Severe Storm Outlook Narrative (AC)
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ACUS11 KWNS 192001
SPC MCD 192000

Mesoscale Discussion 0188
NWS Storm Prediction Center Norman OK
0200 PM CST Sun Feb 19 2017

Areas affected...Parts of north TX and southern OK

Concerning...Severe potential...Watch possible

Valid 192000Z - 192230Z

Probability of Watch Issuance...60 percent

SUMMARY...The severe threat is expected to continue increasing
through 22-23Z (4-5 PM CST), with large hail and perhaps a couple of
tornadoes the main threats expected. Watch issuance is possible.

DISCUSSION...Low-level convergence is weak along an ill-defined
dryline/cold front feature extending north-south across much of west
TX early this afternoon. Regardless, as a southern-stream shortwave
trough/low approaches the southern Plains from the west this
afternoon and evening, large-scale forcing for ascent should
gradually increase across north TX and southern OK. Although
widespread cloudiness has hampered diurnal heating somewhat south of
a warm front located across north TX/southern OK, continued
low-level warm air advection and earlier clearing this morning has
allowed surface temperatures to generally warm into the upper 60s to
low 70s across this area. Adequate low-level moisture to the south
of the warm front, with surface dewpoints in the low to mid 60s,
will support convective development/maintenance.

A modestly steepened mid-level lapse rate plume will persist over
the discussion area through this evening, and combined with
sufficient low-level moisture, a corridor of weak to moderate
instability (500-1500 J/kg MLCAPE) is present across much of
central/north TX into southern OK, decreasing with northward extent.
Largely unidirectional southerly flow strengthening with height is
occurring across the same general area in association with the
approaching mid-level trough/low. Effective bulk shear values of
40-60 kt will easily support supercell structures. Some backing of
the low-level wind field to southeasterly will be possible across
parts of north TX into southern OK per latest RAP guidance, as a
southerly/southeasterly low-level jet is forecast to strengthen
across the eastern fringes of the discussion area by early evening
(around 00Z).

Initial convective development will likely occur by 22-23Z as
convective inhibition erodes with continued modest daytime heating
and low-level moistening. This activity should be mainly discrete
supercells posing a large hail threat, with some initial storm
splits likely given the largely unidirectional flow. If storm mode
can remain mostly discrete into the early evening hours while
remaining surface based, then the possibility for a couple tornadoes
would increase as the low-level jet strengthens and effective SRH
likewise increases. Eventual upscale growth into a QLCS with some
wind threat appears probable by mid to late evening.

..Gleason/Guyer.. 02/19/2017

...Please see for graphic product...


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