Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 192310

Area Forecast Discussion
National Weather Service La Crosse WI
610 PM CDT Thu Oct 19 2017

.SHORT TERM...(Tonight thru Friday night)
Issued at 230 PM CDT Thu Oct 19 2017

Main fcst concerns period are highs and winds on Fri, returning SHRA
chances Fri night.

18Z analysis showed an upper level ridge continuing to build across
the region, with northwesterly flow aloft becoming more zonal. At
the surface, the pressure gradient was tightening to the west,
providing increasing southerly winds and warm air advection. The
upper level ridge axis will slide through the area tonight, with the
surface pressure gradient continuing to tighten, keeping winds up
through the night and leading to relatively mild lows in the mid 40s
to lower 50s.

Warm air advection will continue on Friday, with fairly strong
southerly winds in the 10 to 15 kt range and gusts over 20 kts for
most locations. Do expect a warmer day than Thursday, but warmest
925 mb air is over the area during the morning and then cools as
mixing takes place. Also anticipating some cirrus over the area
during the day, which would keep highs down a bit.

Low-level moisture transport will then ramp up Friday evening/night,
ahead of a long wave trough approaching from the west. With this,
most guidance hints at a few showers impacting the area, especially
west of the Mississippi River, by early Saturday morning.

.LONG TERM...(Saturday thru Thursday)
Issued at 230 PM CDT Thu Oct 19 2017

For Saturday thru Sunday night: main fcst concerns this period are
SHRA/TSRA chances Sat/Sat night.

Model runs of 19.12z in good agreement on the mid level trough to be
crossing the plains Sat, moving into MN/IA by 12z Sun and into the
great lakes by 00z Mon. Overall trend is toward splitting the
trough, with a faster northern portion while holding some of the
southern portion of it in the lower MS valley into Mon. Good
agreement for a shortwave trough to be dropping into northern plains
Mon night. Fcst confidence for Sat-Sun night is generally good.

Models still on track to bring the sfc-850mb front/trough to near
the western border of the fcst area by 00z Sun then across the area
Sat night. Model now look to bring the bulk of the lower level
thermo-dynamic forcing with the front across in a tighter band.
This effectively slowing the onset of frontal induced precip into
the area Sat while accelerating its exit later Sat night/Sun
morning. After the late Fri night/Sat morning SHRA chances as the
850-700mb moisture transport/theta-e convergence in the elevated
layers moves across, the east and south sides of the fcst area may
well stay dry thru Sat afternoon. However will leave the consensus
Sat afternoon rain chances as is for now to keep a good blend with
the neighbors. SHRA chances in the 60-90% range for Sat evening,
tapering off from W to E after midnight also look good with the
tighter frontal/lift passage consensus. With the tighter forcing
signal along the front and 925-700mb drying quickly spreading in
behind the front later Sat night, locations west of the MS river may
well be precip free after 09z Sun, with this drying/end of rain
chances quickly spreading east across the fcst area in the 09-15z
period. Will also leave the later Sat night/Sun morning precip
chance grids as is to keep them blended with the neighbors. Models
continue with a signal of 250-500 j/kg MUCAPE spreading into the
area ahead of the front Sat/Sat night as PW values increase into the
1.25 to 1.5 inch range. Chances for TSRA, especially Sat afternoon/
evening as the front passes thru, continue to look good.

Low level cold advection to spread across behind the front later Sat
night/Sun morning but 925mb temps for Sun afternoon still in the 9C
to 12C range. With what should be mostly sunny skies by the
afternoon and west winds, raised Sun highs a bit more into the low-
mid 60s, still about 5F above the normals even behind a cold front.

For Monday thru Thursday (days 4 - 7): main fcst concerns this
period are small SHRA chances much of the period and cooler temps

Between model and run-to-run consistency in the details for Mon-Thu
period has been less than desirable. However, among the medium
range model runs of 19.00z/19.12z the overall trend is maintained
for a stronger NW flow shortwave to drop into/across the Upper
Midwest Mon/Mon night and carve out a rather deep longwave trough
over the eastern CONUS for Tue into Thu. Fcst confidence in the day
4-7 period is average this cycle.

Stronger shortwave to drop in Mon brings with it a cold front,
increased moisture and a round of lift. The consensus 20-40% SHRA
chances spreading south across the area Mon/Mon night remain
reasonable. Mid level cold pool and deepest of the cyclonic flow is
then progged over the Upper Midwest Tue as the troughing deepens
over the eastern CONUS. Column will be cold but trend is for lower
levels to remain warm enough for any precip Tue/Tue night to fall as
-RA/-SHRA. Some weak instability is even possible Tue under the
steep mid level lapse rates/cold air aloft. Some 20-40% SHRA chances
continuing Tue/Tue night quite reasonable for now. Models are
progressive with the trough Wed/Thu, with some shortwave ridging
aloft looking to build in Wed, ahead of another stronger NW flow
shortwave to drop in Thu. Plenty of detail differences by Wed but
signal is for some warm advection and drier air to spread in from
the central plains on Wed ahead of the next shortwave, then followed
by another round of stronger cold advection and -SHRA chances with a
stronger cold front on Thu. Trend of Highs near to a bit above
normal on Mon ahead of the first NW flow wave, then below normal
temps for Tue-Thu, looks good.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 610 PM CDT Thu Oct 19 2017

Still VFR early this evening, and that won`t change the next 24
hours as the area remains under the influence of upper level high
pressure. Increasing winds aloft this evening will deliver a
period of low level wind shear for both LSE and RST (a bit more
marginal at RST given stronger surface winds), with those winds
mixing down to provide gusts of 20-28 knots later Friday for all
areas as cirrus gradually thickens from the south.


.FIRE WEATHER...(Friday)
Issued at 230 PM CDT Thu Oct 19 2017

As fuels/crops continue to dry with natural maturity and progression
of fall, some concern for heightened fire weather conditions on
Fri. pressure gradient to tighten over the area on Fri, for south
winds 10 to 20 mph gusting 20-30 mph many locations by Fri
afternoon. However, as the S-SW lower level flow deepens/
strengthens lower level moisture and increasing sfc dew points are
expected quickly spread NE across the districts Fri morning.
Minimum humidities Fri afternoon expected to be in the 30-40% range.




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