Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 200356

Area Forecast Discussion
National Weather Service La Crosse WI
1056 PM CDT Thu Sep 19 2019

.SHORT TERM...(This evening through Friday)
Issued at 238 PM CDT Thu Sep 19 2019

Early afternoon analysis showed diminishing showers to the east of
the forecast area as some shortwave energy continued across the
region. Farther south, a stationary boundary was located near the
IA/MO border. This boundary is progged to slowly work northward as a
warm front through this evening and overnight, bringing additional
shower and storm chances to portions of northeast Iowa and far
southwest Wisconsin. Trends in short term guidance have been slower
with the northward advancement of the front, so may not see quite as
much rain tonight as initially thought. In addition, the strongest
moisture transport will be well west of the forecast area. That
said, still looking like some elevated instability along the front
tonight, so definitely think at least some showers and storms will
be around. Something to certainly watch given the ongoing flooding
in far southern portions of the forecast area, as any stronger storm
could lead to additional localized flooding. Could also see some fog
tonight, given the moist airmass in place and weak flow.

Showers and storms could linger along the front during the day on
Friday, especially with a bit of shortwave energy approaching the
area. This (or just clouds) could have a bit of an impact on
temperatures, but a warm and humid airmass will still be in place,
with temperatures in the upper 70s to mid 80s.

.LONG TERM...(Friday night through Thursday)
Issued at 238 PM CDT Thu Sep 19 2019

The main forecast concern in the long term period is additional rain
chances, especially over the weekend.

An upper level trough will eject eastward out of the Pacific
Northwest and spin up a surface low over North Dakota, with the low
eventually moving into Canada. An associated surface cold front will
work its way through the area Saturday and Sunday, with plenty of
moisture moving in ahead of it to produce showers and storms.
Precipitable water values in the 1.5 to 2 inch range and warm cloud
depths approaching 4 km indicate the potential for heavy rain once
again. Raw model QPF output looks to favor southeastern portions of
the forecast area for the heaviest rain, so concern for additional
flooding is certainly elevated. In addition, 0-3 km shear will be on
the increase Saturday, which could lead to a few stronger or even
isolated severe storms, but as mentioned by the previous forecaster,
the strongest deep layer shear looks to lag behind the front and
questions remain on how much destabilization will occur with clouds
in the area.

Following the cold frontal passage, temperatures will return to more
seasonable values with highs in the mid 60s to mid 70s through the
middle of the work week. High pressure will provide a dry day on
Monday, but precip chances return for Tuesday/Wednesday as southerly
flow behind the high brings in moisture ahead of another cold front.
Confidence is somewhat low on exact timing of the precip though, as
models differ on timing of the frontal passage.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1055 PM CDT Thu Sep 19 2019

Main aviation concern will be potential for fog and low stratus
through Friday morning. Already seeing some valley locations
reporting fog with visibility down to 1/2 mile or less at this
hour. Dewpoint depressions currently ranging from 0 to 5 degrees
reveal the very moist airmass in place, and near surface winds are
very light or calm. Therefore confidence has increased over the
last few hours for reaching IFR conditions early Friday morning,
perhaps even briefly LIFR. The main question is whether SE winds
>10 knots just off the surface will translate into more of a low
stratus event for the TAF sites with less impact from fog. This
looks to be more the case for RST than LSE, so have trended
visibility lower at LSE with this issuance. Conditions should
mostly improve by late morning, but there are hints that MVFR cigs
may persist into the afternoon at RST. Have delayed VCSH until
after 00Z Saturday when stronger forcing will arrive.


Issued at 238 PM CDT Thu Sep 19 2019

River levels remain elevated generally along and south of
Interstate 90, with several sites above flood stage. With
additional rounds of heavy rain possible through the weekend, the
flooding threat will continue.




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