Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 210004
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
704 PM CDT Mon May 20 2019

...Updated for 00z Aviation Discussion...

.DISCUSSION.../Tonight through Monday/
Issued at 348 PM CDT Mon May 20 2019

GOES-East Clean IR Water Vapor imagery shows the upper
level low over northern Arizona at the base of the upper level
trough over the western US. There is plenty of convection downstream
in the diffluence region of upper air pattern over the central
Plains. Closer to Iowa, fair weather cumulus clouds were drifting
southward across the state with the highest concentration of these
low clouds over southern Iowa. Meanwhile, GOES-East Day Cloud Phase
Distinction RGB showed high level clouds from the convection over
Kansas moving northeastward into the state this afternoon. This will
result in sunshine increasingly fading behind these clouds with
regional radar showing a broad area of rain slowly moving
northeastward. No rain yet in our forecast area as of 3 pm, but that
will be changing as strong theta-e advection lifts over the state
helping to sustain/expand the area of rain into Iowa through
tonight. HRRR might be a bit fast on arrival so have blended
CONSShort to slow down the timing slightly. Still, by midnight
expect rain over the southern part of the state and fairly
widespread showers by daybreak Tuesday. Thunder looks to be limited
overnight, but still could be some peals of thunder, especially over
southern Iowa.

Tuesday will be a raw May day. As the upper level low rotates into
Nebraska, strong theta-e advection phased with strong Q-vector
convergence will yield periods of showers and thunderstorms. One
period will likely be in the morning with another period late in the
afternoon. The latter should be fairly progressive across the state
with a narrow ribbon of precipitable water values topping 1.5
inches. Given the progressive nature of the rain and flash flood
guidance values just shy of or above 2 inches at the 3 and 6 hour
time steps, no widespread concern for flash flooding. However, with
expected rain totals averaging between 1 to 2 inches this will
certainly cause a rise on area waterways. As for the severe threat,
forecast soundings show a low level inversion that should limit
severe potential. Weak, elevated instability and a freezing level
around 13 to 14 kft may result in a few storms that produce near
severe sized hail. While SPC has maintained their slight risk over
far southern Iowa, the best chance for severe storms will likely be
south of the Iowa/Missouri border. High temperatures tomorrow will
be close to typical lows for the date and within a few degrees of
record low highs. Further, slightly deepening low pressure over
western Kansas and high pressure over Ontario will tighten the
pressure gradient over Iowa yielding strong winds from the east.
BUFKIT soundings from the NAM/GFS/RAP show mixing limited due to the
aforementioned inversion with wind gusts peaking between 35 to just
shy of 40 knots. GFS soundings show stronger winds at the top of the
mixed layer in the low 40s so will need to monitor for possible
increase in wind gusts. The strongest winds are expected over
northern Iowa and have issued a wind advisory for those areas
tomorrow  afternoon into tomorrow evening due to the strong,
sustained winds.

Wednesday looks to be the brightest day at this point and a day to
do any outside activities. Winds will not be as strong as Tuesday
as surface low pressure passes north of the state, but it will
still be a breezy day and much warmer as well with highs mainly
into the 70s. Sunshine should become plentiful by the afternoon.

The next chance for storms will come later Thursday into Friday as a
new upper level low drops into the western US trough and begins to
lift out over the Plains. At the surface, a warm front will lift
over the state, but looks later in the day than previous model
solutions. While parameters are not as impressive, would still
expect strong thunderstorms in an environment characterized by up to
1000 J/kg of MUCAPE and strong, supportive deep layer shear with mid-
level lapse rates of 7 to 8C/km. Obviously, there will be some
timing changes as was noted so will keep a watch on this period for
severe risk over Iowa. While a cold front will try to move through
the state, it looks likely to stall over the region and keep chances
for showers and storms into at least part of the Memorial Day
weekend.

&&

.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 641 PM CDT Mon May 20 2019

Multiple forecast "hazards" in this forecast... Precipitation,
Winds, lowering CIGs.

Precipitation... High confidence in precipitation occurrence
overspreading Iowa from south to north throughout this TAF period.
Left thunder mention out of TAFs now... cannot rule our a stray
lightning strike or two tonight, but very minimal
coverage/potential of thunder not high enough to warrant separate
FM group mentioning thunder at this time.

Winds... High confidence in strong winds Tuesday morning into
Tuesday evening... especially across northern Iowa... KMCW and
KALO... with peak winds occurring during Tuesday afternoon.
Several FM groups likely will be needed for increasing and later
decreasing winds... some gusts of which could exceed 40 to 45 kts
during Tuesday afternoon. Winds at KMCW should be a solid 10 kts
or so higher than at KDSM.

CIGs... Along with the arriving precip, CIGs will be gradually
lowering throughout the TAF period. Challenge will be timing IFR
CIG arrival... High confidence in IFR... so a matter of when, not
if. The when appears to be around sunrise at KDSM, late morning
around KFOD, and KALO, and then early afternoon at KMCW. Will
obviously be refining these times in future updates. As of now,
LIFR does not appear to be likely, but LIFR cannot be ruled out at
KDSM sometime around early afternoon at KDSM. Will monitor this
also.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 1 PM to 10 PM CDT Tuesday for IAZ004>007-
015>017-023-024-033-034.

&&

$$

DISCUSSION...Ansorge
AVIATION...Kotenberg


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