Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 231920
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
220 PM CDT Sun Sep 23 2018

..SHORT TERM...(Tonight and Monday)
Issued at 220 PM CDT Sun Sep 23 2018

Main short-term fcst concerns include returning low level
moisture/clouds Monday, warmer temps both tonight/Mon.

Data analysis at 18z had broad high pressure to our east and low
pressure moving into central SD. Pressure gradient between the 2
responsible for increasing south winds across much of MN/IA/western
WI. Airmass remains dry over much of the region, with sunny skies
over much of IA/WI/south half of MN early this afternoon. Southerly
lower level flow pulling some warmer 925-850mb air northward as
well, with 18z temps some 5-10F warmer than those of Sun. A
rather pleasant late Sept Sunday in progress across the area.

23.12z models initialized well and offer quite similar solutions as
stronger troughing deepens into the northern plains this period and
the flow aloft becomes SW. Short-term fcst confidence is generally
good this cycle.

The quiet, dry weather to continue this period, but the the
generally clear skies looking to not last past tonight. Low/mid
level airmass over the area to remain dry tonight for mostly clear
skies. South gradient sfc-850mb flow to continue pulling warmer
air northward ahead of the low in the plains, plus keeps the BL
stirred for some warmer lows tonight. Blend of guidance lows
mostly upper 40s to lower 50s appear reasonable. By late
tonight/early Mon models begin to advect the lower level
moisture/clouds over eastern OK/SE KS northward into the area.
Models mixed on extent of this moisture manifesting as lower
clouds over the area as it spreads north in/under an inversion.
Trended grids toward more of an increase of clouds later in the
morning and afternoon, when higher level moisture/cirrus clouds
would be thickening over the area as well. Mixed 925mb temps
supporting highs as warm as upper 70s across the SW half of the
fcst area but given thickening clouds for the afternoon, blend of
Mon guidance highs mostly lower-mid 70s looks good. Lead lower
level thermo-dynamic forcing ahead of the approaching sfc-700mb
front/trough, along with some deeper moisture and weak CAPE moves
into the NW end of the fcst area late Mon afternoon. Some small
SHRA/TSRA chances there toward Mon evening reasonable at this time
and blend well with neighboring grids.

.LONG TERM...(Monday night through Sunday)
Issued at 220 PM CDT Sun Sep 23 2018

Main forecast concern in the long term remains periodic shower and
storm chances - Monday night/Tuesday, Thursday, and the weekend.

The cold front will finally make its way into the forecast area
Monday evening/night. With a warm, moist airmass in place ahead of
the front, expecting showers and storms to move through much of the
area. However, given later timing of the front and only 500 J/kg of
MUCAPE, along with lack of strong, deep forcing in the forecast
area, thinking severe weather threat will be pretty much non-
existent, with the main threat from any storms being lightning.

Models then continue to show more shortwave energy sliding through
during the day on Tuesday, with frontogenesis increasing along the
front, which will be in or near far southeastern portions of the
forecast area. Should lead to continued showers and maybe some
storms for these areas during much of the day, but thinking thunder
will mainly be south and east where better instability will reside.
Otherwise, expect high temperatures to be in the 60s behind the
front on Tuesday.

The rain will move out of the area Tuesday evening, with
temperatures cooling further for Wednesday as winds generally stay
northwesterly and longwave troughing builds in. Highs will be in the
50s to lower 60s. Trough remains in place Thursday as another cold
front and associated rain move through. Still looking like very
little, if any, instability with this front, so not anticipating
thunder at this time.

Model solutions then diverge regarding precipitation chances for the
weekend. The GFS has been consistent over the last few runs bringing
in more rain on Saturday, while the ECMWF has trended the opposite
direction (until the 23.12Z run), and holding off until Sunday.
Bottom line, looks like there could be some rain over the weekend,
but difficult to say when/where. High temperatures look like they
will remain in the 50s and 60s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1203 PM CDT Sun Sep 23 2018

SW to SE lower level flow will persist across the area this period,
between high pressure over Ont/great lakes and low pressure in the
central/northern plains. Winds to remain S to SE this period 8-
15kts, stronger/gusty to 15-24kts during the afternoon hours today
and later Mon morning. Good VFR looking to continue thru tonight as
a drier low level airmass remains over the region. However the
persistent southerly low level flow is progged to pull the lower
level moisture/clouds over eastern OK/SE KS northward and into the
area after 12z Mon. Introduced some sct MVFR clouds 11-12z, then
MVFR cigs after 14-15z Mon morning to account for this moisture
return.

&&

.HYDROLOGY...
Issued at 220 PM CDT Sun Sep 23 2018

Routing continues in the area river system with the Mississippi on
the rise through next week. Water levels in the tributaries are
receding. Only one flood warning remains, at Gays Mills along the
Kickapoo River in SW WI.

&&

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

$$

SHORT TERM...RRS
LONG TERM....CA
AVIATION.....RRS
HYDROLOGY....DB/RRS


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