Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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949 FXUS63 KARX 040957 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 457 AM CDT Thu Jul 4 2024 .KEY MESSAGES... - We`ll have some of nature`s own fireworks today as shower and thunderstorm chances increase through the day from west to east. Severe weather potential risk level is Slight/level 2 of 5. - Localized heavy rain possible due to storms; multiple rounds, thus storms may repeat over the same area. - Seasonably cool for the weekend into next week. Rain chances increase again Saturday night into Sunday; probability for .5" or more is 20% to 40% toward DBQ. && .DISCUSSION... Issued at 400 AM CDT Thu Jul 4 2024 Overview: Latest water vapor satellite imagery, 500mb heights, and lightning showed a messy convective pattern with a large 500mb trough/areas of storms over Saskatchewan/Manitoba southward into the Northern Rockies into the Central Plains. Also, storms were noted across the Ohio River Vally into Kentucky and southern Missouri on the northern fringe of the upper level ridge. Meanwhile, a large ridge continued over California. Late evening 3-hr precipitation amounts were variable across Nebraska from a few tenths to over an inch at McCook and Cambridge. In the western High Plains, some gusts 40 to 50kts and an impressive outflow boundary were noted with the storms where dewpoints in the 50s dropped to the mid 40s. Locally, it was a seasonably warm day with temperatures topping out in the 80s. The showers and a few storms generally popped up across northern WI and northern MN into northwest Iowa. The latest WSR-88D radar mosaic showed the organized convection across KS/MO with a shortwave trough and scattered convection farther northward closer to the area of closed 500 mb low pressure over the Dakotas. Today through Friday: A 500mb trough of low pressure over the Central Plains will shift eastward with lead energy moving across the local area this afternoon, however the area of low pressure closes off over MN/Upper Mississippi Valley overnight, slowly moving across Wisconsin during the day Friday. The deterministic models the NAM/GFS/EC have two 850mb circulations over a short distance which makes the precipitation distribution a little messier to forecast. The 850mb front lifts into northeast Iowa and west central Wisconsin during the afternoon, stalling then shifting east across Wisconsin Friday morning. Shower and thunderstorm chances increase through the day from west to east. Highest confidence is from the afternoon into the evening with the approach of the upper level trough and heating during the day with moistening (precipitable water increasing to 1.4 to 1.6"). A seasonably strong 95kt upper level jet tracks across Nebraska by 18Z with left front exit region support across the forecast area. Drier air shifts in from the west, however less confident on where isolated-scattered thunderstorm re-development occurs ahead of the dry slot over parts of southeast MN/northeast IA during the evening. The closed low remains in the area for Friday with wrap-around showers and thunderstorms. Much of our local area is in a Slight Risk/risk level 2 of 5 for severe storms. Surface pressure is somewhat ill-defined with low pressure to the west. By 18Z, the RAP shows surface low pressure over southern MN with a warm from across southeast MN and the cold front pushing across Iowa. The hi-res models show an uptick in stronger storms as the afternoon progresses with an increase in coverage. Generally, it appears one area of storms lifts northeast with additional storms developing closer to the cold front, followed by more isolated development behind the main area during the evening. HREF mean MUCAPE increases late morning through mid afternoon, with deep layer shear increasing during the afternoon. Cloudcover will hinder maximum heating, however we still have highs forecast in the upper 70s to mid 80s. Forecast soundings show dry air initially in the lower levels with longer hodographs. MLCAPE appears modest 500-1200J/kg. With the front in the area and 0-1km shear of 10-20kts and deep layer shear of 50-60kts will want to monitor local boundaries, where instability is being maximized and any thermal gradients. Strong to severe storms possible with large hail and damaging winds possible. Locally heavy rain is also possible with a 20-40% probability of 2" or more per the EPS/GEFs/Canadian ensembles from Winona to Trempealeau Co. Comparing total precipitation from model to model there is a slight lowering of total rainfall across northeast Iowa and a potential of 1 to 2" along and north of I90. The EC was a bit farther northeast more north and east of I94. Considered a Flood Watch due to multiple rounds of storms and recent heavy rainfall. The current forecast has higher amounts 1 to 2" forecast where the 6hr FFG is greater, along and north of I90. There is lower confidence for general higher amounts south of I90 where FFG is lower. Interestingly, the HREF does have higher 1hr rainfall rates this afternoon just south of the local area. The great probability for Friday exceeding .5" is across Clark Co. At this time, storms appear progressive, however, should they get tied to the 850mb boundary north of I94, may need to reconsider the watch. Farther south confidence is low in the storms putting down the general amounts needed for a watch. Will continue to monitor Flood Watch potential trends due to saturated soils. Seasonably cool for the weekend into next week. Rain chances increase again for Sunday. The probability for .5" or more has increased to 20% to 40% toward DBQ: Saturday night into Sunday and into early next week, the unsettled weather pattern continues with a weak wave, then a deepening trough working into the area. The EPS/GEFs/Canadian ensembles showed a slight increase in higher rain potential for northeast Iowa 20-40% chance of .5" or more. The series of cool fronts continues into midweek. Highs Saturday through Wednesday look to remain near or cooler than normal with highs in the 70s to lower 80s. && .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 400 AM CDT Wed Jul 3 2024 We continue to monitor river trends with the recent heavy rains with most rivers continuing to fall, however flood warnings continue for the Mississippi River and the Kickapoo at Viola. General rains of .5 to 1.5" are forecast with the greater confidence along and north of I90. Some spots per the 04.00Z HREF could see 2"+ with a 20-40% chance for Winona/Trempealeau Co. 6-hr flash flood guidance is generally 2 to 3 inches for flash flooding. Cannot rule out localized flash flooding with the storms later today and tonight if storms repeat over the same area. Be weather aware with outdoor activities and if traveling, be aware of your surroundings. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Zapotocny AVIATION...EMS HYDROLOGY...Zapotocny