SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0643 AM CST Thu Dec 17 2020 Valid 171300Z - 181200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Isolated non-severe thunderstorms will be possible today in south Florida and in California. ...Synopsis/Discussion... A progressive, rather amplified mid/upper-tropospheric pattern will cover the CONUS this period. A compact shortwave trough -- initially located over the northern Mid-Atlantic region and related to very heavy snows there -- will eject across New England today and Nova Scotia overnight. The associated occluded surface low was drawn at 11Z south of Long Island, with trailing cold front moving southeastward over the south-central FL Peninsula. Isolated thunderstorms may develop briefly over land in south FL, as the front encounters a moist boundary layer destabilized by late-morning to early-afternoon surface heating. However, any thunderstorms that do form should be short-lived and disorganized, given weakening frontal lift, limitations on buoyancy imposed by weak lapse rates, and lack of substantial low-level shear. Greater convective depth/coverage should be offshore and eastward across the northern Bahamas. Farther west, a series of shortwaves now over and near the Mississippi Valley, and related area of broadly cyclonic flow, will move to the East Coast by the end of the period, atop the continental/Polar air mass left behind the front. An upstream trough -- apparent in moisture-channel imagery from southern BC southward across northwestern CA -- consists of phased northern and southern components. These will split apart through the period, with the southern vorticity lobe -- now crossing northern CA -- digging east-southeastward over the Sierra today. As this occurs, DCVA and related steepening low/middle-level lapse rates will occur atop marginal low-level moisture/theta-e and weak CINH. This will contribute to the potential for convection. MUCAPE should remain under about 150 J/kg, but potentially extending deep enough into suitable icing layers for low-topped cumulonimbi and isolated lightning production this morning into midday. ..Edwards.. 12/17/2020
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