Area Forecast Discussion
Issued by NWS La Crosse, WI

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758
FXUS63 KARX 272233
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
530 PM CDT Thu Jul 27 2017

.SHORT TERM...(Tonight thru Friday night)
Issued at 242 PM CDT Thu Jul 27 2017

The main fcst concerns this period are late night/early morning
valley fog potential both nights and temperatures.

Data analysis at 18z had a broad area of high pressure/ridging from
central Ont to western Neb. North/NE winds across much of MN/IA/WI
south of the ridge axis, slowly ushering a drier airmass southward
across the region. Vis imagery showing plenty of diurnal cumulus
across the region early this afternoon under the cooler 850-700mb
temps aloft. WV imagery showed a rather compact shortwave over far
NW MN, moving steadily SE. Some deeper cumulus and sct -SHRA ahead
of this feature, but confined to the NE 1/4 of MN and SW Ont. Even
with the cooler N/NE low level flow, early afternoon temps were near
the late July normals.

Model runs of 27.12z initialized well. Solutions in good agreement
for the northern MN shortwave to drop quickly SE to Lk MI/lower MI
by 12z Fri, but overall trend is toward slower of earlier runs. Good
agreement for hgts to rise/shortwave ridging to build east over the
Upper Midwest Fri/Fri night. Short-term fcst confidence for a
relatively quiet weather period remains good this cycle.

In the short term, slower movement of the northern MN shortwave
delaying the SHRA chances into the area this afternoon. Short-term
forecaster has already pushed them past 21z this afternoon, with
main chances now with the shortwave passage mainly this evening.
Continued the small SHRA chances mainly near/NE of I-94 thru this
evening. NAM and a couple of the hi-res/CAMs models would continue a
SHRA chance over parts of the east side of the fcst area past
midnight, but given strong diurnal nature of the cumulus and SBCAPE
field today, and a mdt/strong inversion near/just above 700mb, will
leave the overnight hours SHRA-free. Mostly clear skies, light winds
favorable for radiational valley fog formation again tonight. Not
the most ideal set-up for radiational fog with light wind layer
aloft on the shallow side and some NE winds around 5kts in the BL.
Fri night to see a deeper light wind layer aloft, along with BL
winds more in the light/variable category, prone to decoupling/calm.
With moist/wet soils and full evapo-transpiration Fri, some dew
point recover/BL moisture loading expected Fri evening. Late Fri
night/early Sat morning looking more favorable for radiational fog
formation than tonight and added a patchy valley fog mention to the
grids these periods. Blend of the guidance lows/highs tonight thru
Fri night look good, though did trend toward cooler of guidance lows
in normally cooler low laying areas along/NE of I-94 Fri night.

.LONG TERM...(Saturday through Thursday)
Issued at 242 PM CDT Thu Jul 27 2017

For Saturday thru Sunday night: main fcst concerns this period are
late night/early morning valley fog Sat night and temperatures
thru the period.

27.12z models in good agreement for hgts to continue rising Sat, a
weak shortwave to ripple thru the ridge later Sat night/Sun morning,
then for hgts to continue rising/building east across the Upper
Midwest later Sun/Sun night. Fcst confidence for this period is
generally good.

Can sfc high pressure to slowly build SE across the Upper Midwest
this period, with the weekend continuing to shape up as dry, quiet
and comfortable. GFS the most robust bringing some shortwave energy
thru the building ridging later Sat night into Sun, along with
potential for some small SHRA/TSRA chances Sun into Sun evening.
Given the deep and rather dry high pressure over the region, left
the Sun/Sun night periods dry at this time. Discounting the GFS, the
sfc-850mb ridge axis is over the fcst area Sat night. This for a
rather deep layer of light winds under clear/mostly clear skies,
again favorable for radiational valley fog formation. Added a patchy
fog mention to 08-13z Sun grids for the river valleys and usual low
laying areas along/NE of I-94. Sfc-850mb ridge axis moves little for
Sun night and may yet need a valley fog mention 08-13z Mon. Based on
mixed 850mb temps, blend of guidance highs Sat/Sun mostly in the
upper 70s to lower 80s look good. Light winds, mostly clear skies
and a drier low level airmass, blend of guidance lows near to a few
degrees below normal Sat/Sun nights also looking good.

For Monday thru Thursday (days 4-7): main fcst concerns this period
are returning SHRA/TSRA chances mainly Wed/Thu.

Medium range model runs of 27.00z/27.12z showing rather good
agreement for shortwave ridging to continue building over the region
Mon/Tue. Except for some timing differences (as expected) model then
in general agreement on a stronger shortwave to drop SE across the
western great lakes in the Wed/Thu timeframe. Overall day 4-7 fcst
confidence is average to good this cycle.

Under the shortwave ridging aloft, ECMWF/Gem hold the sfc ridging
over the region Mon/Tue for a continuation of the dry/quiet weather
into early next week. GFS in contrast brings some shortwave energy
thru the shortwave ridging later Sun into Mon. This with a weak sfc
trough and some SHRA/TSRA chances. Given the general sfc-mid level
high pressure over the region, GFS looks to fast/wet with this and
will side with the EC/Gem for Mon/Tue. Though they differ on the
timing details, a rather consistent signal for moisture increase and
a sfc/low level trough/front to accompany the shortwave across the
western great lakes Wed/Thu. Consensus 20-30% SHRA/TSRA chances Wed/
Thu reasonable at this point. Lower level gradients remain on the
weak side Mon/Tue for little in the way of advections but lower
level airmass should continue to modify in place under plentiful
late July sunshine. Mon/Tue, even Wed, looking to be some warmer
days with near/above normal temps, before a bit of a cool down
under more clouds by Thu.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 530 PM CDT Thu Jul 27 2017

Cigs: high pressure will keep skies skc/sct into Sat. The one caveat
is the potential for fog and/or stratus in the river valleys the
next couple nights. See some potential for a sub 1 kft cig at KLSE
if the scenario plays out. Not confident enough to add to forecast
at this time.

WX/Vsby: potential for river valley fog the next couple nights that
could impact KLSE. T/Td spread and sfc winds at 22z favors the
potential. Depth of the light wind layer varies between the
NAM/RAP/HRRR per bufkit soundings - the nam fairly breezy through
the evening, increasing the light wind layer overnight. THe hrrr/rap
are a mixed bag and don`t show a reliable trend at the moment.
Approaching the climatological favored time frame for valley fog at
KLSE. For the moment, will go with BCFG to relay the potential
threat at KLSE.

Wind: north/northeast through Friday night. Mostly light, with a
little diurnal bump for Fri afternoon.

&&


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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....Rieck



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