Sunrise 6:20 AM   Sunset 8:25 PM Moonrise 12:42 AM   Moonset 9:06 AM

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FXUS63 KFSD 291725 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 1225 PM CDT Mon Apr 29 2024

KEY MESSAGES

- Remnant rain showers will be ending throughout the morning hours, with clouds moving out during the afternoon giving way to partly/mostly sunny skies.

- Severe weather threat increases Tuesday afternoon through early evening with a Level 2 of 5 risk. Large hail to the size of golf balls and damaging wind gusts to 70 MPH are the main threats, with a tornado and/or minor flooding from heavy downpours being secondary threats. Main period of concern is from 1 PM through 8 PM.

- Cooler than or near normal temperatures are favored for Wednesday and beyond. A few periods of low temperatures in the 30s and lower 40s suggest frost/freeze concerns will linger into early May.

- The cooler temperatures on Wednesday/Thursday will be accompanied by additional showers and thunderstorms. While severe weather is not expected, minor flooding issues are possible.

DISCUSSION
Issued at 404 AM CDT Mon Apr 29 2024

As our surface and upper level lows continue to slowly drift over/out of the region, early morning satellite imagery shows abundant cloud coverage remains in place. 2 AM radar imagery shows remaining rain showers are moving northwards into northeastern SD and southwestern MN, with surface temperatures in the upper 30s to lower 40s along and west of I-29, while areas east of I-29 are reaching up into the upper 40s and lower 50s. Can't rule out some light additional rain showers into the mid-morning hours, especially for those along and north of the highway 14 corridor, given sufficient moisture and positive vorticity advection from the exiting upper level low.

By the late morning hours, dry air begins to filter into the region from west to east, and will be slowly decreasing cloud coverage for the rest of the day. As a result, highest temperatures today will be occurring for areas in south-central South Dakota and those along the Missouri River valley getting up into the lower 60s, while the rest of the region will reside in the lower to upper 50s. As the surface pressure gradient (SPG) remains tightened, expect semi-breezy west- northwesterly to continue before they weaken this evening as they transition to southeasterly winds overnight into Tuesday.

The change in wind directions is due to our next elongated surface low pressure moving in from the west, with the affiliated shortwave moving along with it coming off the Rockies. The warm front will be pushed northwards across the area throughout the morning hours, bringing temperatures in the lower to mid 60s for areas west of the James River Valley before the cold front moves through during the morning hours, and warmer mid 60s to lower 70s expected elsewhere well ahead of the eastward progressing cold front. While there are minor nuances amongst the models, most bring 1-2 kJ/kg MUCAPE and 40 to 50 knots of 0-6km shear straddling the I-29 corridor shortly after noon, with everything shifting eastwards as the cold front progresses eastwards into the early evening hours. Moisture return behind the warm front is one area we'll need to keep an eye on as a point of failure, as less moisture would lower the available instability, but for now convection allowing models would suggest that isn't a problem for the time being.

So, we'll be watching morning showers and thunderstorms west of I-29 moving eastwards ahead of the cold front, with the greater instability moving into the area throughout the morning hours.
Current thinking is the event will start off between 1 to 3 PM with supercells given the favorable deep layer shear, with the primary threat being large hail to the size of golf balls and damaging wind gusts to 60mph in the vicinity of the I-29 corridor. As storms collide into a linear/cluster mode with a well established cold pool, the main threat switches towards strong damaging wind gusts to 70 mph, though can't rule out storms creating large hail on the leading edge of the line. Given 0-1km helicity of 100-200 m2/s2 out of the south-southwest across northwestern Iowa and southwestern Minnesota, can't rule out isolated tornadoes for any portion of the line that is moving more northeastwards. Severe weather threat looks to end in the early evening hours for our area as the cold front clears our eastern counties by around 8 PM. In addition to the severe threat, can't rule out some flooding concerns given the already high soil moisture content and rising rivers. 1 hour flash flood guidance is sitting in the 1-1.5" range, so storms that are producing rainfall rates in excess of 1.5"/hr will need to be monitored closely.

Wednesday and Thursday will see cooler conditions as the surface thermal gradient resides off to our southeast, keeping the warmer temperatures out of reach for the area. Wednesday itself looks to start off dry, but as shortwaves from a cut-off upper level low along the international border move across the area we'll see precipitation chances increase throughout the day, with the best rainfall chances occurring overnight into Thursday. Again, given the high soil moisture content, minor flooding concerns are possible and will need to be monitored, but severe weather looks unlikely as we are on the wrong side of the thermal gradient.

The main wave pushes through late Thursday which looks to end most of our precipitation chances, though the NBM holds onto light rain chances as some ensemble members show rainfall chances continuing.
Near to below normal temperatures look to continue into Saturday, with temperatures and precipitation chances becoming less clear as model solutions diverge into the beginning of next week.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/
Issued at 1225 PM CDT Mon Apr 29 2024

Current satellite shows widespread low level stratus across the area this afternoon. The stratus is currently sitting at MVFR levels and should stay there before scattering out later this afternoon to early evening. West/northwest winds will remain breezy through the afternoon with gusts up to around 20 knots before waning this evening and mixing ceases. While winds will be light during the evening hours, winds will turn out of the southeast during the overnight and become marginally breezy. A few models have suggested the potential for patchy fog tonight across parts of eastern South Dakota, southwest Minnesota, and northwest Iowa. However, think this is a low probability (<20% chance) so have kept out any mention of fog in all TAFs. Light rain is possible west of I-29 tomorrow morning along with strengthening southeast winds to finish the TAF period.

FSD WATCHES/WARNINGS/ADVISORIES
SD...None.
MN...None.
IA...None.
NE...None.

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