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FXUS63 KARX 041141 AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 641 AM CDT Sat May 4 2024

KEY MESSAGES

- Another round of rain moves through this morning and early afternoon, some thunderstorms possible south of I-90. Fog possible tonight along and east of the Mississippi River.

- Dry and pleasant weather for Sunday and Monday with highs climbing back into the upper 60s to mid 70s.

- An active stretch of weather from Monday night through at least Thursday as multiple rounds of showers move through.
Severe weather risk for Tuesday is trending south.

DISCUSSION
Issued at 330 AM CDT Sat May 4 2024

Today into Tonight: Cooler, Showers Especially this Morning

Early this morning, a surface cold front stretched from Omaha, NE to Ashland, WI and steadily marches southeastward today ahead of a parent northern stream upper tropospheric trough quite evident in water vapor imagery over the Dakotas. An MCV developed across eastern Nebraska overnight as convection from the previous day congealed into a larger MCS that stretches down to central Oklahoma. This complex lifts along the frontal boundary this morning, and while expected to gradually weaken through the morning, should bring widespread precipitation with its passage. Indeed, every EPS/GEFS member has measurable precipitation and every CAM shows the system holding together with its passage, with minor timing differences driving down the PoP values in the blended CONSShort data. Therefore, leaned more aggressive with PoP coverage this morning with the notion that everyone should see rain at some point this morning.

The MCV drags up a narrow moist sector of 55-60 F dewpoints today (currently south of Iowa), but confidence is not terribly high with how far north this airmass advances. Additional convection is progged to develop along the outflow-reinforced front in eastern Iowa this afternoon, which could easily cut off the moisture transport and leave us with little additional afternoon storm activity. The timing of this morning's storms will drive what transpires in the afternoon, but any threat for storms would exist over southwest WI before exiting before sunset. The combination of the morning rain and low clouds lingering for the afternoon will keep highs confined in the 50s to low 60s on the cool side of the boundary.

Looking ahead to tonight, low clouds in the wake of this system clear out from west to east from 00-06Z with surface high pressure building in its wake. The setup is somewhat similar to what we saw yesterday morning with the widespread fog, but rainfall amounts today should be less and the clouds clearing sooner. This may limit the fog threat to east of the Mississippi River, and indeed the 00Z HRRR has (noisy) 20-40% progs for visibilities under a mile in this region. Did include the mention of fog with this forecast update to account for this signal. Lows in favored sandy soil regions bottom out in the low to mid 30s tonight as well, which may necessitate targeted frost headlines.

Sunday and Monday: Dry, Warming back Up

Upper level ridging builds in during the day on Sunday and lasts well into Monday, keeping the area devoid of any precipitation. Did increase cloud cover for Sunday over the raw NBM guidance with the 03Z RAP and 06Z HRRR both showing convective temperatures being reached areawide by midday given the cooler airmass aloft and steep boundary layer lapse rates.
This should facilitate the development of a cumulus cloud deck for the afternoon, though with general subsidence through the column and a stout 40-50C dewpoint depression dry wedge above the boundary layer, any clouds will be quite flat in nature.
These types of sky coverage scenarios are very difficult for the blended guidance to depict and thus increased sky coverage just enough to get some mention of clouds in the worded forecast.

Temperatures climb back into the upper 60s for highs today, with increasing southeasterly flow for Monday pushing highs up into the low to mid 70s. Latest trends in the medium range guidance have been to shift the threat for precipitation back into Monday night, so did remove the last vestiges of the 15-20% multi-run NBM blended PoPs from the southwestern forecast area for Monday afternoon.

Monday Night through Friday: A Wetter Stretch of Weather

Convection over the Central Plains Monday afternoon grows upscale into the evening as it moves into Iowa, gradually eroding the already narrow warm sector that stretches north of I-80. Overall trends in the medium range QPF data show a weakening system upon its arrival in our forecast area, likely after midnight for most locales. The overall severe weather threat for Tuesday hinges on the properties of the upper level wave pattern and whether or not recovery of the warm sector can take place in the wake of the morning MCS. The most probable scenario is that the early morning storms shunt the warm sector south as the upper wave pivots through, keeping any severe threat to the south.

The upper tropospheric longwave trough separates from the flow and morphs into a cutoff low for Tuesday into Thursday, lingering over the northern High Plains during this time. This results in multiple, ill-timed waves of precipitation affecting the region. Temperatures cool down into the 50s to low 60s by the end of the week under this pattern, with the global guidance spread increasing in how long this cutoff low lingers beyond Thursday into next weekend.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/
Issued at 641 AM CDT Sat May 4 2024

A band of rain and embedded scattered storms will move east northeast through the area. Currently thinking that the thunder will stay mainly south of the TAF sites, so I kept it out of the forecast for now. This precipitation is currently moving into KRST and it will move out of the area between 04.16z and 04.19z. It will move into KLSE around 04.15z and exit the area between 04.17z and 04.21z. This rain will lower ceilings to IFR/MVFR and visibilities will become MVFR.

As high pressure builds into the area this evening, skies will quickly clear. There could be some fog tonight, but confidence was not high enough to add it at this time.

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

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