Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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810 FXUS63 KARX 021623 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1123 AM CDT Tue Jul 2 2024 .KEY MESSAGES... - The expected first round of efficient rain-producing showers and storms are ongoing overnight and expected to continue through this morning. A second wave of showers and storms, mainly focused on the southern half of the CWA, is expected this afternoon into this evening. Heavy rain and associated flooding remains the main threat with totals across both rounds of 1-2" total with localized 4"+ amounts across the area on swollen waterways and wet soils. Northeastern Iowa is the area of relatively higher concern this afternoon and evening for flooding. - July 4th has shower and thunderstorm chances increasing from west to east as the day progresses, with most areas seeing rain at some point during the afternoon and evening. Showers and thunderstorms may affect Independence Day outdoor activities. - Unsettled weather into the weekend with periodic rain chances and below normal temperatures. Heavy rain and severe weather chances look minimal at this time. && .UPDATE... Issued at 1122 AM CDT Tue Jul 2 2024 The Flood Watch has been extended for southwest Wisconsin for this afternoon and evening with another round of showers and storms under a highly-efficient rain-producing air mass. Warm cloud depths and precipitable waters are very favorable and with another southerly surge in the low-level wind field expected to harness growing instability to the south, mainly elevated storms are expected to form and provide 1-2" of rainfall to the Flood Watch area before shifting east of the area by mid-late evening. The key window seems to be 4-8 pm for flash flooding threats to present themselves. Most of the Flood Watch area had around an inch of rain last night to pre-condition the soils. One area which seemed to be keyed in on was the Wisconsin river valley southwest toward Oelwein in the latest CAMs. && .DISCUSSION... Issued at 400 AM CDT Tue Jul 2 2024 Monday Night into Tuesday Night: Heavy Rain Threat, Potential Severe 07z WV satellite shows an upper high over the ArkLaTex and upper troughing over the central/northern Rockies, with resulting flow helping to pump Gulf and some E Pacific moisture northeastward to the central Plains and Upper Midwest. RAP-based SPC mesoanalysis suggests a plume of 2"+ PWATs is present centered around E NE and W IA. Ahead of a shortwave currently advancing northeastward over Sioux City, an area of showers and thunderstorms developed in north central IA and swept eastward, with heaviest rain totals of 1-2.5" - mostly falling within a single hour - focusing in Mower-Floyd and Fillmore-Chickasaw counties before the convection outran axis of strongest elevated instability and rain rates reduced accordingly. Expect the ongoing moderate rainfall to briefly clear out this morning as the shortwave exits to the east. As the upper trough continues to advance eastward, expect the PWATs described above to be advected northeast to our CWA overnight into this afternoon. GEFS/ENS mean values continue to rise above 2", approaching climatological maximums, with the operational runs of those ensembles` parent models suggesting PWATs will reach 2.25" in NE IA and SE WI. Thus, this afternoon, as our next shortwave trough, currently located over the Rockies and Sangre de Cristos, ejects northeastward, expect an additional round of showers and thunderstorms capable of generating rain rates over 2"/hr. Remaining uncertainty centers around how far north - if at all - appreciable instability develops in our forecast area for those potential blockbuster rates to be fully realized. Given the propensity for heavy thunderstorms to outrun where guidance suggested appreciable (500+ J/kg) MUCAPE was located Monday night and the robust moisture transport expected, am leaning toward more aggressive NAMNest guidance which would place areas southeast of a Charles City to Mauston line most at risk. That said, with less aggressive HRRR guidance suggesting Fayette/Clayton Counties will be on the edge of the axis of instability, have issued a Flood Watch for these counties covering this afternoon/evening, with later expansion possible should other guidance trend toward that aggressive solution. As for severe thunderstorms, whether or not enough surface destabilization occurs remains the all important question as guidance continues to suggest 50 kts of sfc-6km shear will be present as winds aloft increase ahead of the upper trough. While HRRR/NAMNest continue to point toward best surface-based CAPE being located to our south, cannot rule out severe thunderstorms capable of all hazards - sfc-500m SRH could top 100 m2/s2, sufficient for a tornado risk - in our far south, particularly if greater than expected clearing of skies occurs today. Given the high amount of uncertainty, still think heavy rain and flooding are the easily the primary concern this afternoon and evening. Independence Day and Friday: Additional Rain Likely Guidance remains in good agreement that an upper low should slowly churn east over SD to the Great Lakes, bringing the potential for periodic showers and a few thunderstorms Independence Day and Friday. While there remains a good deal of uncertainty regarding timing, latest trends perhaps suggest rain may hold off until the evening of the 4th, keeping rain off parades but ruining fireworks. LREF has trended upward with instability probabilities with now a 20 to 45 percent chance for at least 750 J/kg of SBCAPE, so there may be a few atmospheric fireworks (thunderstorms) at least. In fact, given the increasing southwesterly winds ahead of the upper low, can`t totally write off the chance for a strong to severe storm, but the probability appears low (LREF joint probabilities for sufficient instability and shear are generally under 30%). Saturday through Monday: More Rain Possible With cyclonic flow over CA and the northern CONUS remaining in place, expect an additional upper trough to advance over the Upper Midwest Saturday into Sunday. This may kick off additional convection. At this time, the probability for heavy rainfall and severe thunderstorms appears low as robust low level moist advection looks to be displaced to our south and east. That said, thunderstorms could occur each afternoon given that the probability of SBCAPE building to greater than 500 J/kg is around 50% or higher each afternoon. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 644 AM CDT Tue Jul 2 2024 MVFR/IFR cigs will be the initial TAF concern this morning as a lower stratus deck moving in this afternoon with the NBH probs showing modest probabilities (40-60% chance) for IFR cigs across at both KLSE and KRST late this morning with observations upstream in north/central MN verifying this to some degree. These cigs are expected to persist into the early afternoon ahead of an incoming weather disturbance from the west. As it approaches, showers and storms will overspread much of the region during the evening hours with occasional dips to MVFR/IFR visbys in more intense storms. Conditions will improve to VFR from west to east during the overnight hours. Winds will begin the TAF period from the south at around 10-15 kts before diminishing and to around 5-10 kts and switching to westerly behind the aforementioned weather system. && .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...Flood Watch from 3 PM CDT this afternoon through late tonight for WIZ054-055-061. MN...None. IA...Flood Watch from 3 PM CDT this afternoon through late tonight for IAZ029-030. && $$ UPDATE...Baumgardt DISCUSSION...Ferguson AVIATION...Naylor HYDROLOGY...JAW