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FXUS63 KARX 051124 AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 624 AM CDT Sun May 5 2024

KEY MESSAGES

- Pleasant weather for today and Monday, warming back up into the upper 60s to mid-70s.

- Periods of showers dot the forecast from Monday through Friday. Best chance for rain at this point is late Monday night/Tuesday morning and again late Wednesday.

- Cooler air settles in after midweek, but overall look for seasonal temperatures into next weekend.

DISCUSSION
Issued at 330 AM CDT Sun May 5 2024

Today through Monday: Pleasant!

Fog once again developed across the region in the wake of yesterday's cooler and wetter weather, densest within the river valleys per nighttime microphysics satellite imagery. After this fog burns off, look for a few days of dry and seasonably warm weather. Split flow aloft coupled with weak surface high pressure results in light winds today with the only notable sensible weather coming in the form of some thin daytime cumulus clouds, mainly along and north of I-90 where forecast soundings show convective temperatures being reached under a cooler mid- tropospheric thermal trough. Clouds may be a bit thicker north of I-94 per MOS guidance solutions, but overall expect more sun than clouds.

Winds and temperatures increase for Monday as the upper ridge pushes to the east and southeasterly flow ensues ahead of an approaching upper trough that will end up driving our weather pattern for the remainder of the week. Overall trends in the guidance have been for a slower pattern transition Monday afternoon, resulting in the wind/sky/PoP forecast lowering with successive forecast updates. This latest forecast continues the trend and thus aside from some high cirrus we look to see another nice day in store.

Monday Night through Saturday: Periods of Rain

Convection over the Central Plains Monday afternoon grows upscale in the evening as it moves into Iowa, becoming increasingly elevated as the boundary layer cools and decouples overnight. MUCAPE values fall to around 300 J/kg or less by the time the system arrives after midnight, thus these showers and isolated thunderstorms look to pose little in the way of any risk as they decay during their transit. The negatively-tilted upper trough responsible for this convection takes on a nearly zonal orientation as the upstream longwave closes off into an elongated low stretching from the Rockies to the Great Lakes.
This pattern evolution, coupled with the early morning timing of the convection, nullifies our the risk of severe weather on Tuesday afternoon with the instability axis shunting off to the south/east. Indeed, the latest SPC Day 3 outlook mirrors this thought in its southeastward shift.

Te remainder of the week's forecast hinges on the evolution of the closed upper low that meanders over the region. The 6-hourly precipitation forecast for Tuesday through Friday features nearly a continuous smattering of 20-40% PoPs owing to subtle/difficult to resolve undulations within this cyclonic flow pattern driving the risk for rainfall. We certainly are not expecting a washout by any means and with the deeper moisture transport shunted well to the south, any showers will be of lower impact. One break in the precipitation looks to come late Tuesday night into Wednesday morning as low amplitude ridging builds in the wake of the northward-lifting zonal shortwave.
Convection developing along the central Missouri River basin will lift into the region Wednesday night, with 80+% of the EPS/GEFS members bringing measurable rainfall with this complex.

Forecast confidence in rainfall timing degrades beyond Wednesday evening as the upstream omega block pinches off and energy associated with a Hudson Bay vort lobe interacts with the bifurcating low and keep some semblance of cyclonic flow in place through Thursday. However, there is good agreement that such a pattern will funnel cooler air southward and drop temperatures back into the upper 50s to lower 60s for highs to round out the week. The pattern does trend drier for the weekend, but there exists plenty of variability in the longwave pattern and such stagnant setups have a tendency to linger longer than initially advertised.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/
Issued at 623 AM CDT Sun May 5 2024

Valley fog at KLSE will likely dissipate by 05.13z. Otherwise skies will be mainly clear and visibilities will be VFR.

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

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