Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 180808

Area Forecast Discussion
National Weather Service La Crosse WI
308 AM CDT Fri Aug 18 2017

.SHORT TERM...(Today through Sunday)
Issued at 308 AM CDT Fri Aug 18 2017

Water vapor satellite this morning shows a couple of short wave
troughs upstream of the region that will move across the area
today. The first was over South Dakota with the second over far
northwest North Dakota. The first short wave trough looks like it
will move across the area late this morning into the early
afternoon with the second coming across during the late afternoon
and evening. The first short wave looks like it will be weakening
and is expected to produce a band of weak to at time moderate pv
advection in the 500-300 mb layer. The second short wave trough
looks stronger with moderate pv advection in the 500-300 mb layer.
The low level moisture transport never becomes very strong with
either wave and looks to remains focused primarily south of the
local area. With the first wave weakening as it comes in, the
isentropic up glide on the 305K surface starts out around 2 to 4
ubar/s across the west this morning but then quickly diminishes to
2 ubar/s or less by late morning into the afternoon. With the
second wave taking a more southern track, almost all the
isentropic up glide looks to remain south of the local area. The
instability looks very limited this morning with ML CAPE values of
500 J/Kg or less. Most of the area should have up to 500 J/Kg of
ML CAPE for the afternoon with maybe up to 1000 J/Kg across the
far south for the late afternoon and this should then quickly
diminish to less than 500 J/Kg during the evening. This should be
enough to allow for a few storms to occur this afternoon and
evening. The shear actually looks pretty decent for the afternoon
with up to 40 knots in the 0-3 km layer, but the general lack of
instability should limit the severe potential. Will spread 20 to
30 percent rain chances in across the western section this morning
and then have these same values across the area south of
Interstate 94 for the afternoon and evening.

Once the rain moves out of the area tonight, the skies are
expected to start clearing out. With an area of high pressure
building in at the surface, the concern becomes whether fog will
be able to form or not. The threat for fog looks to be conditional
and would not consider it a given that it will form. Forecast
soundings from the 18.00Z NAM would suggest that there may only be
a couple hours of mostly clear skies right before sunrise. The
winds just above the surface look to remain at or above 10 knots
until 19.10Z or later and will be out of the northwest, which is
not a favorable direction for fog formation. The forecast sounding
suggest saturation at the surface may occur for only an hour or
two right around 19.12Z, maybe a little longer over southeast
Minnesota, so have drop the mention of dense fog and cut back to
just some patchy/areas of fog for late tonight into early
Saturday morning.

The rest of the weekend then looks to be rather quiet as the high
pressure drifts across the region and then off into the Ohio River
Valley Sunday. A few showers/storms could pop up Sunday afternoon
for locations west of the Mississippi River as moisture starts to
return to the area ahead of an approaching cold front.

.LONG TERM...(Sunday night through Thursday)
Issued at 308 AM CDT Fri Aug 18 2017

The early part of this period looks to have the potential to be
wet. The upper level flow looks to start out zonal with a short
wave trough moving across the Sunday night and Monday morning and
then quickly followed by another for Monday night. The flow should
then transition to northwest with an upper level low forming over
northern Ontario. A short wave trough rotating around this low
will have the potential to move across the region Tuesday. A weak
cold front will move into the area with the first short wave
trough Sunday night and should then remain over the region until
the last wave Tuesday finally sweeps it out of the area. The front
looks like it will serve as a focus area for shower and storm
development with each of the short wave troughs. Right now, the
best chance for rain looks to be Sunday night and again Monday
night, but there will also be a chance for rain Monday with lots
of clouds when the eclipse will be occurring. Ridging aloft and at
the surface look to build in over the Upper Midwest for the
middle of the week for a couple of dry and seasonable days.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1110 PM CDT Thu Aug 17 2017

Lower stratus that has encompassed the area for the better part of
the past 24 hours continues to gradually diminish from west to
east late this evening. That trend will slowly continue through
the overnight hours, with mainly VFR conditions in stratus giving
way to just some increasing mid/high clouds from the west at RST
and LSE through the morning hours. A few weakening showers remain
possible at RST though with no impact to ceilings or visibility
expected, while LSE might see a sprinkle at worst through early
afternoon. Additional scattered showers and thunderstorms are
possible during the afternoon hours, though at this time, the
greatest risk looks to be west and south of the terminal sites.
Winds will remain from the northwest early this morning, shifting
westerly and then becoming light tonight. With that light flow,
some fog appears likely to develop, though mainly after the
current TAF period ends at 06Z.




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