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Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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739 FXUS63 KARX 131115 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 615 AM CDT Tue Aug 13 2024 .KEY MESSAGES... - Widespread showers/storms likely later Wednesday into Thursday with locally heavy rainfall possible. This looks to be the highest chance for rain over the next week with medium (40-70%) chances for at least 1 inch. - Seasonable temps generally expected over the next week. && .DISCUSSION... Issued at 303 AM CDT Tue Aug 13 2024 Today-tonight: With light winds under the surface ridge, some fog has been developing early this morning. Expect fog in typical valley/low-lying areas with guidance also favoring parts of northeast Iowa and southeast Minnesota. The fog should lift by mid-morning. Otherwise, high pressure will continue to drift across the Upper Midwest today as mid-level ridging works eastward from the plains. Temps will continue a slow upward trend from the past few days with highs in the upper 70s to mid 80s. Like the past few days, an isolated shower/storm could form (10-25% chance) within a diurnal cumulus field across central Wisconsin where modest instability and cooler mid-level temps are expected. Otherwise, the ridge axis will slide east overnight tonight, but wind fields look to remain light enough to support early morning fog development in valleys and perhaps parts of central Wisconsin, but this will depend on timing/thickness of high level clouds increasing from the west. Wednesday-Friday: The highest chances for impactful rainfall over at least the next week remain on track for later Wednesday into Thursday as a broad upper level trough gradually organizes from the plains eastward toward the Great Lakes. As surface ridging pulls east on Wednesday, an increasing low-level jet will support strong northward moisture transport with precipitable water values increasing to near 2" in spots, 175% of normal or higher. Widespread showers/storms should spread east later Wednesday into Thursday in conjunction with strong mid-level warm advection. The high moisture content and deep warm cloud depths will promote efficient rain production. NBM probabilities for at least an inch of rain have increased to 40-70% with lower (20-40%) probabilities for 2". However, the caveat at this time of year is that rainfall amounts likely will be quite variable given the convective mode. There are also notable differences among ensembles, with the majority of EPS members showing a wetter solution compared with the GEFS. With the somewhat disjointed nature of the larger trough, deep layer wind fields are modest and the probabilities for at least 1000 J/kg of SBCAPE drop off considerably northward across southwest Wisconsin and northern Iowa towards I-90. These, combined with remaining uncertainty in the evolution of the trough/surface features limit confidence in severe storms, but stronger storms cannot be ruled out on Thursday. Showers/storms remain possible into Friday within cyclonic flow beneath the upper level trough, but the risk of heavier rains will mostly be focused late Wednesday into Thursday. Saturday-Monday: For the weekend into next week, upper level ridging is forecast to strengthen across the Rockies into the plain with troughs centered across the Pacific Northwest and over the eastern Great Lakes. While this general pattern is supported broadly across global ensembles, differences exist in the amplitude and degree of progression of these synoptic features. Regardless of these differences, a northwest flow pattern is likely across the Upper Midwest this weekend into next week with generally near to below average temps favored. Overall, relatively dry conditions are expected through this period, but some shower potential could linger into the weekend until the trough pulls east. Any rain chances beyond this time would be dependent on low predictability shortwaves within the broader flow. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 615 AM CDT Tue Aug 13 2024 VFR expected for most of the 13.12Z TAF period. River valley fog potential returns late tonight into early Wednesday morning. While it is too far out to hammer out any details, a more moist airmass increases likelihood of saturation. Otherwise, shower, storm, and flight impacts move from southwest to northeast through Wednesday into Wednesday night. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JM AVIATION...JAR