SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0807 AM CDT Fri Sep 14 2018 Valid 141300Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER EASTERN NORTH CAROLINA... ...SUMMARY... A few tornadoes are possible across eastern North Carolina over the middle/outer northeastern sector of slow-moving Hurricane Florence, today through tonight. ...Synopsis... In mid/upper levels, a positively tilted mean trough will persist, with minor internal/shortwave variations in geometry, from a cyclone over Nunavut southwestward across BC then generally southward west of the U.S. Pacific Coast. Strong upper ridging will persist from New England to KS to northwestern MX. A deep-layer trough will move from the western Gulf northwestward across south TX, increasing general-thunderstorm support in that area. At this time, the low-level wind profiles are progged to produce shear too weak for an unconditional severe threat. Meanwhile, Hurricane Florence is forecast to move generally west-southwestward over the eastern Carolinas. At the surface, a cold front was analyzed at 11Z across north-central MN and eastern SD, becoming quasistationary southwestward to a low over southwestern NE. The front is expected to stall through the period across northeastern MN, eastern SD and western NE. ...Florence... See tornado watch 373 and related mesoscale discussions for the latest near-term information on the tornado threat. See NHC advisories for tropical watches, warnings, and all forecast track/intensity guidance related to Florence. A low-grade, discontinuous, but long-lasting tornado threat will exist over the outlook area, in a corridor of favorable low-level shear and large hodographs but modest instability. The main limiting factors will be messy and mostly nonsupercellular convective modes, and lack of more robust buoyancy. However, isolated and episodic supercell development may occur, primarily outside the inner bands of stabilizing rain. An arc of relatively rich low-level theta-e and relatively maximized 0-3-km CAPE has been evident persistently in the outer north- central/northeast sector from offshore of the HSE area across the northern Outer Banks and westward/inland, gradually diminishing toward RDU. Meanwhile larger hodographs are present slightly more inward over the southern Outer Banks, southern Albemarle Sound, and east-central NC. The greatest CAPE/shear parameter space is in the overlaps zone, which has been longstanding and has encompassed the supercells that have been noted ever few hours between the Outer Banks and areas east and northeast of RDU. This regime is expected to translate slowly west-southwestward in step with the motion of Florence's center that is predicted by NHC. Any expansion of the outer wind fields also may effectively maintain a tornado risk against the translational vector of the center away from the area. Diabatic destabilization should be very gradual/weak under a dense overcast, but onshore theta-e advection and moisture transport should help to maintain at least marginal buoyancy: MLCAPE 200-800 J/kg where SRH is most favorable. ...Upper Midwest... Thunderstorms are expected to develop this evening -- and perhaps as early as late afternoon -- in a zone of elevated low-level warm advection and moisture transport poleward of the frontal zone. This activity should increase in coverage and intensity through the evening, offering isolated severe hail. A dearth of upper-level support should continue beneath the southwesterly flow fetch; in fact, slight height rises are progged as a shortwave trough ejects from southern MB. However, increasing theta-e in the projected inflow layer and warm-advection-related ascent should weaken MUCINH and result in progressively more parcels being lifted isentropically to an LFC. That process, along with midlevel lapse rates around 7 deg C/km, should support MUCAPE around 2000 J/kg. With 45-55 kt effective- shear magnitudes forecast, any relatively sustained/discrete cells should offer the risk of at least isolated large hail. Upscale clustering and messier storm modes with time overnight, roughly from about 6Z onward, may lessen the hail threat with time. ..Edwards.. 09/14/2018
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