Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221120
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
620 AM CDT Sat Apr 22 2017

.SHORT TERM.../Today through Tonight/
Issued at 340 AM CDT Sat Apr 22 2017

Quiet weather is expected during the short term as high pressure
dominates the upper Midwest including Iowa.  Will see expansive
cirrus shield currently over Iowa slowly pull south and east this
morning with sunshine gradually becoming widespread by afternoon
across most locations.  Thermally, will see warmest temperatures at
850mb over northwest Iowa by this afternoon and anticipate this area
of the state will also see some of the warmest highs and forecast
generally reflects this.  Surface ridge will slide across the state
tonight with winds becoming southwest by daybreak on Sunday morning.
With the increase in warm advection overnight, anticipate lows to be
warmer than current readings by a few degrees despite the clear
skies and relatively light winds.

.LONG TERM.../Sunday through Friday/
Issued at 340 AM CDT Sat Apr 22 2017

Monday into Tuesday...
Confidence: High

07z Sat water vapor imagery picking up on an upper low spinning in
the Pacific about 1100 miles off of the northern California
shoreline. This low will continue to propagate eastward... nearing
NW Iowa between 18z Mon-00z Tue. There is very little model spread
with the track of this low, leading to high confidence forecast.
With broad cyclonic flow solidly entrenched across the southeastern
CONUS, the gulf will be closed off ahead of this system. Not much
westward tilt with height as sfc reflection makes it into SW
Minnesota by 00z Tue. An attendant cold front dropping southward
through Iowa will serve as a sufficient focusing mechanism for
convection. However, with this system being "moisture-starved"
not much CAPE to work with. Further analysis shows models are even
over- initializing sfc dewpoints in source region, degrading
strong/severe potential even more. By the time this boundary
reaches the western edge of the DMX CWA, the cap will likely win
out, and any remnant storms should dissipate as they traverse
eastward into an increasing unfavorable environment.

Tuesday into Wednesday...
Confidence: Low-Medium

Somewhat new to the past few model runs, solutions are hinting at
a panhandle sfc low riding into Iowa along the heels of the above
mentioned system for Tuesday into Wednesday. Interestingly, the
GFS suite has popped this feature first on the 12z Thu run, and
the ECMWF appears to be playing catch-up for once... so have
leaned heavily to GFS solution. Still some variance in
track/timing of low...so while confident of the feature, low
confidence in precip/impact placement. The aforementioned low over
the SE CONUS will have propagated northeastward into the Atlantic
far enough to open the gulf up with this system. At this time, it
appears the LLJ/850-300mb moisture transport winds will have
enough of a southwesterly orientation to where best moisture will
be just south and east of Iowa...possibly clipping the far SE
portion of the DMX CWA. Will have to watch this closely as track
not locked in yet.

Thursday and Beyond...
Confidence: Very Low

Models degrade significantly in terms of run-to-run consistency
and clustering after the Wednesday low passes east of the Midwest
by 12z Thu. Like yesterday at this time, not worth getting too far
into mesoscale details at this point with so much uncertainty at
the synoptic-scale. Since the 12z Thu model runs, long-range
models have had another panhandle low crashing somewhere through
the Midwest in the Thursday-Friday timeframe. All long-range
models in agreement with broad anticyclonic flow over the Gulf of
Mexico, centered off the western Florida coastline. This flow will
definitely help drive gulf moisture into the Midwest with this
low. The ECMWF has been faster and further south/east with this
low than the GFS. The 00z Sat GFS solution advects PWATs over 1.8
inches into Iowa Friday morning... while simultaneously, the 00z
Sat ECMWF has this moisture over Illinois/Indiana.

Starting with this most recent model run, the GFS and ECMWF have
started to converge ever so slightly towards a middle-of-the-road
solution between the two. At the very least, this supports cooler
temperatures and heavy rain potential in our far south/southeast.
Severe wx cannot be ruled out in these locations either. Again, a
lot of green pasture in between current confidence level and even
a slight level of fcst certainty to where we can begin to ramp up
messaging.

&&

.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 620 AM CDT Sat Apr 22 2017

Cirrus will slowly erode away by this evening with clear skies
tonight. Ceilings and visibilities will be VFR for the duration of
the forecast with light north to northeast winds becoming variable
overnight as surface ridge slides overhead. Overall, a good period
for general aviation across Iowa.

&&

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

&&

$$

SHORT TERM...Cogil
LONG TERM...Kotenberg
AVIATION...Cogil



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