Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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827 FXUS63 KARX 040415 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1115 PM CDT Wed Jul 3 2024 .KEY MESSAGES... - July 4th has shower and thunderstorm chances increasing from west to east as the day progresses, with most areas seeing rain at some point during the afternoon and evening. There was another uptick to the severe storm threat with the area now in a slight risk /level 2/ for wrn WI, nern IA, and sern MN, mainly in the afternoon. - Showers still on track for Friday with the most rainfall occurring north of I-90. Amounts should remain behaved with little impact. - Unsettled weather into the weekend and early next week with periodic rain chances and below normal temperatures. Heavy rain and severe weather chances look minimal. && .UPDATE... Issued at 700 PM CDT Wed Jul 3 2024 A somewhat conditional fog forecast exists for tonight. The first hurdle to overcome are clouds with multiple bands of upstream mid to high level clouds present. However, these are expected to dissipate later this evening before clouds thicken up after 3-6am, presenting a window of opportunity for fog formation. ADJLAV and GLAMP are quite aggressive with the fog formation along and south of I-90 (even showing some 1/4 sm values towards sunrise), but stand apart from the various model- driven visibility progs from the CAMs that are devoid of any fog. Forecast soundings, if we stay clear, may indeed support fog with winds expected to become light under a weak surface ridge and soils still saturated from recent rainfall. For now, have added patchy to areas of fog to the grids and the coverage of the fog will need assessing through the night. && .DISCUSSION... Issued at 245 PM CDT Wed Jul 3 2024 Periodic Showers and Storms on July 4th, Severe Storms Possible While there remains some differences in the details of the forecast, the bigger picture is becoming more clear with consistent signals of a dynamic mid-level trough deepening into the longwave trough over the central U.S. with surface cyclogenesis deepening a low over the area through Friday. GOES water vapor and lightning strike data is impressive over MT right now with this incoming trough. This system leverages the moist pool to the south with warm advection and modest moisture transport convergence during the day Thursday. Instability is becoming more agreed on which is also modest at 750-1000 J/Kg of MLCAPE. With increasing mid- level flow impinging on the area, there is a slight uptick in the severe storm potential. SPC has upgraded to a slight risk /level 2/ for near and west of the Miss river. While there might be isolated severe storm risk, there are still questions on the low level wind shear profile /weaker?/, shortening the hodograph, and storm organization potential. But, with dynamics coming in, good wind shear above 2km, and surface-based instability growing in the afternoon, severe storms are possible. Damaging wind and hail to 1" would be possible /higher freezing level doesnt help/. Rainfall rates will be good, but certainly not the high-end rates per the more tropic-like air mass we were in Tuesday...with rain amounts around 1/2 inch most probable. The morning may have a few showers around before clearing, mainly west and north of La Crosse. Some inconsistency in the CAM solutions for what appears to be only a weak mid-level shortwave trough in the southwest flow ahead of the main larger low. This is spawning the TSRA activity currently over SD/NEB. There isnt much low-level support for the rain chances. During the evening of the 4th, tried to build some better details into the precipitation chances and honor some of the CAMS suggesting a drying trend coming in from the west. While there is still spread in the CAM solutions for Thursday night, it appears WI will be the place for higher rain chances with more scattered to widely scattered activity over sern MN/nern IA. Showers Continue Friday One noticeable trend in the guidance is to clear the weather eastward as Friday afternoon progresses and having a dry evening in most areas. Until the low pulls east and the clearing occurs, a well-established deformation band wrapping northeastward around the low will favor the northern forecast area for heavier rainfall amounts The I-94 corridor and north amounts look to be 0.25-0.50" Friday, diminishing southward /less than a tenth/. Instability is negligible so showers will prevail and there are still some differences on how this pivots as the low ejects east...the southern forecast area looks like it could have lower rain chances than currently forecast. It will be a much cooler day in clouds and showers...built for frogs...with highs in the 60s mainly. More Rain Sunday Looking Likely, Showers into Next Week Longwave troughing over the central CONUS is persistent into next week with a series of shortwaves troughs shifting through. The next is shifting in Saturday night and Sunday making for what looks like another shower and thunderstorm day. More consistency in the timing is occurring with good agreement on the system ejecting in from the southwest with low-level moisture transport and warm advection. Signals are low for over an inch of rainfall Saturday night and Sunday (5-20%) at this time. Into early next week, more shortwave trough activity shifting through the longwave trough means slightly cooler than normal with shower chances. These do not look like heavy rain makers at this time. A pattern shift is seen in the extended forecasts by mid-end of next week to possibly warmer and drier. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1115 PM CDT Wed Jul 3 2024 Main TAF concerns through the period continue to revolve around fog potential overnight and showers/thunderstorms during the day (Thursday). Similar thinking as the previous forecast for fog potential. Current satellite imagery shows clear conditions over much of the area right now, but uncertainty comes with timing of incoming clouds and if that may hinder any fog development. Will maintain the BCFG at KLSE with this issuance and monitor near-term trends through the night. Showers and thunderstorms are expected to develop and move through the forecast area during the day. With some variability still on exact details such as timing, have maintained the VCSH and the PROB30 highlighting a period for TS potential. Otherwise, some signal remains in guidance for MVFR ceilings to develop southward towards the TAF sites late in the evening and overnight. && .HYDROLOGY... Issued at 245 PM CDT Wed Jul 3 2024 The main changes from this river forecast cycle were to refine the Kickapoo river levels. Most river forecasts remained the same, however, it should be noted that ALL of the rainfall expected Thursday and Friday was NOT in the river forecast. Thus, it is expected that this evenings forecast will see a bump in the river levels as Thursday rainfall is accounted for. In reviewing the tributaries, it seems 2-3" is needed to move these rivers to higher level flooding...and these are above current forecasts. The 03.12Z HREF suggests a 10-20% chance of 2"+ through Friday morning, on and north of I-90. Current forecasts are 1-1.25" for that same area. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ UPDATE...Skow DISCUSSION...Baumgardt AVIATION...EMS HYDROLOGY...Baumgardt