Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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848 FXUS63 KARX 251040 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 540 AM CDT Sat May 25 2024 .KEY MESSAGES... - Limited Reprieve In Precipitation & Storms Through This Afternoon, Returning Tonight, Lasting Into Monday. - Elevated Storm Chances Increase Late Tonight Into Early Sunday Morning, Small Hail & Heavy Rain Remain Primary Hazards && .DISCUSSION... Issued at 349 AM CDT Sat May 25 2024 Cold Morning, Normal Temperatures Today: The upper level low responsible for recent storms and precipitation is becoming well wrapped on GOES Water Vapor imagery loops, lifting through the canadian province of Manitoba. The accompanying dry slot can be seen tightening over the Upper Mississippi River Valley on GOES infrared satellite imagery loops. Low level RAOBs (25.00Z) had the accompanying low level cold airmass sinking southeast from the Northern Plains into the Upper Mississippi River Valley, resulting in slightly below normal temperatures early this morning in the 40s; primarily along our northwestern periphery from southeast Minnesota into central Wisconsin. Although, upstream shortwave ridging through the Central Plains has been shunting the west- northwest winds responsible for this colder airmass with the accompanying surface high pressure moving through Iowa. These higher heights and accompanying anticyclonic flow will recover temperatures back to normal today, in the 70s across the Upper Mississippi River Valley. The respite in precipitation chances is short lived however, as the active pattern returns tonight lasting through much of the weekend. Synoptic Setup This Morning & Potential Forecast Impacts: The combination and struggle between the shortwave ridge, the slowly exiting/lingering lifting closed low, and upstream jet streak(s) over the Rocky Mountain West and southern Great Basin are resulting in forecast discrepancies for storm chances tonight into Sunday morning. The lifting low has been filling (i.e., weakening) on PV analysis through the early morning hours. Interesting tidbit, 0.5 PV analysis late Friday night exhibited pressure falls as low as 850 mb near the low center, translating into 50 kt winds near 825 mb at International Falls, MN and Chanhassen, MN 25.00Z RAOBs. More importantly, this lows accompanying PV folds support and suggest its surrounding jet streaks persisting through today. Low Confidence In Precipitation Potential This Evening: Ability of the northern jet streak shunting the shortwave ridge through today will determine the trajectory of the 100+ kt southern jet streak, and local precipitation and storm chances tonight through Sunday. Initial precipitation potential this evening will be dependent on overall influence of anticyclonic flow and subsequent extent of the associated appendage. The result is a low level pressure saddle with an accompanying shrinking axis this evening. Current high convective allowing models place this initial band of west-southwest to east- northeast precipitation from central South Dakota to north- central Minnesota. Given the dependency on the outcome of this skirmish between the aforementioned synoptic forcing, exact location of this narrow band remains slightly challenging. May see these initial, relatively minimal precipitation chances graze our northwest periphery tonight. Given the lower confidence from the limited width of the band and local dry air, haven`t introduced PoPs tonight, but will be something to initially keep an eye on. Increased Storm and Preciptiation Potential Tonight: Higher impact of the synoptic struggle will be extent of higher moisture through the forecast area tonight ahead of increased forcing from the subsequent jet streak. Peak forcing appears late tonight within the polar exit region of the jet streak providing 20^-5/s upper level divergence and 25 m/s + irrotational upper level winds. While best forcing and moisture is expected to remain to our south, the phasing of the low as it progresses northeast provides more widespread precipitation and storm chances locally along the warm frontal boundary. More exuberant model solution suggest the 55 degree isodrosotherm quickly advecting northeast through the forecast area while pessimistic models keep sub 50 degree dewpoints. Highest confidence will be in our south-southwest counties from northeast Iowa into southwest Wisconsin. Potential Storm Impacts & Hazards Into Sunday Morning: The northern extent of the higher dewpoint temperatures also impact amount of available instability. The strong upper level divergence provides strong mid level ascent with steeper mid level lapse rates, potentially allowing some pulse storms to come to fruition. While instability is questionable, combination of low freezing level and hail growth zones of 100-150 mb, small hail cannot be ruled out. Given the frontal boundaries, shear profiles remain stout although strongest at the surface which most likely will not be realized. Therefore, hodograph profiles become quite messy within any available, transient instability. Again, small hail and heavy rain remain most likely hazards. Storm & Precipitation Potential Through Sunday: As the surface low closes, it bifurcates and lingers across the southern half of the local forecast area. The 1" precipitable water isohyet advects through the area, providing perpetual moisture for heavier precipitation potential. The accompanying surface warm front drapes west to east from the Iowa-Minnesota border and points east, locally resulting in higher precipitation potential initially Sunday morning along our south. Precipitation shifts north through the late morning as the low slowly trudges east, with subsequent precipitation chances falling from northwest to southeast into the evening hours from an additional pressure saddle and shrinking axis. Amount of available moisture with this latter precipitation band becomes a forecast discrepancy as highest moisture is expected to follow the slowly trudging east surface low. Storm chances will be highest late morning through the early afternoon along the zonally draped warm front before storms outrun this quasi-stationary boundary. High resolution convective allowing models mostly disagree on phase of the low as select models suggest occlusion and a narrow filament instability while others drape a widespread 1000 J/kg+ of SBCAPE along and south of the warm front. Potential impacts include locally heavy rainfall near 1" through Sunday and small hail. Low confidence at this time and will need to be ironed out further in coming forecasts. Storm & Precipitation Potential Into New Work Week: These precipitation chances linger through Monday due to very slow shift of the surface low. While precipitation chances do return Tuesday, confidence remains low due to, again, the high dependency on the exit of the low pressure. As a result, quite a discrepancy between long term global ensembles (GEFS/EPS) for 0.01"+ of 24 hour rainfall. GEFS exhibits 70-100% confidence while the EPS broad brushes 40-60% across most of the central CONUS. Therefore, have continued with National Blend precipitation chances, constrained mostly along our east in central Wisconsin, aligning with one of the 100% confidence GEFS bands. Through Midweek: Otherwise, the trough passage results in temperatures 5 to 10 degrees below normal for Wednesday (80-100%; GEFS/EPS) before an omega resembling block causes an amplified ridge to build through the end of the week. Discrepancies in long term models (GEFS/EPS) relate, again, to the exit behavior of this weekend`s low(s). Will be subsequent forecast detail with precipitation potential into next weekend. && .AVIATION /12Z TAFS THROUGH 12Z SUNDAY/... Issued at 503 AM CDT Sat May 25 2024 Passing high pressure to the south provides VFR conditions across the forecast area from northeast Iowa, southeast Minnesota, and western into central Wisconsin through the 25.12Z TAF period. As the high pressure passes into this evening, a surface low churns northeast through the Central Plains into tonight. As a result, winds turn counter clockwise (cyclonically) this evening, becoming out of the south as warm air advects north. Initial precipitation impacts and accompanying potential flight restrictions expected to remain northwest of the local forecast area from south-central Minnesota to west-central Wisconsin this evening. Near the end of this (25.12Z) TAF period, precipitation and storm chances spread from west-southwest to east-northeast, bringing flight restrictions along with. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAR AVIATION...JAR