SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0751 AM CDT Thu Sep 30 2021 Valid 301300Z - 011200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF WEST TEXAS AND EXTREME SOUTHEASTERN NEW MEXICO... ...SUMMARY... Large hail and severe thunderstorm gusts are possible today into this evening, over parts of west Texas and extreme southeastern New Mexico. ...Synopsis... A highly amplified mid/upper-level pattern over midlatitude North America features two slow-moving synoptic-scale cyclones, each located south of splitting northern-stream flow: 1. A strong vortex centered over ME, and forecast to follow a loopy path across eastern ME and New Brunswick through the period. Associated troughing will shift farther offshore from the Mid- Atlantic and southern New England. 2. A weaker, broad area of cyclonic flow (intermittently displaying closed-low characteristics at 500 mb) over the central/southern Rockies and Four Corners States. Primary vorticity maxima/embedded mesoscale lows were evident in moisture-channel imagery over northern UT and central/southern AZ, connecting the main height trough. The trough should drift eastward to western portions of CO/ NM through the period, with a primary 500-mb low probably evolving from the southern vorticity max, and reaching western NM by 12Z tomorrow. At the surface, 11Z analysis showed a cold front across eastern NE, south-central KS, western OK, to near LBB, west-southwestward over southern NM. A dryline was drawn from near PRS-INK then northeastward to a very oblique intersection with the cold front east of LBB. The cold front will move slowly southward today -- aided on the mesoscale by outflow from frontal/post-frontal convection -- reaching eastern KS, central OK, northwest TX and portions of the Permian Basin by 00Z. ...West TX/southeastern NM... Scattered to numerous thunderstorms will offer occasional large hail and strong-severe gusts across parts of west TX and southeastern NM today. The most-favorable parameter space will exist over the 15%/"slight risk" area, though isolated severe may occur in weaker ambient flow and shear northward as far as southwestern OK. A broad area of large-scale lift in low/middle levels, and related mid/upper difluence -- was evident in satellite loops across parts of northern Chihuahua, far west TX and adjoining southern NM. This, along with favorable low/middle-level moisture, has supported episodic areas of thunderstorms all prior night and into this morning, and should shift eastward through the day. Associated cloud cover may restrict intensity/duration of diurnal heating, especially with westward extent, and casts some doubt on the vigor of dryline-related vertical circulations this afternoon. Regardless, with the lack of a strong cap at the base of the EML, MLCIN is expected to be minimal along/east of the dryline and southeast of the front, even under somewhat muted diabatic heating. Surface-based thunderstorms are forecast to develop across the area from midday through at last most of the afternoon, on both sides of the front and dryline. Activity forming behind the dryline may cross it and encounter greater moisture/buoyancy in parts of the warm sector that have not been stabilized too much yet by convection farther east. Coverage should enlarge and become denser through the afternoon, amidst gradually strengthening mid/upper flow shifting cyclonically around the base of the mid/upper trough. Forecast soundings reasonably indicate moist-sector dew points in the mid/upper 60s F and steep 850-550-mb lapse rates contributing to MLCAPE of 2000-3000 J/kg (locally/briefly higher). Buoyancy will decrease westward through the dryline and diminish sharply north of the front. Low/middle-level flow will veer with height, making hodographs that are looped in shape, albeit somewhat small due to lack of stronger absolute speeds. Effective-shear magnitudes of 35-45 kt should develop -- supporting a blend of multicellular and at least transient/messy supercellular characteristics. A channel of 35-40-kt southeasterlies at 850 mb should develop over southwest TX late this afternoon and persist through about 06Z, contributing to continued convective potential into the evening via favorable storm-relative flow of moist air. However, with potential for increasingly messy storm modes and greater outflow coverage into the evening, along with gradual nocturnal/diabatic cooling, overall severe potential should diminish tonight. ..Edwards.. 09/30/2021 Read more LIVE: spc.noaa.gov
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