Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201123
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
623 AM CDT WED JUL 20 2016

.SHORT TERM...(Today and tonight)
Issued at 305 AM CDT Wed Jul 20 2016

Primary concern today is convective potential, but also watching
temperature trends in the short and long term.

07z surface analysis shows ridging from IL up into northern MN,
with low pressure over the western Dakotas. Southerly flow through
the forecast area, with dewpoints in the 60s and lower 70s. Were
watching for the potential for convection overnight, some of which
could lead to Flash Flooding given the warm cloud depths and
available deeper column moisture. Things had been slow up through
2 am, but things are beginning to change. In the last hour, radars
are now starting to show anomalous propagation. Plus, satellite
fog enhancement imagery showing an expanding ACCAS field over
central MN, and that has now translated into convection. This is
the long-awaited convection that many of the models have been
suggesting for hours. Will have to watch trends on these. With the
delay in development, was losing confidence in the need for the
Flash Flood Watch. May still have to eventually cancel it if the
convection does not continue on an upscale trend and start
propagating south.

Outside of the convective threat, should see a continued warming
trend today as southerly flow continues under the building ridge.
850 mb temperatures build into the mid-20s Celsius, leading to
highs in the 85-90 range. With dewpoints in the 70s, should see
heat index values today into the 90s. A few spots over the western
areas could get near 100, but the more significant heat comes
tomorrow.

.LONG TERM...(Thursday through Tuesday)
Issued at 305 AM CDT Wed Jul 20 2016

850 mb temps rise into the upper 20s to near 30 Celsius by
Thursday afternoon, ahead of the cold front that is dropping south
through MN as a result of the short wave tracking across southern
Canada and flattening the ridge. Expecting this to lead to highs
well into the 90s across the forecast area. With dewpoints well
into the 70s, this combination will likely lead to heat index
values in the 105-110 range in places. Thinking we will be in need
of a Heat Advisory on Thursday. However, there are a couple of
things that lead to hanging onto the Watch for now: The front is
forecast to drop into the area during the late afternoon, which is
faster than previous forecasts, and the trend has been faster.
Several models are also now indicating that some convection will
be seen near this front. Model soundings do show a lot of elevated
instability above the cap, but there is a lot of CIN to overcome
before anything could happen. With the uncertainty, will hold with
the Watch for a bit longer. Our messaging is out there that it
will be hot, so this should not be a big deal waiting. This plays
into Friday as well. Temps will only fall into the muggy mid 70s
Thursday night, but Friday highs will depend on the frontal
position. Feel the models are pushing the front too far south as a
result of the convection. If it doesn`t happen, Friday could still
be rather steamy again. But with so many models indicating this,
opted to add some low rain chances just in case.

Another potent short wave trough traverses the international
border region for the weekend, bringing a better chance for rain.
Maintained higher rain chances as a result. Temperatures look to
be more in the normal range from the weekend into early next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 623 AM CDT Wed Jul 20 2016

Scattered thunderstorms are expected across the area today and
they could impact the TAF sites. Periods of heavy rainfall and
lightning would be the main concern at the TAF sites with IFR
conditions possible at times. Showers and storms will end by this
evening then we could see the potential for some weak low level
wind shear, depending on if surface winds become lighter. At this
time it appears that surface winds will stay high enough to not
reach LLWS criteria. Will continue to monitor surface wind trends
closely. Otherwise, plan on VFR conditions through the overnight
hours.

&&

.HYDROLOGY...
Issued at 346 AM CDT Wed Jul 20 2016

Despite the support for heavy rainfall potential with deep warm
cloud depth and PW of 2 inches, the atmosphere just can`t seem to
find a good focus to overcome the strengthening warm layer cap.
The only radar returns to this point has been some light showers.
Do not anticipate this increasing in coverage enough to justify
the need for a Flood Watch any longer, so we will cancel it.

&&

.ARX Watches/Warnings/Advisories...
WI...Excessive Heat Watch from Thursday morning through Friday
     evening for WIZ017-029-032>034-041>044-053>055-061.

MN...Excessive Heat Watch from Thursday morning through Friday
     evening for MNZ079-086>088-094>096.

IA...Excessive Heat Watch from Thursday morning through Friday
     evening for IAZ008>011-018-019-029-030.

&&

$$

SHORT TERM...MW
LONG TERM...MW
AVIATION...WETENKAMP
HYDROLOGY...MW


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