SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1120 AM CST Thu Jan 28 2021 Valid 291200Z - 301200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... The risk for severe thunderstorm appears negligible across the U.S., Friday through Friday nnight. ...Synopsis... To the southeast of blocking mid/upper ridging centered across northern Quebec, large-scale troughing is currently in the process of evolving near the Atlantic Seaboard into the western Atlantic. It appears that this will be maintained through this period, as renewed amplification occurs within the westerlies across the mid-latitude central into eastern Pacific. Models indicate that the latter regime will include deepening large-scale mid-level troughing across the northeastern Pacific, the leading edge of which may at least approach British Columbia and Pacific Northwest coastal areas by 12Z Saturday. In advance of this feature, low amplitude ridging may begin to build inland across California by late Friday evening, as the significant short wave trough currently approaching the California coast is forced east-northeastward. A vigorous smaller-scale perturbation may still be digging within the lead short wave trough, to the west of the southern California coast at 12Z Friday. This feature, and the associated mid-level cold core (with 500 mb temps around -30C), might not turn completely inland of the southern California coast until late Friday afternoon or early evening. However, a more rapid east-northeastward acceleration is forecast Friday night, with the larger-scale short wave trough taking on a negative tilt across and east of the southern Rockies by Saturday morning. In response to the approaching mid-level perturbation, models indicate considerable deepening of surface troughing to the lee of the Rockies, with cyclogenesis commencing across southeast Colorado by late Friday night. While a southerly return flow off the southwestern Gulf of Mexico may be underway by Friday morning across the lower Rio Grande Valley and adjacent southern Great Plains, substantive boundary-layer moistening (including 60-65F+ surface dew points) may not reach lower Texas coastal areas until around 12Z Saturday. Forecast soundings indicate that the depth of this moist layer will remain fairly shallow, beneath very warm and strongly capping elevated mixed-layer air. ...Southwest... Models indicate that the surface cold front associated with the inland advancing mid-level short wave trough may be in the process of advancing into/across the lower Colorado Valley by 12Z Friday, before continuing eastward across the central/southern Arizona deserts during the day. It appears that the front will be preceded by moderately strong (30-40+ kt) south/southwesterly flow in the 850-700 mb layer, but moistening and mid-level cooling sufficient to contribute to thermodynamic profiles conducive to thunderstorm development are forecast to be confined to the post-frontal regime, after this flow veers and weakens. With only very weak CAPE currently anticipated, while some of this activity (which could impact southern California coastal areas into Friday afternoon, as well as the Greater Phoenix area by late Friday afternoon) may be accompanied by small hail/graupel and gusty winds, it is expected to generally remain well below severe limits. ...Parts of the southern Great Plains... Increasing deep-layer large-scale forcing for ascent ahead of the inland accelerating short wave trough is forecast to begin impacting the higher terrain and high plains of west Texas and eastern New Mexico by Friday evening. This will be accompanied by steepening mid-level lapse rates. Although forecast soundings suggest that the most substantive moisture return across this region may be of lower latitude eastern Pacific origin, and based near or above 700 mb, it appears that thermodynamic profiles will become conducive to convection capable of producing lightning. And it appears that this activity will tend to develop northeastward into south central portions of the Great Plains by 12Z Saturday. ..Kerr.. 01/28/2021
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