Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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110 FXUS63 KARX 180336 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1036 PM CDT Tue Sep 17 2024 .KEY MESSAGES... - Unseasonably warm weather will continue through Thursday with highs into the 80s for most areas. - The next risk of storms comes Thursday with a low (5 to 15%) chance for a strong or severe storm, mainly near and west of the Mississippi River during the afternoon and early evening. - Gradual cooling along with low to medium (30 to 50%) rain chances expected for the weekend into early next week, but forecast confidence, including timing of any rain is lower. && .DISCUSSION... Issued at 230 PM CDT Tue Sep 17 2024 OVERVIEW: A blocking ridge with anomalous mid-level heights across eastern Canada will persist through late week with stronger shortwave energy largely deflected north across the northern plains/southern Canada. By the weekend, several troughs embedded within split flow will attempt to move east into the Midwest/Great Lakes, but predictability lowers with the evolution of these troughs, related to uncertainty in the downstream flow pattern/strength of the ridge. Unseasonable mid-September warmth will give way to some cooling through early next week, while shower/storms chances increase ahead of a front on Thursday, and persist through the weekend, although confidence/predictability lower substantially during this time. TEMPS: The trend of warm September days will continue through Thursday with persistent low-level southerly persisting on the western periphery of the ridge. There is high confidence for temps at or above 80 (>70- 80%) for much of the area through Thursday with low chances (~20%) for 90 degree readings, mainly across southwest Wisconsin and in the typically warmer valleys. An increase in high clouds, though, is expected on Wednesday and despite stronger warm advection Thursday with breezy southerly flow, clouds and any showers/storms could also impact temps. That said, overall model spread in temps is low through Thursday. By the weekend into early next week, a gradual cooling trend is anticipated, although the potential range of outcomes increases due to uncertainty in the large scale flow pattern. RAIN/STORM CHANCES: Northward moisture transport will be focused west of the Mississippi River through Wednesday with low-level easterly flow keeping drier air in place eastward across Wisconsin allowing afternoon RH to fall below 30% in spots. As a result, showers/storms will remain focused farther west through this time. By Thursday, a trough will be forced north and east across the northern plains into the Canadian prairies with a southward extending cold front sliding east towards the Upper MS Valley. Warm, moist advection will occur ahead of the front, with some potential for showers/storms during the morning within the area of stronger warm advection. The greater synoptic forcing likely will pass by to the north of the area later Thursday as the primary shortwave gets shunted north and east into Canada, but there is expected to be some overlap between a fairly narrow plume of instability ahead of the front and enhanced low to mid-level flow. Depending on the degree of low-level moistening, capping could be an issue, but storms are probable along the front during the afternoon/evening, then likely weakening as they progress eastward across the area through the evening as instability wanes, given the modest support aloft. The most likely location for any strong to severe storms looks west of the Mississippi River where timing of the front and higher instability would potentially support more intense updrafts. Otherwise, for this weekend into early next week, separate troughs across the southwest US and north across Canada will progress eastward, although the predictability of these troughs is relatively low, given the influence of the downstream ridge and overall uncertainty in the evolution of the flow pattern. Consequently low to medium (30-50%) rain chances extend from Saturday into early next week. These chances will inevitably be refined as predictability increases through the week. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1035 PM CDT Tue Sep 17 2024 VFR conditions and south to southeasterly winds are forecast for the TAF period. Winds tonight range from calm to 5 kts east of the Mississippi River to 5-10 kts to the west. These winds increase to around 10 kts areawide with gusts near 20 kts west of the Mississippi River for the afternoon. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JM AVIATION...Skow