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FXUS63 KARX 281934 AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 234 PM CDT Sun Apr 28 2024

KEY MESSAGES

- Widespread showers continue this afternoon, gradually becoming more scattered this evening and pushing north of the area Monday morning.

- Cooler weather through Monday, with a rebound to above average temperatures for Tuesday and Wednesday. Seasonal temperatures for the remainder of the week.

- Several more rounds of showers and storms are lined up for Tuesday and late Wednesday through Friday. Overall severe threat looks low at this time, but details in the specifics still need to be sorted out.

DISCUSSION
Issued at 230 PM CDT Sun Apr 28 2024

This Afternoon through Monday

A slowly filling 1003-mb surface low pressure system was meandering towards the Siouxland region early this afternoon with an attendant warm frontal structure arcing to the east.
This warm front is rather complex in nature given that an old warm frontal boundary--evident as a 8-10 F cutoff in the surface dewpoint values--lingers around the Highway 20 corridor in northeast Iowa to along the WI/IL border. The surface flow over Iowa does not turn more southerly until one reaches the I-80 corridor with the strongest moisture transport displaced farther east towards northeast Illinois. All combined, this disjointed nature of the kinematic and thermodynamic fields should result in little movement of the low level warm sector into the southern forecast area today. Lapse rates north of the warm front are only around 5-6 C, resulting in almost no upright instability to realize (despite plentiful deep shear and elongated hodographs). This changes little over the course of the afternoon and well-reflected in the shift of the SPC Marginal Risk further south and the removal of most thunder wording from the daytime forecast.

Broad 290K isentropic ascent over the lower tropospheric warm front tapping into the western flank of the deeper Gulf moisture has fueled widespread showers this morning and early afternoon across the region. This deeper moisture shunts off to the east late this afternoon and evening as the mid to upper tropospheric dry slot wraps around the parent trough. While the widespread showers shift to the east this evening, lapse rates steepen enough that any perturbations within the mid-level flow could instigate a line of showers or thunderstorms. Indeed, the RAP/HRRR runs have over the course of the morning started to suggest at a thin line of convection wrapping northeastward out of Iowa during the late overnight hours and have kept slight chance PoPs to account for this scenario.

The upper level trough separates from the mean flow and slows over Minnesota for the day on Monday, resulting in showers possibly lingering north of Interstate 90 through the day, ending before sunset. Low stratus/stratocumulus likewise persist throughout the day on Monday in close proximity to the surface low, thus have kept temperatures on the cooler side, especially over southeast Minnesota.

Tuesday:

There will be a lull in the precipitation Monday night through the day on Tuesday as shortwave ridging slides on through in the broad cyclonic upper level flow. A compact vort lobe races eastward--pushed along by a 110-kt jet--across the the Northern High Plains Tuesday and results in a narrow warm sector expanding northward ahead of an advancing surface trough.
Temperatures on Tuesday should rebound into the 70s for highs as southerly flow strengthens ahead of the trough.

The narrowness of the warm sector, combined with the fast forward motion of the upper level wave, will limit the coverage and duration of any severe weather--but there exists modest spread in the timing of these features that will need resolving over the next few days. At this point, the bulk of the guidance points to any severe weather taking place west and south of the forecast area with waning convection outrunning the warm sector as they arrive in our area.

Wednesday through Saturday:

A zonally-oriented low/longwave begins to rotate and amplify heading into the middle to latter part of the week. This results in a broader and deeper downstream warm sector spreading northward. The model guidance vary substantially on the characteristics and timing of the upper level wave as it amplifies, and likewise with the position of the various surface features. The forecast has broad-brushed PoPs due to this spread and uncertainty, but the likelihood is high that there will be drier stretches during these time windows.

Temperatures also will trend cooler/seasonal owing to the increasing clouds and rainfall potential. The trends in the medium range guidance and ML analogs have been for any severe weather on Wednesday or Thursday to stay south of the area. That being said, there has been a steady increase in the rainfall probabilities over the last 24 hours of model runs, with the Grand Ensemble mean now around 60% for 0.50" and 40% for 1.00" (up 30% from 24-36 hours ago).

AVIATION /18Z TAFS THROUGH 18Z MONDAY/
Issued at 1233 PM CDT Sun Apr 28 2024

Showers continue through this evening, becoming more scattered in nature overnight. Thunderstorm chances continue to decrease at the TAF sites (< 20% chance) so no mention is included for this evening. MVFR/IFR ceilings are a mainstay through this TAF forecast with conditions deteriorating further overnight. At RST, there is a 40-60% chance for LIFR ceilings between 09-15Z Monday. Reduced visibilities are expected during periods of precipitation. Although not explicitly mentioned in the forecast, brief reductions to less than 3 miles are likely during heavier bouts of rain.

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

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