Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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690 FXUS63 KARX 171110 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 610 AM CDT Mon Jun 17 2024 .KEY MESSAGES... - Thunderstorms likely this morning with a locally heavy rain threat, shifting north of the area by later this afternoon. Storms may also produce winds up to 60 mph and hail around 1 inch in diameter, but confidence remains on the lower side. - Continued warm and humid through Tuesday, with at or above normal temperatures for much of the seven day forecast. - An active and wet weather pattern shaping up for mid to late week with repeated rounds of storms that have the potential to bring heavy rainfall. && .DISCUSSION... Issued at 400 AM CDT Mon Jun 17 2024 Today - Tonight: Convective and Temperatures Trends Strong moisture transport and 310K isentropic ascent over a quasi-stationary boundary draped across northern Nebraska sparked an elongated complex of thunderstorms shortly before midnight, with the steering flow and cold pool propagation allowing for the complex to work its way eastward. As the complex transects south-central Minnesota, it will outrun the strongest 850-mb moisture transport and enter a region where the low level flow becomes more boundary-parallel. Therefore, a general weakening trend is expected as the system moves through this morning. That all being said, there has been an uptick in the organization of the cold pool at the leading edge of the now bowing line segment over southwest MN and northwest IA, with multiple wind reports of 60 to 70 mph reported since 2:30am. While the overall deep-layer kinematic environment isn`t ideal for a prolonged wind event, these wind storms can persist for longer than the environment may suggest and has prompted the issuance of a severe thunderstorm watch for this morning across much of the forecast area. In addition, hodographs are more elongated north of the surface boundary (mostly just north of the forecast area), so one cannot rule out the potential for larger hail with any storm that develops ahead of the main line. Given the potential for a period of cell training along the boundary, localized corridors of 1-2 inches of rain may fall over southeast Minnesota into west-central Wisconsin through early this afternoon. A few HREF members even have pockets of 2-3 inches of rain from these cells, but the probability of these higher amounts is very confined in space and time. The various 00-06Z HREF members differ on how much convection remains for the afternoon hours along the lower tropospheric boundary with weak upper level ridging passing through, so have maintained broad-brushed 20-30 percent PoPs through the afternoon. Forecast soundings/hodographs from the RAP/HRRR could support severe weather if there is enough recovery in the wake of this morning convection to realize the 2000-3000 J/kg of SBCAPE. As the boundary finally marches northward in response to the next subtle upper tropospheric wave ejecting from the Four Corners region, the threat for precipitation shifts northward and out of the forecast area by late this evening. High temperatures today--most notably along a line from roughly Charles City, through La Crosse, and up to Wisconsin Rapids-- hinge substantially on convective trends and whether the cold pool from the morning MCS can push the synoptic boundary any further south. Recovery of the airmass won`t take place until late in the day, so locales north of the front will likely stay 10-15 degrees cooler than points just to their south. Tuesday: Hottest Day of the Week Short and medium range solutions are locked in on Tuesday being the warmest day of the forecast with a very tight clustering of NBM members around the 90 degree mark. Did nudge highs a few degrees warmer in favored valleys, which bring heat indices to around 100 degrees at peak heating. Will assess any need for a targeted advisory on upcoming shifts. Tuesday Night - Saturday: Repeated Rounds of Storms and Rain An ejecting PV lobe drives the surface boundary back south Tuesday afternoon and night as a cold front. The mid to upper level flow should take on a more line-parallel orientation as the front sags southeastward, promoting a slower frontal passage and more training of individual cells along the front. Deep moisture will be plentiful ahead of this front with a wide open Gulf enabling PWATs to exceed the 97-99th percentile of the ENS model climatology with values of over 2 inches possible. Such a setup would be favorable for heavy rainfall, with the greatest amounts early on in the event before the cold pool begins outrunning the convective updrafts as complex nears the Mississippi River. Upper level ridging remains steadfast east of the region through the week, even retrograding westward Thursday into Friday. This causes the front to eventually stall out and meander for Wednesday and Thursday before lifting back northward on Friday, then swinging through as a cold front on Saturday. Where the front sets up for midweek and subsequent timing details remain a source of variability in the medium range guidance and are heavily driven by convective influences each day, yet nearly half of the NBM guidance have rainfall amounts exceeding 3 inches over southeast Minnesota and western Wisconsin between today and the weekend. Each round of storms will further saturate the soils and could increase the risk of river and flash flooding through the week. While the passage of the cold front will bring some relief from the heat, highs in the upper 70s to mid-80s remain on the docket for the rest of the week with dewpoints in the 60s still making it feel very much like summer. Sunday into Early Next Week: Cooler and Drier Indications at this stage in the forecast is that the upper level ridge finally flattens with the arrival of a northern stream wave over the weekend with either zonal or northwesterly flow in its wake. Guidance is understandably well-spread in the various details of the forecast, but only about 20 to 30 percent of the members have highs above 80 degrees for Sunday and Monday. Drier weather sets in with the passage of the surface high pressure cell, but the timing of this dry period hinges on the flow pattern as a more zonal setup would usher the ridge through faster versus a more amplified pattern. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 600 AM CDT Mon Jun 17 2024 Widespread showers and thunderstorms impact mainly southeast Minnesota and west-central Wisconsin early this morning, diminishing in coverage through the morning and becoming more scattered in the afternoon. Expect MVFR to localized IFR conditions with these storms. The storms shift north through the afternoon and evening with VFR conditions for the overnight. Winds will be variable for the morning, switching to the south and increasing to 10-20kts with higher gusts west of the Mississippi River. LLWS is possible at LSE if winds decouple at sunset and have included in the TAFs at this point. && .HYDROLOGY... Issued at 400 AM CDT Mon Jun 17 2024 Repeated rounds of heavy rain are expected between today and the upcoming weekend, with each round of rain increasing the risk of flash flooding and river flooding. Much of southeast Minnesota into western Wisconsin will likely see over 3 inches of rain by the end of the week with a 20-30 percent chance of 4-5 inches. River flooding concerns, especially along the Mississippi River and its Minnesota tributaries will need to be monitored closely. Many locations along the Mississippi River will approach flood stage towards next weekend (50-70% chance) if rainfall unfolds as forecast. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Skow AVIATION...Skow HYDROLOGY...Skow