FXUS63 KDMX 200855

Area Forecast Discussion
National Weather Service Des Moines IA
355 AM CDT Mon Aug 20 2018

.SHORT TERM.../Today through Tonight/
Issued at 355 AM CDT Mon Aug 20 2018

Impacts already being felt across central to SW Iowa from well-
advertised cyclonic system propagating through the Midwest. At the
06z Mon 6-hr obs, Lamoni reported 1.08 inches of rain during the
pvs 6 hour period. From 06z-08z Mon, Lamoni measured exact 1.0
inches of rain, bringing their total to over 2 inches. Radar
analysis shows t the rainfall rates/intensity of this precip is
not out of the ordinary across central to southern Iowa. Thus,
have boosted QPF across the CWA. 3-hr Flash flood guidance is
around 3 inches across most of the CWA, which is slightly low, if
anything, based upon current drought conditions which are not
accounted for correctly in FFG. So, while ponding of water is
likely in locations where ponding typically occurs during heavy
rain events, widespread Flash Flooding does not appear to be
likely at this time. Have opted to hold of on any flood-related
headlines at this time. River and small stream rises have already
occurred, but given the drought and low start for most to all of
them, river flooding likely not imminent either.

Will spare the fine-detail mesoanalysis at this point since heavy
rainfall clearly already occurring. For system track, model
consensus sufficient as spread has narrowed considerably at this
point. Center of sfc low easy to pick up on GOES-R imagery
crossing into Page/Taylor counties in southern Iowa at 09z. This
low will then slowly propagate eastward throughout today, making
it near SE Iowa by around 03z Tue. Very broad area of rotation
with this system as 850mb winds still have a SE component to them
all the way to the MN/IA/WI border. This area is very well-painted
with a deformation zone that would probably bring notable snow
accumulations if this system were present in January. At any rate,
have bumped up QPF across northern Iowa as a result of this high-
confidence stream of precip. Northern Iowa much more moist than
southern Iowa, so will need to monitor flash flood potential
especially during the 20z-00z Tue or so timeframe.

By 09z Tue, precip should be east of the DMX CWA. Short range models
struggling with how to handle fog/stratus in NW Iowa. Certainly will
be plenty of moisture present. Winds, however, will likely be 10 to
15 kts at the sfc. So at this time, it seems a low stratus deck
would be most likely outcome vs dense fog. Will need to monitor this
as well.

.LONG TERM.../Tuesday through Sunday/
Issued at 355 AM CDT Mon Aug 20 2018

The upper low will transition to an open wave late tonight and into
Tuesday and will shift quickly northeast. This system will be
quickly followed by another upper short wave passing to the
northeast Tuesday and this system will reinforce the northwest flow
over the state. Expect breezy northwest winds and relatively cool
conditions with highs in the low to mid 70s with a lack of mixing
with the boundary layer remaining below 850 mb. Expect some sun on
Tuesday though could be filtered sun with the northwest flow
bringing the return of smoke.

The thermal trough will be over Iowa on Wednesday, however, deeper
mixing and more sunshine should lead to temperatures a bit warmer
than Tuesday. Warm advection commences late Wednesday night into
Thursday. Another strong upper level system will impact the state
Thursday night into Friday and will bring the next chance for
Thunderstorms to the state. It does appear to have at least an
isolated severe weather potential with this system so will need to
monitor that situation. Beyond that, the Southern Plains sub-
tropical high will excerpt itself next weekend into Monday with very
warm temperatures expected. Currently highs in the mid 80s to near
90 may still not be capturing the potential high temperatures. This
also seems to be a prime setup for ridge riding thunderstorm
complexes which can tend to drop further south than expected this
far out. Any thunderstorms and associated cloud debris would further
complicate the temperature forecast.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1210 AM CDT Sun Aug 19 2018

Showers are widespread over the southwestern half of Iowa with
little if any thunder as low pressure moves near Kansas City at
midnight. Forecast TAF sites are all VFR at 5z with some MVFR
restrictions over southwest Iowa where there is moderate rain
falling. Expect more MVFR conditions to spread over the terminals
tonight through Monday morning, but have pulled back on IFR
conditions at this time choosing low end MVFR. There will be a
break in the rain over southeast before redevelopment in the
afternoon. Winds will increase from the northeast and north and
become gusty on the back side of the low at most terminals by the
end of the TAF period.




SHORT TERM...Kotenberg
LONG TERM...Donavon

NWS DMX Office Area Forecast Discussion