Area Forecast Discussion
Issued by NWS La Crosse, WI

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713
FXUS63 KARX 242345
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
645 PM CDT TUE MAY 24 2016

.SHORT TERM...(Tonight thru Wednesday night)
Issued at 241 PM CDT Tue May 24 2016

Main fcst concerns this period are SHRA/TSRA chances and any severe
potential with them.

Data analysis at 18Z had low pressure over the TX/OK panhandle with
troughing to a weak low in west-central MN. A front extended east
of this low to another east of lk Superior. Area radars and vis
imagery showed main cumulus field and remaining SHRA/TSRA moving
into far eastern IA, eastern WI and U.P of MI as moisture plume
and weak shortwave energy moves east across the region. Shortwave
ridging aloft building over the central/west parts of the region,
with sunny to partly sunny skies and temps mostly in the 70s.

No glaring issues noted with 24.12Z model initializations. GFS sfc
dew point initialized much better today. Models in good agreement
for some hgt rises/shortwave ridging to build across the region
late this afternoon/tonight. This ahead of a stronger piece of
shortwave energy to lift across the central plains tonight, then
across the fcst area Wed. Improving agreement for hgt falls over
the area Wed night as stronger northern plains energy moves into
northwest MN by 12Z Thu. Trend favors stronger of the earlier
models with the ridging tonight then shortwave to pass Wed. With a
tighter model consensus on the details tonight/Wed, short term
fcst confidence is generally good this cycle.

For the short term: Generally quiet (non SHRA/TSRA) weather expected
thru this evening with the weak shortwave ridging building across
the area and the area remaining under the right exit region of a 60-
80kt 300-250MB jet max. Lower level moisture transport hits a
minimum over the area around 00Z this evening. Moisture progs also
showing a decrease of PW over the area thru this evening as the
shortwave ridging builds across. Only potential trigger for any
SHRA/TSRA looks to be a weak sfc front sagging south into the area
this evening. Some MUCape progged to remain over the south half of
the fcst area this evening, however any shear there expected to be
quite weak. Will leave a small -SHRA/TSRA chance across the
southeast end of the fcst area this evening.

All this changes rather quickly after midnight with approach of the
next shortwave. Moisture starts to return/increase rather quickly
after midnight then spreads northeast across the fcst area Wed
morning. This with area under the more favorable right entrance
region of an 80-100kt 300-250MB jet max, PV advection with the
shortwave, PW values in the 1 to 1.5 inch range by late Wed
morning and the increased moisture transport/theta-e convergence.
MUcape of 500-1000 J/KG in the inflow airmass late tonight,
building to 1K to 2K by late Wed morning. Increasing confidence
for a band of SHRA/TSRA to move across the fcst area Wed. Of
course some meso-scale timing difference of this band among the
models. An increase of 0-6KM shear spreads across the area as well
Wed. Enough so that SWODY2 looks good at this time. If faster
models are more correct with the SHRA/TSRA band Wed, SWODY2 for
the area would be a bit overdone with many TSRA all competing for
the same limited energy Wed morning. If slower models are more
correct, the slight risk of severe may intrude more into the fcst
area for the late morning/afternoon hours. One last possibility is
portions of the fcst area getting split by the convection, with a
northern portion following the stronger shortwave/synoptic forcing
and a southern portion feeding on the moisture transport closer to
the greater cape pool just south of the fcst area. Will stay with
50-70% precip chances on Wed for now. SHRA/TSRA chances diminish
Wed night with passage of the shortwave and the moisture plume/
stronger moisture transport being pushed east of the area. If
faster models are more correct, much of Wed night may end up dry.
For now used a model consensus of SHRA/TSRA chances Wed night
given the meso-scale timing differences. Blend of guidance
lows/highs for tonight thru Wed night looks well trended.

.LONG TERM...(Thursday thru Tuesday)
Issued at 241 PM CDT Tue May 24 2016

For Thursday thru Friday night: Main fcst concerns this period are
the continued SHRA/TSRA chances.

24.12Z models in good agreement for rising hgts/shortwave ridging
aloft over the region Thu as the northwest MN shortwave tracks to
north of lk Superior and the next stronger energy from the rockies
troughing ejects into the southern plains. Improving agreement for
troughing to deepen in the central/southern plains Thu night/Fri,
with this trough to then begin lifting toward the Upper Midwest Fri
night. Trend of the model consensus is slower/stronger with the
plains troughing Thu night thru Fri night. Fcst confidence for Thu
thru Fri night is average to good.

Thu would see a relative lull in the SHRA/TSRA chances with weaker
forcing, some drying of the column and the rising hgts/weak
shortwave ridging over the region. However, the airmass progged to
remain unstable with potential for a weak sfc boundary to linger
in/near the area. NAM/GFS progging Thu afternoon dew points into the
70-75F range over much of IA into southern WI, and produce MUcapes
in the 4K-6K J/KG range. This seems unrealistic at this point with
little in the way of greenness in the corn/soybean fields across
IA and nearby states. ECMWF appears much more realistic with its
sfc dew points and cape over the region Thu. Did continue a small
mainly afternoon SHRA/TSRA chance Thu. System moving into the
plains spreads increasing moisture/lift across the fcst area
already Thu night then continues this into Fri night. Consensus
SHRA/TSRA chances in the 40-60% range Thu night thru Fri night
okay for now. As this time period approaches and the meso-scale
details sort them selves out, the entire Thu night thru Fri night
period is not likely to end up wet. Too early to determine much
for details on a severe risk with any of the Thu thru Fri night
TSRA. However a round of stronger to potentially severe storms in
there somewhere cannot be ruled out at this point. Blend of the
guidance highs/lows for Thu thru Fri night looks good for now.

For Saturday thru Tuesday, (days 4 to 7): Main fcst concerns this
period are the SHRA/TSRA chances thru the period in the continued
southwest flow aloft.

Medium range model runs of 24.00Z/24.12Z in reasonably good
agreement for a stronger shortwave trough to be lifting thru the
central plains into Upper Midwest Sat, then across the region Sat
night/Sun. Some timing differences by the day 4-5 time frame (and
for days 6/7), but this not unexpected. Some consensus for shortwave
ridging over the region behind this trough for Sun night/Mon. By
Tue, modest consensus for yet another shortwave trough in the
southwest flow to be lifting northeast toward/into the region as
strong troughing remains over the western CONUS. Given reasonable
model similarity in the longwave pattern, but the shortwave timing
differences, fcst confidence for days 4-7 is average this cycle.

Under southwest flow aloft, the unsettled pattern continues days
4-7. Present consensus timing would spread another surge of Gulf
of Mex moisture northward and a round of deeper layered forcing/
lift across the area Sat/Sat night. The deeper moisture/stronger
forcing would then exit northeast of the area Sun with shortwave
ridging and little forcing for Sun night/Mon. However there are
the timing differences. Consensus SHRA/TSRA chances in the 40-60%
range Sat, lowering into the 20-30% range by Sun night/Mon okay
for now. Next increase of Gulf of Mex moisture and lift would
arrive ahead of the next shortwave trough thru the flow Mon night/
Tue. Consensus 30-40% SHRA/TSRA chances Mon night/Tue reasonable
for now. With a warmer and more moist airmass over the area,
model/ensemble consensus highs/lows remaining a category or 2
above normal for Sat-Tue also looking well trended at this time.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 645 PM CDT Tue May 24 2016

Thunderstorms are expected at the TAF sites Wednesday, mainly from
the mid morning to the mid afternoon. Brief MVFR to IFR conditions
are possible near the stronger storms. Also, south to southeast
winds will increase to 10 to 14 kts on Wednesday with gusts to
around 25 kts possible. The strongest winds will be at KRST. Other
than around thunderstorms, plan on VFR conditions through the TAF
period.

&&

.HYDROLOGY...(Wednesday into Saturday)
Issued at 241 PM CDT Tue May 24 2016

Not too worried about potential Wed/Wed night rain amounts at this
point. With PW values approaching 1.5 inches, some of the SHRA/TSRA
could be efficient rain makers. With most of the fcst area well
short of normal on moisture since April 1st, the landscape could use
a good drink of water. If some locally heavier rains materialize
Wed/Wed night, then a prolonged period of more widespread SHRA/TSRA
Thu night into Sat (again could be efficient rain makers), some of
the landscape could become saturated by Fri/Sat with potential
runoff issues and rises on area streams and rivers.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...WETENKAMP
HYDROLOGY....RRS



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