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FXUS63 KMPX 190827 AFDMPX

Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 327 AM CDT Sun May 19 2024

KEY MESSAGES

- Multiple rounds of showers and thunderstorms are expected beginning this evening into Monday morning and Tuesday through Tuesday night.

- Increasing likelihood of strong to severe thunderstorms Tuesday PM across southern Minnesota and western Wisconsin.
SPC day 3 SWO brings an Enhanced Risk (3 out of 5) into SE MN.

- Heavy rainfall is likely and totals may exceed 3 inches through mid-week. This will likely lead to rises on rivers and their tributaries into the holiday weekend.

DISCUSSION
Issued at 326 AM CDT Sun May 19 2024

TODAY THROUGH MONDAY NIGHT... Today will be a tale of two halves.
Sunday will start off pleasant with mostly sunny skies and temperatures climbing into the 70s across the region. The second half will focus on this evening into tonight as the first of multiple rounds of rain & thunderstorms will impact the Upper Mississippi Valley. A few showers or storms will try to fire this afternoon across western Minnesota but will have to battle plenty of dry air currently in place. By this evening a strengthening LLJ will ramp up moisture advection into the area. This will provide a potent setup similar to what occurred Friday night into Saturday morning.
Better forcing and instability will lead to more widespread nature of convection that'll lead to half inch to an inch of QPF by Monday morning. We'll see an active wave train with several shortwaves eject out of the Rockies into the Upper Midwest. We'll have a brief period to dry out Monday afternoon and temperatures will be seasonable in the 70s.

TUESDAY THROUGH WEDNESDAY NIGHT... Our attention turns to the next event that should be the most impressive wave of the week. It's not often we'll see Summer moisture with Winter synoptic forcing, but Tuesday's system will paint a picture of what it would look like.
The sfc low will move from Nebraska into S Minnesota Tuesday afternoon. This places the most favorable area of lift across the MPX CWA with the focus over western and central Minnesota. This is where the cold conveyor belt of northeast flow will wrap into the SFC low and the TROWAL to produce a prolonged steady moderate rain event across this region. The TROWAL will provide plenty of moisture with PWAT values in the 95th percentile per the ECMWF EFI. MSLP is also in uncharted territory per the EFI, roughly once per decade, for the MSLP to drop this low.+

Western and Central Minnesota is where we see the highest QPF, with amounts in ensembles of 2 to 4 inches possible, as mentioned in the previous discussion. This forecast should not change a whole lot as this is driven in the cold sector of the system, not the warm convective portion. However, the convective side of this system looks increasingly likely to impact southern Minnesota and western Wisconsin Tuesday afternoon & evening. The SPC day 3 SWO highlights a rather large enhanced (3/5) risk across portions of the plains into the Upper Mississippi Valley. It places our I-90 counties in Enhanced. The Slight risk covers much of the eastern half of our forecast area or the Twin Cities Metro and points eastward. This system will have plenty of wind shear with impressive forcing. Less certain on the instability front. The ECMWF has been consistent in tracking the low pressure from SW MN to the Twin Cities. It brings 1000 to 1500 j/kg of instability into the region Tuesday Afternoon that should be plenty to trigger severe weather, but other model guidance with further east low tracks would bring much less instability into the area, so for now there is still some uncertainty and lower confidence. That's why the greatest risk area is to the south and east where there is higher confidence of instability/warmer air mass. This severe chance is also seen in machine learning outlooks run by CSU. The CSU day 3 suggests that Tuesday's severe threat could bring the enhanced area further west, but again model spread and lower confidence make the current SPC outlook a great starting point due to that uncertainty. So Tuesday will be the day to keep an eye on for multiple reasons, slight risk of excessive rainfall & enhanced/slight risk of severe thunderstorms.
Rain chances persist into Wednesday as this system moves into the northern Great Lakes. Wednesday will also be the coolest day of the week with high temperatures in the upper 50s to mid 60s with cloud cover and northwest flow to blame.

THURSDAY THROUGH SUNDAY... Forecast confidence begins to decrease beyond mid-week. There are more signals for additional QPF events among the grand ensemble members, but not as clear of a signal and much more spread. This active period seems to continue through the holiday weekend, but confidence on timing of when rain could fall remains low. Temperatures should be comfortable, in the 60s and 70s, dependent on precipitation or not. The heavy rainfall from the Sunday night and Tuesday system will likely lead to rises on the Upper Minnesota river basin and the Crow River basin. The river rises should begin as we head into Memorial day weekend.

AVIATION /06Z TAFS THROUGH 06Z MONDAY/
Issued at 1055 PM CDT Sat May 18 2024

Increasing mid level clouds Sunday morning, then showers/ thunderstorms become possible in the afternoon across western MN, spreading east Sunday evening. Light and variable winds tonight will turn south southeast Sunday.

KMSP...No concerns tonight.

/OUTLOOK FOR KMSP/ MON...Chc AM MVFR/-TSRA. VFR/chc SHRA aftn. Wind SE 5-10 kts.
TUE...MVFR or IFR/TSRA likely. Wind NE 10-15 kts.
WED...VFR. Chc MVFR/-SHRA. Wind WNW 15G25kts.

HYDROLOGY
Issued at 253 AM CDT Sat May 18 2024

The heavy rain event for Tuesday currently highlights the potential for a large footprint of 1 to 3 inches of rain, with some locations pushing 4 inches to hit much of the upper MN River Basin. Gauges at Montevideo, Granite Falls, and Morton are still at or near minor flood stage and if the kind of rainfall amounts we are currently seeing were to fall in the upper MN basin, then renewed flooding along the Minnesota River upstream of Mankato will be possible as we head into Memorial Day weekend.

MPX WATCHES/WARNINGS/ADVISORIES
MN...None.
WI...None.

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