####018004276#### FXUS63 KGRR 070010 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 810 PM EDT Mon May 6 2024 .KEY MESSAGES... - Potential for strong/severe storms Tuesday - Wet and Cool Weather for the end of the week && .DISCUSSION... Issued at 308 PM EDT Mon May 6 2024 - Potential for strong/severe storms Tuesday Severe storms are just developing over the Plains in response to strong dynamics being produced by low pressure over the northern Plains along a warm/occluded frontal band. These storms...in weakening mode...will move east tonight and move toward Lower MI Tuesday morning. These storms will be weakening as they move across the cwa Tuesday morning due to drier air and limited instability. Once the initial showers/storms move across by late morning, we'll likely see a lull of a couple of hours and with more storm redevelopment during the mid afternoon. CAPE values increase dramatically behind the frontal boundary and will be helped out by any clearing that we see. An approaching strong short wave that's ejected out of the Plains upper low will support convective redevelopment. Dewpoints surge into the lower 60s behind the front. Strong shear in the 55 kt range coupled with 1k+ j/kg SBCAPE, 6.5-7 C/km mid level lapse rates 40kt LLJ and a 75kt mid level jet will support organization. In this case, the strongest dynamics will remain south of the cwa, but will be strong enough to produce a wind and hail threat from roughly 3pm-8pm. SPC has highlighted a Slight Risk for severe storms generally south of I-96 with a Marginal Risk for most of the rest of the cwa. Any lingering showers/storms should be out of the cwa by 06z Wednesday. - Wet and Cool Weather for the end of the week A positive tilted mid level wave will be tracking through the Great Lakes region later Wednesday and into Thursday. PWATs climb as this feature tracks through topping an inch in southern parts of the CWA. This is a good setup for period of steady rain, which will also act to keep the temperatures down. The models do vary considerably with the location and amounts of rain. The GFS has been steady in showing a swath of heavier rain for Muskegon River basin, while the ECMWF and GEM are aiming the precipitation further south and trending further south. Another mid level wave comes down from the Canadian Prairies on Friday night. This wave will bring down another surge of cool air and there could even be some lake effect rain showers with this second wave. While qpf is projected to be light, given the enhancement of the moisture from Lake MI, we will nudge up the POPs for Friday night into Saturday to account for this. With 850 mb temps struggling to top 0 deg C, the daytime temperatures are likely to stay at least several degrees below normal for both Friday and Saturday. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 752 PM EDT Mon May 6 2024 VFR conditions with generally east or southeast winds will continue across the area this evening and overnight. Lower clouds start to move into the area in the morning ahead of a line of showers and possible rumbles of thunder that come off Lake Michigan sometime in the late morning/midday timeframe. Latest model guidane is suggesting these low clouds might stick around for the afternoon even after the rain showers move out, which may limit the strength of some of the thunderstorms that are expected to form in the afternoon - especially down in the Jackson TAF site area. && .MARINE... Issued at 308 PM EDT Mon May 6 2024 Southeast wind gusts will increase to around 20 knots tonight which may touch on advisory criteria for a few hours Tuesday morning, but given the offshore flow and the short duration, we'll hold off on a headline for now. Thunderstorms are possible Tuesday morning and again Tuesday afternoon, which the afternoon storms being potentially strong to severe. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...MJS/04 AVIATION...AMD MARINE...04 ####018006663#### FXUS63 KFSD 070013 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 713 PM CDT Mon May 6 2024 .KEY MESSAGES... - Lines of showers and thunderstorms will move across the forecast area this evening. While an isolated stronger storm is still possible, the risk of severe weather appears to be decreasing. - Periodic rain chances continue for much of the upcoming week into next weekend with the focus on Wednesday. Highs in the 60s to 70s are expected. && .UPDATE... Issued at 713 PM CDT Mon May 6 2024 Still seeing isolated gusts over 40 mph as of 7 PM, but overall expect winds to be on a downward trend through this evening, and thus have allowed the Wind Advisory to expire. That said, with a fairly stout low level jet still over the area, cannot rule out isolated stronger gusts with the showers and thunderstorms moving across the area. While these isolated gusts may be marginally severe (60 MPH), instability across much of our forecast area seems to be lacking given general downtrend in storm intensity as the line of storms in eastern Nebraska moves northeast. Area of greatest concern at this point would seem to be portions of the Highway 20 corridor (Sioux City) and perhaps into Storm Lake. && .DISCUSSION... Issued at 250 PM CDT Mon May 6 2024 Rest of Afternoon and Evening: At a large scale, pronounced mid/upper low is evident on WV imagery across the northern Rockies with surface low deepening across western SD. A warm front is lifting through central NE with a narrow warm sector before a quickly approaching dryline/cold front is crashing eastward. These features will all lift east or northeast into the evening and overnight hours and provide for thunderstorm chances. Severe Storms: Forecast soundings show a ribbon of modest elevated instability lifting from roughly 850:800 mb around 1000 J/kg with higher values as you go south. Surface parcels would obtain closer to 50o J/kg if they can overcome a modest inversion, although current thinking is this may be tough given persistent cloud cover thus far. A robust and dynamic wind field through the entire atmospheric column is helping to drive deep layer shear of 40-50 kts however effective shear for elevated parcels looses a good chunk of the lower directional and speed shear. If a surface parcel can be realized, nearly 30 kts of 0-1 km shear exists with a well curved elongated low level hodograph. With the above stated, well forced dynamics should result in increasing coverage of storms through the 654second half of the afternoon with likely a rapid transition to linear mode. Damaging wind gusts up to 60-70 mph seems the most likely threat with a 50 to 55 kt LLJ at 850 mb. Even less organized convection may be able to tap into this higher momentum air, especially while the boundary layer remains mixed during the daylight hours. Hail would be a secondary risk although again, elevated parcels lack a better effective shear profile. Any tornado threat would be conditional on realizing surface parcels and at this time seems low but should be monitored given the low level wind field. Any more organized bowing segment that can orient from northwest to southeast would also need to be monitored with 0-3 km shear vectors out of the SW AOA 30 kts Heavy Rain: PWATs around 1.25, and near the top of climatology, should provide for a period of heavy rain with the passage of the line of thunderstorms but with a warm cloud layer generally less than 10 kft and the progressive nature of any thunderstorms, any flash flooding threat should remain isolated and probably confined to urban areas. QPF amounts near and east of I-29 are a bit either side of 1-1.5 inches with HREF PMM suggesting pockets of up to 2-2.5 inches, more likely across NW IA. Given saturated soils as of late, renewed issues with ponding of water and river rises will be the more likely impact. Synoptic Wind: Aforementioned deepening surface low to the west will continue to provide for Advisory level wind gusts (45 mph or higher) into the early evening before decreasing BL mixing should result in a decreasing trend closer to sunset. Tonight: After storms exit to our east toward the midnight hour, the remainder of the overnight looks fairly benign. Tuesday and Wednesday: Vorticity ribbons pivot off the filling upper low into midweek. This will bring occasional rain chances with Wednesday currently appearing to be the focus as the main lobe shifts east. With the main upper low overhead, Wednesday would be a timeframe to monitor for slow moving storms capable of locally heavy rain as well at least some potential for low topped funnels. Thursday through Sunday: Transition to upper level northwest flow on the backside of the upper level trough will bring seasonal temperatures for the second half of the week with perhaps a few low chances for a light shower. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 713 PM CDT Mon May 6 2024 Lines of showers and thunderstorms will rotate across the forecast area through this evening. The thunderstorms are expected to push east of the I-29 corridor by 03Z-05Z, and east of the Highway 71 corridor by 05Z-07Z, though an area of showers may linger across southwest Minnesota into the pre-dawn hours of Tuesday. MVFR ceilings and MVFR-locally IFR visibility are expected with the showers and thunderstorms, becoming predominantly VFR behind the lingering showers. Additional stratocu will spread over the region from the northwest on Tuesday. While this is currently expected to be VFR, a brief period of MVFR ceilings is possible. Spotty light showers or an isolated thunderstorm is possible Tuesday afternoon, but chances are too low to include in the TAF at this time. The strongest of the southeasterly gusty winds should diminish with sunset, though occasional gusts over 30kt are still possible this evening. Speeds diminish further as a trough axis slides east behind the showers tonight. Southwest to west winds behind the trough will again increase after daybreak Tuesday with gusts 25-30kt common in most areas, perhaps up to 35kt near/west of the James River Valley. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ UPDATE... DISCUSSION...Kalin AVIATION...JH