SPC AC 171226
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0726 AM CDT Fri Mar 17 2017
Valid 171300Z - 181200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
...SUMMARY...
Thunderstorms are possible through tonight from southeastern
Oklahoma to the Ohio Valley and coastal Tidewater area of Virginia
and North Carolina.
...Synopsis...
The upper-air pattern should trend toward overall amplification
through the period as a pronounced shortwave trough -- now evident
in moisture-channel imagery over the Red River Valley region of
southern MB and the eastern Dakotas -- digs southeastward and
strengthens. By 00Z, the axis of this perturbation should be
located from the MN Arrowhead southward across eastern IA. By 12Z,
it should extend from eastern Lake Superior, through a developing
500-mb low over southeastern Lower MI, to north-central/northeastern
KY.
At the surface, the associated low was located near the MN Northwest
Angle at 11Z, with occluded front to central IA, warm front from
there southeastward across middle TN, and cold front over south-
central KS to northeastern NM. By 00Z, the low should be located
over northwestern ON northwest of Thunder Bay, with occluded front
to the southern Lake Michigan area and cold front southwestward from
there across the Ozarks, south-central OK, and the southern South
Plains or northern Permian Basin of west TX. By 12Z, the main low
should evolve beneath the developing mid/upper cyclone over
southeastern MI, with warm front across the Delmarva area and cold
front over southwestern PA, western VA, northern MS, north-central
TX, and the Trans-Pecos area of west TX.
...TN Valley to southeastern OK...
A loose band of ongoing elevated thunderstorms, initially from the
STL area over central IL, should proceed east-southeastward through
the remainder of the morning hours, offering isolated hail before
weakening. Widely scattered to scattered thunderstorms also should
form near the low-level front this evening and overnight, moving
eastward across the eastern, middle and southern parts of the
outlook area as the frontal zone itself continues penetrating
southeastward.
While lightning is possible virtually anywhere within the general-
thunder area, the strongest convection should occur overnight in the
corridor from middle/western TN across AR, perhaps backbuilding
briefly into southeastern OK. There, the most mature low-level
return-flow and warm-advection regime of the period, in a narrow
corridor near the surface and 850-mb fronts, will supply overnight
convection with PW between 1-1.4 inches and mean mixing ratios
11-12.5 g/kg, beneath EML air characterized by 8-8.5 deg C/km
midlevel lapse rates. This reasonably yields elevated MUCAPE 1000-
1500 J/kg in assorted models' forecast soundings.
Despite barely/technically surface-based effective-inflow parcels in
some forecast soundings across parts of southeast OK and AR, a
shallow stable layer reasonably appears near the surface and the
vertical geometry of the LLJ in progged cross sections. Also, flow
at the surface should be rather weak, near the pressure col crossing
the front, limiting surface lift. As such, inflow should be at
least slightly elevated, constraining an effective-shear layer off
the surface and yielding around 35-kt vector magnitudes. This may
support brief pulsing of the most intense cells to intensities
supporting hail approaching severe limits. However, given the
frontal forcing and likely quick evolution to messy storm modes, as
well as the tendency of the strongest 850-mb winds to shift well
east of the Mississippi River and over the central/southern
Appalachians where buoyancy will be much weaker, potential appears
too uncertain and low to introduce a 5% unconditional hail area at
this time.
..Edwards.. 03/17/2017
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z
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