Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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406 FXUS63 KARX 261114 AAA AFDARX Area Forecast Discussion...UPDATED National Weather Service La Crosse WI 614 AM CDT Sun May 26 2024 .KEY MESSAGES... - Heavy Rain & Low Storm Chances Move Northeast Across The Forecast Area Today, Rainfall Amounts Above 1.5" In Spots. - Storm & Rain Chances Abate Tonight, Only To Return Monday Into Monday Night - Slightly Below Normal Temperatures Through Midweek Become Slightly Above Normal By The End Of The Week && .DISCUSSION... Issued at 339 AM CDT Sun May 26 2024 Early Morning Storm Chances: A surface low seen lifting through the Central Plains towards the Upper Mississippi River Valley early this morning is providing preciptiation chances from southwest to northeast. The responsible upper level perturbation can be seen on GOES Water Vapor imagery, collocated with upper level divergence of 10-20 ^-5/s through the Central Plains in RAP analysis. A general weakening trend in forcing has resulted in the surface low slowly filling as it lifts northeast. As a result, storm potential has been on a weakening trend as it reaches the local area. Primary hazards will be heavy rainfall and small hail as storms have a limited amount of effective shear, pulsing up then weakening shortly thereafter. Rain & Storm Chances Through Today: Rain and storm chances spread northeast through the morning hours as the surface low traverses our southern peripheral counties from northeast Iowa into southwest Wisconsin. Local instability as the storms pass will be limited to our far south where the moistest airmass can penetrate. Additionally, little to no recovery in temperatures will limit any lower level instability building, limiting overall instability to 500 J/kg of MUCAPE. Shear profiles become increasingly messy, further decreasing any confidence or potential for stronger or sustained storms. Therefore, as higher precipitable water values near 1.25" push northwest, heavy rainfall becomes the primary concern through the afternoon. Current confidence has the 1.25" PWAT isohyet bifurcating the southeastern half of the forecast area as the low trudges east-northeast. With dry air wrapping northwest of the surface low, a sharp cutoff in heaviest rainfall is expected. High resolution model guidance slightly disagrees on exact location of rainfall gradient due to differences in location, strength, and quickness of deepening surface low. Overnight Preciptiation Potential: The surface low further occludes tonight, pivoting through east- central Wisconsin during the overnight hours. As a result, an accompanying low level Fgen band northwest of the low center will increase precipitation potential through central Wisconsin. Current confidence keeps the heavier rainfall amounts to the east in central Wisconsin due to the progressive pattern preventing the low from lingering. Will be important to monitor exact location of surface low and subsequent Fgen band on our doorstep for additional heavy rain potential. Storm & Preciptiation POtential Monday: The progressive pattern also provides a subsequent perturbation Monday morning into the afternoon. The main moisture source will be wraparound leftovers from today`s low pressure, limiting overall heavy rainfall potential. Instead, steeper lapse rates being worked on by cold air advection increases confidence in storm and small hail potential. Shear profiles remain weak, resulting in storms pulsing up/down. While storm life will be limited, a low freezing level near 5k ft and thick hail growth zone could allow a pulse storm to provide small hail. Monday Night Storm Chances & Colder Temperatures: While these storm and precipitation chances will be short lived primarily through Monday afternoon, a final appendage of the wobbling upper level low will provide a final round of potential precipitation Monday night into Tuesday. Available moisture will be further limited as northwest flow will be quite dry. Cold air advection behind this final trough will introduce slightly below normal temperatures from Tuesday into Wednesday night. These below normal temperatures don`t sustain through the work week as long term global models (GFS/GEFS/ECMWF/EPS) exhibit similar solutions in amplifying an upper level ridge across the Central CONUS. Synoptic Pattern Through Midweek: However, it will be quite an peculiar upper level synoptic pattern through the work week. An extremely amplified upper level ridge, currently extending along the Atlantic Coast on GOES water vapor imagery, is expected to close along its northern periphery, eventually regressing through Central Canada. While the immediate ramification will be assisting in the departure of the aforementioned area of everlasting closed upper level heights, higher impact will be influence and amplification the subsequent upper level ridge. The initial synergization of this anticyclonic flow with the mean pattern is expected through Monday with rapid amplification of upper level heights through Wednesday across the Northern Plains. As a result, a quasi-blocking omega synoptic pattern is expected to setup across the Upper Mississippi River Valley through the week. Global models (GFS/GEFS/ECMWF/EPS/GEM/GEPS) all exhibit this quasi- blocking pattern through midweek whilst differentiating on location and intensity of the ridge axis. Additionally, mostly all intra (ensemble) model dProg/dT has exhibited increasing confidence (+10- 20%) for higher heights over the Upper Mississippi River Valley through the week. In other words, potential for a slower, longer residing blocking pattern is increasing. Certainty (100% confidence) in the 576 dam isoheight at 500 mb brushes the IA/MN/WI border in the GEPS while enveloping the local area in the EPS, leaving the GEFS somewhere in between. Slightly Above Normal Temperatures, Slower Precipitation Return: As a result, the most boisterous EPS solution is the most optimistic (60-80% confidence) for slightly above normal temperatures by the end of the week while GEFS & GEPS remain less keen. All models are certain of highest temperature anomalies (+10) in southern Canada, along the ridge axis` northern periphery. Ensemble meteograms vary from highs in the 60s (GEFS) to 70s (EPS). Complementary impact will be a potentially dry forecast into the weekend. Again, as one would expect, highest confidence is exhibited in the EPS from the strongest ridge with lower confidence in the GEFS & GEPS. This also carries into the weekend, with the slowest solution for precipitation onset in the EPS into Saturday afternoon. Given the confidence, have slightly decreased precipitation potential from National Blend Thursday Night through Friday. In collaboration with the neighborhood offices. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 615 AM CDT Sun May 26 2024 Precipitation can be seen spreading northeast through the Upper Mississippi River Valley on radar imagery early this morning. This rainfall will continue spreading northeast into the afternoon hours as the surface low pressure progresses from northeast Iowa into western and central Wisconsin. Highest, albeit limited, thunder chances graze locally southern counties from northeast Iowa into southwest Wisconsin this morning and afternoon. South of both TAF sites (KRST & KLSE) where there are very limited thunder chances. Otherwise, heavy rainfall resulting in lower visibilities are the near term forecast detail. Heaviest rainfall amounts today expected southeast nearing 1.5" in spots. Otherwise, ceilings reduce near IFR later this morning and are expected to linger that way into MVFR through the 26.12Z TAF period. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAR AVIATION...JAR