Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 200857

Area Forecast Discussion
National Weather Service La Crosse WI
357 AM CDT Thu Apr 20 2017

.SHORT TERM...(Today and tonight)
Issued at 357 AM CDT Thu Apr 20 2017

Main fcst concerns this package include lingering -SHRA chances
(mainly north) today and cooler temperatures.

Data analysis at 06z had a sfc low near Waterloo IA, moving steadily
E-NE as stronger shortwave energy rounded the base of the mid level
trough over the northern plains. The warm front SE of the low
extended across east-central IA with a roughly 15F temp change
across it. MUCAPE remained rather limited across the fcst area, with
the `warm sector` and warm front remaining south of the area. Main
SHRA/TSRA band was moving east of the area. However, well behind the
leading line, lift ahead of the shortwave and continued moisture
transport of 1+ inch PW airmass into the area were producing sct
SHRA/TSRA across much of the fcst area early this morning.

No issues noted with 20.00z model initializations. Solutions very
similar on mid level troughing moving across the Upper Midwest today
then rising hgts tonight as it moves east of the region. Short-term
fcst confidence is good this cycle.

For the short term, weather quieting down this period as the mid
level trough exits east of the region. Some lift/deeper moisture
near/north of the 850-700mb low/trough remains over the north parts
of the fcst area this morning, for some -SHRA chances across mainly
the north 1/3 of the fcst area. These diminish thru the late
morning/afternoon as the moisture become shallower and the mid level
trough axis passes. Plenty of moisture in/under an inversion near
875mb progged to remain over the area today into this evening. With
an expansive area of low clouds upstream early this morning, skies
looking to remain cloudy today. Combine this with some low level
cold advection and W-NW gradient winds of 10-20 mph, today is going
to feel more like a late March day vs. April 20th. Consensus highs
today mostly in the mid 40s to mid 50s appear well trended.
Reasonable model consensus for the lower clouds to decrease from
west to east during the late evening/overnight hours as high
pressure builds in and the sfc-850mb flow slowly becomes anti-
cyclonic. Coldest of the 925mb airmass in over the area later
tonight, but winds looking to remain mostly in the 5-10 mph range to
keep the BL stirred as the clouds decrease. A chilly night expected
tonight with a blend of guidance lows in the 30s quite reasonable.

.LONG TERM...(Friday through Wednesday)
Issued at 357 AM CDT Thu Apr 20 2017

For Friday thru Saturday night: main fcst concerns this periods are

Model runs of 20.00z in good agreement for rising hgts/ridging aloft
to build across the region Fri thru Sat night, for what continues to
shape up as a dry, generally cool period. Fcst confidence for Fri
thru Sat night is good this cycle.

Hgts rise and Can high pressure settles into the Upper Midwest for
Fri thru Sat night. Coolest low level airmass is over the area Fri
morning with slow modification thru the period under what should be
plenty of late April sunshine both Fri and Sat. Even so, Fri
night/Sat morning looking to be the colder of the next couple
mornings with the sfc ridge axis nearby/overhead and clear/mostly
clear skies for strong radiational cooling. After highs back to near
normal Fri, consensus lows mostly in the low-mid 30s look good. A
few lows in the upper 20s in the normally colder low-laying areas
along/NE of I-94 not out of the question. With plenty of sunshine
Sat and continued modification of the low level airmass, blend of
the guidance highs/lows near normal for Sat/Sat night look good.

For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period are returning SHRA chances centered on Mon night and again
Wed, temperatures thru the period. Fcst confidence is good Sun
trending to below average by Wed.

medium range model runs of 20.00z in decent agreement for mid level
ridging and sfc high pressure to remain over the region Sun, to
round out what should be a rather pleasant late April weekend.
Plenty of sunshine and modification of the BL airmass continues,
with the consensus highs above normal on Sun looking good as the
main storm action remains north and south of the area. Consensus
remains good progging a shortwave/trough into the northern plains
Mon then into/across the region Mon night/Tue. SHRA chances mostly
30-50% centered on Mon night when the sfc-mid level trough axis
would pass are quite reasonable. MUCAPE prog for Mon-Tue remains
marginal to none across the area so will leave TSRA mention out of
the grids these periods. Model solutions diverge for Tue night/
Wed but some loose consensus for more troughing into the central
plains late in the day 4-7 period. Confidence not high but some
SHRA chances later Tue night/Wed OK for now. May yet need a TSRA
mention for Wed as some signal for an increase of instability by
day 7.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1147 PM CDT Wed Apr 19 2017

Late this evening, the surface low was over north central Iowa with
the warm front extending southeast into central Illinois. The
showers and storms north of the low were progressing steadily east
and have already move past KRST and will be past KLSE soon. Plan to
only show an additional hour of rain at KLSE before going dry for
the rest of the period. Lots of IFR ceilings back over Minnesota
that will get drawn in over the area as the low moves off to the
east with these remaining through Thursday morning before improving
to MVFR. Good low level lapse rates will develop behind the low for
Thursday to allow gusty west to northwest winds from mid morning
through the afternoon for both airports. These gusts should
dissipate around sunset as the lapse rates stabilize.




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