SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0239 PM CDT Wed Jul 11 2018 Valid 112000Z - 121200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER MS VALLEY... ...SUMMARY... Severe thunderstorms are expected this evening into tonight across northern Minnesota and vicinity. ...Discussion... No appreciable changes are warranted to 1630z outlook. Strong short-wave trough is currently ejecting east across SK/MB region and the southern influence of this feature should influence northern MN later this evening. Warm advection convection has now retreated to areas north of the international border and the primary concern for severe development is focusing near the ND/MN border. However, substantially more boundary-layer heating is required for surface parcels to reach their convective temperatures. For this reason it appears deep convection should be delayed for a few hours and likely displaced downstream over MN. ..Darrow.. 07/11/2018 .PREV DISCUSSION... /ISSUED 1128 AM CDT Wed Jul 11 2018/ ...MN area this afternoon through tonight... A midlevel shortwave trough will continue moving east-northeastward from southern SK across MB through early tonight. An associated surface cold front will progress eastward from ND to northwestern MN this evening, as weak height falls will brush northern MN. A remnant MCV over eastern ND and residual cloud cover will tend to slow surface heating some today. The timing of the MCV suggests it will interact with the warm front this afternoon in MN, though residual clouds may slow surface heating. Also, mainly weak/shallow ascent is expected across northern/northwestern MN this afternoon along the surface warm front and cold front. These factors cast some doubt on the timing of storm initiation and subsequent storm coverage prior to sunset, which could limit the potential for surface-based supercells and related tornado potential. The mesoscale environment will otherwise be conditionally favorable for tornadic supercells across northern MN in the zone of greatest overlap of buoyancy with deep-layer and low-level shear. The more probable scenario may be for later storm initiation (near or after dark) across northern MN, with storms transitioning to a cluster mode fairly quickly. Large hail and damaging winds will be the main threats, though a couple of tornadoes will be possible if any discrete storms form by this evening. ...Southern VA/northern NC this afternoon through late evening... One embedded mid-upper speed max over southern PA/MD will move southeastward over Chesapeake Bay this afternoon, as an upstream wave moves from the upper Great Lakes to northern VA and vicinity overnight. A weak pre-frontal surface trough will also move slowly southeastward across VA this afternoon, providing some focus for thunderstorm development. Potentially moderate buoyancy (MLCAPE up to 1500 J/kg), DCAPE near 1000 J/kg, and effective bulk shear around 25 kt will support multicell clusters capable of producing isolated damaging outflow gusts for a few hours this afternoon/evening as storms move southeastward into north central and northeastern NC.
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