Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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657 FXUS63 KMPX 241736 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 1236 PM CDT Mon Jun 24 2024 .KEY MESSAGES... - Slight Risk (Level 2/5) of severe thunderstorms this evening. Uncertainty remains due to a conditional setup, but if storms develop they`d likely have significant severe weather given the environment. - Active pattern continues with another round of widespread rainfall Thursday afternoon through Friday night and another system for the start of July. - 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. && .UPDATE... Issued at 1236 PM CDT Mon Jun 24 2024 Confidence is increasing on thunderstorm evolution this afternoon and evening, such that a Flood Watch for the potential for flash flooding as been issued for portions of eastern MN and western WI. 17Z SPC mesoanalysis highlights a pronounced instability gradient from Alexandria to Rochester. It is also along this line where the H7 +12C isotherm is located, which is generally the level that convection has trouble developing (capped). Elevated ACCAS/light showers and a few thunderstorms are ongoing across western MN, although ceilings are greater than 12kft. As low pressure treks eastward across South Dakota this afternoon, very rich Theta-e air will advect northward into western and southern MN. Mid level lapse rates in excess of 8.5 C/km will spread over this moisture-rich airmass and result in extreme instability likely exceeding 5000 J/kg. In addition, the 850 mb jet will strengthen to 40 kts and impinge on the surface front near the Mississippi River. Despite marginal H7 temps, it is believed this front and LLJ will initiate convection late this afternoon across east central MN. CAMs have trended this way in big fashion today. The initial cluster will form and propagate southeast along the instability gradient into far western WI and southeastern MN. Given extreme instability and favorable shear, very large hail and destructive winds are possible. SPC has upgraded this area to a level 3 Enhanced Risk. Following the initial cluster, additional clusters may redevelop again upstream with the LLJ orthogonal to the front. The LLJ will replenish the atmosphere and maintain a healthy pool of available instability. The renewed cluster(s) will also have the potential for very large hail and destructive winds. Further, potential training of these clusters appears likely and multiple CAMs highlight an increasing likelihood of localized totals exceeding 3 inches from the Twin Cities metro area, southeast toward Eau Claire, Rochester, and La Crosse. Given the state of the ongoing flooding, recent rainfall, and thus low flash flood guidance values, a Flood Watch is necessary for these areas this evening. Outflow from previous clusters could displace subsequent clusters slightly farther southwest, so the watch was given a wide berth to the southwest to account for this potential. H7 temps may limit this to some degree, however. && .DISCUSSION... Issued at 329 AM CDT Mon Jun 24 2024 All eyes are on today and the potential for a high-end severe weather event across MN/WI. Water vapor imagery shows a series of shortwaves this morning from Montana up into Saskatchewan. In response to these features, we`ve seen both a very warm EML and Gulf moisture getting pulled north up into the Dakotas. Within this environment, we`ve seen a severe cluster of storms drop out of Saskatchewan into north central NoDak. These storms have not been handled well by guidance (including CAMs) and represents the first bit of uncertainty for this forecast. The CAPE gradient and 850- 300mb thickness pattern would say this cluster of storms will make a run toward Fargo the rest of the night, with the big question being how deep into MN does this cluster make it as it encounters an environment with less instability and increasingly stronger capping (as h7 temps push to upwards of 14+ C). Given the increasingly hostile environment for convection, this activity is expected to be not much more than some disorganized showers (if they`re even still producing rain) into central MN by late this morning. However, how this activity modifies our environment in terms of what the northward extent of the capping h7 thermal ridge and surface warm front end up being are still uncertain for our potential severe event late in the day. For later today/tonight, the warm front is expected to at the very least clear the upper MN River. Dewpoints behind this warm front will increase into the mid and possibly even upper 70s, this combined with air temperatures in the upper 80s to lower 90s sends forecast heat index values into the 100 to 105 range from Fairmont and Mankato up along the MN River to the SoDak border, so have issued a Heat Advisory along the upper MN River Valley for today. h7 temperatures will be increasing to between 14C and 16C. These warm h7 temps will allow for dry adiabatic mid-level lapse rates to develop above a surface environment with observations in the ballpark of 90/75. This combination will create a thermodynamic environment with extreme instability, with mlCAPE values likely to exceed 5000 j/kg in the warm sector. This instability is the reason for the "boom" or "bust" messaging we`ve had. If updrafts form in this environment, they will be strong to violent (boom), but those toasty h7 temps also mean we have the bust in play given the capping. When looking at the HREF the non-HRRR members all produce a forward propagating MCS, but differ on where that develops. The northern outlier is the Fv3, which keeps its MCS north of the Arrowhead. At the other end of the spectrum, you have the ARW/NSSL WRFS which bring the MCS through central MN and western WI (including the Twin Cities). Given the convective debris we`ll have today, we favor that more southern solution, with storms developing in west central MN in the late afternoon, quickly growing upscale and racing southeast along the CAPE gradient. This is a high end severe environment given the degree of instability with the curved hodographs and 40-50 kts of deep layer shear. There`s still enough uncertainty on convective initiation that the SPC maintained a slight risk, but the hatched wind area they have is a pretty good indication for where the most likely region to see a significant wind event evolve resides. For Tuesday, some instability lingers across southern MN, but the main surface front by the afternoon will reside from roughly Des Moines to Milwaukee, so other than some lingering showers in southern MN Tuesday morning, Tuesday continues to look like a day to dry out with mostly sunny skies and highs in the low to mid 80s, very similar to what we just saw on Sunday (minus the isolated showers in western WI!). Wednesday will be dry as well as high pressure moves through, but as we`ve seen going back to the second half of March, dry stretches tend to not last all that long! The next period of active weather continues to be Thursday through Friday, with Thursday night/Friday morning looking to be the most likely window for seeing more showers/storms. If you`re looking for some good news, models overnight started backing down some on the heavy rain signal as this system is looking a bit more organized. The 00z guidance has started trending toward the idea that we see a double warm front structure develop with this system. One down in Iowa will have the tropical airmass (and dewpoints in the 70s, with another up along the international border (with dewpoints in the 60s). We`re kind of stuck between the two, with lesser QPF resulting for the MPX area (with heavier precip focused to the north and south). Still, we will have favorable upper diffluence with a LLJ moving through, so the likely PoPs we have Thursday night are still warranted, we`ll just hopefully be able to avoid the worst of the heavy rain with this one. Behind this system, the last weekend of June looks to be absolutely spectacular, good news for once if you have any outdoor plans. However, in a growing theme for this warm season, the nice/dry weather will be short lived with the ensembles and deterministic models showing another seasonably strong cyclone impacting the upper MS Valley in the July 1-2 period, but this looks to be another opportunity for severe weather and excessive rainfall. When it rains it pours... && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1230 PM CDT Mon Jun 24 2024 Our main concern continues to revolve around the potential severe storms with destructive wind gusts to form this evening. Forecast confidence has increased to include in the 18Z TAFS. Thunderstorm initiation is now expected after 21Z across central Minnesota before congealing into a line of severe thunderstorms that will track southeast. Potential for impacts include STC/MSP/RNH/EAU. Latest TAFS use the 12z CAMs to support the earlier timing for TS development. There`s still some uncertainty with where storms develop, so kept the TS in a TEMPO for STC for now until there`s better confidence on if/when/where the cap breaks. Storms remain possible overnight for AXN/RWF/MKT, but chances at these terminals are low enough to keep the TS mention out until later updates. KMSP... Increased confidence that thunderstorms will develop over central MN after 21Z and tracking across the metro this evening. Opted for prevailing -TSRA between 22z-04z with a TEMPO of +TSRA and strong wind gusts (hurricane force gusts possible) from 23z to 01z. There is still variability in timing of TSRA but TSRA now appears likely for MSP. /OUTLOOK FOR KMSP/ 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 from 5 PM CDT this afternoon through late tonight for Anoka-Carver-Chisago-Dakota-Freeborn-Goodhue-Hennepin- Isanti-Le Sueur-Ramsey-Rice-Scott-Sherburne-Steele-Waseca- Washington-Wright. Heat Advisory until 8 PM CDT this evening for Blue Earth-Brown- Chippewa-Faribault-Lac Qui Parle-Martin-Nicollet-Redwood- Renville-Watonwan-Yellow Medicine. WI...Flood Watch from 5 PM CDT this afternoon through late tonight for Dunn-Eau Claire-Pepin-Pierce-Polk-St. Croix. && $$ UPDATE...Borghoff DISCUSSION...MPG AVIATION...MPG