Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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010 FXUS63 KARX 181048 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 548 AM CDT Tue Jun 18 2024 .KEY MESSAGES... - Warm and muggy today with heat indices around 95 to 100 degrees. - Next round of thunderstorms moves through tonight. Severe weather threat looks low, but locally heavy rain of 1-2" expected. - Periods of showers and storms through Saturday night. Multiple rounds of locally heavy rain Thursday through Saturday may increase flooding concerns. - Cooler and drier early next week. && .DISCUSSION... Issued at 320 AM CDT Tue Jun 18 2024 Today: Warm and Muggy The surface boundary that lingered over the Driftless region yesterday and served as the instigator for multiple waves of storms has started to wash out in response to a 50-60-kt low- level jet developing west of I-35 ahead of a 990-mb low pressure cell lifting through the eastern Dakotas early this morning. A +11C 700-mb cap has likewise expanded over the region with this deepening warm air advection, shunting any overnight convective threat further north into central and northern MN/WI. While today is shaping up to be the warmest of the seven day period, it is by no means a slam dunk temperature forecast. Periods of mid level clouds may develop by sunrise, followed by diurnal cumulus areawide and thicker afternoon mid-level clouds towards the I-35 corridor. All told, these variables have the potential to take the edge off our our high temperatures, but even the coolest of the NBM inputs are only a few degrees below our forecast highs. Therefore, no matter how you slice the forecast, it looks to be warm and muggy. Given the clouds and heat indices staying below advisory criteria (or briefly touching 100 degrees for an hour), have refrained from any headline issuance. Tonight: Rainfall/Storm Details The surface low continues lifting northeastward ahead of a PV lobe noted in water vapor imagery across the northern Front Range that reaches western Ontario by Wednesday morning. A surface cold front slides southeastward in the wake of the departing low, serving as the focus for convective initiation around mid to late afternoon in western and central Minnesota. A more favorable kinematic and thermodynamic environment at initiation will favor some storms reaching severe levels west of the forecast area, but with the storm motion along the boundary given the line-parallel steering flow, the aggregate outflow boundary/front is expected to steadily undercut the convection through the night as it steadily works through the Driftless region from northwest to southeast. If anything, any severe weather threat in the evening may come in the form of localized damaging winds as emphasized by the last few runs of the HRRR NCAR Neural Network output. While the convective evolution does not favor a severe weather threat lasting long past sunset (and staying mainly to the west), the environment will still be characterized by anomalously high precipitable water values of 1.75 to 2 inches, above 97-99% all other PWAT values observed around this time of year. The slower storm motion, despite the likely fragmented convective structure, will still lead to localized pockets of 1-2 inches of rain as depicted by the 18.00Z HREF max QPF progs. It is worth noting that some HRRR/RAP forecast soundings go nearly uncapped in the warm sector ahead of the main line during the afternoon today (especially when adjusted for the virtual temperature correction). At this stage, it is looking more likely that the inhibition will be strong enough withstand CI attempts, but the environment ahead of the front will be favorable for organized supercell updrafts given the 2500 J/kg of MLCAPE and veering low-level hodographs--IF a storm can initiate. Wednesday: Cooler, Rainfall Threat Briefly Slides South The forward progression of the front steadily slows as its upper level support wanes--its forward progression by Wednesday morning driven in part by the strength of the cold pool from the previous night`s storms. Most global and convective-allowing models bring the boundary to along a line from south-central Iowa to south-central Wisconsin for the daytime hours on Wednesday. Subtle upper level ridging slides through during the day with only weak 300K isentropic ascent over the now quasi- stationary boundary, greatly limiting any forcing for stronger storms. There still look to be periods of light to moderate showers along the surface to 850-mb front driven the moist atmosphere during the day, but any heavy rain threat looks muted and south of the region. Expect more clouds than sun during the day in the wake of the front, tempering highs in the low to mid- 70s. Thursday - Saturday: Warmer, Repeated Rounds of Heavy Rain Stout upstream ridging along the Eastern Seaboard retrogrades westward for Thursday and Friday and in doing so allows the aforementioned surface front to advance back northward into Minnesota and Wisconsin by Thursday afternoon. A series of weak undulations in the zonal flow will instigate repeated rounds of storms along the frontal surface Thursday through Saturday, increasing the flooding risk with each passing wave. The details of exactly where the heavier rain sets up and the timing of the storms remains in flux, but it is safe to say that some areas may see another 2-4 inches of rain before all is said and done on Saturday. With the strengthening ridge, warmer temperatures in the 80s become increasingly likely for the latter part of the week. However, there exists a 10 degree spread in the NBM inter-quartile guidance, indicative of the continued convective contamination that has plagued the temperature forecast for the past few days. A stronger mid-latitude trough passes through on Saturday night, once again shunting the baroclinic zone southeastward as a cold front. Sunday - Monday: Cooler and Drier A shot of Canadian air arrives in the wake of the cold front for Sunday and Monday, dropping dewpoints below 60 degrees and highs to around 75 to 80 degrees. The potential for diurnal showers in the post-frontal cyclonic flow necessitated lower PoPs on Sunday, but otherwise this period features the best potential for drier conditions with surface high pressure sliding through. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 540 AM CDT Tue Jun 18 2024 A SCT to BKN CU deck flirting with MVFR to VFR levels develops early this morning and lifts to a low VFR CU deck for the afternoon. A line of thunderstorms arrives after 00Z from west to east and should bring widespread MVFR restrictions with localized pockets of IFR visibilities. These storms gradually exit the area between 06Z and 12Z. Winds will be from the south at 15-25G25-35kts today, backing to the W/NW behind the storms/cold front overnight at 5-10 kts. && .HYDROLOGY... Issued at 320 AM CDT Tue Jun 18 2024 Multiple rounds of storms capable of heavy rain remain in the forecast for tonight and then from Thursday through Saturday. Localized rainfall amounts of 1-2+" are possible with each round of storms. There exists a 50 percent chance that any given location in southeast Minnesota and west-central Wisconsin will see storm total rainfall amounts exceed 3 inches by Saturday and a 20 percent chance of those totals exceeding 5 inches. Each successive round of rain will increase the risk for flash flooding and river flooding. River flooding concerns, especially along the Mississippi River and its Minnesota tributaries will need to be monitored closely. Many locations along the Mississippi River will approach flood stage towards next weekend (50-70% chance) if rainfall unfolds as forecast. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Skow AVIATION...Skow HYDROLOGY...Skow