Area Forecast Discussion
Issued by NWS Hastings, NE
Issued by NWS Hastings, NE
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847 FXUS63 KGID 251031 AFDGID Area Forecast Discussion National Weather Service Hastings NE 531 AM CDT Sat May 25 2024 .KEY MESSAGES... - Mainly dry, mild and breezy to start the holiday weekend today, but chances for thunderstorms increase to 20-50% this evening, and 40-70% overnight. - Some of this activity may be strong to severe with risk levels ranging from Marginal N of Hwy 91 (level 1 of 5) to Enhanced S of Hwy 24 (Level 3 of 5) per the SPC. Main concerns are large hail and damaging wind gusts, though isolated tornadoes and flash flooding are also possible. - Isolated to scattered non-severe thunderstorms may redevelop Sunday afternoon, but Memorial Day itself looks to be dry, albeit a bit breezy out of the NW. - Dry conditions continue through Tuesday daytime, but off and on thunderstorm chances return Tuesday night and continue through the end of the week. && .DISCUSSION... Issued at 515 AM CDT Sat May 25 2024 Today-Tonight: Inserted some mention of sprinkles for various portions of the forecast area this morning, but most areas will just experience some incr mid to high clds and incr Srly flow. Gusts up to around 30 MPH are possible throughout the day, which will make the water on area lakes a bit choppy. Highs should range from mid 70s N where clds will be thickest, to low to mid 80s far S, closer to retreating warm front and thinner high clouds. Outside of the rogue sprinkles, not seeing much for chances of rain before 5pm - so not a bad start to the extended weekend. Main focus of the short term period will be incr chcs for thunderstorms this evening, beginning around 6-8PM W/SW of the Tri-Cities, then 9PM-midnight for the rest of the forecast area. So the majority of the severe threat will come around or after sunset. Models remain in good agreement of the general pattern evolution today, but vary with some key details that leads to considerable uncertainty on exact coverage and severity. There`s been little to no change in the convective outlook from the SPC and risk levels still vary from Marginal Risk (level 1 of 5) over the N, to Enhanced Risk (level 3 of 5) closer to Hwy 24 and I-70. Still think the main risks will be large hail up to golf ball size with initial and/or discrete activity, then transitioning mainly damaging wind gusts of 60-70 MPH from possible upscale growth into lines or clusters. Isolated tornadoes will be possible with possible discrete supercells in the Enhanced Risk area this evening, OR from QLCS spinups along potential strengthening line of storms that moves W to E overnight - somewhat similar to Thu night. Heavy rain is not a widespread concern and most activity should move along at a decent clip, but can`t rule out some isolated/minor flooding over far SE zones. Most thunderstorm activity should end from W to E around 06-09Z. With the general forecast thoughts covered above, will attempt to provide some greater details of possible scenarios on convective evolution. Will start by saying that this still appears to be a fairly late show for the local forecast area due to timing of upper shortwave and delayed return of moisture. I think our severe threat will emanate from some sort of combination of two convective "regimes": 1) discrete or semi- discrete supercells developing along retreating warm front over central KS and 2) gradual upscale growth of convection that originates over the High Plains of NE Colorado and Nebraska Panhandle, then moves E/ESE along nose in intensifying LLJ and theta-e advection. 1) This regime is probably the more uncertain of the two given model discrepancies in the latitudinal placement of the retreating warm front and associated timing of moisture return as it relates to the arrival of incoming shortwave. Various CAMs offer a relatively small window from around 00Z to 03Z for possible iso supercell development somewhere in the area of I-70 to around Hwy 24. If a storm or two can develop, then supercells are likely - possibly quite intense - given combination of strong deep layer shear and moderate instability on the nose of sfc dew points incr from the 50s into at least low-mid 60s. Margin of error so to speak for this scenario is small as sfc based parcels will probably remain capped beneath H7 temps 9-11C if moisture return is delayed even slightly as nocturnal BL cooling will set in especially beyond 03Z. However, this time frame also offers a potentially significant juxtaposition in time and space of enlarging low level hodographs in response to backed sfc flow and developing LLJ, lowering LCLs, and arrival of better upper ascent - all potentially favorable for very large hail and a few tornadoes. Gut feeling is that this regime will probably remain S/SE of the forecast area, but it could be close if CAM solutions such as 00Z HRRR/ARW/NSSL are accurate. 2) This regime feels like a safer bet as nearly all CAM guidance depicts at least scattered high-based convection developing over the High Plains in environment of very steep low to mid level lapse rates on the nose of sfc thermal ridge and beneath increasingly favorable upper forcing/height falls. Seems probable this activity would then propagate to the E/ESE into central NE/KS during the late eve and overnight within zone of strengthening theta-e advection on nose of LLJ. Magnitude of severe potential from this regime is a bit uncertain as MLCAPE will be limited initially by poor moisture and the better instability doesn`t arrive until around or after 03Z for mainly slightly elevated parcels. However, if a significant cold pool develops, then an organized MCS/QLCS could remain sfc based while tapping into the greater theta-e with time and SEward extent, as well as enlarging hodographs thanks to LLJ. Again, think some version of this regime is most likely, with the main lingering questions being the magnitude/coverage of mainly a wind threat. Similar to Thu night, though, some QLCS tornadoes could also be possible if the line remains sfc based given favorable 0-3km shear/CAPE. Regardless, with it being an extended weekend at the beginning of summer, there will likely be increased activities at area lakes/campgrounds which can be especially vulnerable to overnight intense convection with tornado/damaging wind risk. The forecast will continue to update/refine throughout the day, so those folks will want to remain informed to those updates. Rest of the Holiday Weekend: Most areas should wake up to dry conditions Sunday AM, but redevelopment of at least iso to widely scattered thunderstorms appears probable by early aftn as area remains under the influence of upper shortwave and cool mid level temps will support uncapped, tall/skinny CAPE. Deep layer wind shear is quite weak, so not expecting anything more than heavy downpours. Model guidance is mixed on coverage and W/SW extent of the potential, but all areas will have at least a chc, and this could again affect outdoor activities with a lightning threat. Cloud and cool NW flow will keep temps in the 70s, perhaps even 60s if the aftn convection is more than isolated. Fortunately, appears the rest of the holiday weekend will be dry and seasonably mild, though a bit on the brzy side Memorial Day Monday, which could affect those trying to get out on the lakes. Rest of the Forecast: Dry conditions continue through the daytime Tuesday with mild temps in the 70s/80s. Warm air advection could kick off some showers/elevated storms Tue night for areas particularly W/S of the Tri-Cities. This looks to mark the beginning of another modestly active period of off and on thunderstorm chances that continue through the end of the week. However, any significant return of instability looks to hold off until at least Thu - and no particular time frame looks unusually ominous from a severe weather perspective. A stronger trough/frontal passage is progged in the ensemble guidance for sometime Fri-Sat, which should provide the greatest chance/coverage for showers and thunderstorms in the days 3-8 time frame. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1245 AM CDT Sat May 25 2024 VFR forecast through the period. Clear skies currently will give way to incr high clds through the morning. SCT to BKN high clouds expected much of the day. Scattered showers and thunderstorms will become possible around and esp. after sunset as a cold front moves in from the NW. Have introduced VCSH around 03Z-04Z for now as coverage and exact timing is still uncertain. Lgt Srly flow now will give way to breezy conditions after sunrise. Sustained winds will incr to 15-20kt, with gusts up to around 30kt. Winds will back to the SE and E Sat eve ahead of the cold front. Confidence: Medium. && .GID WATCHES/WARNINGS/ADVISORIES... NE...None. KS...None. && $$ DISCUSSION...Thies AVIATION...Thies