Full Notification

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0759 AM CDT Tue Mar 21 2017

Valid 211300Z - 221200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM MIDDLE
TENNESSEE TO PARTS OF NORTHEASTERN AL AND NORTHERN GA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM THE
EASTERN OZARKS TO NORTHEASTERN GEORGIA AND WESTERN SOUTH CAROLINA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND EXTENDING INTO NORTHEASTERN OKLAHOMA AND THE
CAROLINAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE CALIFORNIA CENTRAL VALLEY...

...SUMMARY...
Hail and damaging thunderstorm gusts are possible today from the
Ozarks across the Tennessee Valley to the southern Appalachians. An
enhanced corridor of damaging-wind risk now appears more probable
from middle Tennessee to northeastern Alabama and northern Georgia.
An isolated severe storm or two, capable of hail, damaging gusts
and/or a tornado, may develop across portions of the California
Central Valley.

...Synopsis...
The synoptic/upper-air pattern this period will feature progressive
troughs over eastern Canada (extending into parts of the Great Lakes
region) and offshore from the West Coast. In between,
high-amplitude mid/upper ridging will shift slowly eastward across
the Rockies and High Plains. A cyclone aloft is evident in
moisture-channel imagery west of the OR coast near 135W. This
feature will devolve quickly to a weakening open wave and eject
northeastward toward Vancouver Island and the Olympic Peninsula,
amidst height falls preceding another cyclone digging southeastward
off the Gulf of AK. As this occurs, two closely spaced shortwave
perturbations and associated vorticity lobes will move ashore: the
first oriented north-south and now located just offshore OR and
northwestern CA, the second extending southward from the low at this
time. Each will be weakening but still potentially influential for
convective purposes while moving northeastward and inland through
this evening.

Elsewhere, a weak mid/upper-level perturbation/vorticity lobe in the
southern stream -- currently evident in moisture-channel imagery
over east-central/northeastern NM -- is expected to move across much
of southern KS and northern OK through afternoon, reaching
northeastern OK by mid/late evening.

At the surface, a wavy frontal zone was analyzed at 11Z from the
lower Chesapeake Bay region across eastern KY, northwestern TN,
northeastern OK, the southern TX Panhandle, and north-central NM.
This front will move slowly southeastward east of the Mississippi
River today, while being quasistationary farther west. Overnight, a
reinforcing cold surge now over the northern Plains, upper
Mississippi Valley and upper Great Lakes will overtake the front.
By 12Z, the combined cold front should extend from southern SC
westward across central/northern MS, the Arklatex region, through a
frontal-wave low near Lake Texoma, southwestward to the Permian
Basin region.

...Ozarks to southern Appalachians region...
At least three recognizable convective episodes offering
severe-thunderstorm potential may evolve over this corridor, with
some fill-in linkage or spatial overlap possible between them:

1. Ozarks to Tennessee Valley, morning through afternoon:
Widely scattered, ongoing, elevated thunderstorms across this area
will post a threat for isolated severe hail as they move eastward
through the western/central Ozarks region this morning. See SPC
Mesoscale Discussion 300 for near-term meteorological details. Some
combination of this early convection and additional development over
the region by midday should move atop a progressively more unstable
boundary layer with time, near the Mississippi Valley, and become
surface-based. This is expected to occur as diurnal heating and
near-surface thetae advection continue along and south of the
surface front. The front itself, and associated instability
gradient, may offer a preferential propagational corridor for any
resulting upscale-organizing storm complex to travel into the
Tennessee Valley region. An enhanced corridor of wind-damage
potential now appears more probable as the activity gets into
favorable moisture/buoyancy in middle TN, where well-mixed subcloud
layers and steep low/middle-level lapse rates exist south of the
front. Thermodynamic profiles will support 1500-2500 J/kg MLCAPE
atop a strongly mixed boundary layer.

2. Tennessee Valley to southern Appalachians this afternoon:
By mid/late afternoon, preconvective parts of this area will reside
in a regionally optimized zone of weak CINH and relatively maximized
lift, where frontal and orographic ascent contributions juxtapose
with diurnal heating of higher terrain. This also should be along
the northeastern rim of a field of favorable prefrontal low-level
moisture -- even after boundary-layer mixing processes --
contributing to the development of 1500-2000 J/kg MLCAPE, also atop
a well-mixed boundary layer. Although low-level winds will be
light, the unidirectional character of the forecast vertical wind
profile supports quick evolution to a clustered,
forward-propagational convective character that, in concert with its
passage across some of that mixed layer, supports damaging gusts.
Similarly to the first area, some of this regime may experience a
relative concentration of convective-wind potential as well, but
also may be overtaken by the western complex.

3. Western Ozarks region later today:
Isolated strong to severe storms cannot be ruled out this afternoon
into early evening farther west near the front, across parts of
northeastern OK or northwestern AR, as the aforementioned weak
southern-stream perturbation aloft crosses over the zone of frontal
lift. Coverage will be limited by capping related to the eastern
part of the persistent EML. However, a combination of frontal lift
and weakened CINH from diurnal/diabatic heating may suffice for
isolated convection to survive long enough for a brief risk of hail
or damaging wind.

...CA Central Valley...
A largely north-south band of convective-scale lift associated with
the leading mid/upper-level perturbation, and a weak frontal zone,
will pass across this region throughout today, obliquely crossing
the northern/western central parts of the Central Valley through
late morning and the rest through this afternoon. This regime will
encounter relatively maximized inflow-layer buoyancy on its southern
fringes early-mid afternoon, mainly southeast of the SAC area, while
in its wake, the boundary layer destabilizes as well amidst clearing
conditions. MLCAPE near the upper reaches of an expected 300-800
J/kg range may develop in areas where insolation is sustained for a
few hours and boundary-layer lapse rates accordingly are maximized.

Meanwhile, surface winds will be backed orographically, rendering
veering with height from low-middle levels and low-level
hodographs/SRH at least marginally suitable for supercells.
Forecast soundings suggest around 30-35 kt effective-shear
magnitudes, which is on the low end of what we would expect for
supercells; however, numerical progs likely are underplaying the
magnitude of surface winds as well as directional backing, as usual
in these regimes.

...Southeastern OR/northern NV/southwestern ID region...
Widely scattered thunderstorms are expected to move rapidly
northeastward across this region during mid/late afternoon, along
the northern reaches of the same leading ascent plume noted in the
CA section above. Some of these may produce strong winds. Limiting
factors will include extent and magnitude of favorable
insolation/driven boundary-layer mixing/destabilization, given
presence of antecedent clouds/precip. Forecast surface temps mid
60s F in lower elevations of southwest ID, around the Snake River
Valley, yield nearly dry-adiabatic low-level lapse rates in forecast
soundings with dew points low-mid 40s F and MLCAPE 100-300 J/kg. At
this time, the severe potential still appears too isolated and
uncertain for an outlook area, but one may be needed in an update if
mesoscale trends more clearly favor sustained/substantial subcloud
mixed-layer development.

..Edwards/Gleason.. 03/21/2017

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