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FXUS63 KMPX 192013 AFDMPX

Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 313 PM 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 312 PM CDT Sun May 19 2024

Radar returns from western to central MN have not been able to reach the surface thru this afternoon due to very dry low-level air in place (evidenced in MPX's 12Z sounding). Returns near the MN/IA border have had more success reaching the surface, as southerly flow has begun to advect greater moisture into our region. Isolated to scattered showers will become more common as we go thru this evening in response to moisture advection and a strengthening LLJ moving in from the west. Eventually, clusters of thunderstorms will develop near midnight along the nose of the LLJ, likely over south-central to southern MN, and move east-northeast thru WI into Monday morning.
We're not expecting much in terms of severe weather (owing to the minimal CAPE and marginal wind shear), but the 12Z HRRR LPMM forecasts most of southern MN and into western WI receiving at least 0.5-0.75" of QPF tonight thru Wednesday. There are also several embedded streaks of QPF over 1.25" to upwards of potentially 2", which seems possible if localized areas can receive repeated rounds of storms. Rain should mostly exit to our east by mid-Monday morning, while overcast low stratocumulus will be slower to break up. But, some larger pockets of clearing should occur by the afternoon allowing temperatures to warm into the low to mid 70s.

Most guidance favors keeping us dry through at least the first half of Monday night. However, the HRRR, and to extent the GFS, ECMWF, and GEM, do try to develop convection and QPF over southern MN (mainly along I-90) where there is a reservoir of 1000-2000 J/kg MUCAPE. This precip would theoretically develop near a stationary boundary and in response to an approaching upper- level trough and jetstreak. While shear again looks marginal, likely leading to a more multi-cell storm mode, a severe wind gust or hail stone cannot be ruled out. This region has also been outlined as a Marginal risk (1 out of 5) in SPC's 1730Z Day 2 Convective Outlook. Tuesday basically looks like a lock for widespread, accumulating rainfall from the aforementioned trough developing a strong, synoptic cyclone as it crosses the Rockies. NAEFS and EPS climatological guidance both show the surface low becoming abnormally deep and having anomalously high moisture transport as it travels into the Upper Mississippi Valley into Tuesday evening. The first round of showers and thunderstorms looks to occur during Tuesday morning/early afternoon, moving northeast out of our CWA by mid-afternoon.
This precip should develop underneath divergent upper-level flow and within the right-entrance region of a downstream jetstreak.
CAMs show a brief break in the precip occurring before another round of showers and thunderstorms develops over IA (close to the surface low and frontal boundaries) and moves northeast Tuesday afternoon/evening. This round will eventually affect the entire CWA by Tuesday evening leading to more accumulating precipitation. Another focus with this system will be the potential for severe weather later Tuesday. With such a strong synoptic system forecast, the kinematics will be in place for strong to severe thunderstorms within the impending buoyant warm sector. Current consensus within the guidance places the warm sector from southeastern MN into western WI. Indeed, forecast severe variables and soundings show strong shear and veering wind profiles, especially close to the warm front. Where the uncertainty looms is how much daytime heating and thus destabilization can occur? Mostly cloudy skies should be overhead from the copious amounts of moisture and lift, so breaks in the clouds will be necessary to increase CAPE and improve low-level lapse rates. If some heating can occur, we could have a potentially dangerous severe weather environment to monitor. SPC has also recognized this possibility and issued an Enhanced risk for much of the Midwest, including southeast MN and southwest WI. A Slight risk extends farther north due to the uncertainty of the northern extent of the warm sector.

Returning to storm total rainfall with this system through Wednesday afternoon, the NBM has a mean of 1.25-2" of QPF for most of the MPX CWA Increasing to the 75th percentile of the ensemble results in MPX's area seeing at least 1.75" with localized amounts approaching 2.5". The 90th percentile is where things get really interesting where area-wide amounts generally rise to well over 2.75", showing the high precip potential of this system. Would not be surprised to see localized streaks of even higher amounts, depending on where rainbands set up. This will also have be monitored as flooding could become an issue, especially for low-lying and urban areas.

Guidance shows the cyclone pushing off to our east on Wednesday, cooling our highs into the 60s and causing precip to taper off.
However, temperatures should recover towards normal to end the week as another shortwave crosses the Rockies by Friday. This shortwave looks to bring another good chance of rain to our region during Friday, though likely not as significant (in terms of amounts)
compared to Tuesday. Looking beyond the period, guidance hints a couple more chances of precip through the end of May as a continued train of shortwaves is favored. But, model differences grow during this time leading to decreased forecast certainty.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/
Issued at 1157 AM CDT Sun May 19 2024

Radar returns across central MN are still not making it to the surface, however returns near the MN/IA border are within the more moist environment. Isolated to scattered showers will become more common into this evening before LLJ arrives, causing more strong forced SHRA/TSRA likely from midnight through the early morning as the convection moves northeast. Have kept pre- existing TEMPO TSRA group but have delayed onset time by about 1 hour. Cigs will remain VFR until the heavier precip arrives, with expected drop to IFR/LIFR left in its wake. Cigs should slowly improve from west to east during Monday morning. Winds will be southeasterly around 10 knots (slightly weaker in WI)
into early Monday morning before slowing to at or below 5 knots and becoming west-northwesterly.

KMSP...Could see some light showers this afternoon but more sustained showers will arrive late this evening. 06-10Z continues to look like the best timing of organized SHRA/TSRA.
Post rain stratocumulus will cause cigs to fall to IFR before improvement to MVFR occurs by late Monday morning, and then VFR during Monday afternoon.

/OUTLOOK FOR KMSP/ TUE...MVFR or IFR/TSRA likely. Wind SE 10-15G25kts.
WED...MVFR bcmg VFR. Chc -SHRA. Wind W 15G25kts.
THU...VFR. Wind W 10 kts.

HYDROLOGY
Issued at 655 AM CDT Sun May 19 2024

A heavy rain event remains on track for Tuesday, with a large footprint of 1 to 3 inches of rain, with some locations pushing 4 inches. The greatest rainfall totals are expected across southwest, west central, and central MN. Exactly where and how much rainfall we see will determine where a threat for river flooding will exist going into Memorial Day weekend, but given current rainfall forecasts, flooding during the holiday weekend may be possible in the Cottonwood, Redwood, Minnesota (upstream of Mankato), and Crow river basins.

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

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