SPC AC 181953
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0253 PM CDT Sun Mar 18 2018
Valid 182000Z - 191200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
NORTH-CENTRAL TEXAS EAST-SOUTHEASTWARD ACROSS THE ARKLATEX...GULF
COAST STATES AND SOUTHERN GEORGIA...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
THE SOUTHERN AND CENTRAL PLAINS EAST-SOUTHEASTWARD INTO THE
ARKLATEX...LOWER MISSISSIPPI VALLEY AND SOUTHEAST...
Thunderstorms with large hail and a tornado risk are possible
especially across North and East Texas late this afternoon into
evening, with additional severe thunderstorms possible across other
parts of the south-central Plains to Southeast States.
Several changes have been made to the outlook for this issuance. The
first change is to remove the slight risk area from near the Red
River in northern Texas/southern Oklahoma extending eastward across
most of Arkansas into north-central Mississippi. Instability is very
weak along this corridor and the stronger instability is forecast to
remain to the south. The second change to the outlook is to expand
the slight risk area westward into north-central Texas where cumulus
is developing along an axis of moderate instability. The current
thinking is that surface-based convective initiation will take place
just to the west of the Dallas/Fort Worth metro, see MCD 0133.
Moderate instability and strong deep-layer shear will support a
conditional threat for supercell formation and hailstones greater
than 2 inches in diameter. The tornado and hail probabilities have
been realigned along the corridor with the greatest potential for
supercell development. The final change is to remove the marginal
from parts of north-central Kansas into southeast Kansas, northern
Arkansas and northern Mississippi where instability is not expected
to be strong enough to support marginal severe threat.
.PREV DISCUSSION... /ISSUED 1156 AM CDT Sun Mar 18 2018/
...North/East Texas and far Southern Oklahoma to ArkLaTex...
The potential exists for locally intense storm development later
this afternoon/early evening, but a number of questions remain
regarding the likelihood and locations/coverage of deep convective
development. Along these lines, various convection-allowing models
have shown semi-dramatic sub-regional spatial shifts between 00Z and
12Z of preferred severe corridors later today, while various HRRR
runs have also exhibited considerable run-to-run variability
(supercells vs. essentially no development) this morning.
Scattered showers and some thunderstorms along with multi-layer
cloud cover remain prevalent across the region at midday, related to
east/northeastward-transitioning southern-stream forcing for ascent
embedded with a strong (100+ kt at 250 mb) subtropical jet stream.
In the wake of this scattered precipitation/thicker clouds, cloud
breaks and some additional low-level moist influx should allow for
relatively rapid destabilization especially across
north-central/east-central Texas to the near/east of a dryline and
along/south of an east/southeastward-extending front.
But given some hindered heating through early afternoon and the
early timing of the aforementioned southern-stream impulse, the
likelihood and extent of deep convective development later today
remains questionable, particularly given that areas such as
north-central Texas may be influenced by subsidence aloft during
peak heating. Accordingly, the overall scenario seemingly remains
largely conditional, with the notion that any sustained
surface-rooted development could evolve into supercells capable of
very large hail and some tornado risk in the presence of steep
mid-level lapse rates, moderate buoyancy, and favorable hodographs
for large diameter (potentially significant 2+ inch) hail.
Further outlook refinements/adjustments can be expected with the 20Z
...Lower Mississippi Valley to Southeast States...
A weak impulse or two (and/or MCV) should semi-focus thunderstorm
clusters across the region today, such as is occurring across
southern Mississippi at midday. While buoyancy will not be overly
strong, 40+ kt effective shear could support some bowing segments
and possibly even a transient supercell or two. Damaging winds and
possibly some hail should be the primary concerns.
...Portions of Texas Panhandle/western Oklahoma into Kansas...
At least some severe-weather potential should exist later this
afternoon through early evening with anticipated low-topped storm
development across the region. Very limited moisture will exist
within/north of a narrowing warm sector, but steep lapse rates and
moderately strong wind profiles could support some strong to severe
storms across the Texas Panhandle/northwest Oklahoma northeastward
into southern Kansas.
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z