SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1125 AM CST Sun Jan 03 2021 Valid 041200Z - 051200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Scattered thunderstorm activity is possible along Pacific Northwest coastal areas, mainly late Monday afternoon, with lightning also possible along the northern Sierra Nevada. ...Discussion... Much of North America will remain under the influence of branching westerlies, downstream of a persistent strong zonal jet across the southern mid-latitude and subtropical Pacific, which may nose as far east as 140-145 W latitude during this period. Stronger surface cyclogenesis associated with this feature will remain focused across the northeastern Pacific, but an emerging mid-level perturbation and associated occluding surface front are forecast to progress inland of the U.S. Pacific coast Monday through Monday night. The front appears to be preceded by a plume of moisture emanating from the subtropical latitudes, and models suggest that moisture influx will become maximized across the northern California coast into the northern Sierra Nevada during the day Monday, before becoming cut-off by the inland advancing cold front. Downstream, it appears that mid-level troughing may undergo some further amplification east of the Mississippi valley into the western Atlantic. This will include a number of embedded short wave perturbations, including one advancing east of the New England coast, where it will provide support for further deepening of an offshore surface cyclone. As this occurs, the trailing surface cold front, and narrow plume of moisture return emanating from the southern Gulf of Mexico, are expected to shift south of the Florida Peninsula and Keys, with considerable further mid/low-level drying across much of the Gulf of Mexico. This will contribute to the maintenance of generally dry and/or stable conditions across most areas east of the Rockies. ...Pacific Coast states... Models suggest that the development of thermodynamic profiles most conducive to convection capable of producing lightning will be late Monday afternoon, around 05/00Z (plus or minus an hour or two), along the Oregon into Washington coast. It appears that this will take place within the post-frontal regime, and coincident with the inland progression of the mid-level thermal trough axis (with temps at or below -30C at 500 mb). Otherwise, models suggest that mid-level cooling may steepen lapse rates sufficiently to support convection capable of producing lightning within the moist pre-frontal regime across northern California coastal areas Monday afternoon. Convection capable of producing lightning may also develop into the northern Sierra Nevada, aided by intensifying pre-frontal lift supported by low/mid-level warm advection and orography, which may become focused near or to the west of the Tahoe Valley vicinity by 05/00Z. ..Kerr.. 01/03/2021
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