Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 121028

Area Forecast Discussion
National Weather Service La Crosse WI
525 AM CDT Sun Jul 12 2020

.SHORT TERM...(Today through Monday)
Issued at 246 AM CDT Sun Jul 12 2020

The shortwave trough which brought convection to the local area
yesterday afternoon/evening continues to dive further southeast into
Illinois/Indiana early this morning. Ridging aloft and surface high
pressure will build in its wake, leading to mostly dry conditions
today. Some hints that a few diurnally driven showers or storms may
develop this afternoon into early evening, though if any do they
would be be fighting dry air. A few weak ripples in the northwest
flow ahead of approaching upper ridge may steepen lapse rates just
enough to help spark some showers/storms, with any small chances
being confined mainly along and east of the Mississippi River (east
side of the ridge axis). High temperatures today will be seasonable,
ranging from the upper 70s to low 80s. Dry and clear conditions
likely tonight, with lows in the 50s to low 60s.

High pressure remains in control going into Monday, with a mostly
sunny day expected under large-scale subsidence. High temperatures
will be slightly warmer tomorrow, with values in the low to mid 80s.

.LONG TERM...(Monday Night through Saturday)
Issued at 246 AM CDT Sun Jul 12 2020

Upper-level ridging and surface high pressure will get booted east
going into Monday night as an upper low swings across southern
Canada/the far Northern Plains states. At the surface, a cold front
will approach the area, bringing our next chance for showers and
storms late Monday night into Tuesday. The front looks to bisect the
local area from roughly northern Wisconsin southwestward into
northern Iowa by midday Tuesday. If enough instability can build out
ahead of the front, there will be a chance for some stronger storms
come Tuesday afternoon/evening. There are still some differences
with instability and shear values between the models, with the GFS
having the strongest parameters thus far. The 12.00Z GFS has 1500-
2500 J/kg of CAPE (due to dewpoints in the mid 70s - too high) and 0-
3 km shear of around 25-30 kts. Whereas the 12.00Z ECMWF and NAM
indicate lesser instability (lower dewpoints in the upper 60s) and
similar to slightly better 0-3 km shear values. Either way this
plays out, the forecast parameters and forcing with the front should
be sufficient enough for some stronger or isolated severe storms
Tuesday afternoon into evening. Because of this, SPC has included
much of the area in a marginal risk for the Day 3 convective
outlook. The threat for locally heavy rainfall is also something to
watch on Tuesday as better moisture surges back northward ahead of
the frontal passage, resulting in deep warm cloud depths of 3.5-4 km
and PWATs of 1.5-2 inches.

The front looks to stall out across the south or just south of the
area going into Wednesday, which could result in continued
precipitation chances for some. By the end of next week and into the
weekend, flow aloft looks to becomes more zonal, though the ECMWF
suggests a more low amplitude ridge setting up overhead as a Pacific
low moves onshore out west. This would favor a slight warm up in
temperatures (a few degrees above normal values) by Friday/Saturday.
Confidence in how the flow aloft evolves is still a little uncertain
at this time. Occasional chances for showers and storms will be
possible from late week into the weekend with any shortwave trough
that moves through the large-scale flow.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 525 AM CDT Sun Jul 12 2020

Cigs: a few afternoon cu followed by mostly skc conditions through
the night. Some hints that the cu could go BKN/VFR for a time, but
bufkit soundings trend more dry and won`t add cigs for now.

WX/vsby: a few of the meso models suggest MVFR BR could develop
later tonight, and with light wind field/SKC conditions - some
support for that. Going to hold P6SM for now and monitor.

Winds: light north becoming light south/southeast this evening.




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