Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210457

Area Forecast Discussion
National Weather Service Des Moines IA
1157 PM CDT Tue Jun 20 2017

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Wednesday/
Issued at 319 PM CDT Tue Jun 20 2017

A modest surface trough has progressed steadily southward across
Iowa today, and currently stretches from just south of Dubuque
over to roughly Des Moines and down to the southwest corner of the
state. Weak cold air advection behind this trough is resulting in
cooler temperatures across northern Iowa. Meanwhile scattered
showers and a few thunderstorms have been generated along the
boundary across far eastern Iowa this afternoon, between about
Iowa City/Cedar Rapids and Dubuque. Any additional convection this
afternoon/evening should stay mostly east of our forecast area,
but may just clip our southeast corner from about Grinnell down to
Centerville/Bloomfield. Any storms that do occur would be
isolated and non-severe and only slight chance POPs are included
in the forecast accordingly.

The surface trough will wash out across northern Missouri
tonight, with return flow quickly becoming established over the
plains of Kansas and Nebraska after midnight as a broad thermal
ridge builds in from the west. The warm air advection aloft
associated with the leading flank of this ridge should be
sufficient to generate additional showers and thunderstorms across
western Iowa and states to our north and west late tonight into
Wednesday morning, mainly across northern Iowa, with a low severe
weather threat. There is some uncertainty as to how long this
activity will manage to persist after sunrise Wednesday, with in
turn introduces uncertainty regarding the degree to which we may
destabilize during the day.

By late Wednesday afternoon a weak mid-level impulse approaching
from the west will likely generate a reflective weak surface low
pressure center somewhere over northeastern Nebraska up to around
Sioux Falls, with a trough trailing across eastern Nebraska into
Kansas and an effective warm front setting up somewhere near or
north of the Iowa/Minnesota border. The triple-point and trailing
trough should serve as foci for convective initiation late in the
day Wednesday, but there is uncertainty as to the location and
timing of this initiation, and its resulting eastward evolution,
dependent on the amount of convective overturning/contamination
earlier in the day from the aforementioned morning showers/storms.
For now will maintain chance POPs across northern Iowa during the
day, with the better chances coming after 00Z Wednesday night.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 319 PM CDT Tue Jun 20 2017

The main concern will be precip chances Wednesday night, as well
as other times during the long term period.

Expect convection to possibly be ongoing during the beginning of
the long term period. It will continue into the night as
isentropic forcing appears favorable for it to continue. Northern
CWA is most favored for the precip.

Cold front slowly sags southward on Thursday and hangs up across
the CWA. The front should help provide a good focus for
thunderstorm development during the day on Thursday. Instability
and shear parameters will be sufficient for severe weather so will
have to continue monitoring. Heavy rain parameters suggest heavy
rainfall is a possibility Thursday night.

The cold front will push south and east of the CWA on Friday with
cooler, drier airmass to end the work week. The European model
shows a deepening wave across the northern U.S. Saturday night
into Sunday. Model solutions do begin to diverge during this time
with timing of various features. They do tend to agree, however,
on northwesterly flow in the upper levels bringing near to
slightly below seasonal values. Warmer weather is expected to move
into the CWA by the end of the long term period as the northern
U.S. trof slowly migrates eastward.


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1156 PM CDT Tue Jun 20 2017

VFR conditions will prevail through the entire TAF period. A few
thunderstorms are possible late tomorrow morning into the
afternoon hours and have VCTS mentioned across the northern TAF





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