Area Forecast Discussion
Issued by NWS North Platte, NE
Issued by NWS North Platte, NE
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650 FXUS63 KLBF 171708 AFDLBF Area Forecast Discussion National Weather Service North Platte NE 1208 PM CDT Tue Sep 17 2024 .KEY MESSAGES... - Storm Prediction Center has upped the latest outlook for Tuesday to an Enhanced Risk (level 3 of 5) for areas west of Highway 61 with a Slight Risk (level 2 of 5) extending east to Highway 83. - Strong to severe thunderstorms are expected to roll off the higher terrain late today with the primary threat being damaging wind gusts of up to 80 mph and secondary threats of isolated large hail and heavy rain. - Strong southerly winds gusting up to 50 mph across much of the region Tuesday will promote elevated fire weather conditions. - Active weather looks to return this weekend into early next week as another trough within southwest flow aloft tracks across the area. At this time, precise details remain somewhat uncertain. && .SHORT TERM /THROUGH TONIGHT/... Issued at 424 AM CDT Tue Sep 17 2024 Morning satellite analysis shows broad southwesterly flow across the central CONUS as deep troughing continues to track through the Great Basin. Further east, broad southeast ridging remains in place with an occluding system situated across the Carolina`s impeding its influence for a select few stretching from the Smokey Mountains through the Delmarva Peninsula. Through today, the deep h5 cutoff centered over Nevada will begin to quickly lift east-northeast through western Wyoming and onto the Northern Plains by late tonight. Upper-level dynamics remain strong with this feature: strong 6-10 decameter (dam)/12 hours height falls will increase to 8-14 dam by late tonight as the main PV anomaly tracks across the Bighorns of northern Wyoming and into the western Dakotas by early Wednesday. Rapid cyclogenesis under strong upper- level divergence will foster a sub-995 hPa low tracking southwest to northeast out of the central Rockies and through the Nebraska Panhandle. NAEFS advertises this as below the 5th percentile and generally only something the area sees once every 2 to 5 years according to the Return Interval values. With deepening low pressure in the lee of the Rockies but broad surface high pressure further east, a constricting pressure gradient will yield strong southerly winds through the day. Already this morning we`ve seen gusts nearing 40 mph with VWP data indicating strong flow off the surface that will likely continue through the day. An area of concentrated WAA is helping drive some isolated rain showers over central Nebraska. Various short-term guidance is in good agreement that this activity will be quick to depart to the north and east, leaving the bulk of the daytime dry. With the strong southerly flow, moderate mixing should be ample to tap into anomalous h85 and h7 temperatures with nearly a 3-5 degC climb from Monday`s values. This lends confidence in boosting today`s forecast highs into the upper 80s to middle 90s across the area. With the strong winds and afternoon temperatures nearing 15 to 20 degF above normal, am concerned about a fire weather threat today. While afternoon humidity values look to remain above critical levels, dried fuels and strong winds would be favorable for the erratic and rapid spread of any fire starts so extreme caution is advised. Utilized a blend of NBM90 for wind gust speeds which closely lined up with momentum transfer of multiple NWP solutions. Forecast soundings all generally highlight mixing up to h7, or about 2km up, with an increasing LLJ late in the day supporting gusts in the 40 to 50 mph range for late afternoon and early evening. While strong kinematics and the subsequent mixing will work against the moisture influx, afternoon dew points will still range from the middle 50s west to middle 60s east. As steep lapse rates remain in place aloft, ample instability will build during the day with MLCAPE values progged to reach at or near 1500 j/kg along a narrow corridor in our western zones. H5 flow will ramp up at the same time, with an increasingly sheared environment through the latter half of the day. Inhibiting this somewhat will be the negative tilt of the h5 trough and the highly meridional flow aloft. All this is moot however as a cold front quickly dives east from Wyoming and into west central Nebraska by this evening. Shear will be maximized in close proximity to this boundary and will be more than enough for organized convection. While the increasing LLJ will lead to long/looping hodographs in the 0-3km layer the modestly high LCLs, lack of line normal 0-3km shear vectors, and expected rapid progression of cold pools limits confidence in seeing a tornado threat materialize. As mentioned though, merging cold pools along and ahead of strong frontal forcing should lead to a primarily damaging wind threat. This is the main driver for SPC`s upgrade to an Enhanced Risk, primarily west of Highway 61. Dry sub-cloud layers with inverted-v profiles will be a good environment for merging cold pools and the damaging wind gusts. Can`t rule out a few gusts nearing 80 mph and indeed, various CAM solutions depict just this for the Nebraska Panhandle and far northeast Colorado. While hail is always a possibility with any severe storm, maximum expected hail size will be more limited owing to expected storm mode, marginal instability aloft, and lack of greater shear aloft. Still, can`t rule out a few storms capable of Ping Pong ball size hail but most reports will likely remain at 1.00" in diameter or less. Storms will be fairly progressive, with anticipated storm motions of 20-30 mph. This should limit the heavy rain threat overall even though PWAT values remain anomalously high. Forecast values surpassing 1.00" are nearing +2-3 sigma in NAEFS climatology. Higher NBM percentile values suggest some areas could surpass 1.00" and the HREF generally agrees with this potential remaining on an isolated basis. While ensemble mean values are generally limited to a half inch or less, Probability Matched Mean values show more coverage of 0.75-1.25" amounts for the late evening. All this to say, flooding is not expected to be a major issue but a few instances of low-land flooding cannot be ruled out. Behind the sweeping cold front, drier air will settle into our western zones while low-level mugginess lingers overnight into early Wednesday for our east. As lapse rates aloft diminish, convection should wane by Midnight and through Wednesday morning. HREF guidance echoes these thoughts well with decreasing probabilities of > 40 dbz towards sunrise. The going forecast has a dry forecast by sunrise Wednesday for areas west of Highway 183 but maintains < 20% PoPs for areas east into the daytime. With the expected clouds and breezy conditions, lows will struggle to fall much with forecast values ranging from the middle 60s east to low 50s in the far west. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Issued at 424 AM CDT Tue Sep 17 2024 Wednesday...A dry forecast for most is expected though lingering low- level moisture ahead of the stalled frontal boundary will be a feature to watch for central and eastern Nebraska. Deep surface low will drift across eastern Montana and into North Dakota with and occluded front stretching north and east and the cold front splitting the Plains near the Highway 281 corridor by late in the day. To the west of this, dew points will fall into the upper 30s to low 50s. This will likely preclude any instability concerns as high temperatures only climb into the middle 80s. Further east, MUCAPE values appear likely to reach the 1500-2500 j/kg range but upper- level dynamics having pushed further north will limit overall shear and forcing in the area. Some CAM guidance does manage to convect during the afternoon but all advertise east of our forecast area so will maintain a dry forecast for most if not all locations. Thursday and beyond...a brief period of dry weather returns to the forecast. Southern Plains ridging will build north into the area and subsequent height rises will quell activity across the region. PoPs increase slightly late Friday into Saturday as moisture recovers on the backside of departing weak high pressure. NBM 90th percentile values paint QPF across much of the area Friday night into early Saturday. This is as the next mid-level disturbance tracks out of the southwest and onto the Central and Southern Plains. Variability in NWP guidance casts some doubts on precise evolution of this system, especially as the preceding system looks to linger across the Canadian Prairies if not the far Northern Plains. Ensemble guidance varies considerably by early next week as well, further casting doubt on expected outcome. General thoughts are for a progressive h5 low to track out of the central Rockies and into the local area beginning Saturday. Progression may be stymied by downstream disturbance so track may be slow through the area. This supports a long duration rain event across at least portions of the area along with cooler temperatures. NBM probabilities of > 0.25" in a 24 hour period increase quickly through the day Saturday and peak Sunday morning (for the previous 24 hours) across eastern Nebraska where a bullseye of 70% centers over the LNK/OMA corridor. While values decrease with westward extent, probabilities still exceed 50/50 for much of the local area so potential remains nominal locally. NBM probabilities decrease through the day Monday and this coincides with the greatest h5 negative anomalies quickly filling in across the central CONUS. Thereafter, uncertainties exist with evolution of upper-level pattern. Generalizing, ensemble solutions suggest ridging in the west and troughing into the east with trends strengthening each respective feature. This favors above normal temperatures and drier than normal conditions, which closely matches the latest Climate Prediction Center 6-10 and 8-14 day outlooks. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 1205 PM CDT Tue Sep 17 2024 Primary aviation concerns this period will be strong southerly winds ahead of an approaching cold front, followed by potential for severe storms this evening. Expect winds will be increasing through this afternoon before peaking at TAF sites just ahead of thunderstorm arrival, generally in the 35 to 40kt range. Given strength of gusts at the surface will omit LLWS from the TAFs as winds aloft are not much stronger. Will use a 02Z-04Z window for timing arrival of thunderstorms and potentially severe outflow winds with a quick swing of direction to the west. Will reevaluate timing and peak winds once radar trends become established. Will bring conditions to MVFR with passage of the thunderstorms though a brief period of IFR is possible, especially given potential for blowing dust to accompany arrival of outflow winds. Favorable flight conditions return by daybreak with VFR expected through the latter portion of the valid period. && .LBF WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...NMJ LONG TERM...NMJ AVIATION...MBS