Sunrise 6:26 AM   Sunset 8:20 PM Moonrise 9:36 PM   Moonset 6:02 AM

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FXUS63 KFSD 250857 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 357 AM CDT Thu Apr 25 2024

KEY MESSAGES

- Rain and storms return this evening and night and persist through Friday. Strong to severe storms are possible on Friday.

- Beneficial rainfall is expected tonight with a broad quarter to three quarters of an inch of rain expected, highest over northwest Iowa.

- Chances for rain continue through the weekend with probabilities increasing to 30-50%+ for exceeding a half an inch of rain on Sunday.

DISCUSSION
Issued at 355 AM CDT Thu Apr 25 2024

Sprinkles to very light showers begin the day today. These sprinkles and light showers are expected to dissipate through the rest of the overnight hours. Today will be a mostly quiet day as cloud cover builds into the region. This will hold high temperatures similar to what they were yesterday with highs into the mid to upper 60s but could be a touch cooler. An upper level wave will begin to eject into the Plains which will set the stage for rain this afternoon, evening, and night. Lee cyclogenesis will take place in the front range of the Rocky Mountains as the wave ejects. Low level pressure gradients will tighten ahead of the developing surface low which will result in a strengthening low level jet (LLJ) and southeast gusts up to 30-40 mph. Warm air advection (WAA) will strengthen in response to the LLJ and bring the chance for isolated showers this afternoon. Sounding profiles are mostly dry aside from a moist layer between 800-700 mb so any rain that does fall during the afternoon will be light as only a few hundredths to possibly up to a tenth.
The LLJ will further strengthen into the evening and overnight hours, continuing gusty southeast winds and rain chances during this period of time. A surface warm front will push into Kansas but the elevated front will move into central Nebraska. While still south of our area, a mid level wave along with a 500 mb speed max look to encounter the elevated warm front and produce convection on it.
Elevated instability is expected ahead of this elevated warm front but the instability will be weak on the order of about 500 J/kg of CAPE. Given the weak instability, severe weather is not expected but isolated small hail is possible. The other aspect to this overnight precipitation is beneficial rainfall. Moisture transport will strengthen between 925-850 mb which will allow for a widespread quarter to three quarters of an inch of rain to fall, highest over northwest Iowa. Isolated higher amounts are possible.

The upper wave pushes into the forecast are on Friday which will bring the chance for strong to severe storms with it. However, this chance for severe storms remains conditional as it remains uncertain how overnight rain and storms will affect the environment. If the low levels remain stable via low level stratus blanketing the area as the occluding system pushes into the Missouri River Valley, then severe weather chances will be low and would be confined to elevated convection with a large hail threat. However, if the low levels are able to clear out and destabilize as the wave approaches, then severe weather will be possible. In this scenario, the surface warm front will rapidly advect northwards and push into the Missouri River Valley and allow convection to develop as the main wave and left exit of an upper level jet streak couple with the baroclinically generated vorticity along the warm front. With surface dew points moistening up to around 60 degrees F, sufficient instability will be realized up to 1,000-1,500 J/kg of CAPE. Wind shear will be sufficient at 30-40 knots of deep layer shear. Hodographs reveal that there is clockwise curvature in the low levels with 0-1km storm relative helicity (SRH)
values up to 150-200 m^2/s^2. This along with 0-1 km shear values between 20-30 knots and low LCLs would support an environment capable of tornadoes along with large hail and damaging winds. In fact, storm relative inflow looks to be aligned with 0-500 m vorticity vector, signaling the potential for nearly streamwise vorticity to be ingested into the storms that develop. Taking everything into consideration, think that the the most likely scenario is for the overnight rain and storms to prevent the low levels from destabilizing during the afternoon hours and thus keep low level stratus across the area.
This is supported by hi-res ensembles as they all show barely a 30% chance for 2-5 km updraft helicity exceeding 75 m^2/s^2 and widespread cloud cover throughout the day. The main timeframe for storms is between 4 pm to 10 pm. Outside of storm chances, Friday will be a cooler and more humid day as high temperatures only warm to the upper 50s and 60s while dew points moisten to the 40s and 50s. Winds will remain gusty out of the southeast with gusts up to 30-40 mph expected, strongest along and east of I-29. Low temperatures will remain on the mild side, only falling to the upper 40s and 50s overnight.

An active pattern remains aloft for Saturday as another upper level wave ejects into the Plains. A cold front will lie southeast of the forecast area thanks to Friday's wave. Given the front's placement, severe weather is not expected but light showers may be possible.
High pressure will slowly be pushing into the area from the northwest resulting in marginally breezy northerly winds. High temperatures will warm to the upper 50s to upper 60s from northwest to southeast across the area while low temperatures will fall 40s and 50s.

The same upper wave from Saturday will push into the Northern Plains on Sunday. Medium range guidance varies on how the surface low will track along with where the surface front will lie. If the front was to back further to the northwest, then higher theta-e air could be introduced to the area and may introduce chances for storms. If not, then deformation precipitation on the back side of the system would be more likely. Ensembles only show about a 30% chance for CAPE values to exceed 500 J/kg. They also show a widespread 30-50%+ chance for exceeding a half an inch of rain. This is further supported by cluster analysis which has all clusters showing a broad 40-60% chance for exceeding a half an inch of rain. Aside from rain chances, high temperatures look to remain in the 50s and 60s with lows falling to the 30s and 40s.

Nearly zonal flow sets up for most of the week next week. With westerly flow aloft, next work week look to be mainly dry with highs in the 50s to 70s, coldest on Monday, and lows in the 40s and 50s.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/
Issued at 1051 PM CDT Wed Apr 24 2024

Mid level ceilings may be bring occasional sprinkles overnight, but coverage is too low to have enough confidence to include in the TAF at this time. VFR conditions prevail tonight with southeast winds slowly strengthening.

An increasing southerly low level jet may introduce periods of low level wind shear west of the I-29 corridor and near Huron overnight. Southeast winds will ramp up quickly with sunrise, resulting in gusts in the 20s and 30s through the day. Scattered showers and isolated storms spread in late Thursday afternoon and evening.

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

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