Area Forecast Discussion
Issued by NWS La Crosse, WI

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258
FXUS63 KARX 261935
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
235 PM CDT Wed Jul 26 2017

.SHORT TERM...(Tonight thru Thursday night)
Issued at 235 PM CDT Wed Jul 26 2017

Main fcst concerns this period are lingering SHRA/TSRA chances this
evening then small mainly SHRA chances Thu afternoon/evening.

18z data analysis had a cold front from NE WI to near KLSE to KOMA,
pushing slowly SE across the region. Per WV imagery and radar
reflectivity, a shortwave was moving east across IA. Abundant cloud
cover across IA, and most of WI and southern MN has limited CAPE
development over much of the region today. Thus, in spite of it
being July, convection with the front and shortwave has generally
decreased since 12-14z. Models and SPC meso-analysis indicating a
general lack of moisture transport/theta-e convergence across WI/IA
and nearby areas this afternoon, thus mainly sct SHRA and isolated
TSRA these areas.

No problems noted with 26.12z model initializations. Models in good
agreement for the IA shortwave to quickly pass east of the area this
evening. A stronger shortwave to then drop into northern WI by 00z
Fri and toward lower MI by 12z Fri. Trend favors a tighter consensus
of earlier runs on timing/strength of this feature as it drops SE
across the western great lakes. Short-term fcst confidence is
generally good this cycle.

In the short term, the IA shortwave energy moves well E/SE of the
area by 03z with the weak cold front slipping SE of the area as
well. With limited CAPE today due to the cloud cover and decreasing
forcing/lift, trend among models (including hi-res/CAMs models) is
for the bulk of the SHRA/TSRA to be exiting/out of the fcst area by
or shortly after 00z. Short-term forecaster has already reduced
SHRA/TSRA chances tonight, and depending on mid afternoon WSR-88D
trends, may need to reduce them more. Given a mostly cloudy/cloudy
day, some rain most areas, then a decrease of clouds tonight, patchy
late night/early morning BR/FG seems reasonable. Some increase of
pressure gradient/BL winds later tonight as the main sfc low
migrates to northern IL, but this may not be enough for BL mixing
and will leave the patchy fog in the 06-13z grids as is for now. Can
high pressure builds in Thu/Thu night, but some moist BL air (sfc
dew points in the 60s) progged to be slow to scour out. And there is
the shortwave dropping into northern WI by 00z Fri. Depending on how
much BL/low level moisture remains Thu afternoon, models progging
some MUCAPE over much of the fcst area around 00z Fri. Soundings
showing this mainly capped between 700-600mb. With approach of the
shortwave in the afternoon, continued some small SHRA chances across
the NE end of the fcst area. May yet need some small SHRA chances
across the NE side of the fcst area into Thu evening but will it dry
for now to maintain a good blend with neighboring grids.

925-850mb cold advection spreads across the area for Thu. With a
decent amount of sunshine and mixing to about 850mb, the blend of
guidance highs mostly in the upper 70s-lower 80s looks good.
Considered adding a mention of valley BR/FG to late Thu night/early
Fri morning grids but area remains on the leading edge of the high
thru 12z Fri, with some gradient sfc-925mb winds to keep BL stirred.

.LONG TERM...(Friday thru Wednesday)
Issued at 235 PM CDT Wed Jul 26 2017

For Friday thru Saturday night: main fcst concerns this period are
temperatures.

Model runs of 26.12z continue to advertise rising hgts/shortwave
ridging to build across the Upper Midwest Fri thru Sat night, for
what is expected to be a dry, quiet, seasonable period. Fcst
confidence remains on the good side for this period.

As the ridging aloft builds in, drier and cooler Can high pressure
builds SE across the region under it, with the center of the
high/ridge axis near/over the area Fri night thru Sat night. Model
soundings showing diurnal mixing to around 850mb both Fri/Sat,
with this mixed lower portion of the column supporting highs
mostly in the upper 70s to lower 80s. These temps near the late
July normals, along with an influx of drier BL air and sfc dew
points mostly in the 50s, to make Fri/Sat a couple of rather
comfortable days. The one other concern this period may well be
late night/early morning valley fog. A well watered landscape,
plus evapo- transpiration near its seasonal max and the center of
the high near/ overhead, parameters would be favorable for valley
fog formation these nights. Plenty of time to detail any fog
possibilities as Fri night/Sat nights approach.

For Sunday thru Wednesday (days 4-7): main fcst concerns this period
are small SHRA/TSRA chances much of Sun afternoon thru Wed.

26.00z/26.12z medium range model runs in rather good agreement on NW
flow aloft to remain over the Upper Midwest Sun thru at least Wed.
What they disagree on, which is no real surprise, are various
shortwaves to be rippling SE thru this flow during the day 4-7
period. Confidence is above average for temps to be near normal Sun-
Wed, but given the detail differences with shortwaves, confidence is
average at best on any of the SHRA/TSRA chances this period.

Under the NW flow aloft, 925/850mb temps over the region remain
seasonable with the warmer air held off to the west over the
rockies/western plains. Consensus highs/lows a few degrees either
side of normal for Sun-Wed looking well trended at this time. Model
consensus holds the sfc high over the region Sun/Mon, with the high
then being pushed off into the OH valley Tue/Wed as a stronger
shortwave and a sfc front would approach from the NW. For Sun/Mon,
main moisture plume on the west side of the high generally remains
west of the fcst area. However a model run here and there does try
to drop a shortwave in later Sun or Mon, bringing the moisture plume
and a small chance of SHRA/TSRA east and SE toward the area.
Stronger approaching shortwave and sfc trough/front Tue/Wed offering
a bit more consistency to bring the moisture plume into MN/IA/WI for
the middle of next week. With little confidence in the shortwave
details, the small consensus SHRA/TSRA chances later Sun thru Wed a
reasonable place to start at this point.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1247 PM CDT Wed Jul 26 2017

Scattered showers will persist across the area through much of the
afternoon. The strongest showers could produce brief MVFR
conditions. Cannot rule out isolated TS, but it is looking more
likely this threat will stay south of the TAF sites. As chances for
showers dissipate early this evening, an MVFR cloud deck is forecast
to develop behind a southward advancing cold front. This cloud deck
should slide off to the south during the overnight hours. If skies
clear earlier tonight, patchy areas of fog could develop. However,
the potential for fog seems to be mitigated by lower than expected
rainfall amounts so far today. VFR conditions are expected through
the remainder of the period.

&&

.HYDROLOGY...through Wednesday night
Issued at 235 PM CDT Wed Jul 26 2017

Bulk of the locally heavy rain threat this afternoon/evening has
passed or shifted south of the fcst area. Any additional rains
thru this evening no longer expected to create renewed hydro
issues.

Flood Warnings continue along the Kickapoo River from Gays Mill to
Steuben. River levels at these sites are expected to fall below
flood stage within the next day or so.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....MH
HYDROLOGY....RRS



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