Area Forecast Discussion
Issued by NWS North Platte, NE
Issued by NWS North Platte, NE
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367 FXUS63 KLBF 261143 AFDLBF Area Forecast Discussion National Weather Service North Platte NE 643 AM CDT Wed Jun 26 2024 .KEY MESSAGES... - Threat for strong to severe thunderstorms continues with a Slight Risk (level 2 of 5) across the area each day Wednesday and Thursday, and a Marginal Risk (level 1 of 5) for Friday. - Outside of severe chances, recurring rain and thunderstorms will pose a threat of locally heavy rain and at least a low- end risk of flooding for portions of western and north central Nebraska. - Temperatures will generally favor near to below normal values for the end of the week into the upcoming weekend before warming early next week. && .SHORT TERM /THROUGH THURSDAY/... Issued at 423 AM CDT Wed Jun 26 2024 Northerly low-level flow has established itself across the area as surface boundary that was responsible for Tuesday`s convection has cleared the state and currently is tracking into Kansas. Surface high pressure across the far Northern Plains will encroach on the area but will remain north of the area. This should keep the cooler temperatures bottled up to areas north of Interstate 90. As this high pressure translates east, low-level flow will veer to easterly and eventually to southeasterly across much of western Nebraska. This will occur as mid-level height falls begin in earnest later today with an approaching upper trough into the Pacific Northwest. The result will be a flattening of mid-level flow and developing surface low pressure across the central and northern Rockies. With increasing southeasterly flow along the Front Range, orographic lift coincident with improving upper-level dynamics will again promote afternoon and evening thunderstorm potential for much of the region. Afternoon highs will range from the lower 80s along the NE/SD border to low 90s across far southwest Nebraska. The expected southeasterly flow will favor modest moisture recovery with various short-term solutions depicting a return to upper 50s to middle 60s for much of the local forecast area by late afternoon. With mean h5-h7 flow largely out of the southwest, persistence is expected with regards to steep lapse rates or roughly 7-8 degC/km. Overtop adequate low- level moisture, instability should quickly build this afternoon with MLCAPE values climbing into the 1000-2000 j/kg range. The southeasterly surface flow and almost due westerly h5 flow will combine to yield sufficient deep layer shear for updraft organization as 0-6km BWD values reach 40-50 knots. Capping should preclude more widespread coverage, at least initially. This cap should be overcome in closer proximity to the Front Range for reasons previously mentioned, however, with thunderstorm chances arriving in the local area as early as late afternoon but likely holding off until the evening. Forecast soundings show limited directional shear early on with long straight hodographs. This would favor splitting supercells with large hail begin the greatest concern. Eventually, with merging cold pools within deeply inverted- v profiles, will likely see a transition to more damaging winds in the evening. This is also supported by 1000+ j/kg DCAPE and 0-3km delta theta-e values approaching -25 degC. Even with increasing flow off the surface leading to large, looping hodographs in the evening, the lack of appreciable instability with meager lapse rates arriving should preclude any long lasting severe threat as well as any tornado threat. Latest HREF guidance indicates the greatest signal for more robust thunderstorm activity to be located across southeast Wyoming into the Nebraska Panhandle between 06/26 23z and 06/27 05z (or 6pm to Midnight CDT Wednesday). Subsequent runs of individual CAMS echo this idea fairly well. The latest Storm Prediction Center (SPC) outlook for Wednesday shows a Slight Risk (level 2 of 5) stretching along an Ellsworth to Palisade line and points west. A Marginal Risk (level 1 of 5) buffer extends from Merriman to Halsey to Oconto and west. In general agreement with this placement though will need to be mindful of potential northward expansion with subsequent updates. Increased capping and decreased instability with eastward progression should allow the severe threat to wane quickly through the evening, especially with the lack of more appreciable theta-e advection in the low/mid-level. Lows tonight, under mostly cloudy skies with scattered rain and thunderstorms, should only reach the 60s which will be around 5 to 10 degrees above normal for late June. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Issued at 423 AM CDT Wed Jun 26 2024 Thursday...Fairly notable uncertainties exist regarding Thursday across the region. Northern stream disturbance will continue to dive southeast out of the Pacific Northwest, bringing continued strong upper-level dynamics supporting additional rain and thunderstorm chances across the area. Deepening surface low across southeast Montana will promote strong southerly flow through much of the day as the area becomes wedged between the surface low and developing lee trough and stagnant high pressure anchored near Lake Michigan. This strong southerly flow will support continued WAA and low-level moisture advection. Various NWP solutions depict low-level fog within the moisture advection with a transition to stratus that lingers through much of the day. Latest HREF guidance does suggest mostly cloudy conditions in the morning but a gradual waning of cloud cover probabilities through the afternoon. Given fairly high disparity between various solutions, NBM percentile spreads for daytime highs remain large given only 24 to 36 hours out. For now, utilized a mix of NBM 50th Percentile and CONSRaw guidance plus bias correction. This lowered forecast highs slightly in the area with middle 70s in our northeast and upper 80s to low 90s in our west to far southwest. This generally lines up with Tuesday evening MAV guidance and roughly averages out the MET/ECS guidance, the former suggesting more widespread 70s and the latter showing more widespread 80s to middle 90s. Given discrepancies in forecast highs, this will play a role in magnitude of instability during the afternoon. With continued modest h5 flow overtop nearly southerly low-level flow, deep layer shear will again be supportive of organized convection, but with spreads in HREF members of 750-1200 j/kg of SBCAPE questions remain about magnitude of severe threat. Latest SPC outlook highlights the entire forecast area in a Slight Risk (level 2 of 5) and believe the ceiling of this setup, should the higher instability values be realized, would more than support it. This would include the threat of large hail, damaging wind gusts, and a tornado or two possible as the evening LLJ increases markedly which will elongate hodographs within decreasing LCLs. Will need to closely monitor this potential with subsequent forecasts. Storms, regardless of severe or not, will be occurring in an anomalously moist environment with forecast soundings depicting deep saturation with skinny CAPE profiles and deep warm cloud layers. While storms will likely remain somewhat progressive with anticipated storm motions around 20 to 30 knots should offset a more widespread flooding concern, recent heavy rainfall in the past 14 days will make a few areas more susceptible to a flooding risk as QPF values range from 0.50-1.00" with locally higher amounts possible. Latest Weather Prediction Center Excessive Rainfall Outlook highlights much of our eastern zones in a Marginal Risk (level 1 of 4) with a Slight Risk (level 2 of 4) clipping Frontier/Custer Counties. Trends will be monitored with later forecasts to hone in on where the heavier QPF is likely to set up. Friday and beyond...trough axis will slide through the northern and central plains during the day on Friday, though discrepancies remain between deterministic solutions. The northern stream system will drop a cool front south through the area with timing of frontal passage favoring sometime early morning through early afternoon. Even with this relatively unfavorable timing, moisture pooling within northeasterly flow behind this front should allow for another day of moderate instability underneath a belt of enhanced mid-level flow. This will support another severe weather threat with the latest outlook from the SPC highlighting much of the area in a Marginal Risk (level 1 of 5). The greatest severe threat will likely reside in the warm sector which will be well south and east of the local area, however, believe enough of a local threat exists that have no qualms with SPC thinking. Temperatures will again range from the lower 80s to lower 90s north to south. For the following weekend, expect steady westerly to quasi-northwesterly flow to remain established across the central and northern plains. Upper- level ridging will strengthen across the southern Plains with increasing heights aloft locally. Continued southerly low-level flow should support at least another low-level moisture to feed modest instability which would allow low-end precipitation chances to continue daily. It isn`t until early next week around Monday or Tuesday that a larger system departs the Pacific Northwest and ejects onto the central and northern Plains. Overall agreement is fairly good on timing and placement of greatest upper-level features. This points to cooler temperatures and increased precipitation chances to our north with some questions of how far south these extend. For now, believe the inherited forecast hedging the higher side of the forecast envelope with temperatures is the way to go. While precipitation chances will be prevalent, day to day predictability outside of Monday and Tuesday will be fairly limited confidence wise. && .AVIATION /12Z TAFS THROUGH 12Z THURSDAY/... Issued at 642 AM CDT Wed Jun 26 2024 Mostly quiet aviation conditions are expected over the next day with the exception of some isolated thunderstorms possible during the evening across portions of the Panhandle and southwest Nebraska. Confidence remains low in development and location, however, KLBF has the best chance to see some scattered convection during the late afternoon and evening hours. Storms should come to an end by late evening, however, ceilings remain at or below 5000 feet through 12Z Thursday. && .LBF WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...NMJ LONG TERM...NMJ AVIATION...Kulik