NOAA Storm Prediction Center

Convective Outlooks

Convective Outlooks
The convective outlooks serve as guidance to the local NWS forecast offices and are used by emergency managers, private sector meteorologists, media, and other weather customers concerned with public safety. Three separate risk areas (slight, moderate, and high) are used to describe the expected coverage and intensity for the categorical severe weather threat on days 1-3 along with severe weather probabilities for the potential threat.

Day 1 Outlook Day 2 Outlook Day 3 Outlook Days 4-8 Outlook

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

ACUS01 KWNS 191301
SPC AC 191300

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0700 AM CST Wed Dec 19 2018

Valid 191300Z - 201200Z


Isolated, marginally severe thunderstorms may affect parts of
extreme southeast Texas and southwestern Louisiana through early

A high-amplitude, progressive mid/upper-level trough is forecast to
develop over the central CONUS and western Gulf by late tonight.  An
initial/southern-stream perturbation -- apparent in moisture-channel
imagery from southern OK across central TX to northeastern MX with a
vorticity max near SJT -- will eject northeastward and weaken
considerably.  However, several currently weaker vorticity lobes now
located over the northern/central Rockies will dig southeastward and
contribute to overall wave amplification, increasingly in phase with
northern-stream shortwave perturbations over the northern Plains to
Upper Midwest.  By 12Z the 500-mb trough should extend from
southeastern MB across eastern NE, central OK, the lower TX Coast,
and the extreme western/southwestern Gulf.

At the surface, a weak/complex area of low pressure was analyzed at
11Z over the middle/upper TX coast, with quasistationary front
southward just offshore northeastern MX.  A strengthening
marine/warm frontal zone was drawn from the northern part of the
low-pressure area, offshore GLS, east-southeastward over the
north-central/east-central Gulf.  The low is expected to organize
gradually while generally shifting eastward along the northern Gulf
Coast throughout the period.  To its south and southwest, a cold
front will move eastward over the northwestern/west-central Gulf
while strengthening.  By 12Z this front should extend south of the
western FL Panhandle across the central to east-central Gulf.  The
marine/warm front should move slightly inland over extreme southeast
TX and southern LA today.  The latter boundary also should shift
northeastward across the northeastern Gulf, its progress altered on
the mesoscale by areas of convection and related precip.

...West-central Gulf Coast...
Widely scattered thunderstorms are expected to form the next few
hours over portions of southeast TX and the adjoining Gulf, moving
east-northeastward to northeastward, with the risk of damaging gusts
or a brief tornado.

This potential will develop as large-scale lift related to the
mid/upper trough impinges upon increasing, mostly elevated low-level
moisture transport and warm advection.  Through midday, a narrow
wedge of return-flow boundary layer that is partly modified, yet
still buoyant for surface-based parcels, should cross the outlook
area from west to east along and south of the marine boundary. 
Surface dew points low 60s F and roughly 1.25-inch PW should
characterize this air mass, with MLCAPE 500-1000 J/kg developing. 
Low-level flow will be modest to the east of the still-organizing
low, limiting hodograph size.  However, strengthening winds aloft
will contribute to favorable deep shear, with forecast soundings
suggesting 40-50-kt effective-shear magnitudes.  Any convection
accessing the warm sector or warm-frontal zone long enough to
mature, before crossing the front and becoming weaker/elevated, will
have the potential to become organized multicells or transient
supercells.  The threat will subside through late afternoon as
1.  The initial shortwave ejects/weakens, and in response,
2.  The most favorable low-level mass response (including warm
advection) likewise weakens, and ejects away from the warm front.

...Southwest FL, extreme western Keys, late tonight...
Overall, a favorable unconditional severe potential reaching
categorical thresholds still is not expected to affect the mainland
or lower Keys until at least early in the day-2 period.  Some of the
most aggressive guidance indicates the potential for supercells over
the southeastern and extreme east-central Gulf, developing ahead of
the main frontal/prefrontal convective band over the central Gulf. 
Such activity could approach the southwestern peninsular coastal
waters and lower Keys to Dry Tortugas area late overnight or early
tomorrow morning.  Any such convection would occupy a destabilizing
environment in and near the eastern limb of the marine warm front,
characterized by temporal increases of both deep shear and low-level
theta-e.  Any faster/more-consistent potential that becomes apparent
in future outlook cycles may compel greater concern for the last few
hours of day-1 (i.e., after about 09Z).

..Edwards/Goss.. 12/19/2018