Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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610 FXUS63 KARX 081918 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 220 PM CDT Wed May 8 2024 .KEY MESSAGES... - Heaviest rainfall for tonight continues to shift south with the highest rainfall amounts across northeast Iowa where 1 to 2 inches are expected. Probabilities for 0.1" or greater drop dramatically north of I-90. - Scattered showers, a few storms, tracking southeast across the area Fri afternoon/evening, mostly for WI. - Periodic rain chances from the weekend into next week, but mostly "low end" (20-40%) at this time. Temps trending 5 to 10 degrees above the seasonable normals. && .DISCUSSION... Issued at 220 PM CDT Wed May 8 2024 * REST OF TODAY AND TONIGHT: Storms, Heavy Rain Shifting Further South Pleasant conditions are expected for the rest of the afternoon as fairly robust diurnal mixing will allow for some clearing in sky cover which should allow temperatures to warm into the 70s for much of the region. As a surface low moves south of the region, a mid- level frontogenesis band will entrain itself across the southern portion of the local area which when combined with some 850mb moisture transport and precipitable water values of just over 1" in northeast Iowa as shown in the recent deterministic model runs (NAM/GFS/RAP). This will allow for fairly efficient rainfall where the frontogenesis forcing is maximized. One recent forecast trend that has created some challenge is the CAMs beginning to shove much of the higher probabilities for heavier rainfall further south. Currently, probabilities in the 08.12z HREF show modest probabilities (40-70% chance) for 1 inch of QPF or greater across southern portions of the local area in northeast Iowa. Additionally, the probabilities in the 08.12z HREF of 0.1" or greater sharply drops off north of I-90 (roughly 40% at La Crosse) where much of the 08.18z CAMs show a tight gradient in the northern edge of the frontogenesis band. As a result, confidence remains low with how far north more substantial precipitation will get but areas where the forcing is best realized across northeast Iowa and points south could see 1 to 2 inches of rainfall with locally higher amounts possible. The strong to severe storm threat with convection appears to be fairly minimal across our region with forecast soundings showing only a few hundred J/kg of MUCAPE, so while some rumbles of thunder are likely, much of the severe threat will be outside the local area. * FRIDAY/FRIDAY NIGHT: another round of showers, isold storms The medium/long range guidance has been in good agreement with dropping a shortwave trough south/southeast out of southern Canada, sliding it across the upper mississippi river valley Friday afternoon/night. Preceding warm air advection is weak with not well defined Fgen along the leading edge of the shortwave. The system brings enough saturation with it to fuel rain chances. Decent shear with longish, straight line hodographs, but only about 250-500 J/kg of MUCAPE. So a smattering of storms with the showers, but not expecting anything too perky. Dry sub layer does suggest some enhanced gustiness possible, depending on timing of any storm (noctural sfc inversion acting as a deterrent). * WEEKEND-EARLY NEXT WEEK: periodic rain chances, seasonable Active, fairly zonal flow sets up for the weekend into the middle part of the new week. EPS and GEFS continue to push various ripples in the upper level flow across/near the region - with some expected differences in timing/location. Nothing too organized at the moment (locally), nor much for a tap into southernly moisture. Plenty of saturation though to fuel occasional rain chances. For now, the model blend will paint low end chances (20-40%), which are reasonable given the time frame/strength of the shortwaves/model variances. Enough instability to spark some storms too, but currently no day is holding much for a severe threat (all subject to change of course). For temps, the flow points to a return to normal to a few degrees above. GEFS and EPS have trended this way, although the GEFS has remained the cooler of the two with 75% of its members generally at or below the early May normals. The grand ensemble of members suggests a 30-40% for 70+ highs for next week, but driven up by a warm Canadian solution. Meanwhile, the NBM tops the suite of model guidance (5 to 10 degrees above normal), riding some of the top 10% of the EPS and GEFS which push temps into the 80s. No anomalous fetch of warm air on the horizon per the NAEFS and EPS. The NBM isn`t "out to lunch" if we have mostly sunny days and keep any rain chances outside of the afternoons. For now, will ride with the NBM...but not completely sold on its output yet. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1228 PM CDT Wed May 8 2024 VFR conditions are expected for much of the period with MVFR cigs sneaking into the region by the morning hours on Thursday. A weather disturbance will approach the region this later this evening bringing some chances for shower into the region. Currently, short- term guidance has been continuing to trend the heavier precipitation further south, suggesting that areas along and north of I-90 will likely receive minimal rainfall. With both TAF sites right on the sharp cutoff for more substantial rainfall, opted to hold onto a VCSH with confidence being lower on exact timing of any showers that may occur this evening and overnight. Otherwise, as this system begins to depart, some wrap around moisture will allow for cigs to drop during the later morning hours on Thursday to MVFR. Winds will remain fairly light throughout the TAF period at around 5-10 kts. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Rieck AVIATION...Naylor