Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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736 FXUS63 KARX 020422 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1122 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - Two waves of showers and storms are expected overnight and again later Tuesday into Tuesday evening. It still looks like the main threat is heavy rain of 1-2" total /with localized 3"/ across the area on swollen waterways and wet soils. - July 4th has shower and thunderstorm chances increasing from west to east as the day progresses with evening fireworks in question. The timing and coverage can still change, but it looks like most will get wet in the afternoon to evening. - Unsettled weather into the weekend with periodic rain chances and below normal temperatures. Heavy rain and severe weather chances look minimal at this time. && .DISCUSSION... Issued at 258 PM CDT Mon Jul 1 2024 Two Waves of Rainfall into Tuesday Night and Renewed Flooding Risk GOES water vapor at 1830Z is showing the trough axis from the morning IA convection now on an FSD-DVN line with radar echos diminishing as they head east but the core remaining just west of DSM. This also matches the low-level moisture gradient with very dry air to the northeast. The second more vigorous shortwave trough is really visible coming through WY which will act to really fire up the low-level return flow in concert with the low-level ridge shifting east at ~1025mb. The air mass is still quite dry over the area with dewpoints in the 40s, while the 70 dewpoint line is still in KS. GOES precipitable water is depicting an incredible 1.8-2.2 inch tongue through eastern KS with the 01.12Z Topeka KS soundings showing 2.14 inches. WOW. This evening the low-level jet ramps up with 45-50 kts at 850 mb with strong moisture transport convergence ramping upward isentropically along its trajectory from DSM->EAU. MUCAPEs increase into the 600-900 J/Kg range near and south of I-90 in concert with the forcing, so SHRA/TSRA should erupt in the later evening as the airmass transitions to more moist. The very tropical air mass previously described to the south doesnt fully engage over the area until later in the forcing period tonight toward pre-sunrise. Thus, while it does look favorable for heavy rain with warm cloud depths just below 4km, the very high precipitable water is in place for wave #2 for Tuesday and Tuesday evening. Amounts tonight look to be near an inch with only a 5-20% chance of 2"+ from the 01.12Z HREF as it remains progressive shifting east in the morning. After a mid-day break which the CAMS are all in great agreement on, stronger forcing again shifts in ahead of another Pacific NW trough shortwave trough ejecting out of WY toward the area in the afternoon. While questions remain on exactly the recovery that occurs and where the CAPE pool will evolve northward too, the air mass in place ahead of the wave is now tropical with near record precipitable water. With preconditioning tonight, soils will be primed for flash flooding with super-efficient rain rates. By mid-afternoon, a cold front should be on about I-35 with the main CAPE pool south of the area but 750-1000 J/Kg suggested across swrn WI and nern IA and along the cold front. This would bump rates and cause some flash flood concern as moisture transport convergence ramps up and another round of convective storms erupts in the afternoon. The most intense rainfall rates would be closer to the instability pool which the CAMS currently disagree on for placement. The latest HRRR runs suggest swrn WI would be in play for strong storms, while others suggest the strongest storms are south of the area. Have considered a Flood Watch internally and with NWS neighbors, but think tonights rain will be tolerable and a pre- conditioning for Tuesdays second wave. Plan at this time is to wait on a watch and continue to assess the Tuesday ingredients. There could be some strong storms around with maybe a gust to 40-50 mph and small hail, but rain is the main threat with soils/rivers. See Hydrology section below for more. Also of note is the strong low-level jet in place tonight which could cause wind gusts of 30-40 mph west and north of La Crosse. July 4th Looking Wet After a dry and mostly sunny day Wednesday, yet another weather system is shifting in Thursday. While some differences on the timing details exist, the big picture is that shower and storm chances increase through the day as a moderately strong trough over the Dakotas moves in with moderate 300-500mb QVector convergence shifting over the area later afternoon and then evening into WI. There is still quite a bit of spread on the details of how the near- surface trough and instability evolve Thursday with a wide range of SBCAPE suggested by 7 pm Thursday /200-1000 J/Kg 25-75th percentile/. Also, the trough placement and orientation is critical to the wind shear profile. Should a more NW->SE trough occur, southeast low-level winds would favor a bit better wind shear. But the majority of guidance would suggest the lowest 3 km of wind shear appears to be weak in the 01.00Z Grand Ensemble, with instability leveraged into showers and generic storms spreading in from west to east across the area during the late afternoon and evening. 12-hour rain amounts through midnight look to average around 0.25". Rain chances have been increased with this forecast. Unsettled into the Weekend There is excellent agreement on modest amplitude, longwave troughing over the northcentral U.S from Friday through Monday. Friday would feature a lingering low-level trough axis across the area which, should there be any clearing and instability increase, would be the focal point for storms. 01.00Z LREF suggests a 25% chance of SBCAPE values above 700 J/Kg so there isnt a great deal of instability to work with. While the 01.00Z Grand Ensemble is in agreement that high rain chances exist on that trough axis, noticeable gradients exist on its north and south sides suggesting the location will drive precipitation chances. Should that location of the trough shift, Friday could turn out drier for some. Shortwave ridging builds in overnight into Saturday morning. An unsettled northwest flow with less predictable timing and shortwave trough energy takes over through Monday. Each day looks to have some chance of showers or thunder. Heavy rainfall is not favored in this pattern and shower and storm chances are tied to both trough passage and diurnal heating. With details hard to extract, the forecast for the Saturday through Monday is splashed with 30-50% rain chances. These days should have plenty of dry hours though. Temperatures through the period should be slightly below normal. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1122 PM CDT Mon Jul 1 2024 The forecast looks to remain on track as a couple rounds of showers and thunderstorms develop/move across the forecast area. Have maintained the TEMPO MVFR/IFR vsby and ceiling conditions with this first round of storms for now. Further, increasing signal in model guidance suggests MVFR (NBH probs 70-90%) to potentially IFR (NBH probs 40-60%) ceilings prevail for Tuesday morning. There then looks to be a brief mid-day break in storms before additional showers and storms are expected tomorrow afternoon/evening, exiting towards the end of the TAF period. With lower confidence in details will maintain the VCTS and let the later issuances refine further. Otherwise, increased southeast/south winds continue overnight with wind gusts 20-25+ kts into the early morning. Winds become more south and ease some late in the day. && .HYDROLOGY... Issued at 258 PM CDT Mon Jul 1 2024 he Mississippi River will remain in flood stage through at least the next week and likely into mid-July as additional precipitation is expected to fall across the Upper Mississippi River basin. The bulk of this precipitation is expected to fall tonight through Tuesday night and again Thursday evening into Friday. Exact crests at each location are dependent on which Mississippi tributary receives the rainfall. Tonights rainfall is expected to fall along and near the Black, Root, Cedar, Upper Iowa, and Turkey River basins. All of these rivers flow into the Mississippi River at different locations, which brings uncertainty to Mississippi River crests around Genoa southward. With this additional precipitation, the Hydrologic Ensemble Forecast Service (HEFS) suggests a roughly 20% chance of Minor Flooding at Black River Falls, which would take roughly 2.25 to 2.5 inches of rain through Tuesday night. The Cedar River at Charles City has a 10 and 15% chance of Major and Moderate Flooding respectively. The rainfall amounts that drive these chances range from 3 inches (Moderate) to 4 inches (Major); however, the latest HREF probabilities show only an 11% chance of receiving 2.00 inches of precipitation, so the HEFS probabilities may be a touch too high. As for flash flooding, the current thinking is that the moisture transport and precipitation tonight will prime the atmosphere and soils for a higher flash flooding threat with the second round of precipitation Tuesday/Tuesday night. The area with the most precipitation forecast remains along the Clayton and Grant county border across NE IA and SW WI. Communities whose stormwater drains into the Mississippi will be more susceptible to urban flash flooding as storm drains may have a reduced capacity. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Baumgardt AVIATION...EMS HYDROLOGY...JAW