Sunrise 5:28 AM   Sunset 8:36 PM Moonrise 6:01 PM   Moonset 3:15 AM

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FXUS63 KMPX 210455 AFDMPX

Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 1155 PM CDT Mon May 20 2024

KEY MESSAGES

- Rounds of thunderstorms across southern Minnesota tonight - some could be severe with large hail and damaging winds.
Locally heavy rainfall is also possible.

- A more substantial severe risk, although still conditional on early day convection, will unfold Tuesday afternoon and evening across southern/eastern MN and WI. Damaging winds, large hail, and tornadoes are possible.

- Widespread heavy rainfall and gusty winds expected across central and western MN Tuesday.

DISCUSSION
Issued at 321 PM CDT Mon May 20 2024

A quiet day today, but conditions will be changing as early as tonight. A weak cold front from northwest WI to south central MN and eastern NE is passing through and will stall over northern IA tonight. A trough beginning to approach the Rockies will send multiple short waves eastward tonight and early Tuesday, bringing the potential for several rounds of thunderstorms across the area. This complex scenario may unfold a number of ways.

As the evening progresses, a strong 50 kt LLJ will develop across the central Plains and moisture transport will increase across the region. Several CAMs develop clusters of thunderstorms this evening across southern MN, becoming more numerous overnight as the nose of the LLJ reaches IA. There could be a few strong or severe storms occasionally, maybe perhaps an elevated supercell, but the instability could be reduced if activity becomes fairly widespread. Another area to watch will come from upslope convection today from the front range of the CO Rockies and western NE. An MCS is likely to evolve from this activity this evening, which will track along the nose of the LLJ to southern MN and IA early Tuesday morning.
One thing to watch will be for the preceding convection’s augmentation of the front and where the best forcing for the early Tuesday round will exist. If convection this evening becomes decently organized, it will likely send the that round south to northern IA, such is the case with the 18Z HRRR.
Showers and a few thunderstorms will still overspread much of the area in all likelihood, but the most robust convection will be displaced south.

Attention then turns to the main round Tuesday afternoon and evening. An anomalously deep surface low will track northeast from the Omaha vicinity midday to the Twin Cities vicinity early evening. Given the synoptic setup, the early day convection should be progressive enough to allow for recovery to take place throughout the afternoon. However, the continuance of a 40-50 kt LLJ through the day and seasonably high pwats around 1.5 inches does give one some pause that maybe a clear break may not occur with continued development. Working on the assumption that there is a break, destabilization should occur quickly with the arrival of 7.5-8.0 C/km mid level lapse rates east of the low in the warm sector as most solutions suggest. Robust thunderstorm redevelopment would take place by mid afternoon across southwestern MN into western IA, then continue northeastward as supercells, clusters, and eventually a QLCS into Tuesday evening. Strong wind shear profiles and moderate instability will lead to all severe hazards being possible, including strong tornadoes (especially in southeast MN). This final round will be quite progressive, except near the surface low where strong forcing/convergence overlap with sufficient instability, while an orientation of convection also leads to some training. This is where the best potential is for heavy rain to fall - 2 or 3 inches. HRRR highlights this area nicely with a distinct heavier corridor from Mankato to the TC Metro and St. Cloud. Its probability-matched mean QPF has some pockets even surpassing 4 inches. Farther west, moderate to heavy showers associated with the deformation band will also lead to heavy rainfall totals, but persistent convection won’t be as much of a threat and should keep those areas from receiving excessive rainfall rates and thus higher totals.

A Flash Flood Watch has been issued for southern and eastern MN and parts of far western WI where the heaviest corridor of precipitation is expected. 6-hr flash flood guidance values generally range from 2.5 to 3 inches, so there may be some issues if the higher totals pan out. The watch begins tonight across southern MN with the overnight activity and then expands northward for Tuesday’s.

Finally, strong winds will develop on the backside of the cyclone. Forecast soundings indicate a shallow mixed layer up to about 3 kft, but very strong winds at the top of the channel - in some cases 40-50 kts. A Wind Advisory has been issued for west central MN, but will probably need to be expanded eastward with time.

A largely chaotic pattern indicative of the warm season will follow for the rest of the week into the weekend. Several disturbances could bring chances for showers and storms, but the predictability of any of these remains low. Temperatures will hold near climatological norms in the low 70s for highs and low to mid 50s for lows.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/
Issued at 1150 PM CDT Mon May 20 2024

Showers and thunderstorms have developed across much of the region, and these will become more widespread overnight. There could be a break at times, but the overall threat for thunderstorms will continue through Tuesday. A surface low will track across the region, with strong southeast wind gusts ahead of it, and northerly winds developing behind it Tuesday night.

KMSP...A prolonged period of thunderstorm chances will begin overnight and last through Tuesday. Continued to try to narrow the thunder timing, but the overall threat does continue from 06Z tonight through probably 02Z on Wednesday. Winds will be southeast, and become southeast and eventually northwest Tuesday night.

/OUTLOOK FOR KMSP/ WED...MVFR bcmg VFR. Chc -SHRA. Wind W 15G30kts.
THU...VFR. Wind W 10 kts.
FRI...VFR/MVFR chc -SHRA/TSRA. Wind SE 10-15kts.

HYDROLOGY
Issued at 321 PM CDT Mon May 20 2024

Given the setup for widespread heavy rainfall, we’re certain to see most rivers in the area end their recent recession and begin rising once again. Extended probabilistic guidance (HEFS)
shows the potential for decent rises on area rivers should rainfall exceed 2.5 inches or so over large areas, something that is in the realm of possibility. The upper Minnesota and tributaries, as well as the Crow river basin, are probably the most susceptible to seeing rises to near or above flood stage by later in the week should this occur. Urban/small stream/overland flooding is also possible during and shortly after some of the heavier downpours Tuesday and Tuesday night.

A reminder: The NWS hydrology/river forecast webpage (AHPS) at water.weather.gov is going to be discontinued next week (May 28th). The new site (NWPS, the National Water Prediction Service) is located at water.noaa.gov … now is the time to replace bookmarks and familiarize yourself with how the new site looks and feels. Resources for learning more are available at the bottom of the this page: https:/ www.weather.gov/owp/operations

MPX WATCHES/WARNINGS/ADVISORIES
MN...Flood Watch from 7 AM CDT Tuesday through late Tuesday night for Anoka-Benton-Carver-Chisago-Dakota-Hennepin-Isanti- Kanabec-Kandiyohi-McLeod-Meeker-Mille Lacs-Morrison- Ramsey-Renville-Scott-Sherburne-Sibley-Stearns-Todd- Washington-Wright.
Wind Advisory from 4 PM to 11 PM CDT Tuesday for Chippewa-Lac Qui Parle-Stevens-Swift-Yellow Medicine.
Flood Watch through Tuesday evening for Blue Earth-Brown- Faribault-Freeborn-Goodhue-Le Sueur-Martin-Nicollet- Redwood-Rice-Steele-Waseca-Watonwan.
WI...Flood Watch from 7 AM CDT Tuesday through late Tuesday night for Pierce-Polk-St. Croix.

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