Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0745 AM CDT Mon Aug 28 2017 Valid 281300Z - 291200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF SOUTHERN LOUISIANA AND EXTREME SOUTHEAST TEXAS... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE GULF COAST SLIGHT RISK... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE COASTAL CAROLINAS... ...SUMMARY... The eastern parts of Tropical Storm Harvey will pose a tornado threat today and tonight from extreme southeast Texas to near the coast of southern Louisiana. A marginal tornado threat also may develop near the coastal Carolinas this evening and overnight. ...Synopsis... A seasonally high-amplitude upper-air pattern is expected through the period, anchored by a large anticyclone over the Great Basin. A smaller cyclonic vortex, centered over WI, is expected to pivot slowly southeastward/eastward across southern Lake Michigan through the period, with troughing southward to parts of the Mid-South and Tennessee Valley regions. A lack of favorable instability and shear will preclude an organized severe threat with the associated convection. The main two features for this outlook are (or are becoming) tropical in nature and associated predominantly with some tornado potential: Tropical Storm Harvey near the upper Texas coast and Potential Tropical Cyclone (PTC) 10 for the eastern outlook area. Refer to latest NHC advisories for forecast track/intensity information regarding each of the systems responsible for the outlook areas below. ...Southeast TX, southern LA... Harvey has assumed a decidedly asymmetric convective distribution across the eastern semicircle -- the best-organized convective/convergence band and low-level wind fields supporting potentially tornadic supercells each becoming rather far-removed from the circulation center. A pronounced dry slot has wrapped around the southern semicircle behind the primary convective band, which was analyzed at 12Z from southwestern LA southward across the Gulf to near buoy 42002. Northward-moving embedded supercells and bow/lewp features may offer the risk of brief tornadoes and damaging gusts as the band itself continues to shift eastward. Most progs show the main convective band continuing to move outward through the circulation envelope -- in other words, translating faster than the center, and overtaking discrete to semi-discrete supercells to its east, as has been the case the last 8-10 hours. If this occurs, the main band may outrun the most favorable low-level shear by this afternoon, assuming it has not weakened greatly or dissipated already. In the meantime, more precursory supercells may form over the Gulf and pose a tornado risk as they move ashore in southern LA. Later in the period, this evening and tonight, additional development is possible across the Gulf and inland into the outlook area, with favorable low-level shear still in place to support some tornado threat. Refer to SPC tornado watch 473 and related mesoscale discussions for the latest near-term info on Harvey's tornado threat. ...Coastal Carolinas... NHC guidance shows PTC 10 developing into a tropical storm by tonight, moving northeastward near and parallel to the Carolinas coast. Under this scenario, the isallobaric response to a tighter, closed circulation may permit development of enough easterly boundary-layer flow component to advect relatively high-theta-e marine air, now offshore, across at least a sliver of near-coastal areas and southern Outer Banks before the end of the period. Such a regime would be juxtaposed with backed low-level flow and favorable SRH to support some supercell potential. Right now the tornado threat looks sparse and conditional enough to maintain just the marginal probabilities from the previous outlook. However, since this isn't even a tropical cyclone with a single coherent center yet, and early-development track/size uncertainties abound, note that a wide variety of tornado-outlook changes remain possible, from probabilities increased to dropped altogether or shifted laterally. ..Edwards/Smith.. 08/28/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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