Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 290452
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1152 PM CDT THU JUL 28 2016

.SHORT TERM...(Tonight thru Friday night)
Issued at 314 PM CDT Thu Jul 28 2016

Main fcst concerns this period: small mainly SHRA chances across the
south half of the fcst area thru the period, cooler temperatures.

Data analysis at 18z had low pressure over central IL with an
inverted trough to the northwest near a KDBQ-KLSE-KEAU line. WV
imagery showed a shortwave spinning slowly east across southern MN.
Combo of the lower level circulation, mid level trough and shortwave
producing a rather classic looking `comma` head cloud across the
Upper Midwest today. Model soundings/KMPX RA-OB from this morning
showing the column nearly saturated to 700mb. The lower level
circulation and shortwave aloft doing some lifting putting the
squeeze on the airmass, with KARX 88D showing at light light returns
over much of the fcst area. With this airmass, even the lightest of
return producing -RA/DZ today. This precip and the low clouds/cigs
doing a number on temps today with early afternoon temps only in the
60s to low 70s, some 10-20F below normal.

No issues noted with 28.12z model initializations. Solutions rather
similar as the MN shortwave rotates across the fcst area later this
afternoon/evening, then as the energy over western Neb passes mainly
across IA/IL Fri/Fri night. The mid level trough axis remains near/
over the area thru tonight/Fri with the passing shortwave energy,
then is replaced by weak shortwave ridging Fri night. Larger scale
similarities would say short-term fcst confidence is good but some
subtle smaller scale detail differences are there to impact the
sensible weather outcomes. Overall short-term fcst confidence is
above average this cycle.

In the short-term: an inverted sfc-925mb trough and the 850-700mb
circulation remain near/over the area thru tonight into Fri evening
as the MN shortwave passes this evening then the stronger western
Neb shortwave passes just south of the area Fri. Sfc-700mb portion
of the column remains quite saturated tonight into Fri evening,
especially over the southeast half of the fcst area. Until the
trough axis and sfc-700mb moisture clear the area, will continue
with some 20-30% -SHRA chances where the deeper moisture lingers.
Generally limited an isolated TSRA mention to the afternoon/early
evening hours when any CAPE would be greater. Plenty of clouds/lower
level moisture looking to keep lows from falling too far tonight.
Clouds to limit diurnal warming again Fri, with warmer of highs Fri
across the north end of the fcst area where a bit more sunshine is
expected during the afternoon hours. Passage of the mid level trough
axis and some rise of hgts/shortwave ridging building in Fri night
should put an end to the -SHRA. However, clouds may be slow to clear
over the south/east portions of the fcst area as 850-700mb moisture
looks to wrap around the west/north side of the slowly departing
circulation. Had considered adding some radiational valley fog to
favored areas around 12z Sat, but potential cloud decks lingering,
left this out of the grids for now. Stayed with a blend of the
guidance lows/highs for tonight thru Fri night.

.LONG TERM...(Saturday through Thursday)
Issued at 314 PM CDT Thu Jul 28 2016

For Saturday thru Sunday night...

Main fcst concerns this period are any lingering SHRA chances Sat,
temperatures.

28.12z model runs in reasonable agreement for a northwest flow
shortwave to drop toward the fcst area Sat and a weaker mid level
trough axis to pass Sat night. Detail difference by Sun/Sun night
but trend is for rising hgts/shortwave ridging into build into the
Upper Midwest to end the weekend. Fcst confidence in the Sat thru
Sun night period is average to good this cycle.

Models showing a narrowish plume of moisture and perhaps 500-1000
/KG MUCAPE to sneak into the fcst area Sat, around the west side of
high pressure centered over the great lakes. This ahead of the
shortwave approaching from the northwest. Small consensus SHRA/TSRA
chance for much of the fcst area Sat afternoon reasonable at this
point in time. Great lakes sfc high then progged to build west Sat
night/Sun morning before retreating back to the east Sun afternoon/
night as low pressure advances across the northern plains. Rising
hgts aloft and building high pressure make for a dry fcst Sat night/
Sun. Sun night should be dry too, but reasonable model agreement for
850-700mb moisture transport with PW values around 1.5 inches to be
spreading into the west end of the fcst area later Sun night. left
the small consensus SHRA/TSRA chance over the southwest end of the
fcst area after 06z Sun night as the deeper moisture and some lower
level thermo-dynamic forcing would be arriving. Model soundings
showing mixing to near 875mb Sat/Sun, with mixed 925mb temps
supporting highs a few degrees below the late July normals Sat then
near normal Sun. Blend of numerical temp guidance looks to have the
highs/lows for Sat thru Sun night well trended at this time.

For Monday thru Thursday (days 4-7)...

Main fcst concerns this period are warming temperatures and
SHRA/TSRA chances thru the period.

Medium range model runs of 28.00z/28/12z in decent agreement for
shortwave ridging to build across the Upper Midwest Monday, which is
quickly pushed east Mon night by a stronger shortwave tracking
across south-central Can. Timing/strength differences among the
models on this shortwave, and subsequent shortwaves to come thru the
quasi-zonal flow over the north-central CONUS Wed/Thu. Given the
usual timing/strength differences among the models on shortwaves in
the day 5-7 time-frame, fcst confidence for Mon-Thu is average this
cycle.

Monday should be one more quiet/dry day as the shortwave ridge axis
moves across the region. However GFS/ECMWF already spreading
increasing lower level thermo-dynamic forcing/lift and increasing
moisture/CAPE ahead of troughing in the northern plains into the
area on Mon. Small SHRA/TSRA chances mainly west of the MS River on
Mon okay for now. Wave coming across south-central Can Mon night/Tue
would drag the troughing/front in the plains eastward into/across
the region during that time-period. Even with some model detail
differences, increasing SHRA/TSRA chances Mon night/Tue as the
trough/front would move into the area reasonable. Next shortwave
trough across south-central Can Wed/Thu would then push another
lower level trough/front into/across the area (depending on the
timing) Wed into Thu, for continued SHRA/TSRA chances. Not all the
periods from Mon night thru Thu likely to end up wet, but which
would be the dry periods difficult to know given the model timing
differences. By Tue-Thu NAEFS indicating 850mb temps of 1 to 2 std
deviations above normal, with potential for highs to be back in the
90s for the middle/end of next week. With potential for clouds/SHRA/
TSRA to limit diurnal warming any of these days, model/ensemble
consensus highs/lows for Mon-Thu look good at this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1152 PM CDT Thu Jul 28 2016

MVFR clouds have been tough to erode this evening and with light
easterly flow overnight, see no reason why they won`t stick around
for at least a few more hours, especially at KRST. Should finally
see a return to VFR ceilings by sunrise Friday morning. A few
showers remain across the area tonight, but if they happen to
move over an airfield, expect little to no impact, so will keep
forecast dry. For 29.12Z through the end of the period, expect
VFR conditions with most if not all additional rainfall south and
east of KRST/KLSE. Winds will be light from the northeast.

&&

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

$$

SHORT TERM...RRS
LONG TERM...RRS
AVIATION...ROGERS


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