Area Forecast Discussion
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Issued by NWS
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624 FXUS64 KLUB 181731 AFDLUB Area Forecast Discussion National Weather Service Lubbock TX 1231 PM CDT Wed Sep 18 2024 ...New AVIATION... .SHORT TERM... (Today and tonight) Issued at 316 AM CDT Wed Sep 18 2024 Satellite imagery at 2 AM revealed the tail-end of an upper trough lifting northeast across the Panhandles well south of a stacked cyclone in southern Montana. In the wake of this trough, modest height rises will unfold over our area today within weak westerly flow aloft. Low-level moisture remains respectable today on S-SE flow ahead of a surface trough and diffuse dryline that shifts to near an AMA-HOB line by mid-afternoon. Finer res models are in good agreement that temps in the low/mid 90s near the boundary will push CIN to negligible values, so NBM`s dry PoPs won`t make the cut today. Combined with MLCAPEs around 1200 J/kg and upwards of 35 knots of cloud depth shear, the stage will again be set for another round of severe storms including supercells. Higher LCLs today will fuel even greater DCAPEs in excess of 1500 J/kg, so downbursts remain the primary concern followed by some severe hail potential. Limited background forcing looks to keep most storms from running too far beyond sunset, so will keep PoPs dry after 06Z. Another mild and rather muggy night awaits on steady S-SE winds with clearing skies. && .LONG TERM... (Thursday through Tuesday) Issued at 316 AM CDT Wed Sep 18 2024 The mid/upper-level pattern will remain amplified at the start of the period as the cyclonic gyre located over the western U.S. begins to dissipate. Dissipation of this gyre will occur after the ejection of the negatively-tilted trough pivoting over the northern Great Plains as it becomes absorbed into a longer-wave trough over far northern Canada. Farther southwest, a positively-tilted trough will continue to dig into the Pacific coastal region and begin pivoting over the Sierra Nevada Mountains as a mid- and high-level closed low forms; and will be accompanied by 250 mb and 500 mb jet streaks near 100 kt and 50 kt, respectively, rounding its base. The blocking pattern over the Lower 48 will remain somewhat intact as a remnant, post-tropical low over the eastern U.S. becomes absorbed into an open trough, while the center of the subtropical ridge over central Mexico shifts northward into southern Texas via modulation of the amplifying trough to the west. Large-scale subsidence in the mid- levels will increase further from the prior day, with a nearly 100 mb deep subsidence layer advecting over the CWA as the northern periphery of the subtropical ridge eclipses West Texas. At the surface, a quasi-stationary front is forecast to be located west of the TX/NM state line, with a weak, lee cyclone rotating across southeastern New Mexico and the dryline extending southward from the low. The quasi-stationary front should bend northeastward into the TX PH/north of the CWA, connecting to another lee cyclone along the KS/OK state line before branching northward as an occluded front that extends to the vertically-stacked low in southern Canada. The CWA will remain in the moist sector on Thursday, and despite the positive geopotential height tendencies, leeward pressure falls will be maintained with surface winds remaining slightly veered of south. Intense heating is expected with a deeply-mixed boundary-layer profile developing, with PBL heights ascending to above 700 mb before encountering the deep subsidence layer aloft. Convergence at the surface and low-levels is forecast to be displaced to the west and north of the CWA (e.g., nearest the front), and while the development of a cu field should occur by peak heating, the lack of foci for convergence and the magnitude of the mid-level subsidence layer will prevent cu from transitioning into cBs. The forecast remains dry for Thursday despite the QPF signal from the HREF as all indications otherwise point towards the suppression of deep convection. The positively-tilted trough to the west of the CWA is forecast to pivot into the Desert Southwest by late Friday, further modulating the amplitude and position of the subtropical ridge over the State of Texas. There remains excellent agreement among the guidance with respect to the superposition of the 250 mb and 500 mb jet streaks over the Intermountain West and into the northern Great Plains, as these features will become elongated via the effects of coupling from a northern-stream jet streak emerging over southern British Columbia. The subtropical ridge over the region will attain a southwest-to-northeast-oriented position as the upstream trough begins its transition to a neutral-tilt. Scattered-to-numerous thunderstorms should form along the lee of the southern Rocky Mountains, but the south-southwesterly-to-southwesterly component of the steering flow is expected to keep convection west of the TX/NM state line on Friday. PoPs were removed for Friday evening storms should be delayed in arriving into the CWA until the early morning hours Saturday as geopotential heights begin to fall. PoPs generated by the NBM have increased on Saturday compared to the previous assessments, and further adjustments to the PoPs were made to expand the 40-percent contour into the I-27/HWY-87 corridors as confidence continues to increase in at least scattered convection forming across the Caprock Escarpment on Saturday. By this time, the neutrally-tilted trough will begin to emerge over the southern Rocky Mountains while the mid- and high-level closed low remains intact, with indications of the trough becoming negatively-tilted by the afternoon hours. Rapid-intensification of the 250 mb flow will occur throughout a 12-hour period as the core of the related 100 kt jet streak translates over northeastern New Mexico, resulting in intense divergence and isentropic ascent. Low-level convergence and ascent will be further aided by frontogenetical forcing from the cold front to the west, with an expectation for scattered to potentially numerous thunderstorms to develop and form into a broken line on Saturday afternoon across the Caprock Escarpment. Some storms may be severe on Saturday, with a primary hazard of damaging wind gusts. It appears that the highest coverage of storms should be across the far southern TX PH, with storms becoming more-scattered with southward extent as large-scale forcing for ascent lessens due to the negatively-tilted trough pivoting poleward into the west-central Great Plains heading into Saturday night. A further increase in PoPs may be necessary in forthcoming forecast cycles as mesoscale guidance comes into range. The system is forecast to dampen in amplitude heading into Sunday as begins to dissolve into an open trough over the central Great Plains. The position of the cold front may be farther southeast than what the current NWP guidance is indicating, especially due to the likelihood of its movement being convectively-reinforced by outflow from the night prior. The dry forecast generated by the NBM has been maintained, but depending on how incoming guidance handles the frontal position, an introduction of low PoPs may be necessary for the southeastern zones on Sunday. Thereafter, global NWP guidance has significantly diverged on the state of the upper air pattern for early next week compared to the previous prognostications. Low PoPs are reflected in the forecast by mid-week, and have been maintained as predictability becomes limited by the end of the forecast period. Sincavage && .AVIATION... (18Z TAFS) Issued at 1227 PM CDT Wed Sep 18 2024 VFR expected to prevail through this TAF period. Potential still exists for some iso-sct TSRA to develop during the mid to late afternoon hours mainly north of a line from LBB to CDS, but confidence in eventual areal coverage of this convection remains too low to include TS mention in any TAF at this time. Strong and erratic downburst winds should be expected in the vicinity of any TS, with convection diminishing after sunset and VFR continuing overnight through Thursday. && .LUB WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...93 LONG TERM....09 AVIATION...30