Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 311918
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
218 PM CDT TUE MAY 31 2016

.SHORT TERM...(Tonight through Wednesday)
Issued at 218 PM CDT Tue May 31 2016

Obvious focus for tonight is on-going convection and related severe
weather threat. Idea of marginal threat still on track with less
organized storm structure, but could not rule out possible severe
weather depending on location and intensity.

Closed upper-low is making steady progress across Northern Plains
with well occluded surface front structure into western Great Lakes.
Cold front has passed Interstate 35 and moving into western forecast
area at mid afternoon, with occluded or warm front arcing into
western Wisconsin, and perhaps some type of trough feature marking
higher dew points closer to Mississippi River.

Narrow area of higher dew point air sandwiched in between all this
producing 1000-1500 J/Kg most unstable CAPE values. Meanwhile shear
is very minimal in this corridor, which you can see with gust front
progress ahead of convection when it formed earlier in day. Only
exception to shear diagnosis is along trough near Iowa-Wisconsin
border where some low-level shear could be materialized as storms
cross.

HRRR runs have been very consistent popping up convection along main
cold front in better low-level lapse rates, but deeper shear is well
post-frontal so besides local interaction organization, just do not
expect very significant storms. Could see an occasional warning or
two with lots of marginal activity, including some hail and gusty
winds. Other storms are expected closer to trough, mainly in
Wisconsin but going into this evening, storms will likely merge as
cold front sweeps east, ending threat from west to east.

Even though front will progress east, will need to watch southwest
to northeast training of cells as well leading to localized heavy
rain threat. Could see some urban or minor problems from time to
time. Still fairly dry in most places so guessing some rain can be
absorbed before troubles result.

Although area will be post-frontal Wednesday, proximty of upper low
could be close enough to generate showers, mainly along and north of
Interstate 94 where lapse rates could be steeper.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 218 PM CDT Tue May 31 2016

Post wave and front will bring cooler, but quiet stretch from
Wednesday into Thursday with short term ridging and drier air moving
in.

After that, next upstream wave will buckle nearly zonal flow across
northern states with warm air advection returning by Thursday night.
Wave deepens flow but not sure enough moisture return is expected to
aid with instability. Would guess severe weather threat might remain
low to marginal even with this feature...but still several days out.

Even cooler behind this feature as cooler drier likely to come out
of Hudson Bay high so likely quiet going into early next week but
temperatures residing below normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1232 PM CDT Tue May 31 2016

A line of SHRA/TSRA still on track to move across the the TAF sites
this afternoon into early this evening. SHRA/TSRA were already
developing into north-central IA, with distance-speed tool bringing
them into the KRST area around 20Z. Little fuzzier timing into KLSE,
but in the 22Z-01Z window remains quite valid. Added a tempo group
to both TAFs for MVFR cigs/vsbys, and some gusty west winds, as the
band of SHRA/TSRA would pass across each site. Outside of this
period of convection with the frontal and upper trough passage,
generally good VFR conditions expected at the TAF sites early this
afternoon and again later this evening through Wed.

A concern late tonight/early Wed morning remains potential valley
BR/FG at KLSE. Depending on how much rain falls with the convection
later this afternoon/early this evening, if skies clear overnight
and winds decouple, conditions would be favorable for valley fog
formation at KLSE in the 09Z-13Z time-frame. left this out of the
KLSE taf for now, but will pass along concerns to later shifts to
continue monitor the KLSE BR/FG potential.

&&

.ARX Watches/Warnings/Advisories...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Shea
LONG TERM...Shea
AVIATION...RRS


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