Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 252308

Area Forecast Discussion
National Weather Service La Crosse WI
608 PM CDT Mon Sep 25 2017

.SHORT TERM...(Late this afternoon through Tuesday)
Issued at 300 PM CDT Mon Sep 25 2017

As of 25.20Z, a weak cold front was draped along a line from
Ladysmith to Rochester to Mason City. Areas to the west of this
line have remained under thick cloud cover all day, but mostly
sunny skies to the east within the warm sector have allowed
MUCAPES to climb to near 1000 J/kg. Although deep layer forcing
and shear is weak, there is enough low-level convergence ahead of
the front to spark a rather narrow line of showers and
thunderstorms. Some of the stronger storms have produced gusts to
40-45 mph, but severe weather is not expected through the rest of
the afternoon as this line moves east-northeast.

For this evening, a second area of showers and scattered
thunderstorms currently across southwest IA are expected to lift
northeastward along the eastward advancing cold front and across
the forecast area. Additional rainfall amounts likely to be a
quarter inch or less. Coverage of rain should then diminish
somewhat overnight. Overcast skies will keep temperatures
elevated despite the cooler air mass, with lows from the upper 50s
to lower 60s.

Aforementioned cold front will be across the far eastern forecast
area by 12Z Tuesday and then into eastern WI by mid-afternoon.
Deep layer shear increases across the area Tuesday, but minimal
CAPE in the post frontal air mass will confine additional
thunderstorms to mainly southwest WI. Dropped POPs below 60
percent, but could still see some scattered showers through the
afternoon before finally drying out in the evening. Temperatures
Tuesday will be noticeably cooler, ranging from the mid 60s across
the northwest to mid 70s across the southeast.

.LONG TERM...(Tuesday night through Monday)
Issued at 300 PM CDT Mon Sep 25 2017

Broad high pressure builds across the region Tuesday night through
Thursday, resulting in dry conditions with partly cloudy to
mostly sunny skies. It will feel much more Autumn-like, however
with highs in the 60s on Wednesday only moderating slightly into
the upper 60s/lower 70s for Thursday.

Thursday night through Monday...pretty good model agreement
through the period. For Thursday night, a short wave trough will
move through the Great Lakes region, which could lead to some
precip in the northeast part of the forecast area, though the
25.12Z GFS and 25.12Z ECMWF keep a majority of the rain out of the
area. Ridging aloft and surface high pressure then look to
dominate Friday and Saturday, keeping the area dry. Meanwhile, a
short wave trough will move out of the Pacific Northwest, with
decent agreement of a low cutting off over the Montana/Canada
border and an associated cold front/precip swinging into the area
Sunday night into Monday. Temperatures through this time period
look to be more seasonable, with highs in the 60s and lows in the
40s. Friday night looks to be the coldest, with some of the
favored cold spots in central Wisconsin possibly seeing some
patchy frost as high pressure moves directly overhead.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 608 PM CDT Mon Sep 25 2017

Variety of flight categories to deal with given scattered convection
moving through this evening and IFR to MVFR ceilings being observed
along and behind passing cold front.

Have tried to time best threat for impactful convection but storms
transforming quickly as they advect northeast with short wave.
Higher storm impacts will be this evening with cold front passage
leading to period of IFR to eventually MVFR ceilings.  Might even
see some lingering shower activity into the overnight before
activity gradually works east so tried to account for that as well.




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