Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0155 PM CST Sat Jan 21 2017 Valid 212000Z - 221200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO TONIGHT ACROSS PARTS OF THE AR-LA-MISS REGION... ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS TONIGHT ACROSS PARTS OF SOUTHERN AND CENTRAL ALABAMA AND ADJACENT PORTIONS OF THE FLORIDA PANHANDLE INTO SOUTHWESTERN GEORGIA... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE GULF INTO SOUTH ATLANTIC COAST STATES... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE SOUTHEAST... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS IMMEDIATELY SURROUNDING THE SLIGHT RISK AREA... ...SUMMARY... Severe thunderstorms remain possible across parts of the Southeast late this afternoon, and are expected to develop across parts of the lower Mississippi Valley into the Southeast this evening through the overnight hours. This includes a risk for supercells capable of producing tornadoes, particularly across parts of southern and central Alabama, and adjacent portions of the Florida Panhandle, into southwestern Georgia. ...20Z Outlook Update... Severe weather potential has been complicated considerably by at least a couple of short waves impulses, emerging from a strong mid/upper jet which continues to migrate inland across the southern tier of the U.S., in advance of one much more significant perturbation still evolving near the southern Rockies. The lead impulse is now in the process of shifting across/northeast of the southern Appalachians. This feature has provided support for a severe mesoscale convective system which maintains considerable strength as it shifts across southern Georgia and portions of the lower Savannah River Valley, coincident with a 40-50 kt southwesterly 850 mb jet. Severe probabilistic lines have been adjusted some to account for the progression of this feature. Upstream, another impulse is already in the process of shifting through the Ark-La-Tex region, with the main amplifying upper trough not likely to reach the lower Mississippi Valley until late tonight, though another smaller scale impulse or two may still emerge from it, and progress toward Gulf coastal areas ahead of it. At the same time, the boundary layer from the vicinity of the north central Gulf coast region into the Ark-La-Tex is still in the process of recovering, as a convective outflow boundary and developing warm frontal zone begin to advance northeastward. Based on the variability that has been evident in short term model forecast guidance, the details of the potential convective evolution across the lower Mississippi Valley into the Southeast through the remainder of the period is far from certain. However, based on current observations and model output, severe categorical and probabilistic lines across the lower Mississippi Valley have generally not been changed, and appear to reflect potential that currently exists. Farther east, confidence continues to increase concerning severe weather potential associated with restrengthening of west southwesterly 850 mb flow (50+ kt) across portions of southern Alabama into southern Georgia, late this evening into the overnight hours. As the boundary layer moistens and destabilizes across this region, and low-level hodographs enlarge, supercells capable of producing tornadoes (some strong) appear increasingly likely. ..Kerr.. 01/21/2017 .PREV DISCUSSION... /ISSUED 1027 AM CST Sat Jan 21 2017/ ...AL/FL/GA/SC This Afternoon... A fast moving and intense squall line is moving rapidly eastward across parts of the FL Panhandle, extreme southeast AL, and southwest GA. This line will progress across GA and into SC later today. Transient rotating updrafts have been occurring along the line this morning, producing locally damaging wind gusts and tornadoes. This threat will persist as activity moves eastward due to strong southerly low level winds just ahead of the storms maintaining enhanced low level shear and helicity values. Some breaks in the clouds and modest afternoon heating may further destabilize the area and maintain the Enhanced risk of severe storms. ...AR/LA/MS This Afternoon and Evening... Water vapor loop shows a lead shortwave trough moving eastward across the Gulf Coast states. Subsidence behind this system will help to suppress deep convection over the ArkLaTex region for a few more hours. However, another shortwave trough over NM and West TX will lead to increasing lift and cooling aloft by mid/late afternoon. Present indications are that scattered thunderstorms will develop over southeast OK and northeast TX around peak heating and track eastward across parts of AR/LA and into MS this evening. Forecast soundings show steep lapse rates, cold mid level temperatures, and favorable deep layer shear values to promote a widespread risk of severe hail. CAM solutions also indicate discrete supercells will be favored, further increasing the risk of very large hail and a few tornadoes. Therefore have upgraded portions of this area to MDT risk for the hail threat. ...LA/MS/AL/GA This Evening and Tonight... Considerable uncertainty exists regarding the evolution of the remnant outflow boundary from this morning's convection and the re-development of thunderstorms tonight over this area. Forecast soundings indicate that the combination of sufficient instability, ample low level moisture, and significant low level vertical shear will promote a tornado risk in any storms that form. A few models suggest another active round of severe storms affecting areas similar to this mornings storms. Therefore will maintain the Enhanced Risk area with minimal changes.Read more
from SPC Forecast Products http://ift.tt/u32qoI
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