Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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229 FXUS63 KARX 230246 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 946 PM CDT Wed May 22 2024 .KEY MESSAGES... - Limited Scattered & Isolated Rain & Storm Chances Today - Storm Chances Return Late Thursday Through Friday With Strong to Severe Storms Possible - Storm & Rain Chances Return Sunday Night Into Monday && .DISCUSSION... Issued at 244 PM CDT Wed May 22 2024 Storm/Precipitation Chances Today/Tonight: A closed upper level area of lower heights continues lifting north through the Canadian province of Ontario early this afternoon. A meridional (north-south) appendage has been deepening and phasing across the central to southern Mississippi River Valley with the southern periphery collocated with a 100 kt subtropical jet streak, resulting in severe weather near the confluence of Ohio and Mississippi Rivers. Locally, minute pulses within the body of the shortwave appendage have been providing high based scattered precipitation chances. Commercial aviation based ACARS soundings line up with high resolution forecast models concerning a meager low level temperature inversion causing the higher cloud bases. Dry air within this nose and minute pulses within the west-northwest flow limit overall confidence and spatial continuity in precipitation chances. Erosion of the inhibition layer will allow tapping into slight instability and ability to overcome this drier air. The colder airmass in place has lowered the freezing level near 8k ft, resulting in pea size hail reports with the strongest storms. Have continued with scattered and isolated chances similar to previous forecasts. Thursday: Otherwise, drier forecast on tap through much of Thursday with precipitation and storm chances returning Thursday night, lasting through much of Friday. Highest temperatures within return flow and increased low level WAA remains to the west, keeping storm chances and higher temperatures from the 80s into the low 90s well to the west. Storm Chances Return Thursday Night Through Friday: The trough responsible for storm and precipitation chances late Thursday night through Friday currently seen on upper level GOES water vapor imagery closed over the Pacific Northwest. Upstream of this feature, increased upper level winds are expected to initially dig an appendage of this closed low across the Rocky Mountain West before phasing and lifting through the Central Plains into the Upper Mississippi River Valley. The narrow filament of increased low level theta e lobe advects through late Thursday night through Friday morning, challenging initial storm chances given lack of diurnal heating. Storm chances increase through Friday as the surface low lifts through central Minnesota into southern Manitoba with deepening appendages swinging around the main low. Long term global ensemble (EPS/GEFS) loses confidence in surface low location as the low lifts north, challenging local confidence. The occlusion of the surface low will provide a narrow appendage of potential instability through Friday shown in an initial rounded mound of 50-70% chances for 500 J/kg of SBCAPE, becoming a filament of 20-50% SBCAPE probabilities along Mississippi River Valley into Wabasha and Buffalo Counties. Given the diurnal timing, severe storm chances push into the Northern Plains on Day 2 (Thursday) and along and east of the Mississippi River Valley on Day 3 (Friday) with highest probabilities into central Wisconsin. Primary threat would be potential for hail but currently, but the forecast window pushes the end of the high resolution guidance. Therefore, HREF max/min/mean instability, only till Friday at 12Z, shows 1000 J/kg, <500 J/kg, and 0 J/kg; respectively. Much disagreement on shear, highest collocated with the occlusion passing over early Friday afternoon, but very diverse solutions between models. Otherwise, higher impact will be potential rainfall amounts. Have slightly backed off from most recent National Blend forecast model that included 6 hour QPF values near 1" Friday 18Z to Saturday 00Z. Should be mentioned lack of members at that time after the 54 hour forecast window. Upcoming forecasts will increase high resolution member inclusion, increasing confidence in amounts. Otherwise, slight flip flop between long term global deterministic and ensembles, intramodelly. The ECMWF with a slower progression paints widespread 0.5" or higher while the GFS remains transient, limiting overall spatial impacts. This Weekend: A slight reprieve in precipitation chances expected through Saturday, only to become more widespread again through Sunday into Monday (Memorial Day). North to south separation of a phasing upper level trough limits local confidence. Highest confidence lies to the south where synoptic forcing best aligns for a lifting, deepening surface low from the Central Plains into the Mississippi River Valley. Will be subsequent feature to iron out in coming forecasts given the longer forecast window. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 942 PM CDT Wed May 22 2024 High pressure will build across the area tonight and remain in control of the weather into Thursday. West winds will shift to south and southeast on Thursday. Wind speeds will be 5 to 15 knots. && .HYDROLOGY... Issued at 300 AM CDT Wed May 22 2024 MRMS rain estimates and station reports showed a swath of 2 to 4 inches of rain since yesterday from parts of northeast Iowa through central Wisconsin, resulting in rises on some rivers, including the Turkey, Kickapoo, Black, Upper Iowa, and Yellow Rivers. Minor flooding is expected along parts of the Turkey River, and is possible along sections of a few Wisconsin tributaries over the next few days. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAR AVIATION...Boyne/JAR HYDROLOGY...JM