Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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592 FXUS63 KMPX 250542 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 1242 AM CDT Tue Jun 25 2024 .KEY MESSAGES... - Enhanced Risk (Level 3/5) of severe thunderstorms across eastern Minnesota and western Wisconsin this evening. Severe thunderstorms will be capable of producing very large hail, damaging wind gusts, and a few tornadoes. - Forecast confidence is increasing that the "cap" will weaken or break, allowing storm development this evening. Thunderstorms that do develop will quickly strength and become severe. - Given the state of the ongoing flooding, recent excessive rainfall, and thus low flash flood guidance values, a Flood Watch is has been issued for potential flash flooding due to heavy rain across the Twin Cities metro and portions of S MN and W WI for this evening. - Active pattern continues with another round of widespread rainfall Thursday afternoon through Friday night. Local river levels remain on the rise following the recent rainfall. River flooding remains a concern, as several sites have yet to crest in moderate or major flood stage. && .MESOSCALE... Issued at 558 PM CDT Mon Jun 24 2024 Things remain fairly quiet for now thanks to a substantial capping inversion evident in a special 330 PM sounding launched at our office earlier. This cap is the major source of uncertainty heading into this evening, and the big question remains if ascent ahead of an approaching 700 hPa trough will be enough to break the cap and initiate severe storms over the next few hours. Satellite trends indeed show thickening cloud fields over central MN near St. Cloud indicative of such ascent, and many models do eventually produce convection although where this convective initiation occurs varies widely from model to model, anywhere from Mankato to Duluth. However, the aforementioned satellite trends near St. Cloud suggest this may be the primary area to focus on, and several WoFS members are keying in on this area as well. If this does end up being the location where convection develops, it could put areas in and near the Twin Cities metro in the path of potentially significant severe weather later this evening. Very strong instability and low- level shear will be more than favorable significant wind damage and tornadoes. But again, this is highly conditional on storms developing in this area. Should storms fire further east into western Wisconsin, as some models suggest, weaker capping would favor more widespread storms but the environment wouldn`t be as supportive of significant severe storms as it is further west. We will keep a very close eye on trends over the next one to two hours for a better idea on where/if significant severe weather can develop. && .DISCUSSION... Issued at 344 PM CDT Mon Jun 24 2024 The stage is set for an active evening across southern Minnesota and western Wisconsin. What we`ve been calling a "boom or bust" set up the past few days is quickly becoming an apparent "boom". Forecast confidence has increased and supports convective initiation this evening across portions of southern and central Minnesota. Current observations pin the warm front across southern Minnesota and it`s expected to continue to push north through late afternoon. Temperatures have warmed into the mid to upper 80s with sfc dewpoints in the low to mid 70s over SW MN. Overall forecast scenario remains valid with potential for initial supercells to grow upscale into a bowing MCS and track southeast into SE MN and W WI. 20Z SPC mesoanalysis highlights extreme instability building in across eastern Dakotas, SW Minnesota, and western Iowa. The instability gradient is roughly from Alexandria to Rochester and will serve as the "bumper" for any MCS that may develop later this evening. ACCAS remain across portions of southern and central Minnesota with a few light showers (or virga). Mid level lapse rates of 8.5 C/km continue to spread east into Minnesota across a moisture rich airmass that is resulting in the high-end CAPE exceeding 5000+ J/kg. The 20Z day 1 SWO update echoed concerns for potential significant damaging wind gusts (65+ kts) for the eventual MCS. It also expresses the uncertainty for when storms initiate. A special 21Z ROAB at KMPX will hopefully shed light on the strength of the EML with 700 hPa temps 12-14C and if the CIN has begun to weaken in addition to what instability has built in. Unfortunately I do not have that data evaluable yet but the latest ACARS at MSP show a very favorable shear profile that`s supported in the KMPX VAD. Effective shear values of 50+ kts and SRH of 200+ m2/s2 will be enough to support initial supercell storm mode. Low level shear will likely improve as the LLJ strengthens this evening. Several WOFS runs in a row have highlighted two separate areas of initiation this evening. The first is south-central MN from 21-23z and then second is north in central MN 23-01z. Other CAMs have hinted at two different clusters and this scenario would support the increased concern for heavy rain across portions of SE MN given the high-end thermodynamic environment and abundant moisture available. Initial storms will likely be supercellular in nature and be capable of producing very large hail, damaging winds, and possibly a tornado before growing upscale into a MCS with primary concern of damaging winds downstream. If the split initiation scenario is correct, the northern/later area will still have plenty of instability to work with as the strengthening LLJ orientation is favorable to replenish any moisture behind the first cluster of storms. Both clusters will likely grow into MCS and turn SE-ward into the gradient with additional convective development likely across portions of S MN and W WI. Given the recent stretch of excessive rain and flooding, low Flash Flood Guidance values, a flood watch was issued for SE MN and W WI for the potential flash flooding this evening due to repeated rounds of heavy rain/storms. Severe threat will decrease after midnight as the complexes exit to our SE. Any lingering convection should stay sub-severe but an isolated hail threat may exist in additional the the heavy rainfall threat. By Tuesday morning, we`ll dry out and have a few days to recover from this round of storms. Some instability may linger and could support a few showers in the afternoon. Highs remain in the low to mid 80s. Wednesday and Thursday are cooler, in the upper 70s and low 80s, but less humidity. Attention turns to the next system progged for Thursday night into Friday, with Thursday night/Friday morning looking to be the most likely window for seeing more showers/storms. The 12z guidance has plenty of variability/spread that makes sense given we`re 4 days out. The heaviest rain threat appears to favor Iowa at this time, but we`re still on tap for another round of widespread rain, whether or not it`s heavy/excessive will be resolved later on. A tropical airmass (and dewpoints in the 70s) will stay just to our south, but 12z ECMWF highlights PWAT values approaching 2.0" by 12z Friday. This will be capable of producing heavy rain and potential severe given favorable upper diffluence with a LLJ moving through, so we`ll need to remain vigilant as the active pattern continues for the next week or so. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1225 AM CDT Tue Jun 25 2024 Main forecast concern in the short term is with the track of strong to severe thunderstorms across western Wisconsin. As of the 6z TAF issuance, thunderstorms were located RNH with a gradual movement to the southeast towards EAU in the next hour or two. Winds will become light overnight behind the departing storms and will increase out of the WNW by late morning, generally 10-15 kts through the day. KMSP...The threat for thunderstorms has shifted east of the terminal into western Wisconsin. VFR for the remainder of the TAF period, with WNW winds increasing late morning/early afternoon. /OUTLOOK FOR KMSP/ WED...WED...VFR. Wind NW 5-10 kts. THU...VFR. MVFR/-TSRA likely overnight. Wind S 5-10 kts. FRI...VFR. MVFR/-TSRA likely morning. Wind S 10-15 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...Flood Watch until 1 AM CDT early this morning for Anoka-Carver- Chisago-Dakota-Freeborn-Goodhue-Hennepin-Isanti-Le Sueur- Ramsey-Rice-Scott-Sherburne-Steele-Waseca-Washington- Wright. WI...Flood Watch until 1 AM CDT early this morning for Dunn-Eau Claire-Pepin-Pierce-Polk-St. Croix. && $$ MESOSCALE...DPH DISCUSSION...BPH AVIATION...Strus