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FXUS63 KMPX 211743 AFDMPX

Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 1243 PM CDT Tue May 21 2024

KEY MESSAGES

- Enhanced risk (level 3 of 5) of severe weather for this afternoon extends as far northwest as Mankato, the southeast Twin Cities metro, and Eau Claire. Damaging wind gusts, hail, and a few tornadoes are all possible

- Strong synoptic winds late this afternoon and evening in western Minnesota. Wind gusts in excess of 50 mph will be possible.

- Widespread heavy rainfall is still expected, which will likely lead to an increased risk for river flooding going into the holiday weekend.

- Active pattern remains in place. Next for chance arrives Friday, with more unsettled weather possible Sunday and Memorial Day.

DISCUSSION
Issued at 424 AM CDT Tue May 21 2024

All things are still on track to have a dynamic system impact the MPX area today as an unseasonably strong and deepening area of low pressure moves across MN this afternoon/evening. As befitting of a strong and dynamic system, severe weather, strong synoptic winds in the cold conveyor belt, and heavy rainfall are all on the table for later today.

Overnight, thunderstorms have been most prevalent just north of I-80 across Nebraska and Iowa on the nose of the best 925-850mb moisture transport. Farther north, we're more in the MUCAPE gradient, with enough isentropic and lift to get the occasional shower/storm to pulse up and down, but we have not had to deal with the level of convection as Iowa has. As we go into the morning and the Nebraska MCV moves into northwest IA, we should see shower/storm coverage expand across southern MN. As the system starts to deepen today, rapid airmass recovery is expected, as a southerly LLJ builds to over 50 kts in IA, quickly transporting a fresh airmass into the region. Current expectations are that shortly after 18z today, storms will rapidly develop near the center of the surface low and along its attendant cold front. These storms will then quickly develop into an arcing line of storms that will accelerate to the northeast across Iowa, southeast MN and into western WI. From the severe perspective, given the intensifying nature of the surface low, the kinematic environment will be impressive, the bigger question mark comes with the degree of destabilization we see out ahead of the line. From the MPX perspective, we'll see the northwest end/bookend vortex of this QLCS move across our area. The first rendition at the Day 1 convective outlook looks to have a good handle on how far northwest an environment supportive of severe weather will get with the northwest edge of the Enhanced risk that comes up to Mankato, the southeast metro, and Eau Claire. This should be a fast moving QLCS with the primary threat being damaging wind gusts, though the shear environment will also be conducive to QLCS tornado development, think of something like the severe QLCS we saw back in September of 2018.

From the rainfall perspective, there will be two QPF maxima with this system. One will be the trowal region in the cold conveyor belt where synoptic forcing will be maximized. In a different time of year, this would be something that could drop 2 feet of snow, but at almost Memorial Day, it will be a large swath of 2-4" of rain from headwaters of the MN northeast to the Voyageurs NP region. The other rainfall max will come where that bookend vortex tracks, which right now looks to be from Mankato up through the Twin Cities to the Twin Ports. This looks to be more of a 1-3" type of setup, though with much of that falling in a couple of hour period. The current Flash Flood Watch captures the potential path for this second area of heavy rain and left it unchanged.

Finally, but certainly something that can't be ignored, are the strong winds on the west side of the system. HRRR mean wind gusts show a large area of wind gusts exceeding 50 mph developing along the MN/SD border this afternoon. We continued the wind advisory and expanded some out there, though with the HRRR showing winds gusting over 60 mph, an upgrade to an High Wind Warning will be possible for western MN.

For Wednesday, the biggest change we saw with the 00z guidance is a sunnier/drier forecast for our area. The sunnier aspect also meant highs took a step up as well, with 70s progged for areas south of I- 94. Dry weather is also expected to continue into Thursday if you're looking for a time to cut down the jungle your yard has become.

Models continue to show our next shortwave and associated system impacting the area Thursday night through Friday. We continue to see some spread with this system, especially in terms of its strength, but there was a noted strengthening of this system with overnight runs of the ECMWF and Canadian. Rainfall amounts don't look to be as eye popping as today's, but given the upward trend we'll be seeing with our rivers, more rain certainly won't help the situation.

For the holiday weekend, the one area where we are seeing some agreement is Saturday being dry. For Sunday and Monday, models show more shortwave activity rotating through the region, though agreement at this point is pretty poor, though we can at least expect the possibility for rain to cap off the holiday weekend.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/
Issued at 1227 PM CDT Tue May 21 2024

We're seeing a break in the showers and storms early this afternoon before another round starts approaching from the southwest by around 20z. A return to MVFR/IFR conditions is expected as the storms move through, along with gusty winds.
Conditions will improve after 03-04z with only some scattered showers. Our western most terminals will likely see gusts of 40kts or greater by this evening as the low wraps up and pulls away to the northeast. LLWS will also be a concern for these sites. These strong winds will move eastward through the overnight hours and diminish somewhat by sunrise. A return to VFR is expected during the early morning hours.

KMSP...Skies are clearing early this afternoon with thunderstorm development still expected by 20-21z. All severe hazards are possible through 00z before conditions improve. Winds will turn westerly at this time with gusts around 30-35kts possible before sunrise.

/OUTLOOK FOR KMSP/ WED...VFR. 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...Wind Advisory from 4 PM this afternoon to 11 PM CDT this evening for Chippewa-Douglas-Lac Qui Parle-Pope-Redwood- Stevens-Swift-Yellow Medicine.
Flood Watch through late tonight for Anoka-Benton-Carver- Chisago-Dakota-Hennepin-Isanti-Kanabec-Kandiyohi-McLeod- Meeker-Mille Lacs-Morrison-Ramsey-Renville-Scott- Sherburne-Sibley-Stearns-Todd-Washington-Wright.
Flood Watch until 7 PM CDT this evening for Blue Earth-Brown- Faribault-Freeborn-Goodhue-Le Sueur-Martin-Nicollet- Redwood-Rice-Steele-Waseca-Watonwan.
WI...Flood Watch through late tonight for Pierce-Polk-St. Croix.

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