Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 212325
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
625 PM CDT Thu Sep 21 2017

.SHORT TERM...Through Tonight
Issued at 220 PM CDT Thu Sep 21 2017

At 2 PM, visible satellite and surface obs indicated a surface warm
front draped across the western MN/IA border, then extending
southeastward toward northern Illinois. The northward advancement of
the warm front over eastern Iowa was somewhat hampered by an MCS
this morning, which has since dissipated. The big question is
whether any TS/SHRA will be able to develop along the warm front
this afternoon. Model soundings show a 750-850 mb capping inversion
firmly in place across the region, and convective-allowing models
are not too excited about potential development. However, given the
strong moisture transport signal and 1000 to 2000 J/kg of CAPE, will
keep a slight chance of showers and storms along the northward
advancing warm front into tonight. Strong southerly flow should keep
lows tonight in the upper 60s to lower 70s, which would be near-
record territory for warmest daily minimum temperatures.

.LONG TERM...(Friday through Thursday)
Issued at 220 PM CDT Thu Sep 21 2017

Through Sunday...Heat impacts are the main message for this
period. Ridge building takes place and holds through the period.
On Friday, an elevated mixed-layer off the High Plains moves over
the forecast area. These are the real heat producers, with
stronger gradient winds in the boundary layer leading to enhanced
mixing and warming. Feel the forecast is positioned well Friday
for record heat, and heat index values of 95 to 100. The air
aloft is a bit cooler on Saturday/Sunday but still very warm. Have
brought the Sunday highs up to just a smidge. Winds and mixing
drop off a bit each day as well to support this slight cooling.
Dewpoints are also expected to lower a bit each day after Friday
as somewhat drier air is pulled in from the Ohio Valley via the
circulation around the ridge position. Thus, heat index values
will trend lower from Friday to Sunday.

Sunday night through Thursday...
The main forecast concern for the beginning of this period will
be timing of showers/storms associated with a cold front moving
west to east through the area. The GFS remains the fastest model,
with the 21.12Z run bringing precip into the forecast area Sunday
night, while the 21.00Z ECMWF holds off until Monday afternoon.
The GFS is also more robust/widespread with the precip than the
ECMWF, likely due to the precipitable water values around 1.50
inches and a bit more instability. Both models do show enough
instability for thunder through Tuesday, with MUCAPE values around
1000 J/kg. The precipitation looks to be out of the area by
Wednesday afternoon, with a trough axis and much cooler air behind

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 625 PM CDT Thu Sep 21 2017

Primary aviation concern over the next 24 hours will be wind
shear given a strengthening nocturnal low-level jet. Sustained
southerly surface winds through the night will be generally 10 kts
or less. However, south-southwest winds increase dramatically to
near 40-45 kts in the 1500 to 2000 ft agl layer. This will result
in low-level wind shear at both TAF airfields through the night
and into early Friday morning. After sunrise Friday, low-level
wind shear will diminish as surface winds increase, gusting at
times into the low-mid 20 kt range through the afternoon. Cloud-
wise, skies will be mostly clear/sunny through the period. Any
ceilings that do develop will be well above 7000 to 8000 ft agl.

&&

.CLIMATE...Friday through Sunday
Issued at 220 PM CDT Thu Sep 21 2017

Records for Friday-Sunday are posted on our web site at
www.weather.gov/arx/weatherstory or in the Area Forecast
Discussion from 352 AM CDT Thu Sep 21 2017.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...MH
LONG TERM...Baumgardt
AVIATION...Rogers
CLIMATE...Baumgardt



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