Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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409 FXUS63 KARX 011746 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1246 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - Two rounds of convection are expected Monday night into Tuesday night, bringing a threat for heavy rain. While the probability is low, some severe storms could occur late Tuesday afternoon and evening. - Additional rainfall could occur Thursday through Sunday but, at this time, heavy rain and severe thunderstorm potential appears to be low during this period. && .DISCUSSION... Issued at 221 AM CDT Mon Jul 1 2024 Monday Night into Tuesday Night: Heavy Rain Threat, Potential Severe Today, as a surface high, centered to our east as of 06z, will continue to build eastward as an upper trough departs off the mid- Atlantic coast. As this occurs, a second upper trough will advance from the northern Rockies to the northern Plains. This combination will allow for southerly return flow to resume, bringing plentiful moisture to the CWA by tonight. Late tonight into Tuesday morning, as a lead shortwave ejects northeastward, expect widespread showers and thunderstorms to develop given the 850mb moisture transport described above. Progged PWAT values look to approach 2" as the night goes on, near or at GEFS/ENS climatological maximums. Thus, while the system as a whole looks to be progressive in nature - a welcome contrast compared to the repeat generation of convection along a quasi-stationary front seen June 21-22 - thunderstorms should be highly efficient rain producers. Latest NBM 25th to 75th percentile values suggest around 0.5" to 1.5" of rain for most locations with the potential for locally higher amounts as the distribution still has a long tail on the high side, in line with what one would expect with convectively driven heavy rain. Moving ahead to the second main round late Tuesday afternoon into Tuesday night, additional convection should develop ahead of a lead shortwave progged to traverse southern portions of the CWA and ahead of the body of the main upper trough farther north. Trend in guidance has been for weaker moisture recovery following the initial round of convection and a slightly farther south trajectory with the shortwave and have reduced PoPs north of a Wabasha to Black River Falls line accordingly. As for hazards, flooding risk seems similar compared to the initial round, with highly efficient rain production tempered by the progressive nature of the pattern and storm motions of 35 mph or greater leading to an additional 0.5" to 1.5" of rain. Severe thunderstorm risk, while potentially encompassing all hazards given abundant deep and near-surface shear, appears increasingly doubtful as leading edge of the best moisture return and instability continues to trend to be just to the south of the CWA. Nonetheless, will need to continue to closely monitor this potential. Independence Day through Sunday: Additional Chances for Rain Guidance is now in fairly good agreement that an upper low should slowly churn east over SD to the Great Lakes, bringing the potential for periodic showers and a few thunderstorms, in particular Independence Day and Friday. As for hazards, Gulf moisture return continues to be displaced south of the CWA as it has been for several model cycles, so am not too concerned about flash flooding or severe thunderstorms during the latter part of the week. That said, any additional rainfall would serve to delay falls on any rivers in flood due to Monday night into Tuesday night`s heavy rain. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1247 PM CDT Mon Jul 1 2024 VFR conditions are expected into the evening with a trend towards increasing TSRA chances and lower ceilings from 03->07Z. A low-level jet will transport instability and moisture into the area overnight with TSRA expected to break out across the local area and airfields. Confidence was high enough to TEMPO TSRA for a period overnight before the weather tapers off and shifts east around sunrise. A good portion of the morning hours will be MVFR, and currently ceilings are forecast to be on the higher side in the TAFS versus much of the model guidance. Most of the model guidance suggests IFR ceilings are likely from 12-18Z Tuesday. With the GLAMP/NBM guidance usually running too pessimistic, have kept the forecasts in the MVFR range at this time where confidence was higher. && .HYDROLOGY... Issued at 223 PM CDT Sun Jun 30 2024 Minor to moderate flooding continues along the Mississippi River and will continue into this week. Additional rainfall is expected Monday night into Tuesday evening. At this time...rainfall amounts of 1 to 2.5 inches are expected across the area. With the antecedent conditions across the region...water levels will rise on area rivers and flash flooding will be possible. Interests along rivers...streams and creeks should monitor the latest forecast and trends Monday night into Tuesday. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Ferguson AVIATION...Baumgardt/Kurz HYDROLOGY...DTJ