Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 012327
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
627 PM CDT FRI JUL 1 2016

.UPDATE...
Issued at 627 PM CDT Fri Jul 1 2016

Updated the sky forecast as there is a massive amount of cirrus
over South Dakota and Nebraska spreading eastward towards the
forecast area. This cirrus represents a subtropical plume of
moisture from the east Pacific being pulled northward by an upper
low near the four corners. With northwest flow aloft over our
region and a shortwave tracking southeast out of northwestern
South Dakota, this cirrus should spread across most locations
along/south of I-90 tonight. Increased cloud cover as a result and
may have to increase further if it looks like the cirrus will end
up more dense. Low temperatures south of I-90 still look okay for
now, but it is possible these may need to be increased slightly.
Lastly, given the cirrus spreading across, this should help
keep valley fog from getting too dense outside the WI river valley.
Going coverages for fog seem reasonable for now.

&&

.SHORT TERM...(Tonight through Saturday night)
Issued at 250 PM CDT Fri Jul 1 2016

Main fcst concerns this period are valley fog potential tonight and
cooler than normal temperatures thru the period.

Data analysis at 18z had a broad and cool area of Canadian high
pressure centered over MN, building steadily southeast into the
Upper Midwest. Diurnal cumulus was located over much of MN/IA/WI
north/south of the low level ridge axis, but the end result was a
mostly sunny afternoon across most of the region. Even with mostly
sunny skies, being that it is a Canadian high, early afternoon temps
were mainly in the upper 60s to mid 70s, some 8F to 15F below the
July 1st normals.

01.12z models look to have initialized quite well. Solutions quite
similar for tonight thru Sat night as the flow aloft remains north-
west but hgts slowly rise over the region. Improving consistency for
shortwave energy moving out of the southwest CONUS into the western
plains tonight to track east across the central plains into mid MS
valley Sat/Sat night. Short-term fcst confidence is good this cycle.

In the short term, sfc ridge axis drifts across the area tonight
with light/calm winds and clear/mostly clear skies, a favorable set-
up for radiational valley fog formation. Not the most favorable time
of the year for late night/early morning fog formation with the
short nights, but model soundings showing deep layer of light winds
from the sfc to 850mb ridge axis drifts across. Sfc dew points
should rebound to the low/mid 50s as E-T continues for a while into
the evening and the mixing layer shrinks with sunset/nocturnal
stabilization. These dew point to then be chased down by the cooling
overnight temps. Per NAEFS, 850mb temps at 12z Sat 1 to 2 STD
deviations below normal. This cool airmass, the light winds and
clear skies should allow lows tonight to fall into the mid 40s to
mid 50s (coolest in the low laying areas along/northeast of I-94),
and near/below the expected mid-evening dew points. Previous crew
added patches/areas of valley fog to fcst grids in the 08-14z
period. Refined it a bit on both ends of that time period. Model
soundings showing mixing to 850mb Sat afternoon, with the mixed
850mb temps supporting highs mostly in the mid/upper 70s. These
highs still a category or 2 below normal but should make for a
rather comfortable start to the holiday weekend (opposed to the
July 4th 2012 holiday period when highs topped out in the mid 90s
to around 100 for several days).

Sfc ridge axis remains near the area Sat night, with what should
be clear/mostly clear skies. Some increase of winds in the 925-
850mb layer as the lower level circulation with the shortwave
energy coming across the central plains moves into western MO by
12z Sun. May yet need patchy fog mention late Sat night into
early Sun morning, but given a weaker signal for it left this
mention out of the grids for now. Trended toward cooler of
guidance lows tonight then favored a blend of the guidance highs/
lows Sat/Sat night.

.LONG TERM...(Sunday through Friday)
Issued at 250 PM CDT Fri Jul 1 2016

For Sunday thru Monday night: Main fcst concern this period is
temperatures.

01.12z model runs offering a tighter consensus as the shortwave
trough tracks across the mid MS valley Sun. Northwest flow aloft
continues over the region Sunday, then gradually decreases/becomes
zonal/westerly with rising hgts Sun night thru Mon night. Trend
favors more progressive of the earlier runs with the mid MS/OH
valley shortwave Sun/Sun night, then toward higher hgts over the
area Mon/Mon night. Fcst confidence for Sun thru Mon night is good
this cycle.

Dry/quiet pattern continues over the region thru the Sun/Mon portion
of the holiday weekend. High pressure remains anchored over the great
lakes thru Mon night, with ridging westward into MN/WI. A seasonably
cool east-southeast flow is over the area Sun, then southeast around
the ridge axis Mon. Model soundings showing mixing thru 850mb both
Sun/Mon afternoon. Mixed 850mb temps supporting highs in the mid 70s
to near 80 Sun afternoon then in the upper 70s to lower 80s Mon
afternoon. These highs Mon still about a category below the early
July normals. Plenty of sunshine both afternoons will make it feel
warmer than the recorded highs. Light winds and a dry-ish airmass
will allow for another stronger radiational cooling/cooler night Sun
night. By Mon night south/southwest gradient sfc-850mb flow starts
to increase as the high moves off, brining both an increase of lower
level moisture and some winds for boundary layer mixing. Lows Mon
night trending closer to the normals as the holiday weekend comes
to a close. Blend of the guidance highs/lows for Sun thru Mon
night looking to be in the ballpark and well trended.

For Tuesday thru Friday (days 4 to 7): Main fcst concerns this
period are Tue night thru Fri SHRA/TSRA chances and warmer
temperatures thru the period.

Medium range model runs of 01.00Z/01.12Z in rather good agreement
for the mid level ridge axis to move across the Upper Midwest Tue.
Flow aloft then becomes more west-southwest for Wed thru Fri, ahead
of broad troughing to set up over the western CONUS. Some detail
differences with shortwaves thru the flow by Thu/Fri by this not
unexpected at the day 6/7 time-frame. Overall day 4-7 fcst
confidence is average to good this cycle.

Last of the dry/quiet weather days for much of the area looking to
be Tue as the mid level ridge axis moves across the region. Main
axis of moisture returning around the persistent great lakes high as
it finally retreats east progged to remain west/north of the fcst
area Tue, however the small consensus SHRA/TSRA chance across the
north end of the area Tue (mainly the afternoon) okay for now. The
increase of south/southwest low level flow across the region
spreads warm advection across the area, with highs on Tue expected
to trend near/above the normals for the first time in a week or
so. By Tue night/Wed the low is expected to move across the northern
plains as the flow aloft turns west-southwest. Moisture plume coming
north around the retreating high moves east ahead of the low coming
across the plains as well. At this time, trend of SHRA/TSRA chances
spreading in Tue night/Wed looks good. Low pressure then progged
to remain in the central/ northern plains Wed/Thu, moving into/across
the Upper Midwest Fri as a stronger shortwave would come east thru
the flow. Again, plenty of detail differences by Thu/Fri but consensus
SHRA/TSRA chances mostly in the 20-40% range much of Wed thru Fri
reasonable at this point. Lower level warming trend continues as
well. NAEFS 850mb temps around 1.5 STD deviations above normal Wed/Thu,
cooling a bit Fri as the low wound move into the region. Model/ensemble
consensus of highs/lows near/above normal for Tue- Fri appear well
trended. If Wed/Thu would trend drier/sunnier than the present consensus
fcst, some highs these days could easily climb into the lower 90s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 627 PM CDT Fri Jul 1 2016

High pressure situated over the TAF sites will remain in place
through Saturday, providing continued VFR conditions and light
winds. The only concern is valley fog at LSE, though chances for
that have become lower as a shield of cirrus may end up
encompassing the airport through much of the night. Maintained the
VCFG and SCT003 for now but this may be removed with the 06Z
issuance if the cirrus ends up being a thick deck over LSE.

&&

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

$$

UPDATE...AJ
SHORT TERM...RRS
LONG TERM....RRS
AVIATION...AJ


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