Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 151121

Area Forecast Discussion
National Weather Service Des Moines IA
521 AM CST Wed Feb 15 2017

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

.SHORT TERM.../Today through Tonight/
Issued at 416 AM CST Wed Feb 15 2017

No Hazardous WX Impacts Expected
Confidence: High

09z Water vapor imagery picking up on a well-defined upper low
spinning in Canada just east of the Great Lakes. Meanwhile, a huge
thermal ridge that extends well into Canada has been building
over the intermountain west. Sandwiched in between the two,
decently strong NW flow/CAA has been reinforced over the DMX CWA.
850 mb temps have fallen to the -5C to -10C range... a solid 5 or
so degrees cooler than yesterday morning. The leading push of cold
air has now made it through Iowa...meaning that the CAA faucet
will be shutting off, and WAA will begin to take over...with 850mb
temps crossing the 0C barrier in western Iowa by around 00z Thu.

Temps have been trending warmer and warmer with each model package,
so have jumped ahead and upped temperatures over guidance by a few
degrees. Now looking like 850mb temps will near +10C by Thursday
afternoon. Though we won`t fully mix this high, given that we hit 52
degrees in Des Moines yesterday with 850mb temps near +0C, it is
looking like Thursday will begin our first day of what could end up
being upwards of 6-8 days in a row of temperatures in the 60s in Des
Moines. The normal max temperature is in the mid 30s for this time
of year.

.LONG TERM.../Thursday through Tuesday/
Issued at 416 AM CST Wed Feb 15 2017

Friday through Sunday...

No Hazardous WX Impacts
Expected Confidence:  High

Run-to-run consistency has been very good and consensus is
continuing to tighten with 850mb temps in the +12C to +15C range for
Friday afternoon. Record to near-record warmth still looks likely
during this time period so have added this wording to central to
southern sites.
For Saturday, models dialed in with sandwiching Iowa in westerly
flow between a cut-off low passing through the Ozarks, and a
somewhat weak shortwave propagating eastward through southern
Canada. Neither of these systems should have mesoscale impacts on
Iowa. 850mb temps "fall" to the +7C to +10C range... so placed
temps near persistence and then knocked 5 or so degrees off
highs. WAA looks to re-build on Sunday...meaning temps should be a
few degrees warmer than persistence from Saturday.

Monday and beyond...

Heavy Rainfall and Ice jams/river flooding possible
Confidence: Low

Confidence in overall evolution of Monday-Tuesday event less today
than it was yesterday at this time. Run-to-run and model-to-model
consistency are still not great with overall synoptic setup. There
appears to be a shortwave that will pass near the Canada/US border
that is somewhat phased with a low that looks to propagate eastward
through the deep south. The 00z Wed model suite has actually slowed
this southern low some...which theoretically should be bad news as
this may support additional time for moisture to advect from the
Gulf of Mexico into Iowa. Moisture transport into Iowa may be off
the charts as 850mb winds reach and exceed 50 kts from a wide-open
Gulf. With confidence so low, won`t get into too much detail...but
the GFS and GEFS Ensemble is much more favorable than the ECMWF for
heavy precip. The overall synoptic setup matches up well through
12z Mon, and then discrepancies arrive thereafter. The ECMWF
strangely does not advect as much moisture into Iowa as other
models. The GFS has PWATs of around 1.4 inches, which is literally
well off the charts for this time of year. Frighteningly, the 00z
Wed GEFS M-climate has PWATs in the +5 to +6 std dev range and
maxes out the return interval. Record Des Moines rainfall for Mon
2/20 is 0.67 inches and for Tue 2/21 is 1.22 inches.

Overall timing has been (not surprisingly) pushed back 6 hours,
with much of the rainfall (at present) slated to occur in the
Monday afternoon to Tuesday morning timeframe. Am hoping today`s
model runs will better resolve this so confidence can be
increased. If confidence can be increased, heavy rainfall wording,
along with POPs and QPF will need to be ramped up. At this time,
cannot rule out ice jam/flooding potential as well...with ice
still being reported along rivers primarily in northern Iowa.


.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 514 AM CST Wed Feb 15 2017

Confidence high in VFR conditions throughout the TAF period. Wind
speed should generally be under 10 kts at the sfc throughout this
TAF period. Confidence generally high in LLWS shear occurring
overnight tonight beginning at KFOD around 05z Thu...and
spreading eastward...making it to KDSM around 06z...and KOTM
around 09z Thu. As of now, it appears KMCW and KALO will not
experience LLWS. Magnitude and timing may need to be tweaked in
subsequent TAF updates.




SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Kotenberg is the U.S. government's official web portal to all federal, state and local government web resources and services.