SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0700 AM CST Wed Dec 02 2020 Valid 021300Z - 031200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF NEAR-COASTAL SOUTHEAST TEXAS AND SOUTHWESTERN/SOUTH-CENTRAL LOUISIANA... ...SUMMARY... Isolated severe thunderstorms are possible over portions of near-coastal southeast Texas and southwestern/south-central Louaisana. ...Synopsis... A highly amplified but very evolutionary upper-air pattern will dominate much of the CONUS this period. The deep, large Northeastern cyclone -- now centered over southern QC -- will move northward to northern QC through 12Z tomorrow. A series of shortwave troughs initially over central Canada will dig southeastward through the peripheral cyclonic-flow field, across northern ON and the upper Great Lakes. Farther southwest, moisture-channel imagery depicts a large cyclonic gyre over the southern/central Rockies and adjoining Great Plains, with primary vorticity maxima over the northeastern TX Panhandle and southeastern WY. In a loose Fujiwhara interaction, the southern circulation will remain over the eastern Panhandles/western OK region through 00Z, while the western one retrogrades southwestward toward the Grand Canyon. Overnight, the TX/OK portion will become the main low and start to pivot/eject eastward over OK, while the other lobe (then an open-wave/shortwave trough) digs south- southeastward over AZ and western NM. The net effect by 12Z should be a 500-mb low between TUL-FYV, with height trough and vorticity ribbon extending west-southwestward across the TX South Plains and southern portions of NM/AZ. At the surface, 11Z analysis showed a low over northwest TX between CDS-MWL, with cold front southwestward across the lower Pecos River/Big Bend region. The low should move erratically over southern OK today and occlude, while the cold front surges southeastward to a 00Z position over east-central TX and the lower- middle TX Coast. A new low may develop along the boundary, overnight, over the Gulf, and move inland over southwestern LA after 09Z. ...Western Gulf Coastal Plain... A plume of precip, with scattered embedded shower and isolated to widely scattered thunderstorms, is expected to expand and become more dense today from the northwestern Gulf northwestward across east TX and southeastern OK. This will occur as warm advection and moisture advection/transport continue in a conveyor related to the deep-layer cyclone, most of it elevated above a relatively stable boundary layer. However, ongoing and future marine airmass modification over the next several hours should support sufficient theta-e advection into near-coastal southeast TX for surface-based effective-inflow parcels to develop inland, perhaps as far north as roughly I-10. MLCAPE of 500-800 J/kg may develop inland. As a result, isolated surface- based supercells may either develop over the Gulf and graze the immediate coastline, or (somewhat less probably) form inland, before the flow near/ahead of the front veers substantially and frontal passage occurs. 45-55-kt effective-shear magnitudes and 150-300 J/kg effective SRH will support supercell and small-bow potential, though the bulk of strong-severe convection should be over the Gulf where the inflow-layer air mass is better optimized. An effective/marine warm front marking the northern rim of the modifying/surface-based parcels may move very slightly inland farther east across southwestern/south-central LA overnight, in support of marginal severe potential. ..Edwards.. 12/02/2020
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