Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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621 FXUS63 KARX 120414 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1114 PM CDT Tue Jun 11 2024 .KEY MESSAGES... - Isolated thunderstorms possible this afternoon, some of which could produce small hail and gusty winds. - Complex severe weather setup for Wednesday afternoon and evening with the potential for multiple rounds of storms. Large hail, damaging winds, heavy rainfall and possibly a few tornadoes are all on the table depending on how storms unfold. - Warmer weather on the docket for early next week with highs pushing into the upper 80s to low 90s from Sunday through Tuesday. && .UPDATE... Issued at 1004 PM CDT Tue Jun 11 2024 Challenging forecast for tomorrow on synoptics and mesoscale evolution and the severe storm threat. Things are still on track with the thinking of a moisture surge into western MN tonight with storms erupting in the pre-dawn hours near the ND/MN/SD triple point. Moisture transport convergence and the instability is really keying on this area in the RAP and latest 12.00Z CAM solutions. We are still firmly entrenched in the pacific air mass overhead right now with low dewpoints and stable conditions. During the morning hours, moisture transport in the low-levels and instability continues to surge northeast across MN, but the rate is in question. Based on the timing of the pre-dawn storms and their southeast motion, the instability would seem to favor a more southerly track, with storms propagating into the instability. All of the 12.00Z CAMS want to track some convective system toward the area arriving by late morning, with questionable instability, mainly elevated, but a ton of warm air and moisture advection into the area. But still, as the complex moves southeast, possibly some severe, it is outrunning instability. And this timing could kill the afternoon severe weather chances by limiting heating. The 12.00Z HRRR delays the complex just an hour or two into the forecast area from the northwest and has the strongest outcome of afternoon storms. I think that speaks to the sensitivity of when these morning storms/showers arrive and if they continue to develop into severe storms or not. Later favors severe storms. The earlier arrival seems to favor a low severe weather chance overall with heating and moisture advection impacted. Either way, the first system seems to really seems to really cap off and stabilize the atmosphere. The CAMS also suggest generally that there is probably only one morning/afternoon system/shot. Overall, the new 12.00Z CAMS seem to produce fewer waves of storms from 18-09Z overall. One pattern we need to watch for is a NW->SE oriented boundary setting up west of a strong convective complex and cold pool. The orientation of the low- level flow and moisture advection into that boundary orientation would favor a training scenario with storms moving southeast along the boundary. So, still details to workout. Will be working to increase the later morning storm chances in southeast MN after 15Z per the accelerated CAMS into that area. Soundings and instability arent the greatest for severe weather that early (as stated above per later HRRR). && .DISCUSSION... Issued at 250 PM CDT Tue Jun 11 2024 This Afternoon and Evening: Isolated Storms Current radar mosaic this early afternoon shows this morning`s showers continuing to exit eastward out of the local forecast area. GOES satellite imagery and RAP progs show an upper level trough continuing to move towards the region, with surface observations showing an approaching (currently draped down through central MN) surface cold front. As the front moves across the area, model guidance continues to suggest the potential for isolated to widely scattered showers and storms to develop this afternoon/evening. However, there still remains some uncertainty in just how much instability will be able to build ahead of the front. Hi-res guidance would suggest the potential for a very narrow axis of instability with CAPE values generally around/under 1000 J/kg. If storms do develop, stronger storms may be capable of producing small to marginally severe hail and gusty winds. However, as we lose daytime heating the storm threat will quickly decrease. There is still quite a bit of variability in solutions on location/timing of initiation of storms, so will be monitoring near-term satellite and radar trends closely through the afternoon and evening. Wednesday Afternoon and Evening: Severe Weather Details A zonal pattern becomes established for tomorrow with a seasonable strong 120-kt jet working over the Dakotas in the wake of subtle perturbation in the flow. The surface boundary sliding southeastward this evening quickly washes out overnight with southerly flow becoming reestablished up through Minnesota by sunrise tomorrow. Steepening mid-level lapse rates and increasing surface theta-e values result in SB/MLCAPE values approaching 2000 J/kg by the mid-afternoon while elongated hodograph will be supportive of organized storm structures. The main question revolves around convective mode and timing. The latest 12Z HREF members are rather locked in on having elevated convection develop on the nose of a weak low-level jet towards sunrise in western Minnesota, with this complex and its convective debris/cold pool serving as the focus for renewed development during the early afternoon hours in central/eastern Minnesota and western Wisconsin. The hodographs would be supportive of supercellular structures and all storm hazards, though the explicit composite reflectivity progs show rapid upscale growth along the line into an MCS that propagates SSE with the Corfidi vectors during the afternoon. However, with the main synoptic forcing still off to the west, renewed development is likely along the western flank of this MCS cold pool that would result in multiple rounds of thunderstorms training over the same area throughout the late afternoon and evening hours along a northwest to southeast axis. This would point to a potential heavy rain and flooding threat somewhere in south-central Minnesota to northeast Iowa, but it remains too early to pin down the exact area as such a threat will rely heavily on how storms behave earlier in the day. Hodographs become quite favorable for tornadoes as we move through the evening hours, but the main question will revolve around the surface airmass thermal characteristics given the passage of the MCS earlier in the day. Any areas that do manage to recover may realize the over 500 m2/s2 of 0-1-km SRH and pose a tornado threat. At this stage, given the trajectory of the afternoon MCS, such a threat might be limited to south-central Minnesota and north-central Iowa. Thursday and Friday: Cooler and Drier Northwesterly flow settles in for the latter part of the week in the wake of our midweek storms and holds temperatures near seasonal norms for this time of year. With low level moisture slow to depart during the day on Thursday, steepening low-level lapse rates may result in diurnal showers developing during the day, with the showers sliding southeastward as drier air filters south during the afternoon. Friday is shaping up to be the coolest and driest day of the forecast period as a 1020-mb surface high pressure slides through, but highs should still top out near 80 degrees. Saturday and Sunday: Next Round of Stronger Storms Return flow builds in behind the departing ridge early Saturday, with surface dewpoints rebounding into the 60s by Saturday afternoon. The medium range solutions are in modest agreement with a sharp shortwave impinging on the backside of the synoptic ridge and thus we should see strong to severe storms somewhere across the Upper Midwest, but enough differences exist in the placement of the surface baroclinic zone make it difficult to nail down any further specifics on threats and locations. How convection unfolds on Saturday and the timing of the shortwave passage will play a role in the threat area for Sunday, though the southwesterly flow is strong enough that the warm sector may have time to recover on Sunday and result in repeated rounds of storms each day. Stay tuned for more details. Next Week: Warming Up Upper ridging amplifies over the central and eastern CONUS for much of next week, resulting in southwesterly flow across the region and with it the first real surge of summertime heat. There exists some modest synoptic differences with the various ensemble members with respect to the strength of the ridge, with the more amplified solutions resulting in warmer and drier conditions whereas the flatter solutions bring periodic bouts of thunderstorms and cooler temperatures as shortwave perturbations ripple northeastward over the Dakotas. The resultant precipitation forecast contains a smattering of 20 to 40 percent probabilities for the end of the forecast period, but these will be honed in over the coming days as the strength of the ridge is better analyzed. What is more certain is that we will see above average temperatures with over 70 percent of the available guidance pushing the mid-80s for highs and 20 to 30 percent of the members exceeding 90 degrees. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1115 PM CDT Tue Jun 11 2024 Forecast confidence after 15Z Wednesday is very low. A complex weather scenario unfolds for Wednesday with abundant moisture and instability, for thunderstorms, streaming into MN. This should cause some later morning showers to the northwest of the area, potentially a few storms depending on the instability location. Since the previous forecast (00Z TAFS), high-res models are suggesting a convective system approaches southeast MN by late morning, shifting southeast. The forecast at the TAF site is mainly VFR, with periods of storms causing the potential for lower MVFR. With confidence lower, have decided to keep the forecast about the same with an afternoon period of storms in the forecast. However, this is likely to change as we see thunderstorms form overnight near Aberdeen SD and move southeast. Thus, a changeable forecast for Wednesday should be expected with this lower predictability system. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ UPDATE...Baumgardt DISCUSSION...Skow AVIATION...Baumgardt