Sunrise 5:36 AM   Sunset 8:44 PM Moonrise 6:10 PM   Moonset 3:23 AM

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FXUS63 KMPX 212351 AFDMPX

Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 651 PM CDT Tue May 21 2024

KEY MESSAGES

- Severe weather threat quickly diminishing this evening.

- Strong synoptic winds near 50 mph late this afternoon and evening in western Minnesota.

- Heavy rainfall is still expected, leading to an increased risk for river flooding going into the holiday weekend.

- Active pattern remains in place with several large systems bringing additional rain Friday and Memorial Day.

UPDATE
Issued at 643 PM CDT Tue May 21 2024

With the line of storms having moved out of Minnesota into Wisconsin the threat for severe weather in Minnesota has diminished greatly.
Rain and lightning will continue with the primary threat being urban and small stream flooding. For the early evening the line will move through western Wisconsin with high winds and an isolated tornado as the threats. Once this line passes the remaining risk in Wisconsin will be the same as Minnesota with rain and lightning.

DISCUSSION
Issued at 350 PM CDT Tue May 21 2024

TODAY AND TONIGHT...Severe weather is still on track for this afternoon and evening across much of south central Minnesota and western Wisconsin. At the surface as of 18z, there are two low pressure centers; one in far southwest Minnesota and a second, stronger one near Omaha. The Minnesota low and its associated warm front have lifted north into central Minnesota this afternoon and has resulted in the warm sector making it north of the Twin Cities metro. Previously, there was a lot of uncertainty regarding how far north this would extend and thus uncertainty with thunderstorm coverage on the northern and western edge of this complex system. Further south with the second, stronger low, a line of severe thunderstorms is currently pushing into western Iowa. This activity should continue to move east/northeast at a pretty good clip through mid afternoon and eventually reach southern Minnesota by around 20z. With the clearing and strong advection of moist unstable air across south central Minnesota, these storms should be able to sustain themselves through western Wisconsin this evening.
CAMs and other guidance continue to highlight three main threats with this event: the risk for all modes of severe weather, heavy rain and flooding, and very strong synoptic winds on the backside of the low.

For the severe risk...as mentioned above, all modes of severe weather are expected this afternoon into this evening with the highest threat across southern Minnesota and west central Wisconsin.
This is highlighted in the SPC SWODY1 discussion as well as PDS Tornado Watch 277. Thunderstorm wind gusts in excess of 80 MPH, isolated very large hail up to 4 inches, and strong (EF2+) tornadoes are possible. A special 20z MPX sounding will hopefully give us more insight to the environment, but CAMs and forecast soundings continue to show very high shear/helicity and moderate MUCAPE values (1000- 1500 J/kg) with higher values 2000 J/kg + in extreme southeastern Minnesota/Iowa/western Wisconsin. UH tracks have been fairly consistent in moving across far southeastern Minnesota into western Wisconsin with a second area further to the northwest across the Twin Cities metro through 00z.

For the heavy rain/flash flood threat...CAMs are showing impressive rainfall rates of 1-2"/hour with the storms as they move through.
Per the MPX 12z sounding, PWs were around 1.2" but have likely increased to near climatological max. The other concern is the training of storms and recent rainfall across southern Minnesota over the past few days. This will likely create an elevated flash flood threat. Additionally, the Twin Cities metro will be susceptible to flash flooding given the potential for impressive rainfall rates. This could coincide with the evening rush hour and will need to be monitored closely.

For the strong wind threat...As the two previously mentioned lows merge and deepen a little bit more by this evening, 45-50kt synoptic wind gusts are possible across western Minnesota. A Wind Advisory remains in effect for this region through 04z. These winds will diminish below advisory criteria as the low pulls away to the west, but gusts of 30-35kts are likely across central Minnesota (including the metro) through 07-08z. Elevated northwesterly winds will remain in place through Wednesday morning.

WEDNESDAY THROUGH MONDAY...High pressure will briefly settle in for Wednesday and Thursday as the departing storm system lifts to the northeast. A few showers will develop in the afternoon along and north of I-94. Thursday will by dry, with northwest winds becoming more southerly ahead of the next system that will bring widespread rain Thursday night into Friday. Convergence along this boundary is quite impressive, with southerly winds south of it, and northeast winds on the poleward side. Did increase precipitation chances toward the higher end of guidance, as it looks like nearly all locations will see rain.

There will be a brief break in this system, but then another shortwave trough will move across the region over the second half of Memorial Day weekend.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/
Issued at 643 PM CDT Tue May 21 2024

Thunderstorms will continue to end from west to east this evening. Northwest winds will develop and continue across the region throughout the TAF period. VFR conditions will develop overnight as ceilings clear. There is a chance for scattered showers during the afternoon and evening at KAXN, KMSP, and KSTC

KMSP...Steady rain and thunderstorms will continue for an hour, but the thunder threat should diminish. Otherwise expect northwest winds to increase overnight as skies clear and VFR conditions settle in. The winds will become more westerly on Wednesday and diminish.

/OUTLOOK FOR KMSP/ THU...VFR. Wind W 10 kts.
FRI...VFR/MVFR with -SHRA/TSRA likely. Wind SE 10-15 kts.
SAT...VFR. Wind SE 05-10 kts.

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 until 11 PM CDT this evening for Chippewa- Douglas-Lac Qui Parle-Pope-Redwood-Stevens-Swift-Yellow Medicine.
Flood Watch until 1 AM CDT Wednesday for Anoka-Benton-Carver- Chisago-Dakota-Hennepin-Isanti-Kanabec-McLeod-Meeker- Mille Lacs-Morrison-Ramsey-Scott-Sherburne-Sibley- Washington-Wright.
Flood Watch until 10 PM CDT this evening for Blue Earth- Faribault-Freeborn-Goodhue-Le Sueur-Martin-Nicollet-Rice- Steele-Waseca-Watonwan.
WI...Flood Watch until 1 AM CDT Wednesday for Pierce-Polk-St. Croix.

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