SPC Mar 6, 2018 1300 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 6 Mar 08:00

SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0657 AM CST Tue Mar 06 2018 Valid 061300Z - 071200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE FLORIDA GULF COAST... ...SUMMARY... Thunderstorms forming along and ahead of a cold front will pose a marginal severe-weather risk over sections of the coastal Florida Panhandle today and west-central Florida overnight. ...Synopsis... The upper-air pattern over the nation will be dominated by progressive, synoptic-scale ridging moving inland from the West Coast, and a broad area of cyclonic flow east of the Rockies. Two primary cyclones aloft are expected to contribute to the latter by merging: 1. Initially centered over western IA, and forecast to pivot southeastward over IL and eastward to OH, effectively becoming an open-wave subsidiary of the next perturbation by the end of the period. 2. Currently evident in moisture-channel imagery over northern ON, and forecast to dig southwestward then southward over Lake Superior, upper MI and WI. By 12Z, these two perturbations should occupy a single, elongated, 528-decameter 500-mb isohypse, as they form one rather irregularly shaped cyclone covering the Upper Great Lakes and vicinity. At the surface, an occluded cyclone was vertically stacked with the mid/upper level circulation center over western IA. This low should migrate/redevelop eastward along the occluded front to Lake Erie by the end of the period. A new cyclone should form along the cold/warm-frontal triple point tonight near the northern Outer Banks of NC, then move north-northeastward over Atlantic waters east of Hampton Roads. The associated cold front -- analyzed initially across eastern TN, north-central AL, southern MS and southeastern LA -- is forecast to move to the Carolinas, central/southwestern GA, central FL Panhandle, and the north-central/west-central Gulf by 00Z. By 12Z, the cold front should reach the east-central to southwestern portions of the FL Peninsula. ...FL Gulf Coast... In addition to convection already occurring near the cold front itself, and even behind the front over deep south TX and TX shelf waters, one or more bands of convection may form over the northeastern Gulf along prefrontal confluence/convergence lines. Isolated damaging gusts remain the primary concern, though a brief tornado may be possible given the favorable low-level shear apparent in some forecast soundings very close to the coast. The northeastern fringes of supportive surface-based instability, with MUCAPE up to about 500 J/kg, may brush the coastal areas of the Panhandle near AAF and PAM, and again overnight around 60-75 nm up and down the coast from the PIE area. Isolated to scattered thunderstorms are possible in these bands, the greatest coverage being over the Gulf through the period, with discontinuous/episodic onshore intrusions of surface-based thunderstorms. Still, favorable instability for severe is now expected to bypass parts of the Apalachee Bay/Coastal Bend region, with optimal near-surface lapse rates and boundary-layer theta-e remaining just offshore. A marginally supportive air mass -- with surface dew points 60s F -- may reach the coast again north of PIE tonight as prefrontal flow veers with the formation of the triple-point low farther northeast. However, that veering process also will contribute to weakening of vertical shear and convergence with time as the front and any prefrontal/convective boundaries work southeastward down the FL West Coast, limiting organized severe potential for any relatively sustained/ discrete storm or small bowing segments that can last long enough from their more-unstable offshore origins to move inland. ..Edwards/Smith.. 03/06/2018 Read more



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