Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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550 FXUS63 KARX 051943 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 243 PM CDT Fri Jul 5 2024 .KEY MESSAGES... - Scattered Showers w/ Isolated Embedded Storms, Waning This Evening - Perpetual Precipitation Chances This Weekend Through Wednesday. - Slightly Below Normal Temperatures Through Wednesday. && .DISCUSSION... Issued at 215 PM CDT Fri Jul 5 2024 Scattered Showers & Isolated Storms Today: Scattered showers and isolated storms have frequented the forecast area early this afternoon as an area of lower heights exits east over the Great Lakes. In its wake, cold air advection has been sufficiently steepening low level lapse rates, tapping into more unstable mid level lapse rates. Steepest off deck lapse rates remain off to the northeast, tied to the mid to upper level cyclone, providing more widespread shower and storm coverage accordingly. As diurnal heating decreases this evening and the upper level low shifts farther east, expect these scattered showers to wane from west to east. Shower & Storm Potential Saturday Night Through Sunday: While CAA weakens Saturday, a tight, well-wrapped upper level cyclone, seen on upper level GOES 16 Water Vapor imagery early this afternoon, continues a southeasterly trajectory into the Upper Mississippi River Valley. As a result, precipitation chances initially lift from southwest to northeast as the low phases from central Iowa through central Minnesota. Resultant, initial precipitation potential grazes our western counties in northeast Iowa and southeast Minnesota before spreading farther east over the forecast area into Saturday night and Sunday. In detail, persistent dry air shunts a narrow filament of increased low level theta e away until Saturday night. A meager lobe of increased moisture is then expected to overcome the lingering dry air Saturday evening, pushing a narrow quasi- meridional filament of increased precipitation potential. Eventually, concurrent with the low level trough passage, the nocturnal low level jet advects the higher moisture filament towards the forecast area into the early morning hours on Sunday. The highest moisture remains shunted to the southwest through the entire event, limiting overall widespread rain potential to near 0.1" with 0.25" to 0.5" possible along our southern periphery and in spots where storms frequent. Periodic Precipitation & Storm Chances Through Wednesday: Periods of precipitation potential fall primarily on Monday, Tuesday, & Wednesday as a quasi-omega block results in perturbations along southwest to northeast flow. As the trough exits to the northeast through Monday, west-northwest flow converges along the moisture boundary through Tuesday before high pressure shifts east along the International border, perpetuating northerly flow towards this boundary. Eventually this boundary is overcome with southeasterly flow through Wednesday night as the high sags over the Great Lakes. Precipitation chances hinge on location of a wavering surface frontal boundary and the exit of the low level trough from Sunday night through Monday. Current confidence places this narrow corridor of increased moisture, oriented southwest to northeast, bifurcating the forecast area from southwest to central Wisconsin. Periodic Precipitation Impacts Through Wednesday: With relative weak forcing and an overall lack of moisture as we remain on the periphery of a meager, narrow moisture filament overall impacts appear limited at this time. Increased lapse rates remain northwest of the moisture boundary while saturation lies to the southeast. In summary, there is meager instability, highest to the northwest, albeit unsaturated. Farther southeast, long, skinny CAPE profiles limit any storm confidence with coincidental saturation. Therefore, storms are possible, primarily within a narrow corridor along the weak boundary that may shift southeast and northwest with diurnal heating and precipitation. Long term global ensemble confidence (30-80%) suggests 1" to 2" of total rainfall from Saturday through Wednesday. Within a 24 hours, confidence (50-75%) for near 0.25" Sunday and near an additional 0.1" Monday and Tuesday. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 1155 AM CDT Fri Jul 5 2024 CIGS: BKN MVFR looks to hang around for the early part of the afternoon with the RAP/HRRR soundings suggesting a gradual lift into VFR as the afternoon wears on. Should see an exit southeast with a scattering into the evening. WX/vsby: scattered -shra will persist through the rest of the afternoon as weak instability works with an upper level shortwave. Isolated thunder possible. -SHRA or VCSH could cover the likelihood/coverage with most vsby impacts holding at P6sm. However, could see a dip to 3SM with any moderate shower. Showers will shift southeast through the afternoon with decrease in areal coverage with loss of daytime heating. With clearing skies and saturated grounds/near sfc layer - some concerns for fog. Not a lot of consensus for where or how widespread the potential would be in the short term guidance, so opting to not add to forecast yet. Will keep a close eye though and adjust as needed. WINDS: north/northwest becoming light for the night time hours. Expecting a shift to the southwest moving into Sat afternoon. && .HYDROLOGY... Issued at 233 PM CDT Fri Jul 5 2024 4th of July rainfall amounts were generally less than an inch across the bulk of the forecast area, however some pockets of 1" to 2"+ did occur. Some of these localized amounts fell over the Whitewater, Root River, Kickapoo, Pine River, and Wisconsin River Basins, and the Mississippi River mainstem. As a result, we saw some river rises that have been recovering through today. Periodic precipitation chances dot the forecast period through Wednesday. Highest amounts are expected this weekend into Monday with 0.1" to 0.25" of widespread rainfall amounts. Higher amounts of 0.5"+ are possible along our southern periphery and where storms frequent. Further precipitation chances through Wednesday are expected to remain light with amounts of near 0.1" over each 24 hour period. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAR AVIATION...Rieck HYDROLOGY...JAR