Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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973 FXUS63 KARX 262326 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 625 PM CDT Wed Jun 26 2024 .KEY MESSAGES... - Rain/Storm Chances Return Early Friday and Continue for Friday and Friday Night. Probability for .1" of rain is 80-90% however, for 0.5" 30 to 60% with the highest toward DBQ. - Cooler Weekend On Tap, Temperatures 0 to 10 Degrees Below Normal For This Time Of Year. - Turning More Active as we Head into Early Next Week. && .DISCUSSION... Issued at 300 PM CDT Wed Jun 26 2024 Overview: Water vapor satellite imagery, 500mb heights, and lightning showed a 500mb trough from the Great Lakes southwest toward the southern Plains. A ridge was over the Rockies and the Four-Corners area. Another area of closed low pressure was heading toward the West Coast. A couple of large mesoscale convective systems were over the eastern Great Lakes/PA/NY and also from southern MO/MS/LA. Sporadic convection was noted throughout the ridge and ahead of the West Coast trough. Locally, we are under northwest flow aloft with surface high pressure building in. Satellite imagery showed sunny to partly cloudy skies. Winds were brisk northwest with temperatures in the 70s to lower 80s with dewpoints in the 50s. Through Friday: A back-door cool front will move southwestward across parts of Wisconsin through tonight with surface high pressure building in. RAP forecast soundings show patchy low level moisture. With the MVFR ceilings upstream. Clouds should decrease after sunset, but will need to monitor any patchy low cloud development. Also, where areas are more clear, there is the potential for valley fog with the weak low level flow. With the drier airmass and short nights, any fog looks to be patchy. Thursday morning, return flow is on the increase on the western periphery of the high pressure system and the ridge aloft tries to build. Moisture transport is on the weaker side, however some 850mb moisture warm air advection will continue to produce clouds and a few sprinkles or a shower could occur Thursday afternoon into Thursday night. The RAP/HRRR both show some activity in parts of the forecast area by 01Z,however do not agree on the area. Moisture transport increases across Minnesota after 06Z Thursday night into Friday, shifting eastward across the forecast area and tends to split north and south. Meanwhile a shortwave trough is forecast to flatten the ridge and move through the westerly flow. As a result, we see showers and thunderstorms become more widespread 80- 90%. At this time, the severe weather threat appears low. Forecast soundings show limited instability with MUCAPE of 250-500J/kg with weak mid level winds. 0-3km shear is substantial though. Should instability increase, the severe weather threat would be more elevated, thus as we approach Friday, we`ll need to watch this. The surface front lags the short wave, thus showers and thunderstorms continue into Friday night. The upper level trough swings through and tends to focus the storms north and south of the forecast area. Before the cold front clears the forecast area, some instability pushes through overnight, but the flow is more veered by then. The EC/Canadian/GEFS ensemble probability for .1" of rain is 80-90% however, for 0.5" 30 to 60% with the highest toward DBQ. The weekend into next week: Saturday, the upper level trough is digging through the Upper Mississippi Valley with surface high pressure building in the the west. Generally dry weather is forecast, however forecast soundings show a spotty shower cannot be ruled out with the heating during the afternoon/low level instability. Surface high pressure is forecast to be over the area Sunday. High temperatures Thursday through Sunday are forecast to be in the 70s to lower 80s, thus near to 10 degrees below normal. Active weather returns for early next week. A trough over the West Coast during the weekend heads toward the Upper Mississippi Valley for early next week. Southerly flow ahead of a couples of embedded shortwave troughs should bring sporadic thunderstorm activity to the forecast area through possible Tuesday afternoon. Warmer temperatures also return with highs in the 80s. && .HYDROLOGY... Issued at 300 PM CDT Wed Jun 26 2024 Several rivers fell below flood stage over the last 12 to 24 hours, but most of this water is still on route to the Mississippi River. The Mississippi River is expected to keep rising through the next 5 to 7 days depending on location and which rivers route into that particular location. Minor to moderate flood stages are expected from Lake City through Guttenberg. While official forecasts from the River Forecast Center do take into consideration all of the water that has already fallen and is on its way into the Mississippi River, it only takes the next 24-48 hours of forecast precipitation. Since the next chance of precipitation for the local area is not until Friday, this will not be accounted for in the official forecasts. At this time, rainfall expected for Friday (0.10-0.70") does not appear that it would substantially affect the Mississippi River forecast. The forecast rain on Friday will likely keep the river elevated beyond the next 7 to 10 days and the river may initially fall some before beginning to rise again as the additional rainfall flows downstream this weekend into next week. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/... Issued at 625 PM CDT Wed Jun 26 2024 Cooler and drier regime holding us in VFR conditions for the time being. Diurnal cumulus will clear this evening as winds slacken. With this surface high building in, will be monitoring risk for any valley fog towards morning but dry layer and wind direction not helpful for anything extensive. Perhaps only other concern might be approach of stratus layer from the north towards morning but even that would keep us in VFR levels. There is a scenario where MVFR ceilings develop but risk for that is 10-15%. Return flow behind departing high will swing winds around and could lead to more diurnal cumulus development later Thursday, as well as increasing high clouds from convection to the west. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Zapotocny AVIATION...Shea HYDROLOGY...JW/Zapotocny