Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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361 FXUS63 KARX 060837 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 337 AM CDT Sat Jul 6 2024 .KEY MESSAGES... - Showers and storms tonight and Sunday (60-80% chance). Rainfall amounts of around 0.1" to 0.25" expected with locally higher amounts possible. - Remaining unsettled for first half of upcoming week with periodic chances for showers and storms from Sunday night through Wednesday. - Temperatures fairly consistent through mid-week with highs slightly below average. && .DISCUSSION... Issued at 337 AM CDT Sat Jul 6 2024 Drier, Below Normal Temperatures Today: GOES-16 water vapor imagery early this morning shows our region sandwiched between two upper-level features caught in the general northwesterly flow. The first wave situated to our northeast that was responsible for our convection over the past couple of days with northwesterly flow behind keeping our temperatures below normal. A second shortwave trough can be noted moving out of the Canadian Rockies and about to eject into the Dakotas. Weak surface and mid- level divergence ongoing tonight and into early tomorrow will allow for modest clearing of cloud cover which when combined with relatively light winds and recent rainfall could result in some patchy fog formation, especially in river valleys and west-central Wisconsin. As we head through the day, upper-level northwest flow should keep us seasonably cooler with highs in the upper 70s to lower 80s with scattered clouds. Showers & Storms Tonight and Sunday: As we head into tonight, the aforementioned upper-level shortwave moves towards our region bringing the likelihood for showers and storms (60-80% chance) by late evening and into the overnight hours. Mid-level Q-vector convergence towards the exit region of the trough would enable some lift which combined with waning MLCAPE in the 06.09z RAP of 500-1000 J/kg would suggest weakening convection moving into the region. Shear profiles seem fairly meager with the 06.06z RAP showing under 25 kts of 0-6km bulk shear overnight. Consequently, minimal severe threat would be expected, maybe an isolated strong storm with gusty winds and small hail in the evening when instability and low-level lapse rates are marginally more favorable. Showers and storms then move through the region throughout the overnight and into the morning hours. Rainfall amounts during this period currently trend lighter as the environment with this shortwave is not as primed for more efficient rainfall with precipitable waters in the 1.4-1.6" range, marginal warm cloud depths of 3-3.5km and a relatively weak 850mb moisture transport nose associated with the convergent forcing. Consequently, probabilistic guidance has very low probabilities for 0.5" or greater (around 10% chance in the 06.00z HREF) with only medium probabilities for 0.1" or greater (40-70% chance in the 06.00z HREF). As a result, current thinking for amounts is generally in the range of 0.1 to 0.25" with some locally higher amounts of up to 0.5" through Sunday morning, possibly in southwest WI/northeast IA where the 850mb moisture transport nose is situated. Cyclonically induced showers and storms likely occur during the afternoon and evening as the wave departs northeast with the recent 06.06z CAMs suggesting this potential, particularly across north-central Wisconsin. With roughly 1000-1500 J/kg of MLCAPE and low- level lapse rates of 7.5-8 C/km, cannot rule out some small hail with stronger updrafts. Additional Shower and Storm Chances into Next Week: As the aforementioned primary shortwave pushes northeast, a secondary shortwave perturbation is noted in deterministic guidance (06.06z GFS/NAM/EC) late Sunday and into Monday morning as a part of a broader synoptic trough. As this shortwave approaches the local area from the south, low-level convergence with this wave will help instigate a broader area of precipitation. Still some uncertainty amounts deterministic guidance with this wave regarding both placement and timing of its approach so holding onto chance precipitation mention for now (30-50%) across southwest WI for Sunday night and Monday. Regardless, probabilities in the 06.06z GEFS/EC ensemble for 0.5" or greater with this wave are very low across southwestern WI (10% or lower) at this moment. Furthermore, with fairly limited instability and shear profiles, not anticipating much of a severe threat. As we head through the first half of the week, additional shortwave perturbations can be noted in this complex synoptic pattern which will continue chances for showers and storms through Wednesday. Overall the 06.00z grand ensemble (GEFS/EC ensemble/Canadian Ensemble) has an inter-quartile range of 0.5" to 1.25" for QPF across the region from now through mid-week which is a sensible spread considering the convective nature of precipitation and generally muted moisture profiles. Towards the end of next week, guidance pivots the aforementioned broader trough eastward resulting in more quasi-zonal to northwesterly flow with a building ridge to the west. Consequently, early indications suggest warming temperatures to near or even slightly above average by the end of the upcoming week and perhaps trending drier as well. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1112 PM CDT Fri Jul 5 2024 Main TAF concern in the short term still revolves around monitoring the potential for fog overnight. Model guidance has backed off some from previously noted runs, with higher focus more east of the MS River. Have leaned towards the mention at KLSE, but there is still some variability in short term model guidance and uncertainties on winds just off the surface. Will continue to monitor trends and observations through the night. Expect short term adjustments will be possible. Winds remain light through the period, turning more west/southwest through the day Saturday. Otherwise, look for shower and storm chances to increase again Saturday night. && .HYDROLOGY... Issued at 337 AM CDT Sat Jul 6 2024 Rainfall amounts with scattered showers and storms today generally were generally around 0.25" or less with pockets of higher amounts of 0.5" to even over 1" at Sparta. Some river responses have been noted over many western Wisconsin basins and the Mississippi tributaries in light of rainfall that has fallen over the past couple of days. However, general recovery in river stage is expected in some basins with lighter rainfall expected into the upcoming week. Off and on shower/storm chances are expected tonight and Sunday which could yield additional rainfall amounts of 0.1" to 0.25" with localized higher amounts. An unsettled pattern is expected to persist through the first half of next week however amounts are generally expected to remain light at this moment, likely around 0.1" or so with each wave that passes through. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Naylor AVIATION...EMS HYDROLOGY...Naylor