Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 281925

Area Forecast Discussion
National Weather Service La Crosse WI
225 PM CDT Fri Jul 28 2017

.SHORT TERM...(Tonight thru Saturday night)
Issued at 225 PM CDT Fri Jul 28 2017

Main fcst concern this period is late night/early morning valley fog
both nights.

Data analysis at 18z had a broad area of Can high pressure centered
over Lk Superior with ridging to eastern Neb. The high was bringing
a drier airmass to the region, with early afternoon dew points
across much of WI/MN/IA in the more comfortable 50s to lower 60s.
Under Sunny to partly sunny skies, early afternoon temperatures
mostly in the mid 70s to lower 80s along with the lower dew points
were making for a day more reminiscent of Late Aug/early Sept than
late July.

No problems noted with 28.12z model initializations. Solutions
similar as hgts rise/shortwave ridging aloft builds over the
region tonight thru Sat night. Short-term fcst confidence is good
this cycle.

In the short term: the stretch of dry, quiet, seasonable weather
continues this period as the ridging aloft builds across the Upper
Midwest and sfc Can high pressure settles over MN/IA/WI. The light
wind layer (less than 10kts) over the area tonight/Sat night progged
to be much deeper than the past couple nights, more favorable for
valley fog formation. Short-term forecaster has beefed up valley/low
laying area fog in grids tonight to areas of BR/FG vs. patchy
wording in most of the valleys. Will do the same with late Sat
night/early Sun morning, as light wind conditions around 12z Sun
trending as much or more favorable that at 12z Sat. Light winds and
clear/mostly clear skies should allow for strong radiational cooling
the next couple nights. NAEFS indicating 850mb 0.5 to around 1 std
deviation below normal thru Sun morning. Blend of guidance lows in
the 50s tonight/Sat night look good. May even see a few mid/upper
40s in the normally cooler, low laying areas near/NE of I-94 both
nights. Blend of guidance highs mostly in the upper 70s/lower 80s
for Sat supported by the mixed 925-850mb temps.

.LONG TERM...(Sunday through Friday)
Issued at 225 PM CDT Fri Jul 28 2017

For Sunday thru Monday night: main fcst concerns this period are
valley fog potential late Sun night and temperatures.

Model runs of 28.12z remain in good agreement for shortwave ridging
aloft to remain over the region Sun into Mon, then for hgts to start
falling Mon night as a stronger wave moves into Man/western Ont and
weak energy ripples SE across the northern plains. Fcst confidence
for this period is on the good side.

Dry, quiet weather looks to continue Sun thru Mon night as broad sfc
high pressure remains centered near the WI/IL border thru the
period. Some southward shift of the high Mon night as a front pushes
into northern MN. Weak ripples thru the ridging flow aloft looking
to stay west/north of the Mon night as do the impacts of the
approaching front. Radiational valley fog potential continues to be
a concern in the 08-13z time-frames, especially Sun night. Sfc high
center is still progged to be near/over the fcst area at 12z Mon,
for another night of light/variable winds Sun night. This with a
very deep layer of winds less than 10kts aloft. Previous shift
already added at least patchy valley fog to the 07-13z Mon grids.
With the high shifting a bit south Mon night and an increase of sfc-
850mb winds, left the patchy fog mention out of 07-13z Tue. Airmass
with the high should continue to modify in place with plenty of late
July sunshine. Trend of the consensus guidance highs warming a few
degrees each day looks good.

For Tuesday thru Friday (days 4-7): main fcst concerns this period
are SHRA/TSRA chances centered on Tue night and temperatures.

28.00z/28.12z medium range model runs offering improving agreement,
(at least in time) for a northern stream shortwave to drop into the
great lakes Tue/Wed. This wave to then drop into and amplify the
eastern CONUS troughing Thu into Fri, setting up a rather strong NW
to N flow aloft over the Upper Midwest for late next week. Day 4-7
fcst confidence is average to good this cycle.

A warmer day yet Tue in SW flow ahead of the cold front the short-
wave dropping into/across the great lakes is set to push south
across the area centered on Tue night. Mixed 925/850mb temps Tue
looking to support most highs in the low/mid 80s, near to a bit
above normal. Increase of moisture/MUCAPE to precede/accompany the
front into/across the area. WIth the tightening consensus in time,
30-40% SHRA/TSRA chance centered on Tue night look good at this
time. Behind this front, another seasonable cool/dry Can high is set
to build south across the region Thu/Fri. Trend of generally dry
Thu/Fri with highs around a category below normal quite reasonable at
this time.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1240 PM CDT Fri Jul 28 2017

VLIFR conditions are expected in valley fog at KLSE from 10Z to
13Z. The visibility is forecast to drop to 1/4SM with a VV of 200
ft. Otherwise, plan on VFR conditions with high pressure in




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