Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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942 FXUS63 KARX 182256 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 555 PM CDT Tue Jun 18 2024 .KEY MESSAGES... - Widespread showers/storms will spread east this evening with localized rainfall amounts of 1-2"+ and damaging winds the primary threats, especially west of the Mississippi River. - Periods of showers/storms continue over the next several days. Localized higher rain amounts and the cumulative impact of multiple rounds of rain will lead to some river rises and possible flood concerns. - Drier, somewhat cooler conditions expected for early next week, although this break may be short-lived. && .DISCUSSION... Issued at 158 PM CDT Tue Jun 18 2024 This Evening-Tonight: Early this afternoon a shortwave trough was ejecting into the northern high plains with a cold front extending southwest from a surface low in northern Minnesota. Ahead of the front a very warm to hot, humid airmass persists across the region with gusty southerly flow. Some gusts around 40 mph could occur through the afternoon for open, unsheltered areas to the west. In addition, a few showers/storms cannot be ruled out through the afternoon/early evening as broad low-level moisture transport interacts with an uncapped atmosphere. The higher storm chances (>80%) arrive towards evening as the front approaches. With a deep warm cloud layer and stronger deep layer shear lagging the front, storms likely will have the tendency to become outflow dominant as they progress towards the Mississippi River through the evening, with localized higher wind gusts (especially with any bowing segments) and high rain rates the primary hazards. The hail/tornado tornado risks, while non-zero, are considerably lower. The HREF 24-hr PMM rainfall amounts exceed 1" for some areas west of the Mississippi River with low (10-30%) chances for 2" or more. Low-level shear will be stronger, but overall, confidence in sustained, balanced cold pool development is lower given the weaker deep layer shear. As a result, the highest risk for severe storms likely will be within the first few hours of initiation near and west of I-35, which agrees well with the area of SPC`s slight (level 2 of 5) risk area today, and with the mid-level HREF UH fields. Wednesday-Friday: The front will sag south on Wednesday and slosh north/south through late in the work week with an unsettled weather pattern continuing. Wednesday will be cooler and cloudier in the post-frontal airmass with little instability present to support much of a thunder threat, but some showers are possible. The boundary likely will remain close to the area within southwest flow leading to periods of showers/storms through Friday and potential for localized heavy rains. Global ensembles continue to indicate 40 to 60 percent chances for 2" of rain this week (through Friday) from northeast Iowa into central Wisconsin, although mesoscale details will impact where higher rain amounts occur. See the hydrology section below for more information as flood risk could increase with repetitive rounds of storms. Saturday: There is a somewhat stronger signal for a deeper trough and cold front to pass across the Upper Midwest this weekend. Ahead of the front, temps may climb back well into the 80s late this week, although frontal placement/timing and associated cloud/precip trends lead to higher forecast spread. Some strong/severe storm risk is possible ahead of the trough with sufficient flow on the southern flank of the trough and potential for destabilization ahead of the front, but considerable uncertainty exists in the evolution of this system yet. The GEFS-ML probabilities show 15% chances for severe weather centered on Wisconsin on Saturday. Sunday-Tuesday: In the wake of the frontal passage, global model guidance indicates at least a transient period of drier weather comes early next week as the flow transitions northwest and a somewhat drier airmass builds in. However, stronger mid-level flow across the northern US into next week within a quasizonal flow could result in a continued active pattern with periodic shortwave passages and shower/storm chances. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 555 PM CDT Tue Jun 18 2024 CIGS: BKN VFR likely to give way to MVFR/IFR as a cold front sweeps across the area later this evening/overnight. Cigs should gradually lift to a SCT-BKN VFR deck in the afternoon with a veil of high/mid left cigs then quickly returning for Wed evening. Models trending toward a return to MVFR by 12z Thu with next area of rain. WX/vsby: broken line of shra/ts developing along a north-south running cold front over MN pushing eastward tonight. Current timing suggests the line could reach KRST btwn 03-04Z. Atmosphere looks uncapped ahead of the line though, so some development is possible pre front moving into the early evening. Coverage is expected to increase as the front moves in, but coverage and intensity of any storm will be on the wane as they move into western WI, through the overnight hours. CAMS clear pcpn east/south between 09-11z. WINDS: staying relatively strong and gusty from the south/southwest into mid evening, swinging west/northwest overnight as the cold front shifts east across the area. Winds should lighten up as that happens, mostly near/below 10 kts. && .HYDROLOGY... Issued at 158 PM 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. Some of these areas have received 2 to 4" of rain over the past 3 days and will be more susceptible than areas to the south and east to flooding. 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...JM AVIATION.....Rieck HYDROLOGY...Skow/JM