Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 252334
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
634 PM CDT THU AUG 25 2016

.SHORT TERM...(Tonight through Friday night)
Issued at 245 PM CDT Thu Aug 25 2016

Main fcst concerns this period: valley fog potential late tonight/
Fri morning, returning SHRA/TSRA chances and heavy rain threat
Friday night.

18z data analysis had broad high pressure centered near the Black
Hills with ridging east into IA. Low pressure was located over Ont.
Between the 2, a W-NW gradient covered the region, with westerly
winds continuing to spread a drier lower level airmass into the
Upper Midwest. However, mid/high clouds associated with shortwave
energy over southern IA to KS were spreading over much of the south
half of the region, while lower level moisture/clouds wrapped behind
a stronger shortwave near the MN/Ont border were spreading across
much of northern MN into NW WI.

25.12z models initialized well. Solutions offer a tight consensus as
the northern MN/southwest Ont shortwave rotates east across Ont
tonight then weak shortwave ridging quickly builds east across the
region Fri. Solution remain quite similar as hgts fall over the
region Fri night, ahead of shortwave troughing/energy moving across
the northern plains. Even with a rather tight larger scale
consensus, some meso/convective scale detail differences by Fri
night that have sensible weather impacts for the area. Fcst
confidence is good tonight/Fri, then more average Fri night.

In the short term the dry/quiet weather tonight/Fri as southern
stream energy remains south of the area and the MN/Ont border
shortwave moves east away from the region and hgts gradually rise
thru 00z Sat. This quiet weather will help rivers in northeast IA
crest then fall, and with clean up and recovery work where rivers
are receding. W-NW sfc-700mb flow continues to push a drier airmass
into area for tonight/Fri. This as high pressure from the northern
plains/southern Can builds east into/across the Upper Midwest.
Lower clouds with the northern wave looking to brush the north end
of the fcst area this evening, otherwise deep layered westerly flow
and north/south stream waves moving east of the area looking to
produce clear/mostly clear skies across the area by later tonight
and for Fri. Light winds, drier airmass, mostly clear skies, sfc
high building over head, a cooler night tonight with strong
radiational cooling and favored cooler of the guidance lows. Layer
of light winds remains rather shallow much of the night, with winds
above the boundary layer N-NW 10-15kts - this not overly favorable
for valley fog formation. For now limited valley fog formation to
the lower WI river valley, where fog most likely to develop on
nights with marginal conditions for it. A comfortable day Fri with
dew points in the 50s and mixed 925mb temps producing highs in the
70s.

Lower level thermo-dynamic forcing and moisture transport/increase
ahead of the troughing coming across the northern plains spreads
into the area Fri night. Reasonable agreement for PW values to
increase into the 1.5 inch range by 12z Sat, but that is where the
lower level similarities by later Fri night end. Plenty of detail
differences among the models on the various lower level lift
parameters, depending on timing/strength of the shortwave energy
coming thru the increasing southwest flow aloft over the region.
Nam/ECMWF lean faster/stronger and would spread higher chances of
more rainfall across most of the area by 12z Sat. Nam with 1/2 to 1
inch for a good share of the area, but it`s on its own with this.
Bulk of other models weak/slower with the lead shortwave energy and
spread less precip chance/amounts into the area Fri night, and keep
the bulk of the CAPE south of the fcst area thru 12z Sat. Given
lesser confidence in the details Fri night stayed near the model
consensus of SHRA/TSRA chances/amounts Fri night.

.LONG TERM...(Saturday through Thursday)
Issued at 245 PM CDT Thu Aug 25 2016

For Saturday thru Sunday night: main fcst concerns this period are
SHRA/TSRA chances/rain amounts Sat/Sat evening, temperatures thru
the period.

25.12z models offering a tightening consensus for the main shortwave
and trough to move into northern MN Sat, continuing quickly
northeast Sat night. Reasonable agreement for hgts over the region
to rise Sun, ahead of weak troughing moving across the central/
northern plains to move toward/into the region for Sun night. Fcst
confidence for Sat thru Sun night is average to good this cycle.

Lower level thermo-dynamic/forcing detail differences coming out of
Fri night continue on Sat. Stronger of the forcing and deeper/higher
of the moisture/PW is over the area Sat morning along with MUCAPE
increasing into the 500-1000 J?KG range. The forcing diminishes west
to east thru Sat afternoon/evening as the 925-700mb trough axis
passes, along with a decrease of PW as deeper westerly flow spreads
drier air into the area. Continued with 70-75% SHRA/TSRA chances Sat
morning then decreasing from west to east thru Sat afternoon/
evening. Threat of locally heavy rains Sat is decreasing as
SHRA/TSRA Sat are looking to be progressive with the 925-700mb
trough axis, but threat cannot be completely ruled out yet. High
pressure and weak shortwave ridging aloft expected to move across
the region Sun. However models want to quickly return southwest
lower level flow and spread some 925-750mb moisture northeast from
the central plains. MUCAPE progged to be in the 1K-2K J/KG range by
Sun afternoon. Continued a dry fcst for Sun (which blends well with
neighboring grids), but confidence not high in this at this time.
With approach of the weak trough and a moist/unstable airmass over
the area Sun night the small consensus SHRA/TSRA chances are
reasonable. Used a blend of the numerical guidance for highs/lows
Sat thru Sun night. Sat highs the trickiest. If SHRA/TSRA/clouds
would move out more quickly/drier of models for Sat end up more
correct highs could be a category or 2 above those in the fcst
grids.

For Monday thru Thursday (days 4 to 7): main fcst concerns this
period include small SHRA/TSRA chances thru much of the period,
temperatures thru the period.

25.00z/25.12z medium range models in good agreement on quasi-zonal
flow across the northern CONUS Mon/Tue with the main shortwave track
across southern Can. By Tue night/Wed, models in reasonable
agreement for the flow to start buckling as stronger troughing digs
along the west coast, with downstream ridging building over central
NOAM. Detail differences with the strength of the ridging by Wed/
Thu and on pieces of shortwave energy try to sneak thru it. Fcst
confidence this period is good for near/above normal temps but on
the low side for any of the small precip chances.

Mid level troughing moving across south-central Can Mon/Tue pushes
the sfc high east and drags a weakening sfc trough/front toward/ into
the region. Increasing southerly flow does spread some increase of
moisture into the area but transport into the area and overall
forcing progged to remain on the weak side. Some small SHRA/TSRA
chances Mon afternoon/Tue reasonable for now with the approach of a
lower level trough. Hgts rise Wed/Thu, but with a moderately moist
airmass over the area and potential for weak shortwaves to ripple
thru the ridge, continued small SHRA/TSRA chances Wed/Thu okay for
now. With southwest lower level flow Mon-Thu and an extended period
of lower level warm advection, 925mb temps by Wed/Thu progged to be
in the 23C-26C. Mixed, these temps would produce highs in the low-
mid 80s as Aug ends and Sept begins. 925mb temps Mon/Tue in the 21C-
24C range, supporting highs near the normal upper 70s/lower 80s for
the start of the work week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 634 PM CDT Thu Aug 25 2016

VFR conditions are expected over the next 24 hours for both LSE
and RST as high pressure and drier air work across the region.
Winds will remain at or below 8 knots through the TAF period, and
may well end up quite variable through the day on Friday with the
ridge axis overhead. One thing we continue to monitor is the
potential for some fog at LSE, though with drying through the
night and winds in the 10-16 knot range just above the shallow
surface inversion, it appears this is not a very good setup for
fog at the airport. With that said, there may be some shreds of
fog in the tributaries surrounding LSE.

&&

.HYDROLOGY...tonight thru Saturday.
Issued at 300 AM CDT Thu Aug 25 2016

Moderate to major flooding ongoing on portions of the Turkey and
Upper Iowa rivers and their tributaries in northeast Iowa.
Warnings continue for much of this area. Some high water also in
Kickapoo River basin in southwest Wisconsin but that is improving.

Dry weather continues tonight/Fri, to aid flood clean up and
recovery where river levels are falling. SHRA/TSRA chances return
for Fri night/Sat. Will have to watch this for the threat of
additional heavy rains. A moist airmass returns for Fri night/Sat,
but appears the more unstable of the airmass and greater threat for
any locally heavy rains may end up more in central IA and northern
IL, south of the area. Even so, potential still exits for 1/2 to 1
inch of rainfall across forecast area from Fri evening thru Sat
evening.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...Lawrence
HYDROLOGY....RRS



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