Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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429 FXUS63 KARX 191915 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 215 PM CDT Thu Sep 19 2024 .KEY MESSAGES... - Showers and thunderstorms are expected to increase for this afternoon and evening (70 to 90%). A slight risk (level 2 of 5) for severe weather continues for much of the area. Main hazards are large hail and damaging winds, an isolated tornado, lightning and local downpours of heavy rain. - Above normal temperatures continue through Saturday before cooling to more seasonal temperatures lasting into early next week. - Chances for rain return Saturday and possibly Sunday as next storm system moves into the region. && .DISCUSSION... Issued at 210 PM CDT Thu Sep 19 2024 Overview: Late morning water vapor satellite imagery, 500mb heights, and lightning showed an area of closed low pressure over the Saskatchewan/Manitoba border with a ridge from parts of the Great Lakes into the Mid-Mississippi Valley to the Southern Plains. A second area of closed low pressure was over the Southwest U.S. Lightning was noted to the east of the upper level trough from parts of northern Minnesota into western Wisconsin/central Iowa toward northwest Missouri. An additional isolated storm was noted near the front in northeast Neb. Surface dewpoints at 16Z were mostly in the upper 50s to lower 60s. Dewpoints in the upper 60s to around 70 were pooled ahead of the surface front in southwest Minnesota. At 17Z...Visible satellite imagery has some clouds locally with a large clearing area to the west with temperatures warming into the 80s. An 18Z radiosonde was launched at MPX. Lapse rates were steep around 8.7 deg C/km with SBCAPE of nearly 3000 J/kg and downdraft CAPE of near 1000 J/kg. The latest reflectivity shows storms growing across northern Iowa with one severe thunderstorm warning issued already. This is an hour or two ahead of some of the CAMs and these storms are more elevated. Strong to severe storm potential with locally heavy rain: The 500mb trough axis is forecast to swing through the upper Mississippi Valley this afternoon into this evening dragging a cold front into the forecast area; gradually exiting the area Friday morning. The axis of instability over western Minnesota and northwest Iowa will shift eastward with increasing moisture transport, an strengthening deep layer shear (pockets of 40-60kts) and 0-1km shear (30-40kts). MLCAPE tends to be strongest 21-03Z (1000-2500J/kg) in the western portions of the forecast area, clipping west central WI. A few of the CAMs (WRFARW/NAMNest) have more convection into early afternoon for portions of northeast Iowa, thus will continue to monitor this area along the moisture gradient and gradient of MUCAPE. Attention then turns to storms increasing ahead of the cold front with upscale growth along the front. Discrete storms will be capable of producing of severe weather with large hail, damaging winds, and isolated tornadoes. Some of the forecast soundings are showing slightly more veered winds versus backed winds, thus the tornado threat will be conditional on the ability to tap the backed flow. The most recent HRRR/RAP show multiple southwest to northeast broken lines and potential for embedded bows or surges with some of the line segments. The HRRR/RAP do try to converge these lines as they move into western WI by 02 or 03Z, but not as much as the other hi-res data. Additional storms develop to the west during the evening and linger until the front pushes through. A severe weather watch was being coordinated and issued for parts of southeast MN, wcntrl WI, and parts of northeast Iowa. Initially, the MPX 19.12Z sounding has a low level inversion and dry air below 850mb. Deeper moisture is arriving though with a 15 deg. C 850mb dewpoint. Precipitable water values at MPX were nearly 1.5 inches and are forecast to increase across the forecast area 1.5 to 2". The 2" axis around 00Z is in the western parts of the forecast area (parts of SE MN/WC WI) exits the area around 06Z with drying from the west. Forecast soundings show a warm cloud depth of around 4km. We have some 10-40% probabilities of 1" per hr. rates on the 19.12Z The 3hr FFG is 2.5 to 3.5". The storm movement is on the faster side 25 to 35kts which is less supportive of flash flooding. However, will want to monitor rain rates and storms moving over the same area. HREF PMM for 3hr shows some isolated spots could see amounts in excess of 2 to 3 inches. Above normal temperatures through Saturday then cooler for the weekend into next week: Surface high pressure builds in for Friday, however 850mb temperatures do not cool significantly until the next cold front Saturday night. Highs Friday and Saturday should range from the mid 70s to the mid 80s. As another 500mb trough moves through central Canada and the closed low over the Southwest U.S. moves into the Four Corners region, moisture returns northward and ahead of the trough Saturday. The forcing appears to be somewhat split, but there is enough to mention rain chances 40-70% Saturday night and possibly into Sunday. There is not great agreement for Sunday on the strength of the push of dry air with the cold front and the strength of the storm system moving through Colorado with the deterministic runs. The ensembles do have 40-70 probabilities of 0.25" or more. We`ll continue to monitor this though with the deterministic EC being drier for Sunday. Similarly for Monday, depending on the closed low tracking into the region, precipitation may affect parts of the forecast area. We are on the cool side of the front Sunday into mid-week, thus temperatures remain seasonable in the 60s to lower 70s. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1236 PM CDT Thu Sep 19 2024 Thunderstorms will be the main aviation concern this evening with IFR category reductions possible at times. As a result, have included a prevailing MVFR conditions with a tempo for IFR visbys where confidence is highest on timing. Winds may shift to westerly with strong wind gusts being possible as storms move through. With convective initiation still 4-5 hours away, likely will need to make forecast adjustments prior to the 00z issuance depending on convective trends. Storms move eastbound through the region during the evening and eventually exit towards midnight. As a front passes through the region, cooling overnight temperatures and recent rainfall in combination with clearing skies will create a fairly favorable fog/low-stratus setup into the morning hours on Friday. However, there remains some questions with how some stronger winds to around 20 kts in model soundings (recent HRRR/RAP/NAM) just off the surface will impact any fog/low-stratus formation. Consequently, held with the 3SM but will likely introduce IFR/LIFR reductions in subsequent issuances as forecast confidence increases. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Zapotocny AVIATION...Naylor