Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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236 FXUS63 KFSD 210426 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1126 PM CDT Thu Jun 20 2024 .KEY MESSAGES... - Multiple rounds of showers and thunderstorms tonight through Friday night will contribute to a heavy rain risk. A broad 2 to 4 inches is expected with isolated higher amounts up to 5 to 8 inches possible north of I-90. Flash Flooding is also possible with the heavy rain. - The Flood Watch has been expanded to encompass the majority of the area and is in effect until 7 am Saturday morning. - Strong to severe storms are again possible on Saturday. Some uncertainty remains in where storms will develop but large hail to the size of golf balls, damaging winds to 60 mph, and a few tornadoes are possible. - Dry weather returns for the weekend but rain chances are uncertain next week. && .DISCUSSION... Issued at 254 PM CDT Thu Jun 20 2024 Scattered showers and embedded thunderstorms continue across the area this afternoon. So far, an additional quarter to half inch of rain has fallen through the first half of the afternoon hours with isolated amounts up to around an inch. Expect another quarter to half an inch of rain through the rest of the afternoon hours. A mid level wave currently over the four corners area will push into the Plains this evening and night, bringing renewed chances for thunderstorms and heavy rainfall. As the wave ejects, the low level jet (LLJ) will strengthen which will also further strengthen warm air advection (WAA) across the area. The warm advective ascent will reside beneath the right entrance region of an upper level jet streak, promoting more then sufficient lift across the area. A mesoscale convective system (MCS) looks to develop across western Nebraska this evening and track eastwards into the forecast area. While the lift will be more then enough to sustain storms, instability is questionable as a surface warm front looks to reside south of the forecast area. This signals that storms will transition from surface based (double check) in Nebraska to elevated in South Dakota. Instability will wane to around 1,000 J/kg of CAPE but long skinny CAPE profiles in a highly moist atmosphere will set the stage for heavy rainfall tonight. More details can be found in the hydrology section. Skinny CAPE profiles will still contribute to a strong to severe storm risk. Shear will be sufficient with deep layer shear values up to 40-50 knots and 50-60 knots of 0-10 km shear. However, with low end instability in place along with waning values of both DCAPE and 0-7 km theta-e difference, think the MCS will most likely be on a down trend as it enters the forecast area. Damaging wind gusts up to 60 mph along with large hail up to the size of quarters is possible along and west of the James River. With weak instability in place, the storms will not completely fizzle out but remain sub severe, further contributing to the previously mentioned heavy rain risk. Low temperatures will only fall to the 60s overnight. No break in storm and heavy rain potential as a Maddox Frontal pattern persists aloft. The warm front will finally push northwards into the forecast area but how far north the boundary pushes remains uncertain. However, the anomalously moist environment will persist which will again yield another day of heavy rainfall. More can be found on this potential in the hydrology section. Turning to the severe weather side of things, the right entrance region of an upper level jet streak will remain overhead while 850 mb flow remains southerly through the day, continuing to advect sufficient moist inflow into any storm that develops. Cloud cover is also a bit uncertain as cloudy skies look to persist north of the boundary with clearer skies south of it. Dew points will further moisten to the 70s while high temperatures warm to the upper 70s along highway-14 to mid to upper 80s along and south of I-90. Although mid level lapse rates will only be on the order of moist neutral, steeper low level lapse rates will contribute to strong instability on the order of 2,000 to 3,000 J/kg of CAPE. With the right entrance of an upper level jet streak overhead, sufficient deep layer vertical shear is expected on the order of 30-40 knots. However, hodograph reveal larges shear values above 6 km which could allow for a large hail threat, potentially up to the size of golf balls in the strongest storm. On top of the threat for large hail, an tornado is also possible along the warm front. Hodographs do not show much low level curvature during the daylight hours but baroclinically driven vorticity on the warm front may allow a tornado to develop. DCAPE values up to around 1,000 J/kg could allow for damaging wind gusts up to 60 mph in any storm that is able to fire in the open warm sector. Another short wave trough will push into the forecast area during the evening hours which will develop additional storms. With a relatively uncapped atmosphere, widespread showers and storms look to develop. This could turn down the severe weather side of this event and focus more on the heavy rainfall aspect. Heavy rainfall will continue through the overnight hours before any chance for rain finally moves east of the forecast area Saturday morning. After remaining showers move out Saturday morning, a dry weekend looks to be in store with highs into the 70s and 80s with lows down to the 60s. Additional chances for rain are possible next week but details are too uncertain to say at this point. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 1117 PM CDT Thu Jun 20 2024 Showers and thunderstorms - some with heavy rain - will expand eastward overnight, with the focus from south central SD and along and north of the Interstate 90 corridor. The activity may diminish on Friday morning, before showers and thunderstorms develop once again along and north of Interstate 90 in the afternoon and evening. Some of these storms could be severe, with heavy rain likely. Visibility and ceilings will be in the MVFR/IFR range with any shower and thunderstorm activity. Other than in thunderstorms, east to southeast winds will be relatively light through the TAF period. && .HYDROLOGY... Issued at 254 PM CDT Thu Jun 20 2024 An anomalously moist airmass will be in place this evening and night as precipitable water (PWAT) values increase up to around 2 inches. This places this airmass in the 90th to 97.5th percentile per the NAEFS ensemble. Warm cloud depths will be more then sufficient up to 12,000 to 13,000 ft, further increasing confidence in heavy rain potential. Latest hi-res guidance has come in with quite a bit of rain with amounts up to 1-3 inches through Friday morning. Isolated higher amounts are possible, especially along and north of I-90 where guidance is consistent in where the heaviest rain will fall. CAMS show this in the incoming MCS but also show a warm advective wing out ahead of the MCS. This will result in the highest rainfall totals that may exceed 3 inches due to a longer duration of heavy rainfall over a specific location. The Maddox Frontal pattern continues through Friday. Although the warm fronts location is uncertain at this time, another round of heavy rainfall is expected as another ridge ridging shortwave trough pushes into the Northern Plains. The environment will only further moisten as PWAT values rise to the 99th percentile of climatology per the NAEFS ensemble. Mean flow will be roughly parallel to the warm front which could result in repeated thunderstorm development along the boundary during the afternoon hours. As the shortwave trough arrives during the evening hours, additional storms are expected as a cold front moves through the area. Another 1-3 inches are expected through the duration of Friday with isolated higher amounts possible. The isolated higher amounts will again be dependent on where the warm front sets up. Rainfall totals for the entire event will be quite high with a broad 2 to 4 inches expected along with the potential for 5 to 8 inches north of I-90. Given saturated soils in place, Flash Flooding is possible. With the expected rainfall, river flooding appears likely but will ultimately depend on where and how much rain falls. Some guidance would suggest moderate to major flooding is possible across portions of the forecast area. FEW && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Flood Watch through Saturday morning for SDZ038>040-050- 052>071. MN...Flood Watch through Saturday morning for MNZ071-072-080-081- 089-090-097-098. IA...Flood Watch through Saturday morning for IAZ001>003-012>014. NE...None. && $$ DISCUSSION...Meyers AVIATION...JM HYDROLOGY...Meyers