SPC AC 282007
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0307 PM CDT Tue Mar 28 2017
Valid 282000Z - 291200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS WESTERN
NORTH TX AND SOUTHWEST OK...
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL OK
TO THE EDWARDS PLATEAU OF SOUTHWEST TX...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
SOUTHERN PLAINS INTO PARTS OF EAST AND NORTHEAST NM...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS VA AND
Numerous severe storms are expected across the southern Great Plains
including parts of Texas and Oklahoma, especially late this
afternoon and evening. Very large hail, damaging winds, and
tornadoes are likely.
Previous forecast remains valid as initial severe storms have
developed earlier this afternoon at the triple point in west-central
TX near LBB and southward along the dryline. Given trends/evolution
of these storms and placement of the dryline per mesoscale analysis
extending southwest into Pecos to Brewster counties, the Marginal
and Slight risk areas and the general thunderstorm line have been
shifted west. The rest of the southern extent of the severe
probabilities and overall severe-weather threat remains valid for
this forecast period.
Meanwhile, ongoing storm interactions of discrete supercells
extending from the TX counties of Floyd to western Kent to Howard
per radar imagery would suggest some diminishing trend in the threat
for a greater coverage of hail. However, ongoing poleward
destabilization (moderate instability into northwest TX) combined
with strong effective bulk shear suggests the moderate risk for hail
(some very large) remains valid at this time. The enhanced tornado
risk persists across northwest TX into southwest OK.
Farther north, the Enhanced risk and significant hail threat have
been trimmed some across parts of the southeast and south extension
of the TX Panhandle, based on trends in radar imagery.
...East-central and northeast NM...
The marginal severe risk area has been expanded some across
east-central and northeast NM as a midlevel impulse moves north
across eastern NM this afternoon into the early evening. Cooling
500-mb temperatures/steepening midlevel lapse rates with this
impulse and height falls ahead of the approaching synoptic trough
will support additional thunderstorm development into the early
evening. Marginal instability should limit the overall coverage of
severe storms, with isolated hail and/or locally strong wind gusts
the primary threats. Reference Mesoscale Discussion 354 for
additional short-term details.
The forecast remains valid for a marginal severe-weather threat this
afternoon with overall risk diminishing by early evening. No
changes needed across this region in the convective outlook.
Reference Mesoscale Discussion 352 for additional short-term
.PREV DISCUSSION... /ISSUED 1130 AM CDT Tue Mar 28 2017/
A combination of 12Z upper-air analysis and water vapor satellite
imagery features an east/southeastward-digging shortwave trough over
southeast AZ/southwest NM at late morning, with a preceding belt of
40-50 kt mid-level southwesterly winds overspreading eastern NM and
west TX. These south-southwesterly mid-level winds will continue to
advect a stout elevated mixed layer east-northeastward today atop an
increasingly moist sector ahead of a dryline and upstream Pacific
front. Regional 12Z soundings and upper-air analysis featured 7.7+
C/km 700-500mb lapse rates across all of west TX, while around 14
g/kg mean mixing ratios were noted at Del Rio and Corpus Christi TX,
with 850 mb dewpoints of 12C+ as far north as Midland/Dallas-Fort
Initially, increasing elevated storms may pose a severe hail risk
early this afternoon across the TX panhandle/northwest TX. Reference
Mesoscale Discussion 350 for additional short-term details.
To the east of the dryline, low clouds remain prevalent at late
morning, although a relatively narrow upper dry slot continues to
steady approach west Texas and low clouds are steadily eroding
across the Permian Basin vicinity. As additional moistening/heating
occurs, moderate to strong destabilization with minimal inhibition
is expected particularly near the surface low/warm
front/dryline-related triple point across the TX south Plains/low
rolling plains, southward to near/east of the dryline as it extends
into the Permian Basin vicinity. Here, aforementioned moisture/lapse
rates should yield MLCAPE in excess of 2500 J/kg.
Current thinking is that at least isolated surface-based severe
storms will develop near the triple point vicinity by around
mid/late afternoon, with additional development possible near the
dry line. Although current (late morning) cloud cover casts some
uncertainty, a somewhat displaced corridor of warm-conveyor-related
storms are expected by late afternoon/early evening farther east
across additional parts of western north TX into southwest OK.
Initially discrete supercell mode should favor large hail and
localized severe wind gusts. Although the upper flow pattern will
become increasingly meridional with time (with general tendency of
backing mid-level winds), and a more linear/mixed mode should evolve
during the evening, enlarged low-level hodographs will support the
potential for tornadoes through at least early/mid-evening. Linear
bands should accelerate northeast into southern/central Oklahoma
during the evening, with attendant risks for damaging winds, hail,
and a couple tornadoes.
Farther south across west-central/southwest TX including the Edwards
Plateau, initially isolated late afternoon/early evening storms near
the dryline should become widespread in coverage during mid/late
evening as the Pacific front merges and overtakes the dryline. This
should result in a predominant risk to large hail transitioning
quickly to strong to severe wind gusts. An extensive squall line
will likely evolve east overnight with at least an isolated severe
risk spreading eastward into central TX including the Hill
Country/Interstate 35 corridor late tonight.
A dampening shortwave impulse over WV will shift off the VA/NC coast
by mid-evening. Ongoing showers and isolated storms will subdue
destabilization early in the diurnal heating cycle. However, by late
afternoon, widely scattered storms should develop ahead of the
impulse. Increased veering of the wind profile with height should
foster mainly discrete cells, some of which should weakly rotate.
Isolated severe hail and damaging winds will be possible.
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