially Mason City, would
need to be quick on trigger for Flash Flooding headlines.

May be somewhat a repeat of today. Will be watching for
an outflow boundary to kick off the overnight convection and sag
southward. The difference in the setup tomorrow is that as the LLJ
cuts into Iowa, this boundary will not drop all the way into
Missouri. Rather, it will take a slightly more NW-SE orientation.
There will be a remarkable temperature difference north/south of the
boundary. Highs south of the boundary will be into the 90s, highs
north of the boundary may struggle to make it into the 70s again.
Heat index values south of the boundary will likely be in the low to
upper 90s... opening the door for possible heat-related issues in
southern Iowa.

Instability south of the boundary very high as MUCAPE values
approach 3000 J/KG. Low-level lapse rates also very steep, so hail
would be a primary threat, along with damaging winds. Like today,
thunderstorms may produce periods of heavy rain.

Thursday into the weekend...
All eyes shift to the cold fropa as a sfc low finally gets its
act together and pushes through the upper midwest. The past
several 12z/00z model runs have trended to slow the progression of
the cold front. This is notable as the timing of the fropa may
now not be until 03z Fri-09z Fri at the earliest... which would
put us out of a lot of the daytime heating convection.
Thunderstorms are very likely with this fropa, including severe
weather. With the focus on the near-term... the Thursday fropa
will be detailed in higher resolution over time.

Seasonable temperatures return to Iowa this weekend as conditions
should dry out through Saturday.


.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 633 PM CDT Tue Sep 18 2018

Main challenge will be onset of next round of convection and
duration through Wednesday. Last area of -shra/-tsra moving
southeast sections of Iowa now and break from storms until 05-07z
expected when storms begin to refire over northwest Iowa.
Meanwhile cigs will begin to lower northwest and along with storms
moving east southeast affecting KFOD/KMCW/KALO and to a lesser
extent KDSM/KOTM from 07z north sites to near 15z southeast sites.
Another break/lessening of activity may follow until end of
period. Will need to evaluate meso trends with each package./rev




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