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Day 1 Outlook Convective Tornado
Hail Wind
Categorical Day1 1300Z Outlook
Images courtesy of the NWS Storm Prediction Center
 Forecast Discussion

ACUS01 KWNS 201301
SPC AC 201259

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0759 AM CDT Tue Mar 20 2018

Valid 201300Z - 211200Z




Damaging thunderstorm winds, large hail and a few tornadoes are
possible today from the Florida Peninsula northward to portions of
the coastal Carolinas.

In mid/upper levels, a substantial synoptic-scale trough in the
eastern U.S., with an anchoring cyclone, should evolve through the
period as the result of the interaction of several shortwave
perturbations.  A small/closed 500-mb cyclone currently near CVG
will weaken and eject eastward into a larger-scale progressive
ridge.  However, the next substantial shortwave trough -- now
positively tilted from the Mid-South across northwestern LA to
central TX -- will phase with a smaller perturbation digging
southeastward across the lower Missouri Valley.  As this occurs, the
southern trough will amplify somewhat and become less positively
tilted.  By 00Z, the net 500-mb trough should reach eastern TN, GA,
the central FL Panhandle, and the northeastern Gulf.  The northern
part will merge with the upstream perturbation to form a mid/upper
cyclone by 12Z over the south-central Appalachians, while the
southern part ejects rapidly northeastward over the Atlantic.

At the surface, 11Z analysis showed an elongated area of low
pressure from WV to extreme eastern TN and extreme western NC, with
occluded then cold front southwestward across west-central GA,
southeastern AL, and the west-central FL Panhandle.  The synoptic
warm front was diffusely evident over northern SC and southeastern
NC; however, strong outflow to its south from an overnight MCS has
shunted the effective baroclinic zone southward.  That boundary --
analyzed over central/southeastern GA and offshore SC, should
retreat northeastward through portions of eastern SC and perhaps
extreme southern NC before passage of the cold front, and/or the
main convective band discussed below.  A weak low appeared to be
forming on the diffuse synoptic boundary over southeastern NC, but
should become better-defined through the day as baroclinicity in its
proximity increases, and it ejects offshore from the Hampton Roads
area.  A secondary low should form along the cold front today over
northern/eastern SC and move offshore from ILM after 00Z.  By 00Z,
the cold front should extend southwestward near the SC/GA coasts
then across northwestern FL.  The cold front is expected to exit FL
entirely by 12Z as the Atlantic cyclone strengthens well east of the
VA coast.

Scattered thunderstorms should develop primarily in prefrontal
low-level convergence zones, offering damaging gusts, large hail and
a few tornadoes.  The activity should take the form of multiple
episodes of southwest/northeast-oriented convective bands, from
later this morning through at least early evening, in broken to
solid form.  Embedded supercells, bows and LEWPs are possible.  The
earliest episodes may evolve through the remainder of this morning
from activity now evident in satellite and long-range radar imagery
over the northeastern Gulf, with additional development expected
over both the Gulf and peninsular Florida through the remainder of
the daylight hours.

Severe potential will be supported by an uncommon juxtaposition of
lapse rates, moisture and shear for this area in late March. 
Eastward transport of a plume of climatologically steep
mid/upper-level lapse rates is expected ahead of the shortwave
trough.  Its trajectories emanated from a southern Plains/Mexican
EML that has been altered only minimally (at most) by convection. 
This plume will overlie boundary-layer southwesterlies advecting
favorably moist air from the Loop Current area of the Gulf. 
Low-level theta-e advection shortly will finish eradicating the
residue of yesterday's outflow air (at and above the surface) from
the Atlantic coastal areas of the central/northern peninsula, making
upper-60s to low-70s F dew points and mean mixing ratios 13-15 g/kg
common.  Thus setup thermodynamically, the final destabilization
process will be diabatic surface heating, which should remove the
small remaining MLCINH and boost buoyancy considerably over the next
few hours, with MLCAPE reaching 1500-2000 J/kg.

Meanwhile, deep shear should strengthen, with effective-shear
magnitudes 40-50 kt already evident at this time in modified RAOB
data and objective analyses, despite the veered/southwesterly
surface winds.  0-1-km SRH will be hindered somewhat by the nearly
unidirectional flow and long, modestly curved nature of the
hodographs; however, effective SRH 150-250 J/kg should remain common
through most of the day in the prefrontal/preconvective areas.  Such
shear should support a mix of convective modes involving storm-scale
rotation.  Into this evening and tonight, with southward extent and
time, shear, lift and instability each will diminish, reducing
convective coverage/intensity.

...Southeastern GA, SC, extreme southern NC coastal areas...
Scattered thunderstorms this afternoon may offer a threat for
damaging gusts, severe hail and perhaps tornadoes.  A mixed-mode,
broken to solid band of thunderstorms is most probable, given the
geometry of the low and middle-level mass fields.  As in FL, the
potential exists for embedded supercells, bows and LEWPs.

Low-level warm advection, as well as surface heating in the wake of
the departing MCS-related cloud cover, will aid destabilization, as
the outflow boundary retreats northeastward and likely diffuses
somewhat.  Meanwhile, cooling aloft is expected ahead of the
approaching mid/upper-level trough.  A narrow inland corridor of
1000-2000 J/kg MLCAPE should result, amidst strengthening deep shear
(e.g., effective--shear magnitudes 45-55 kt).  Nearly unidirectional
wind profiles are expected, but still with enough convergence to
breach the weakened MLCINH layer and sufficient low-level shear to
support storm-scale rotation as well.  Activity should move offshore
this evening.

..Edwards/Peters.. 03/20/2018