Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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465 FXUS63 KARX 080900 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 400 AM CDT Wed May 8 2024 .KEY MESSAGES... - There has been a southwestward shift to heavier rainfall tonight and also a slower onset today...making for a nice day! Heaviest rainfall and any severe/strong storms should remain southwest of the area. - River rises continue across the area and additional rainfall in northeast Iowa will cause further in bank rises. Heavy rainfall over 2" could cause higher rises but this is a low probability outcome (less than 3%) at this time. - Rain showers and isolated thunderstorms likely to shift through mainly Wisconsin Friday afternoon and night. && .DISCUSSION... Issued at 400 AM CDT Wed May 8 2024 Storm and Heavier Rainfall Chances Later Today and Tonight Interesting forecast for later today with strong signals for a deformation rain band in the I-90 to I-80 corridor with increasing moisture inflow from the south for heavier rain amounts into Thursday morning. Later today and into the evening, as the massive longwave trough dominating much of the central CONUS shifts south, a surface/low- level low pressure area travels along the frontal boundary /now from the Ohio Valley into southeast OK/. GOES derived precipitable water indicates the air mass of ~1.50" is beginning to shift north ahead of the surface low on the KS/OK border. By later today, the surface low in the 08.00Z SPC HREF is in southeast IA with mean dewpoints of 60F there, with an inverted trough extending northwest across Iowa. This sets up a nice rainfall environment with moderate to strong east- west 850-600 mb layer frontogenesis, the inverted trough convergence, and rich 1" precipitable water values traveling into that nearly stationary lift region tonight. The 08.00Z HREF 24 hour rainfall mean/PMM/LPMM suggest 1.5"/2"/3" respectively in northeast IA by Thu 7 pm, but most of that rain falls tonight. However, there is a continued southward model trend noted /e.g., the 07.12Z HREF had this max more on the I-90 corridor/. The latest few runs of the NBM QPF are slowly shifting the rain south as well, possibly just south of the forecast area. This trend toward the moisture inflow and instability further south makes more sense and the synoptics have trended south as has the SPC risk for severe storms in MO. Just seeing the 08.06Z HRRR, it slows the timing even more into today (dry day across the forecast area!) but then places the deformation rain band 1" line from KDBQ to KMCW (to the south, 1"+), with noticeable lighter amounts across the I-90 corridor in the drier easterly flow in the 925-850mb layer. Thus, have to step forward cautiously as the trend seems southward yet, but rainfall amounts could be several inches in the forcing. Right now, will have the greatest rain amounts through Thursday across northeast Iowa with around 1" and thinking any heavier rainfall will be southwest. With dry air in the low-levels in the later afternoon think surface-based storms to leverage the looping hodographs is unlikely. Dewpoints could mix out in the afternoon further stabilizing the marginal risk area. MUCAPEs in the latest 08.06Z HRRR are in the hundreds. Thus, pea-sized hail, wind gusts above 40 mph, or funnels look to be located out of the forecast area and are of very low probability. Friday Afternoon and Evening Showers and Isolated Storms Good consistency remains in a highly dynamic shortwave trough rapidly shifting through the area in northwest flow Friday afternoon and exiting Friday evening or so. Model consensus on a 300-500mb QVector dipole of forcing shifts through mainly WI with widespread showers expected. While forcing is good and wind shear is okay, the system is pretty instability starved with most indications of 100-200 J/Kg of MLCAPE in the afternoon. So, thunder is probable but severe storms are not expected. Rain chances are in the 20-55% range from northeast Iowa to northcentral WI,respectively, however as models converge on position and timing differences, would think those chances will increase. Right now it looks like a showery period in at least WI. Weather Trends Toward Above Normal Temperatures Next Week Saturday begins a period dominated by a flatter ridge over the northern states with periodic lower rain chances and warming temperatures. The days should be pretty sunny with above normal temperatures in the 70s and tranquil weather seen. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 950 PM CDT Tue May 7 2024 A brief shower remains possible for a few hours late this evening. Otherwise, VFR conditions are likely into Wednesday with patches of mid/high clouds and winds under 10 kts. Chances for showers and isolated storms will increase later Wednesday/Wednesday night, but with lower confidence in placement of the rain at this time, will not explicitly mention at this point, but may be needed in future updates. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Baumgardt AVIATION...JM