Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 301938
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
238 PM CDT SAT JUL 30 2016

.SHORT TERM...(Tonight thru Sunday night)
Issued at 238 PM CDT Sat Jul 30 2016

Main fcst concerns this period are SHRA/TSRA chances into early this
evening, valley fog potential late tonight/early Sun morning,
temperatures thru the period.

18z data analysis had high pressure centered over the great lakes
with ridging to the southwest across WI to IA. Anti-cyclonic flow
around the sfc-850mb ridge axis slowly clearing out the lower level
moisture over WI, but moisture converging in a narrow band around
the west side of the ridge axis from eastern IA to southeast and
east-central MN. Cooler temps aloft allowing for plenty of diurnal
cumulus development across the region, along with 500-1500 J/KG of
SB CAPE. Sct SHRA, even a few TSRA, in the band of pooled sfc-850mb
moisture. More overall sunshine across the region today was warming
early afternoon temps into the mid 70s-low 80s, only about a
category below the normals.

No issues noted with 30.12z model initializations. Solutions quite
similar as a rather strong mid level low slowly tracks across the
Canadian Rockies thru the period with downstream rising hgts over
the Upper Midwest. Short-term fcst confidence is good this cycle.

For the short-term: one weak shortwave tries to drop across the area
this evening with another due in Sun afternoon. Overall larger scale
forcing of rising hgts over the region looks to negate these waves,
leaving the weather rather quiet for the area tonight thru Sun
night. Any SHRA/TSRA from this afternoon looking to diminish rather
quickly with loss of diurnal heating this evening. After that, the
building ridging Strengthens the high over the great lakes allowing
it to build a bit westward and spreads deeper layered subsidence and
anti-cyclonic flow over the area for much of tonight thru Sun night.
Will continue the trend of clearing skies for much of the fcst area
tonight. Soundings showing a deep layer of light winds to near
600mb. This along with residual boundary layer moisture and the sfc
ridge axis nearby remains very favorable for radiational valley fog
formation late tonight into early Sun morning. Continued with the
mention of patchy areas of fog in the favorable locations along/east
of the MS river in the 09-13z period tonight. A shortwave tries to
sneak thru the ridging and approach the area late Sun night, slower
than earlier model runs. Any increasing moisture/moisture transport
with this now looks to remain west of the area until Mon, with fcst
trending dry and mostly clear Sun thru Sun night. Plenty of sunshine
on Sun along with a bit more south winds should allow highs to climb
to near normal. Blend of guidance lows/highs for tonight thru Sun
night appears good.

.LONG TERM...(Monday thru Saturday)
Issued at 238 PM CDT Sat Jul 30 2016

For Monday thru Tuesday night: the main fcst concerns this period
are the SHRA/TSRA chances mainly Mon afternoon into Tue night.

30.12z models in rather good agreement as the Can Rockies mid level
low moves slowly east thru the period, ending up west of Hudon`s bay
at 12z Wed. This wave flattens the riding over the region a bit Mon
thru Tue. Another mid level low tracks inland from the Pacific and
ends up near the MT/ALB border by 12z Wed, working to rebuild the
ridging over the region Tue night. Some detail differences on smaller
shortwaves thru the flatter flow over the region Mon-Tue however
fcst confidence for Mon-Tue night remains average to good this cycle.

Mid level low tracking across central Can slowly drags a sfc-700mb
trough eastward to near the MN/Dakotas border by 12z Tue, then
into/across the fcst area late Tue/Tue night. Plenty of detail
differences by Tue/Tue night but general trend is for the trough/
front to weaken as it would approach move into the area. NAM appears
the outlier by Tue. However at least GFS/Can Gem bring this trough
in with some weak shortwave energy sneaking thru the flattened
ridging aloft. Models (NAM slower than the rest) bring a plume of
1.5+ inch PW into the region ahead of this trough along with an axis
of 1K-2K J/KG MUCAPE and modest 925-700mb moisture transport in the
flow. Most models generating a `warm advection wing` of SHRA/TSRA on
the stronger initial surge of moisture transport/warm advection/
isentropic lift into the area Mon night, ahead of the 700mb
warming/capping. Models then producing a sct-bkn line of SHRA/TSRA
as the decaying sfc-700mb trough/front would move in/across Tue/Tue
night. Lower level airmass continues to warm Mon/Tue, but this
looking to be off-set by more clouds both days. If drier/partly
cloudy models would end up more correct Tue a few highs could reach
90F. Given the detail differences much of this period, stayed near a
blend of the guidance highs/lows for Mon thru Tue night and
SHRA/TSRA chances of 30-50% centered on Mon night/Tue.

For Wednesday thru Saturday (days 4 to 7): main fcst concerns
this period are SHRA/TSRA chance much of the period, potential for
heat indices near/above 100 Wed/Thu.

Medium range model runs of 30.00z/30.12z in good agreement on
rising hgts/ridging over the region on Wed, ahead of stronger
troughing moving east along the MT/Can border. Decent agreement
for this wave to lift toward Hudson`s Bay/western Ont Thu, but
still lowering hgts over the region as it does. Differences
increase for Fri/Sat but consensus is for rising hgts over the
Rockies/western plains and broad west-northwest flow aloft across
the western great lakes. Fcst confidence is average to good
Wed/Thu then average Fri/Sat.

A warm, moist airmass remains over the area Wed in the broad
southwest sfc-500mb flow. Would think Wed would be a dry, very warm
day under the 850-700mb thermal ridging, but there are model
differences on the strength of the 700mb warming/capping and
strength of some lower level warm advection/isentropic lift into
the area. Can`t completely rule out a SHRA/TSRA chance Wed with
the small consensus POPs looking okay. If Wed trends toward the
drier/warmer of the models, highs could easily reach 90-95F with
concerns for heat advisory conditions. Will stay with the model
consensus temp for Wed until the details get sorted out. Trough
moving across southern Can Wed night thru Thu night drags a sfc-
700mb trough/front and falling hgts aloft into the area. Some
timing differences as usual, but with cooling temps aloft and the
approaching/passing trough/front, SHRA/TSRA chances in the 20-40%
range for later Wed night into Thu night are reasonable. Highs
Thu a big question depending on timing of the front into/thru the
area. Slower front would give Thu the potential to be another day
of highs 90-95F and heat advisory concerns. Behind the front,
Fri/Sat trending drier or dry with temps returning to near the
early Aug normals as a modified Pacific high would be building in
from the northern plains/southern Can.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1235 PM CDT Sat Jul 30 2016

VFR conditions continue through the afternoon with some scattered to
broken cumulus, and likely a few showers and maybe a rumble of
thunder around RST. Any showers that develop will be slow moving, so
it is possible that period of lower ceilings and/or visibility will
occur at RST if any showers develop over the site. Otherwise, high
pressure looks to build into the area overnight, with skies expected
to clear off. With light winds in place, suspect there will be some
fog development, particularly across the river valleys. Not yet
ready to commit LSE to LIFR conditions in fog with low level drying
taking place through the night, but will include a vicinity mention
as fog will probably be nearby. Tomorrow should feature dry weather
with just some diurnal cumulus development, though less than seen
today.

&&

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

$$

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


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