Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 081718
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1218 PM CDT Sat May 8 2021

.SHORT TERM...(Today and tonight)
Issued at 338 AM CDT Sat May 8 2021

At 245 AM, skies were clear under high pressure, with widespread
20s and 30s across the area. Some mid and high clouds were
approaching from the west in advance of our next system.

A lot to cover concerning the weather today through tonight, as
our quiet frosty morning belies the more active weather coming. As
high pressure moves east this morning, temperatures will recover
and we`ll begin to see increasing clouds as shortwave energy
approaches from the west. Accompanying this will be a strong area
of 500-700 mb frontogenesis setting up across southeast MN into
southwest WI during the afternoon. While a substantial amount of
dry air will still be in the column, the strong frontogenetic
forcing should be able to overcome this after a while, producing a
narrow band of showers. With this fgen placement, model guidance
is in better agreement that rain probabilities really fall off
northwest of a Wabasha to La Crosse to Wisconsin Dells line, so
areas near and north of I-94 may not see any rain and have
reflected this in the updated PoPs through tonight.

By evening, a few models suggest a couple of hundred J/kg of
elevated instability could sneak into northeast Iowa, so will have
to watch out for an isolated rumble of thunder. A stronger
shortwave and surface low pressure system will also be spinning up
to our south this evening through tonight. The current forecast
track of the surface low through central MO and southern IL would
favor keeping the heaviest rain/moisture transport/WAA to our
south, but the still-present frontogenesis over our area should
keep showers going across the south overnight until exiting the
area by morning. When all is said and done, amounts look fairly
light outside of the main frontogenesis band, generally under
0.25". Where the most intense portion of the frontogenetically-
forced precipitation band sets up, however, the 08.00Z SPC HREF
Ensemble shows a mean of 0.5" and potential for upwards of 1-1.5"
in a very narrow corridor across northeast Iowa into southwest
Wisconsin. In fact, a very faint footprint in the HREF 6-hr QPF
greater than 1 inch probability field can be seen in this vicinity
between 00-06Z tonight. As is often the case with frontogenesis
bands, this narrow corridor could be subject to small shifts in
placement as newer guidance comes in.

Finally, a few models suggest a brief change over to snow is
possible in far northeast Iowa into southwest Wisconsin given
diabatic cooling of the column early tomorrow morning. Confidence
is lower in band placement and whether precipitation rates will be
able to produce enough cooling for this, but if this does
materialize, it could bring a brief period of reduced visibilities
and minor accumulations mainly on grassy surfaces.

.LONG TERM...(Sunday through Friday)
Issued at 338 AM CDT Sat May 8 2021

The medium range models have the same general idea on the overall
pattern heading into next week but have enough differences to
lead to considerable uncertainty on any precipitation chances. In
the big picture, the upper level low near Hudsons Bay is expected
to move southeast through the first half of the new work week with
a rather complicate pattern following behind it. The upper level
ridging currently in place over western Canada looks to be
undercut by several short wave troughs coming in off the northern
Pacific. Just how much of this ridging will survive as it moves in
behind the departing upper level low along with how strong the
undercutting short wave troughs will be is where the uncertainty
in the models and forecast comes from. The 07.12Z ECMWF is weaker
in general with both the decaying ridge and the initial short wave
troughs over the Midwest until a weak northwest flow aloft
develops and brings some short wave troughs across the region from
Wednesday night through Friday. Even though the 08.00Z GFS holds
the decaying ridge together more than the ECMWF, it is also faster
and stronger with the initial short wave troughs and suggests
these will start moving across the region as early as Tuesday
night. It too then shows several more short wave troughs moving
over the region through the end of the work week. As a result of
these differences, there will be an extended period of some small
rain chances starting Wednesday and continuing for the remainder
of the week. At least temperatures look like they will slowly
moderate with highs expected to approach 70 by Friday and
overnight lows reaching the 40s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1218 PM CDT Sat May 8 2021

VFR conditions to prevail through the period as a storm system
slips south of the KRST/KLSE sites. Could see a few sprinkles in
the 21-03z time frame. Otherwise, winds will be fairly light out
of the east/southeast in the 5 to 15kt range this afternoon,
becoming light and variable overnight, and then ending up out of
the north 5-10kt after 16z.

&&

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

$$

SHORT TERM...Lee
LONG TERM...04
AVIATION...DAS


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