Area Forecast Discussion
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Issued by NWS
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284 FXUS64 KLUB 011911 AFDLUB Area Forecast Discussion National Weather Service Lubbock TX 211 PM CDT Wed May 1 2024 ...New SHORT TERM, LONG TERM... .MESOSCALE UPDATE FOR AFTERNOON SEVERE STORMS... Issued at 1057 AM CDT Wed May 1 2024 Upper air analysis this late morning depicts a shortwave trough digging into the north-central Rocky Mountains, with a well-defined vorticity lobe rounding the base of the trough and beginning to eject into the west-central Great Plains. Farther south, a subtle, shortwave perturbation was analyzed over northern Mexico, and was steadily moving northeastward towards the Big Bend region while the mid/upper-levels are otherwise bereft of any noticeable feature on the water vapor bands. The 12Z objectively analyzed upper air data depicted the left-exit region of the 250 mb jet eclipsing southern New Mexico, evident by a 50 kt wind maximum observed on the EPZ RAOB this morning while the primary jet streak near 80 kt was located further south per the MMCU RAOB (Chihuahua International Airport). Upstream RAOBs from ABQ and EPZ also indicated a substantial elevated mixed layer (EML), and the 12Z AMA RAOB also sampled the EML as the balloon was launched within the dryline circulation. Meanwhile, the 12Z RAOB from WFO MAF sampled a large EML atop a strong cap with mixed-layer parcels yielding >2,500 J/kg CAPE amidst a very moist, yet shallow, boundary-layer, characterized by T/Td spreads of only 6 degrees and a dewpoint of 67 degrees. This airmass was, and continues, to advect northward where LIDAR data out of LBB continues to detect the dispersion of a strong low-level jet (LLJ) with a speed now <=30 kt compared to the near-50 kt LLJ sampled earlier this morning. At the surface, a lee cyclone with a minimum pressure of 1008 mb was analyzed over the OK PH, particularly GUY, with a sharpening dryline extending southwestward across the TX PH and into the extreme southwestern TX PH as per recent West Texas Mesonet (WTM) and METAR data. A reservoir of 60+ degree dewpoints exists to the east of the dryline, with the 65 degree isodrosotherm delineated generally along the I-27/HWY-87 corridor per WTM data. Dewpoints were approaching 70 degrees across the Rolling Plains as well, with a corridor of very high theta-e air (e.g, 350-355 K) spread across the Caprock and Rolling Plains this morning. Visible satellite imagery and webcams reveal a shallow, low-level stratus deck anchored across the eastern Rolling Plains with evidence of billows across the Rolling Plains while a shallow cu field continues to develop and advect northward from the Permian Basin and into the CWA. The billows have been convectively reinforced by outflow from severe convection last night, though the increase in pressure tendencies following the passage of the initial gravity waves were negligible and pressure tendencies have since stabilized. Differential heating along the edge of the stratus deck was most pronounced across the far southeastern TX PH where current temperatures are in the upper 60s while increasing into the middle 70s where the billow field is located. Deepening of the lee cyclone to the north will continue throughout the rest of the day beneath the glancing influence of the shortwave trough ejecting into the west-central Great Plains today, with most guidance indicating gradual pressure falls of 10 mb/12 hr through 00Z this evening. The pressure falls associated with this deepening cyclone will generate an isallobaric response such that the southeasterly winds within the moist sector accelerate to 15-25 mph with gusts to 35 mph this afternoon and evening. The dryline bulge remains positioned to the north of the CWA at this time, and the combination of intense, diabatic surface heating as temperatures breach 90 degrees along the I-27/HWY-87 corridors and into the upper 80s in the Rolling Plains; intensifying low-level storm-relative inflow, and the continued advection of high theta-e air, will serve as a focus for the initiation of a discrete supercell or two in close proximity to the extreme southeastern TX PH and northern Rolling Plains. VWP data from the KFDR WSR-88D (despite the lapse in data from maintenance) continues to observe a substantial component of streamwise vorticity in the 0-1-km layer where storm-relative helicity values are approaching 400 J/kg as of 1518Z. A tornado threat is forecast to focus in proximity to the extreme southeastern Texas Panhandle, including the City of Childress, late this afternoon as convection unzips to the south in the form of a broken MCS as CINH erodes from intense heating in congruence with the belt of weakened mid-level flow. The best timing for this scenario is 5-8 PM CDT (22Z-01Z) as discrete propagation, particularly with a rightward-moving supercell, will be <=10 kt to the southeast in environment of buoyant and unimpeded storm-relative inflow, especially if the full component of the right-moving storm motion vector is realized. Deviant tornado movement can be expected in a situation such as described above, and with high relative humidity within from the surface-to-700 mb layer, in addition to the dampened storm-relative flow at anvil-level, hydrometeor loading of updrafts will occur with HP supercells expected and would lead to cyclic behavior of discrete/semi-discrete storms before upscale growth occurs. Large, bulbous, rear-flank downdrafts would also enhance the potential for wind-damage and further enhance deviant tornado movement during the occlusion process. The slow storm motion associated with right-moving supercells and excessive hydrometeor loading also points towards both the potential for very large hail near baseball size and/or a accumulating hail event as wet-bulb zero heights were objectively analyzed near 8 kft AGL. An additional discussion involving the forecast for the rest of the CWA will be issued with the primary package this early afternoon as the expectation for a broken line/MCS remains intact. Sincavage && .SHORT TERM... (This evening through Thursday) Issued at 127 PM CDT Wed May 1 2024 A shortwave impulse was analyzed on water vapor imagery this early afternoon over northeastern Mexico and was propagating into far southwestern Texas. Meanwhile, shortwave troughing well to the north of the CWA is beginning to emerge over the west-central Great Plains. The former impulse will serve as the impetus for the initiation of thunderstorms across the Caprock and Rolling Plains within the next few hours. At the surface, West Texas Mesonet (WTM) and METAR data have been detecting steady pressure falls, as indicated in the prior discussions, at around 1 mb/1 hr as the lee cyclone located in the OK/TX PH region gradually deepens. The dryline continues to sharpen with a secondary lee cyclone analyzed south of the I-40 corridor in far eastern New Mexico, and the dryline was delineated along the I-27/HWY-87 corridors as per recent WTM data. The moist sector is particularly buoyant with dewpoints beginning to breach 70 degrees across a few WTM sites in the Rolling Plains, and the billow field draped across the Rolling Plains earlier has since dissipated as a deepening cu field advects poleward from the Permian Basin. Congested cu was also evident on visible satellite imagery across the extreme south-central TX PH, with additional congested cu along the edge of the differential heating boundary anchored across the extreme southwestern TX PH where temperatures have risen into the middle 80s on the western periphery of this boundary. This differential heating boundary has also been reinforced by the now-remnant outflow, while a secondary, westward-propagating outflow boundary was analyzed across the southeastern Rolling Plains, though the airmass in its wake is not overturned. The 18Z RAOB from WFO MAF indicated a pristine elevated mixed layer (EML), with 700-500 mb lapse rates of 8.5 deg C/km atop a weakening cap (MLCINH now at -48 J/kg compared to -123 J/kg on the 12Z RAOB). The arrival of the 700 mb trough was also sampled by the 18Z RAOB from WFO MAF, with south-southwesterly flow near 20 kt that continues to advect towards the CWA. Mixed-layer CAPE values were in excess of 3,000 J/kg while most-unstable parcel trajectories yield over 4,000 J/kg, and this reflects the estimates of RAP-derived CAPE values on the SPC Mesoscale Analysis. Explosive thunderstorm development is expected this afternoon across the Caprock and Rolling Plains, with the potential for a bimodal convective situation initially. Thunderstorms are expected to unzip northward along the sharpening dryline as convective temperatures are reached from intense, diabatic heating amidst strengthening low-level convergence; and a more-isolated threat for supercells also exists across the extreme southeastern TX PH where discrete propagation will occur in the event storms remain cellular. Destructive interference from rapid cellular development and the splitting of cells along the HWY-87 and into the I-27 corridors will govern a mixed-mode of embedded mesocyclones initially that will pose a threat for significant (2"+) hail and wind-damage. Inverted-V profiles, excessive hydrometeor loading; and large, upward-directed accelerations from strong-extreme instability will directly result in swaths of wind-damage. However, the dampened mid-level flow should act to temper mesocyclones from remaining intact within the broken line/MCS along the I-27/HWY-87 corridors as it propagates eastward into the Rolling Plains. The amalgamation of cold pools and intensity of the theta perturbations associated with downdrafts will also pose a risk for wind-driven hail, with swaths of 60-70 mph likely with isolated significant gusts to 75 mph possible heading into the evening as the MCS matures. The aforementioned area of congested cu across the extreme south- central TX PH should serve as a focus for the initiation of one or two cells that will rapidly become severe this afternoon. WTM data continues to indicate a corridor of backed, southeasterly flow between 15-25 mph with theta-e values approaching 360 K and upper 60/lower 70 dewpoints. This particular area and east-southeastward into the rest of the extreme southeastern TX PH and northern Rolling Plains is where the potential for tornadoes should be maximized as initial coverage of storms remain isolated. Discrete, right-moving propagation would foster a cyclic behavior to supercells with deviant tornado movement expected as low-level storm-relative inflow intensifies as a function of the isallobaric response from the deepening cyclone in the OK/TX PH. The dampened storm-relative flow at anvil-level, and overall weaker flow throughout the mid-levels as the CWA remains beneath the left-exit region to the 250 mb jet streak, will also result in HP supercells with large and buoyant rear-flank downdrafts that would also result in a potential for localized wind-damage. The depth of the effective inflow-layer should also facilitate a favorable injection region of hydrometeors, with the potential for very large hail and/or swaths of accumulating hail possible. Flash flooding will accompany the slow-moving supercells from extreme rain rates owing to the excessive water loading. Eventual upscale growth is expected area-wide towards dusk with an MCS propagating eastward across the Rolling Plains. The primary hazards near and after dusk should focus towards wind-damage and flash flooding, with the MCS clearing the eastern zones after midnight tonight. Benign weather is expected the rest of the night with southerly flow remaining intact ahead of a southward-moving cold front that will move through the CWA tomorrow morning. Cooler, though still warm, temperatures will follow as northerly winds persist ahead of renewed chances for thunderstorms return to the far eastern Rolling Plains tomorrow evening. Sincavage && .LONG TERM... (Thursday night through next Tuesday) Issued at 127 PM CDT Wed May 1 2024 By late Thursday afternoon, a cold front will still be slowly dipping to the south across the area with a dry line extending to the south out of the Rolling Plains/Big Bend area. There still exists model uncertainty on the exact position of the front on whether it will hang up in the Rolling Plains or be through the FA. Independent of the eventual position of the front, convection will be possible along the front near the dryline triple point and to the northeast along the frontal boundary. The best chances for storms within the FA would be in the southern Rolling Plains where the front has greatest chances of slowing. Convection would be forming within a moisture rich and very unstable environment under steep mid- level lapse rates between 7-8 C/km. Mixed layer instability values will be on the order of 3000 J/kg. Shear still looks to remain fairly weak for May standards with deep layer shear vectors around 25kt. Therefore, the potential for large hail will exist with any convection that develops. Low level moist upslope flow will continue for Friday as the front drags farther to the south. Cloudy and cool conditions will dominate the area under this regime with shallow moisture present leading to cooler temperatures. Light precipitation may be possible on Friday within this post-frontal airmass. The next front will move through Saturday morning bringing even cooler temperatures for the rest of the weekend. Brief upper level ridging will move overhead from late Friday into early Saturday. Upper level winds will then rapidly back to the southwest in advance of the next short wave trough approaching the region. Lift will spread over the area from about 00Z - 12Z Sunday then tapering off after 12Z as the short wave moves east. With strong amounts of moisture within the atmospheric column, there is a potential for heavy rainfall during the overnight hours, especially for areas off the caprock. Sunday afternoon will see rapid drying of the column above the lower levels as subsidence spread overhead in the wake of the departing short wave. Lower levels of the atmosphere will remain moist keeping low stratus in for much of the day. The next short wave trough is currently progged to skirt the area to the north on Monday bringing a return to dry, westerly, and windy flow. && .AVIATION... (18Z TAFS) Issued at 1219 PM CDT Wed May 1 2024 VFR will continue at KCDS, KLBB, and KPVW, for the next few hours before thunderstorm chances increase by late afternoon at KPVW and KLBB. Severe thunderstorms will be possible, and better confidence exists with storms affecting KLBB compared to KPVW. Severe-caliber wind gusts of 50 kt or greater will accompany storms in addition to the potential for large hail surface and aloft. Convection will clear KLBB and KPVW near 03Z tonight. Severe thunderstorms are also forecast to affect KCDS tonight with gusts of 50 kt and large hail, and clearing out after midnight. VFR will prevail thereafter at all terminals with a cold front moving through all terminals by late tomorrow morning. Sincavage && .LUB WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...09 LONG TERM....01 AVIATION...09