SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0635 AM CST Sat Nov 07 2020 Valid 071300Z - 081200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF EASTERN ARIZONA AND WESTERN NEW MEXICO... ...SUMMARY... Strong to isolated severe wind gusts may accompany thunderstorms across parts of eastern Arizona and western New Mexico later this afternoon and early evening. ...Synopsis... In mid/upper levels, a high-amplitude pattern is expected through this period, anchored by longwave troughing over the western CONUS. Within the associated cyclonic flow, a complex synoptic to shortwave evolution is expected. A strong synoptic cyclone -- now centered over northern CA -- is forecast to pivot eastward over the Sierra today, reaching northern NV, then weakening overnight. The associated/basal shortwave trough will split eastward over southern CA to central AZ by 06Z. This perturbation then should eject rapidly northeastward over the Four Corners to central CO by 12Z, while weakening rapidly. Meanwhile, a strong trough -- now evident in moisture-channel imagery over central/southern BC -- will dig southward across the Pacific Northwest as a closed low. This feature will become the main anchoring synoptic cyclone, centered over southeastern OR by the end of the period, with associated cyclonic flow covering most of the West and Great Plains. Elsewhere, a broad area of negatively tilted troughing, with several embedded vorticity maxima, will persist from the Arklatex across the central/southeastern Gulf, influencing both general thunderstorm potential in parts of the Southeast and the winding path predicted for what now is T.D. Eta. Supercell/tornado-favorable wind fields related to Eta, juxtaposed with favorable convection, are not forecast to expand into south FL and the Keys until day 2. Refer to NHC advisories for latest tropical watch/warning info and track/ intensity forecasts for Eta. ...Southwest CONUS... A loosely organized belt of somewhat low-topped convection, with isolated to widely scattered embedded thunderstorms, is forecast to develop mid/late afternoon over southeastern AZ, extending northward over higher altitudes north of the Mogollon Rim as it shifts eastward over the outlook area. The main concern will be strong to isolated severe gusts. A combination of gradual diurnal/diabatic heating and low-level warm advection ahead of the basal shortwave trough will contribute to destabilizing the boundary layer throughout the day. Moisture remains rather scant across much of the outlook area; however, available surface obs to the south in MX, as well as GPS PW data, indicate at least marginally favorable moisture will advect across the region as well, ahead of a cold front now located across southeastern portions of NV/CA. Relatively maximized low-level convergence along/ahead of the front, impinging upon the moistening/ destabilizing layer, should compel convective development by mid- late afternoon. Pockets of 100-400 J/kg MLCAPE are possible in and near the outlook area. Strengthening deep-layer winds -- aligned just to the right of the frontal zone -- will boost speed/bulk shear steadily through the day. Time series of forecast soundings and isotach cross sections show intense mid/upper winds lowering to just a few km AGL before frontal passage. This supports the potential for evaporationally aided downdrafts to convey momentum from those faster-flow areas through a well-mixed layer to the surface, in a few locales. Though minimal, surface-based buoyancy above an inverted-v boundary- layer thermodynamic profile may develop as far north as the Four Corners region late this afternoon into evening, in a brief time window before nocturnal surface cooling at those relatively high altitudes remove the SBCAPE. The wind potential there, while nonzero, still appears too conditional and short-lived to place in a categorical outlook. The threat also will diminish with eastward extent into central NM for similar reasons. ..Edwards/Kerr.. 11/07/2020
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