SPC Sep 14, 2018 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0807 AM CDT Fri Sep 14 2018

Valid 141300Z - 151200Z


A few tornadoes are possible across eastern North Carolina over the
middle/outer northeastern sector of slow-moving Hurricane Florence,
today through tonight.

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

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.

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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