SPC AC 201255
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Mon May 20 2019
Valid 201300Z - 211200Z
...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
NORTHWEST TEXAS AND THE EASTERN TEXAS PANHANDLE TO WESTERN AND
CENTRAL OKLAHOMA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
EASTERN NEW YORK AND NEW ENGLAND...
...SUMMARY...
An outbreak of tornadoes, some potentially long-track and violent,
is expected today into this evening over portions of northwest Texas
into western and central Oklahoma. More-isolated but still
potentially dangerous severe weather, including tornadoes and
destructive winds and hail, is possible in surrounding parts of
Texas, Oklahoma, Kansas, and Arkansas.
--- Technical Discussion ---
...Synopsis...
Two primary features will serve as the main mid/upper-level
influences on convective potential through the period:
1. A synoptic cyclone -- centered initially over NV and embedded
within a persistent western CONUS longwave trough. The associated
500-mb low is forecast to pivot southeastward across the Grand
Canyon region today and the southern Rockies overnight, while the
associated trough at that level evolves from positively to
negatively tilted. This will occur as an upstream, northeastern
Pacific low moves southeastward to western OR. The Southwestern
synoptic wave will yield kinematic fields rarely seen in mid/late
May at their latitudes, for example (by 12Z): 120-130-kt 250-mb jet
max over the TX South Plains/Panhandle region, 90-100-kt 500-mb flow
over northwest TX and western OK, 70-80-kt 700-mb winds over or near
eastern OK, and an evening LLJ potentially exceeding 75 kt across
parts of north TX and OK.
2. A shortwave trough -- now apparent in moisture-channel imagery
over portions of Lower MI and Lake Huron. This feature -- embedded
within the southern belt of cyclonic flow around a more-substantial
perturbation over the James Bay region -- will eject east-
northeastward toward the lower St. Lawrence Valley and Maine through
00Z.
At the surface, 11Z analysis showed a diffuse low over southeast NM
near ROW, with warm front eastward across the DFW Metroplex to
northern LA. A dryline was drawn from southeastern NM into the Big
Bend region of southwest TX. The dryline will mix/advect eastward
across the southern High Plains through the day, reaching the
eastern TX Panhandle and Permian Basin regions by late afternoon.
Thereafter, where not muddled by outflow, the dryline should retreat
northwestward overnight. The warm front is forecast to move/develop
northward across AR, OK and portions of the TX Panhandle through the
day, reaching the Ozarks this evening and overnight. Warm-frontal
progress will be tempered somewhat by the presence of abundant
precip to its north; however, extreme mass response forecast in
advance of the mid/upper-level trough should contribute to enough
warm-sector thetae advection and maintenance to keep the warm-
frontal baroclinic gradient sharp, and the dryline well-defined.
...Southern Plains and vicinity...
For details regarding ongoing hail/wind threats from mainly elevated
storms over the Panhandles/northwest OK area, refer to SPC watches
195/196 and related mesoscale discussions.
A serious outbreak of destructive, tornadic supercells is likely
over parts of this region this afternoon into evening, especially in
the high- and moderate-risk areas. Given the expected fast storm
motions, especially mid-afternoon into evening, a few of the
best-organized supercells may reach an equilibrium with their
already very favorable mesoscale environments long enough to sustain
wide, long-track tornadoes. Very large and damaging hail also is
possible from some of the supercells, along with sporadic severe
downdraft gusts. The hail threat will extend farther north of the
warm front than the wind and tornado potential. The wind-damage
threat may increase this evening as convection aggregates into
clusters or lines, though tornadoes and hail still will be likely
from any relatively discrete storms. QLCS/line-embedded tornadoes
also are possible. Collectively, the multi-episode threat justifies
maintaining a high risk for this outlook cycle.
Scattered to numerous thunderstorms are expected to develop, in
aggregate, across a substantial portion of the southern Plains from
the dryline eastward across OK, perhaps into western/central AR,
southward to the Red River region and northwest TX. Isolated to
widely scattered convection also may develop farther south in north
TX and west-central/southwest TX along/ahead of the dryline this
afternoon and evening. The first episode should involve moist-
sector convection building gradually into potentially tornadic
supercells by early-mid afternoon, between north TX and
central/western OK, perhaps even over eastern OK/western AR, in
subtle confluence/convergence zones. Without a strong EML, MLCINH
will be modest, with only subtle diabatic heating and ambient
theta-e advection necessary to support them.
Thereafter, several supercells should form near the dryline in a
regime of strengthening deep-layer flow and shear. This activity
will move rapidly northeastward into the warm sector, which will be
characterized by large low-level hodographs and buoyancy. With
steep midlevel lapse rates, upper 60s to 70s F surface dew points
and deep moist layers being common, MLCAPE ranging from 2000-5000
J/kg should develop in the moist sector, amidst 55-70-kt effective-
shear magnitudes. Effective SRH 300-500 J/kg should develop --
locally higher, especially near the warm front. Some areas will
experience a rare combination of 3000+ MLCAPE and 400+ J/kg
effective SRH. The parameter space over much of the region today
will support a threat of violent tornadoes, and the likelihood of
multiple significant (EF2+ events).
...Northeast...
Widely scattered to scattered thunderstorms are forecast to develop
this afternoon along/ahead of a surface cold front forecast to cross
the region from west to east. Occasional severe hail and damaging
gusts will be the main concerns, though some supercell potential
exists over the northern parts of the risk area, and a tornado
cannot be ruled out.
Destabilization of the boundary layer is expected throughout the day
and into afternoon, following the passage of a belt of precip now
crossing parts of southern New York and New England in the warm-
advection zone. Both diabatic surface heating and boundary-layer
theta-e advection will contribute to airmass recovery from south to
north ahead of the front. Surface dewpoints 60s F and steep low-
level lapse rates will support 1000-2000 J/kg MLCAPE. Deep shear
will increase northward, supported mainly by speed strengthening
with height, with 35-45-kt effective-shear vectors possible over
parts of New England. The threat should diminish with time this
evening as the remaining inland warm sector shrinks and stabilizes
diabatically near the surface.
..Edwards/Jewell.. 05/20/2019
