Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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337 FXUS63 KARX 210303 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1003 PM CDT Thu Jun 20 2024 .KEY MESSAGES... - Multiple rounds of heavy rain are expected Friday morning through at least Saturday morning. How the bands line up will determine the local flooding threat. Confidence is lower on exactly how the later storms Friday night into Saturday afternoon evolve. - A few storms Friday and Saturday may be severe, with damaging winds the main threat. - A brief respite from the active pattern for Sunday and Monday, but more storms loom for Monday night into Tuesday. Unfortunately, severe storms cannot be ruled out for Monday night into Tuesday. && .DISCUSSION... Issued at 338 PM CDT Thu Jun 20 2024 This Afternoon into Tonight: Decreasing Potential for Thunderstorms Largely as expected, widespread cloudiness across much of the CWA has limited surface insolation, with latest RAP guidance suggesting less than 250 J/kg of MLCAPE is present and, with winds out of the east, am not expecting robust enough low level warm advection to change this. Have therefore continued to limit mentions of thunder to 30 percent or less. Additionally, with 18z WV satellite showing the CWA is largely between two shortwaves - one southeast of the CWA and the second looking to pass just to our northwest, much of the area looks to remain free of precip entirely, with the exception of areas along an Albert Lea MN to Medford WI axis, where the close approach of the northwestern shortwave should be enough for at least a few showers. In either case, am not too concerned about excessive rainfall with any showers this afternoon and evening given PWAT values are generally under 1.65" and 700/850mb moisture transport is modest. Friday through Saturday: Heavy Rain, Flooding, and Severe Storm Potential This evening, moisture transport increases markedly over the central High Plains ahead of an advancing upper trough. As a shortwave ejects over NE, an MCS should develop and translate east- northeastward along a northward lifting warm front to SD and eventually S MN by Friday morning. As for severe thunderstorms, this morning MCS could pose an isolated damaging wind threat in S MN if it remains sufficiently organized. Finally, flood threat with this initial rounds appears to be low as 700/850mb moisture transport remains modest for this initial round. Friday afternoon and evening, low level moist advection should help MLCAPE build to 1000-1500 J/kg as the front, forced southward by the morning MCS, pushes back northward. While 0-6km bulk shear values are generally 35 knots or less, this would be sufficient for perhaps a bit of organized convection. While hodographs are not overly impressive, if enough surface destabilization can occur, there would be at least a small probability for all hazards with damaging winds having the higher relative risk. The primary concern with this activity will be heavy rainfall and flooding as 700/850mb moisture transport will increase in time as the main upper trough advances eastward over the central Plains and becomes oriented out of the southwest, somewhat along the expected east-west axis of the front. PWAT values look to top 2 inches, with these values exceeding the 99th percentile in both the NAEFS and ENS climatology. Thus, expect efficient rain producers with some areas potentially affected by training storms. Given this and the saturated soils from abundant recent rainfall, have issued a Flood Watch beginning Friday afternoon. Main uncertainty Friday afternoon focuses on how far north the low level warm advection pushes the front after it is forced southward by outflow from the morning MCS. Non-CAM guidance suggests the boundary may return to a position just northwest of our CWA while many CAMs have trended toward a solution roughly around Interstate 90. While all guidance tends to struggle with the effects of cold pools, am inclined to learn toward the CAM solutions as they tend to do a bit better. Have therefore trended axis of highest QPF to the south and this adjustment may not yet be enough. Second uncertainty focuses on the timing of the morning MCS - should this arrive sufficiently late, this round could carry a higher risk for severe wind as the morning inversion breaks. Additionally, this would delay airmass recovery and onset of the second round of storms. Moving ahead to Saturday morning, our third round of convection looks to occur as the main upper trough finally approaches. As moist advection continues, PWAT values approach NAEFS/ENS climatological maximums and training risk increases as 850mb moisture transport becomes more oriented along the surface boundary. Have therefore kept the Flood Watch going through midday Saturday. The already high uncertainty in details increases markedly by Saturday afternoon. Should the previous three rounds of convection occur largely as envisioned, the boundary may very well be shunted southeast of the CWA for good. Given this uncertainty, have ended the Flood Watch early Saturday morning. If the boundary remains in our CWA, would need to extend this Watch in time. Finally, if the boundary remains in our CWA, destabilization to the south of this feature Saturday afternoon would lead to another period with severe thunderstorm potential. Given the modest increase in deep shear as mid to upper level flow ramps up ahead of the upper trough, all severe hazards could be in play in the southeastern third of the CWA if this fourth round manifests itself. Sunday through Wednesday: Brief Break from Storms, then Active Weather Returns In the wake of Saturday`s upper trough, northwesterly flow aloft begins to dominate proceedings. With conditions at 700/850mb drying out considerably, most areas should remain free of rain Sunday, with the exception of north central Wisconsin due to a passing upper wave. Next wave Monday night into Tuesday may bring additional rainfall with CSU ML severe outlooks suggesting at least some risk for severe thunderstorms during this period. Admittedly had little time to closely examine this given the abundance of active weather in the short term but will need to keep an eye on Monday and Tuesday. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 955 PM CDT Thu Jun 20 2024 Increasingly challenging Aviation forecast. In the nearest term, upstream observations have varied between LIFR and VFR with thunder slowly trudging east through central Minnesota late tonight. Has been drastically slower with every radar loop however. Confidence increases through Friday for precipitation moving west to east. Confidence remains low on exact location of heaviest rainfall and convective storms. Therefore, have continued VCTS at both TAF sites but needs further refinement as forecast hour nears. & .HYDROLOGY... Issued at 405 PM CDT Thu Jun 20 2024 A Flood Watch has been issued for parts of southeast Minnesota, northeast Iowa, and western Wisconsin from Friday into Saturday. Multiple rounds of heavy rain are expected from Friday morning through Saturday, increasing the potential for flash flooding. Widespread rainfall accumulation of 2 to 4 inches is forecasted with locally higher amounts exceeding 5 inches possible in spots. Highest flash flooding risk remains dependent on exact location of heaviest rainfall bands. Should the heaviest rainfall bands line up over the smaller river basins, there is a 20-30 percent chance that these basins could experience periods of flooding at moderate flood stage. Additionally, the combination of recent heavy rains with the ongoing rounds of storms increase river flooding potential into next weekend. Moderate to high confidence (60-90% chance) for sites along the Mississippi River to exceed flood stage during this time. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...Flood Watch from 7 AM CDT Friday through Saturday afternoon for WIZ032. MN...Flood Watch from 7 AM CDT Friday through Saturday afternoon for MNZ079-086>088-094>096. IA...Flood Watch from 7 AM CDT Friday through Saturday afternoon for IAZ008>011-018-019. && $$ DISCUSSION...Ferguson AVIATION...JAR HYDROLOGY...JAR