Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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070 FXUS63 KARX 161907 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 207 PM CDT Sun Jun 16 2024 .KEY MESSAGES... - Storms This Evening & Overnight. Heavy Rainfall Will Be Primary Hazard With Limited Potential For Hail - Warm Into Midweek - Temporary Break In Rain & Storms On Tuesday, Likely The Warmest Day of The Week && .DISCUSSION... Issued at 1242 PM CDT Sun Jun 16 2024 Observations This Afternoon: The active pattern continues this evening and through the overnight hours with separate rounds of storms. Surface observations show 2-3 surface frontal boundaries this afternoon that will impact precipitation and storms tonight. Pronounced boundary lies to the west near the Red River Valley exhibiting 15 degree dewpoint differential and 5 to 10 degree temperatures differential. This frontal boundary will be the main forcing mechanism for initial storm potential tonight. Another frontal boundary drapes zonally through central Wisconsin, associated with the warm sector. The anomalous airmass within the warm sector, 90th percentile SPC climatology; 2+ standardized anomalies NAEFS/ENS, has quickly increased temperatures after a slightly cooler morning than. Also of note are the increased winds and increased 70 degree dewpoint temperatures within this airmass. While ACARs and high resolution forecast soundings have shown building instability sufficient for potential storms (2000-3000 J/kg) early this afternoon, satellite imagery has shown building billow and cumulus clouds, evident of stable air off the deck. This elevated mixed layer has been building east through the early afternoon, coming into play for storm potential locally tonight. Satellite imagery and surface observations also exhibit minimal shear. Initial Precipitation & Storm Chances Late This Evening: Local precipitation chances are expected to graze the northwester periphery from southeastern Minnesota into central Wisconsin this evening, shifting southeast with the aforementioned more pronounced surface boundary. Concurrently, the driving surface low lifts north through the late evening stranding the increased low level theta e gradient from the west-southwest to east-northeast. The EML, a low level inversion, and little to no CAPE limits initial local impacts from these storms. Primary impact will be isolated areas of increased rainfall with amounts near 0.25" in select areas. Overnight Rain & Storm Chances: Subsequent precipitation chances form on the nose of the intensifying and more meridional oriented low level jet during the early overnight hours. The moisture transport greatly increases with a Midwest Atmospheric River of 500-1000 kg/m-1/s-1. Initial overnight, locally impactful, storm formation occurs along a shrinking-stretching axis from southwest Iowa into east-central Minnesota. An anticyclonic rotation in the nocturnal low level jet and best moisture transport, highly evident in the AR orientation, advects this area of storms from west to east across the forecast area through the overnight hours. The location of the previously mentioned low level theta e gradient suggests highest storm and rainfall potential north of the local forecast area. While instability builds from steepening mid level lapse rates, a low level inversion and EML limits stronger storm confidence locally. The departure of the low level jet limits overall shear as CAPE deepens concurrently in forecast soundings. In summary, expect limited strength and duration for these storms as well. Confidence & Potential Solutions Overnight: Confidence remains low due to the location of the narrow convergence zone and exact location of the frontal boundary. Evidently there is a near split in solutions between high resolution model guidance. Nearly half of the CAMs (NAM4km/ARW/ NSSL) are more progressive with the frontal boundary, advecting a southwest to northeast oriented line of storms from the northwest to southeast early in the overnight while remaining CAMs delay to the early morning hours dependent on the low level jet. Expected Hazards: Primary hazard will be heavy rain as the tropical air mass contains anomalous moisture. HREF paints 3 hour amounts near 0.5" by midnight with an additional 1" grazing our western periphery. Highest confidence (50+%) for 1" per hour rainfall rates also lie along our northwest periphery in south-central Minnesota to west-central Wisconsin. Total amounts vary in the HREF from a maximum of 3+" and a probability matched mean of 2"- 2.5" in a narrow band in these same areas. Confidence in other hazards remain quite limited. Although, with a deeper, stronger CAPE profile, can`t rule out some hail. Higher freezing levels, lack of shear, and anomalous temperatures/moisture resulting in melting hail limit overall confidence. Monday Storm Potential: Storms are expected to linger through Monday morning as the low level jet continues to push north through Minnesota. The associated quasi-stationary surface boundary advects north, resulting in perpetual storms along and north of the local forecast area from west-central into central Wisconsin. Exact location does remain in flux however with the most southern solution lingering locally earlier in the morning. Select CAMs suggest a limited 3 hour window of higher storm potential of deepening CAPE after meeting convective temperatures near 90 degrees. However, there`s lower confidence in temperatures tomorrow given the increased sky cover from morning storms. Also, besides the lowest levels, which likely won`t be realized, little to no shear to sustain stronger storms. Have also decreased surface temperatures for tomorrow away from National Blend in collaboration with neighbors and due to the increased cloud cover. Through Midweek: Through Tuesday morning, an upstream amplified trough is expected to lift through the Central Plains, remaining well west of the forecast area. As a result, a break in precipitation expected locally through Tuesday morning as the increased low level theta e axis pushes into southern Manitoba. This lingering moisture source eventually separates from its Gulf of Mexico source through Tuesday, eventually advecting the narrow corridor of precipitation and instability across the forecast area from northwest to southeast late Tuesday into Wednesday morning. While the short break in precipitation may be beneficial for some, there`s higher confidence for it to be the warmest day of the week. Long term global ensembles, GEFS; EPS, both suggest temperature anomalies greater than 10 degrees. The EPS has a slightly warmer solution, grazing the 90 degree isotherm farther north while the GEFS keeps the same parameter well to the southwest in the Central Plains. Probabilities increase for greater than 0.5" through midweek in both the EPS and GEFS from 50%-80%. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1232 PM CDT Sun Jun 16 2024 Increased southwest winds are expected to continue into this evening across the forecast area from southeast Minnesota, northeast Iowa, and western into central Wisconsin. Highest winds have been observed west of the Mississippi River. Clear to scattered skies also continue into this evening. Initial limited storm chances return this evening, confidence lies primarily in central Wisconsin. Higher confidence storm chances move from west to east during the overnight hours. Thunderstorms likely to impact TAF sites (KLSE & KRST). Have not included flight restrictions given lower confidence but will be subsequent forecast detail to monitor. && .HYDROLOGY... Issued at 646 AM CDT Sun Jun 16 2024 Due to the repeated storms, we`ll need to assess the flood potential with each shift. The 3hr FFG remains around 2 to 3 inches. Although heavy rain fell northwest of the local area Saturday night, locally, amounts were generally less than an inch. Some areas could see another inch this morning before this wave exits the area. We`ll need to monitor the rainfall rates due to the high efficiency. The storms continue lifting northeast at a pretty good clip, thus heavy rain should be localized. Tonight the heavier rainfall is again forecast north of the local area with more rain Tuesday night. With each rain event over MN, the probabilities for flooding increase for the Mississippi River. The Mississippi River should remain elevated for the next couple of weeks. There is a 30% chance that the Mississippi River reaches Minor Flood Stage over the next 10 days through Winona (and 50% chance at Wabasha). && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAR AVIATION...JAR HYDROLOGY...Zapotocny