Area Forecast Discussion
Issued by NWS North Platte, NE
Issued by NWS North Platte, NE
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867 FXUS63 KLBF 171744 AFDLBF Area Forecast Discussion National Weather Service North Platte NE 1244 PM CDT Mon Jun 17 2024 .KEY MESSAGES... - Lingering thunderstorms early this morning will continue to pose a risk of locally heavy rainfall across northern Nebraska, before exiting the area by early this afternoon. - Redevelopment of isolated to widely scattered thunderstorms this afternoon and evening will lead to an increasing threat for severe weather, with very large hail, damaging winds, and a tornado or two possible. - Additional rounds of thunderstorms will persist nearly each day into the end of the week, with at least some continuing threat for strong to severe thunderstorms by late week. && .SHORT TERM /THROUGH TONIGHT/... Issued at 404 AM CDT Mon Jun 17 2024 Currently, an ongoing MCS continues to move eastward across the Sandhills, with an east to west oriented band of robust convection centered north of HWY 20 in Keya Paha and Boyd counties. This line has primarily brought locally heavy rainfall, though multiple gusts of around 40 to 45 miles per hour have been observed as well. This line and its associated outflow will continue to push east early this morning, gradually exiting the area through late morning. Attention then turns to a round of potentially significant severe weather across much of western and north central Nebraska tonight. For the near term, will have to watch the progression of the outflow from this morning`s storms, currently stretching from central into portions of southwest Nebraska. It is uncertain how this outflow will impact the progression of a frontal boundary, currently bisecting from just south of IML to south of OFK, as it slowly lifts northward this afternoon. Given the strong warm advection across the broad warm sector in south central Nebraska, not anticipating the outflow boundary to dampen the front southward much this morning. Aloft, southwesterly flow prevails, with an upper low anticipated to slide eastward into the Intermountain West into this evening. Broad positive vorticity advection and subtle height falls are expected to overspread the area through the day today, with nothing in the way of a more obvious shortwave trough upstream. At the surface, broad lee cyclogenesis will persist into this afternoon across eastern Colorado, with the surface low expected to gradually eject northeastward towards southwest Nebraska by tonight. In response to the increasing surface cyclogenesis, increasing pressure falls and strengthening warm advection should promote the aforementioned surface front to lift northward towards the Sandhills into this afternoon. Along and south of this boundary, dewpoints in the upper 60s to low 70s are anticipated, along with surface temperatures warming into the 90s. Lapse rates aloft will remain very steep, and in combination with the warm/moist boundary layer will send MLCAPE to above 3000J/kg for areas south of the frontal boundary. Moisture pooling near the boundary may promote locally higher instability as well. As temperatures warm this afternoon, residual CIN should erode across the warm sector, with forecast soundings suggesting a largely uncapped environment by late afternoon. That said, a lack of upper level support should limit robust convective initiation to locally enhanced convergence near the aforementioned surface boundary and potentially just ahead of the northeastward lifting surface low in northeast Colorado. This should lead to isolated to widely scattered convective development this afternoon across the area, in closer proximity to surface boundaries. Initial updrafts may struggle owing to dry LFC-LCL relative humidity, and multiple CI attempts may be needed for an updraft to eventually become dominant. This will be aided where TCU can cluster and better shield themselves from dry air entrainment, likely again near the surface front. Mesoscale details will drive the zone of greatest threat today, and boundary progression will need to be monitored closely. Guidance currently paints this front reaching somewhere near the HWY 2 to HWY 20 corridor by late this evening, though will at least mention there has been a gradual southward trend noted in high- res solutions over the last 12-24hrs. Should robust CI occur, the environment will be very supportive of rotating updrafts, with forecast soundings indicating near pure streamwise vorticity in the lowest few kilometers of the hodograph by late evening. This is in combination of strengthening backed southeasterly surface winds (primarily in closer proximity to the surface front) and an increasing southerly LLJ (H85 flow ~40-50kts) that should overspread the Plains near/after sunset. Broad southwesterly flow in the mid-levels will lengthen hodographs with height somewhat as well. This points towards a corridor of all severe hazards should robust supercellular development occur this evening near the surface front. Any tornado threat will likely be maximized near the front should a storm anchor nearby. Instances of large to very large hail will likely accompany any dominant supercells as well, both near the front and further south. It appears there will be at least a short window where storms could maximize a robust severe weather threat, before CIN quickly increases again near/after sunset and updrafts will have a propensity to become quickly elevated. The better signal for CI with respect to high-res guidance is across far northeast CO ahead of the surface low, with this convection lifting into southwest NE and presenting a threat for large hail/damaging winds. Lesser signal exists for robust CI along the surface front. With largely subtle forcing, mesoscale details will drive the difference between just some cumulus development and robust supercells. This will continue to be monitored very closely, and future forecasts should be monitored closely. Convection will largely weaken after sunset and exit the area prior to midnight. The deepening surface low will keep warm advection strong, and should promote increasing synoptic winds overnight. In fact, some high-res guidance paints southerly wind gusts even as high as greater than 50mph overnight. This is largely due to enough mechanical mixing to tap into the stronger southerlies aloft, and confidence in this is not high for now. Still, a period of 40-45mph gusts look likely tonight, and should keep temperatures very mild in the upper 60s to low 70s across southwest and central Nebraska, where they will remain south of the front. The boundary layer looks to decouple to the north of the front, leading to lows in the upper 50s to low 60s across northwest Nebraska. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Issued at 404 AM CDT Mon Jun 17 2024 A surface cold front will quickly push through the area Tuesday morning, as the aforementioned surface low ejects into eastern SD. This front largely looks to clear the area by tomorrow afternoon, keeping a renewed round of convection off to the east. Still, at least some guidance suggests the frontal boundary is slow enough to keep a threat for scattered thunderstorms across far eastern Custer and Wheeler counties. Confidence in the timing of the front remains lowered, and will be monitored going forward. A threat for large hail and damaging winds would accompany storms should the front slow enough to impact more of central and north central Nebraska. The area remains post-frontal on Wednesday and increased cold advection behind the boundary should lead to a day of well below average temperatures in the upper 60s to low 70s. The return of southerly flow late this week should bring with it returning thunderstorm chances, and these look to persist into the weekend. Aloft west-southwest flow late week gives way to northwesterly flow this weekend, and at least some threat for severe weather should return into this weekend. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1232 PM CDT Mon Jun 17 2024 KLBF will shortly be on the warm side of a front lying east to west across the region, with clouds scattering out and CIGs trending towards VFR, continuing through much of the period. Expect KVTN will remain on the cool side of the front, keeping IFR CIGs over the terminal through this evening. Around 21Z, expect CIGs at KVTN will trend to MVFR or VFR. A highly conditional threat for thunderstorms is in place for later this afternoon through the evening. Exact timing/location of convective initiation remains uncertain, thus will leave out mention at both KLBF and KVTN. Future amendments and inclusion may be needed with subsequent TAF issuances. Winds have begun to amplify this afternoon and will become quite robust this evening with gusts 30 to 40kt, especially after 00Z. At the same time LLWS will increase considerably out of the south-southwest at 55 to 65kt and will continue through the 06 to 09Z timeframe. As the front transitions to a cold front, it will sweep from northwest to southeast, resulting in winds to veer towards the northwest around sunrise on Tuesday. && .LBF WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...Brown LONG TERM...Brown AVIATION...Viken