SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0656 AM CST Sat Feb 13 2021 Valid 131300Z - 141200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF CENTRAL/SOUTHWESTERN FLORIDA... ...SUMMARY... Strong to marginally severe thunderstorms are possible today through tonight across parts of central and southwestern Florida. ...Synopsis... In mid/upper levels, a high-latitude blocking pattern will start to break down as 1) a high now over northern Yukon redevelops southeastward across the Northwest Territories and 2) a long-lived cyclone over southern AB moves slowly eastward to northwestern ON. As this occurs, a speed max and associated shortwave trough -- initially near the CA/OR coastline -- will dig southeastward in a strongly difluent upper pattern across the Great Basin today. This perturbation will expand into a synoptic-scale trough by the end of the period, from the Four Corners region southward across northwestern MX. In turn, the mid/upper trough now over southern NM and northwestern MX -- with embedded 500-mb low apparent over Chihuahua -- should pivot across northeastern MX and south TX through the period. By 12Z, this trough should reach the middle TX Coast and western Gulf, while weakening rapidly amidst a broadening cyclonic-flow field related to the expansive upstream troughing. Downstream, a more- subtle shortwave trough -- now evident in moisture-channel imagery over the western Gulf -- will eject east-northeastward over the central/northeastern Gulf, weakening substantially by the time ot reaches FL around the end of the period. At the surface, a quasistationary to northward-drifting frontal zone was drawn across north-central FL just south of a DAB-LEE-CGC line, and is being overtaken from west-east by a convective complex moving off the Gulf. A frontal-wave low was drawn behind the convection near buoy 42039, with slow-moving but strong cold front southwestward across the north-central/west-central Gulf. This low should weaken over time, with a new one forming tonight over the central Gulf along the front, and ahead of the ejecting southern-stream shortwave trough. ...FL... A loosely organized band of thunderstorms over coastal areas of west-central FL -- and adjacent Gulf waters -- will proceed east-northeastward across the peninsula through the rest of the morning, offering the threat for isolated strong-severe gusts. A brief tornado cannot be ruled out near and just south of the frontal zone, where low-level SRH/vorticity will be relatively maximized based on the modified TBW wind/thermal RAOB. See SPC mesoscale discussion 73 for near-term details. This activity has been very poorly progged by most 00Z convection- allowing guidance including HREF members, as well as HRRR runs from then through 10Z, when finally forced by reflectivity input to depict (underdone) convection initially. The atmosphere clearly is not obedient to the models. Observationally, based on satellite and radar trends, associated cloud cover and precip should impede diurnal destabilization over northern and parts of central FL, effectively shunting the baroclinic zone southward on the mesoscale and casting increasing doubt on severe potential with northern extent. An additional round or two of strong/isolated severe convection, with potential for embedded supercells/bow structures, is possible this afternoon and/or tonight over parts of central/southern FL. Some of this activity will interact with the boundary reinforced/ relocated southward by morning convection, while also accessing less-disturbed warm-sector inflow parcels. In the warm sector over west-central/southwestern FL late tonight, forecast soundings indicate that rich boundary-layer moisture will offset slightly warming midlevel temperatures (around 500 mb) enough to yield 500-1000 J/kg MLCAPE, though low-level shear/hodographs will diminish with southward extent. In association with such convection, a relatively concentrated corridor of potential may develop within the broader marginal area; however, mesoscale uncertainties preclude assigning greater unconditional probabilities at this time. ..Edwards/Smith.. 02/13/2021
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