Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212327

Area Forecast Discussion
National Weather Service Des Moines IA
627 PM CDT Sat Oct 21 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 312 PM CDT Sat Oct 21 2017

The primary concern will obviously be convective trends into
tonight. Confidence is high that showers and storms will blossom
eventually due to the deep convergence and deep, strong
thermodynamic and kinematic forcing with the long wave trough
passage, but less so with respect to intensity. Storms developed in
NW Iowa around 18z and looked as if they were going to mature fairly
quickly, but since diminished. Additional convection has also
developed farther south in a zone of strong frontal convergence and
frontogenetical forcing, but this has also been somewhat slow to
strengthen. Western Iowa RAP soundings show a 2km warm layer at the
base of the EML continuing to erode and there may be a slim window
for surface based convection to mature, but it will not last long as
the front quickly undercuts anything resulting in convection being
rooted around 1km. With plenty of stratus over Iowa and the loss of
heating soon, feel much of the convection will be below severe
limits unless trends increase soon.

Shallow and deep shear parameters are both high so there remains a
small potential for supercells and brief tornadoes in addition to
hail and wind. QLCS tornadoes will be possible into the evening with
low level shear persisting, but as mentioned overnight the 0-3km
shear vectors are not oriented favorably: too far to the north vs
east and line/front orientation. Small segments would need to orient
NE to realize available shallow shear and SRH. Moderate to locally
heavy rains may eventually be a greater threat with strong low level
moisture transport ahead of the front. 00Z NAEFS precipitable water,
specific humidities and integrated water vapor transport are all
near climatological extremes. Thus expect widespread efficient rains
in 3.5+km warm cloud depths with 1 amounts over the SE half of the
forecast area possible by tonight.

Skies should clear into tomorrow as the system departs with
temperatures 5 to 10 degrees cooler than today. The biggest change
will be less humid conditions with surface dewpoints no better than
the 30s and 40s.

.LONG TERM.../Sunday night through Saturday/
Issued at 312 PM CDT Sat Oct 21 2017

The long term period begins with significant shortwave trough diving
south through the Upper Midwest early next week. This will send a
strong cold front through central Iowa during the morning and
afternoon hours on Monday. Deep unidirectional winds and efficient
boundary layer mixing behind the front should still be enough to
warm temperatures into the upper 50s and low 60s. Not expecting
anything more than isolated showers with the frontal passage as the
atmosphere is starved of moisture. A reinforcing shot of cold air
advection will lead to much cooler and blustery conditions on
Tuesday. Model guidance remained consistent to the previous forecast
with highs only in the upper 40s and low 50s.

A bounce back to near average temperatures is expected through the
middle of the week as an upper level ridge builds over the Plains
states. The warm up will be short lived as a deep upper level low
digs southeast and dislodges a large dome of cold air to our north.
The GFS and EC remain out of sync with their timing and evolution of
the trough, although the slower EC has trended toward the faster GFS
with the latest run. Regardless of any model discrepancies, there is
high confidence that the coldest air we have seen so far this season
will be ushered in by a strong cold frontal passage in the late
Thursday or Friday time frame. Latest guidance suggests highs will
struggle to get out of the 40s on Friday and Saturday with below
freezing temperatures possible overnight. Light rain and/or a few
snow flakes are also possible Friday night into Saturday as moisture
wraps around the back side of the upper level low. At this time it
looks like the better lift/moisture will slide north of the area.


.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 627 PM CDT Sat Oct 21 2017

Broken line of convection with occasional MVFR cigs moving across
area now...and will exit the far east by 10-12z Sunday. Activity
may increase southeast overnight near KOTM as low level jet
increases aft 03z. Some wind and mainly MVFR VSBY reduction as
storms roll through. Aft 06-10z nearly all areas will be VFR with
either high clouds or skc aft 12z. Winds increase aft 15z with
gusts near 16-18kt until 21z. /rev




LONG TERM...Martin
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