Area Forecast Discussion
Issued by NWS La Crosse, WI

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988
FXUS63 KARX 181917
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
217 PM CDT Sat May 18 2024

.KEY MESSAGES...

- A lower end risk for a few strong/marginally severe storms
  exists across central WI through early this evening (4 to 8
  pm), with hail/gusty winds the primary hazards.

- Another round of thunderstorms Sunday night into Monday

- Increasing confidence for severe thunderstorms on Tuesday
  afternoon across the region. Best chances south of I90.


&&

.DISCUSSION...
Issued at 213 PM CDT Sat May 18 2024

Late this afternoon:

A cold front was progressing into the Upper MS Valley this afternoon
in conjunction with a mid-level trough ejecting northeast from the
North Dakota/Manitoba border region. With the stronger synoptic lift
passing to the north with the wave, convective initiation will be
tied to surface heating and low-level convergence along the front.
CAMs/HREF generally indicate the higher chances for storms late this
afternoon/early evening across central/northern Wisconsin, with more
isolated convection possible farther south where lift is weaker and
mid-levels slightly warmer. Some of the differences in the CAM
output likely are related to the degree of low-level moisture within
the narrow instability axis ahead of front and depth of boundary
layer mixing, with the lower dew points/stronger mixing solutions
(such as the HRRR) verifying best as of early afternoon.

The 18.12Z HREF shows some clustering in the UH fields, particularly
across central into northern Wisconsin, where the risk of a few
strong to severe storms is highest. However, again, the caveat is
that quite a few of the HREF members are verifying too high with
boundary layer moisture, so the overall magnitude of this event
looks quite marginal. That said, although the stronger deep layer
shear lags the front, effective shear of 25 to 35 kts, combined with
1000-1500 J/kg MLCAPE and steep low/mid-level lapse rates would
support small to marginally severe hail with initial more intense
updrafts and strong wind gusts with more robust storms, especially
north and east of I-94. Storms will quickly exit by 8-9 pm as the
front moves through.

Overview: An active and progressive period of weather over the next
week. Severe Thunderstorms Possible Tuesday.

Broad troughing across the North American Rockies starts this period
off this afternoon. This trough is expected to deepen as it drops
south across the western CONUS through the rest of the weekend. As
it does, SW flow takes hold across the CONUS with a downstream
elongated ridge across the Appalachians. This SW flow will allow
PWATs to climb above 1.25 inch on Monday and closer to 1.5 inches on
Tuesday (near the climatological max for May 21 DVN sounding). A
shortwave trough ejects from the Rockies Tuesday morning as a
leeside Colorado Low develops and lifts NE through the Central
Plains. This trough takes on a negative tilt as the surface low is
expected to deepen quickly into a seasonably strong low. This brings
Tuesday as the day to watch this forecast period for the potential
of severe weather. After Tuesday, the upper-level pattern becomes
temporarily more zonal and progressive with another upstream trough
dropping south along the West Coast. This trough moves inland across
the northern Rockies Thursday and through the Northern Plains Friday
through Saturday bringing another surface low and chance for
widespread precipitation somewhere along the Upper Midwest.
Variability among the medium range guidance will bring low chance
PoPs Friday afternoon through Saturday, but expect the timing to
become more precise with time this upcoming week.

This evening: Clearing and Seasonable

Surface trough and associated cold front shift east of the local area
this evening lending to clearing skies and lighter winds. Winds
likely won`t go completely calm as we remain between a passing
surface low to the north and increasing surface high pressure to the
south. Winds 1 to 2kft off the surface will still remain 30 to 40
mph from the west so it`s possible a few gusts (20mph or so) could
reach the surface late this evening before the boundary layer
decouples.

Sunday through Monday: Increasing Clouds Sunday, Thunderstorms
Sunday Night and Monday

High pressure on Sunday morning will bring clear skies for the first
half of the day; however, as warm air advection and isentropic
ascent increase Sunday afternoon from the west, clouds will begin to
increase. Did decrease PoPs for Sunday evening this shift as
upstream convection will likely hinder precip chances until the
better synoptic forcing movings in overnight Sunday into Monday. On
Monday morning, shortwave is progged to be right overhead with
ongoing showers and thunderstorms across the area. By Monday
afternoon, MLCAPE increases  around 750 to 1200 J/kg by Monday
afternoon, so will keep thunderstorm chances in the forecast, but
the shear values look too low at this time for any organized severe
weather threat.

Tuesday: Seasonably Strong Low Pressure. Severe Thunderstorms
Possible

A potent negatively tilted trough and seasonably strong surface low
pressure system are expected to lift NE from the Central Plains into
the Upper Midwest Tuesday, bringing severe thunderstorm chances
along with it. Latest deterministic models bring the surface low as
deep as low as the mid 980s (ECMWF) with their ensemble means
starting to catch on bringing the low into the mid 990mb. SPC has
introduced a Day 4 15 and 30% chance locally across our area, with
the best chances south of I90 at this time. PWATs climbing around
1.5 inches would be near the climatological max for DVN sounding on
May 21, so quite a moist airmass.

As far as instability goes, mean Canadian Ensemble has remained an
outlier on the high side, with mean sfc based CAPE in the 2500-3000
J/kg range, where as the GEFS and EPS solutions (while still
elevated) remain in the 1000-1500J/kg range. With the sfc low
favored to pass to the W of our local area, we are favored to remain
in the warm sector where surface dewpoints in excess of 60F across
our area and in the mid to upper 60s across NE Iowa. At this time it
seems like wind would be the primary hazard as the threat evolves
with time, but initial development of isolated storms could pose a
tornado or hail threat. Continue to monitor forecast updates.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1235 PM CDT Sat May 18 2024

VFR conditions are expected through the TAF period at LSE/RST. A
cold front is passing through the local area from west to east
this afternoon. This cold front will pass through RST around 19Z
and LSE around 21Z. There is a small chance (10%) for a shower
or perhaps a thunderstorm west of the Mississippi this
afternoon, but the better chances (20 to 60% will remain east
of the river and north of I90, with the best chances (40 to
60%) across Clark and Taylor counties and north and eastward in
Wisconsin (Neilsville to Medford). Thunderstorm chances vacate
the local area by 23-24Z this afternoon with clearing skies
through the rest of the period.

&&

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

&&

$$

DISCUSSION...JM/JAW
AVIATION...JAW