Area Forecast Discussion
Issued by NWS La Crosse, WI

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531
FXUS63 KARX 201655
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1155 AM CDT WED JUL 20 2016

.UPDATE...
Issued at 1155 AM CDT Wed Jul 20 2016

have updated the excessive heat watch to an excessive heat warning
for the entire CWA starting at noon Thu. This then extends thru
10pm Thu for Clark/Taylor counties and 7pm Fri for the remainder
of the CWA. Excessive heat for Thu into Fri not as slam-dunk as
it looked a day or 2 ago, but enough confidence and collaboration
with the neighbors for the upgrade late this morning.

Questions starting to crop up on potential convection/rain chances
dropping south into the fcst area late tonight and lingering into
at least Thu morning. This with convective cool pools and debris
clouds, with potential to impact highs Thu afternoon.  For strong
warming/heating and pushing heat indices above the 105-110 range
a dry and sunny/mostly sunny fcst would be more favorable, but
this no longer a given in the 20.12z model suite. Chances for
convection across the area again later Thu night into Fri as a
sfc-850mb trough/front would slide into the area. Clouds/SHRA/
TSRA/debris clouds could well again impact warming/highs on Fri.

&&

.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 18Z TAFS through 18Z Thursday afternoon)
Issued at 1155 AM CDT Wed Jul 20 2016

Generally good VFR expected this period as warm mid level ridging
and mdt sfc pressure gradient build across the fcst area. South
gradient winds around 10kts to keep the boundary layer stirred
thru tonight, and any late night/early morning BR to a minimum.
The warming mid level temps should keep clouds thru the period in
the sct 5k-10k ft range.  However, there are some caveats as a
sfc-850mb trough/front slowly sags south toward the fcst area.
Airmass over the area is quite moist with perhaps a shortwave
ripple or 2 to move thru the rather flat ridging aloft over the
region. Some signals among the models these could be strong enough
to trigger convection that may move into the TAF sites late
tonight into Thu morning. Confidence not high enough to include a
TSRA mention in the TAFs after roughly 09Z at this time.

&&

.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 Warning from noon Thursday to 7 PM CDT Friday for
     WIZ032>034-041>044-053>055-061.

     Excessive Heat Warning from noon to 10 PM CDT Thursday for
     WIZ017-029.

MN...Excessive Heat Warning from noon Thursday to 7 PM CDT Friday for
     MNZ079-086>088-094>096.

IA...Excessive Heat Warning from noon Thursday to 7 PM CDT Friday for
     IAZ008>011-018-019-029-030.

&&

$$

UPDATE.......RRS/DAS
SHORT TERM...MW
LONG TERM....MW
AVIATION.....WETENKAMP
HYDROLOGY....MW



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