SPC AC 190545
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1145 PM CST Sun Feb 18 2018
Valid 191200Z - 201200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF THE SOUTHERN PLAINS...
A few strong to severe storms are possible over a portion of the
Southern Plains, mainly later this afternoon into tonight.
Synoptic upper trough will continue to amplify over the western U.S.
with a building upper ridge over the Southeast States. Between these
two features, several weak vorticity maxima embedded within a
southwesterly flow regime will move through the Southern Plains
during the period. At the surface a cold front from the upper MS
Valley into northwest KS will advance slowly south, possibly
becoming stalled near the KS/OK border. South of this boundary, a
dryline will mix ewd through western OK and western TX during the
afternoon before retreating overnight as a lee cyclone becomes
established over the central High Plains.
...Southern Plains region...
Within a broad southerly flow regime, partially modified Gulf air
with low to mid 60s F dewpoints will advect through the TX and OK
warm sector today. Meanwhile, a modest elevated-mixed layer will
spread eastward and overlap the western fringe of the moist warm
sector contributing to a corridor of 500-1000 J/kg MLCAPE primarily
from western and central OK through southwest TX. However, warmer
temperatures aloft will contribute to a capping inversion over the
more unstable portion of the warm sector during the day. Model
consensus is that showers and a few thunderstorms may be ongoing
early in of the period along warm conveyor belt, primarily from
north central TX through eastern OK into the middle MS valley.
Primary uncertainty this forecast is extent of surface-based
thunderstorm development given presence of widespread clouds,
likelihood of at least a modest capping inversion, and tendency for
the low-level jet to veer and shift eastward during the day.
Potential for heating and mixing in vicinity of dryline may
contribute to at least isolated storms by late afternoon or evening.
Other storms may initiate over the higher terrain of northern Mexico
and spread northeast into southwest TX. Another area of potential
surface-based development will be near the dryline/cold front
intersection over northern OK or southern KS, though any storms
initiating in this area may have a tendency to become elevated as
they move northeast and cross the frontal zone.
Very strong (55+ kt) 0-6 km shear will support potential for
supercells with any storms developing along the dryline or higher
terrain from southwest TX into western OK. Conditional upon storm
initiation, a small window will exist for a tornado during the early
evening, mainly from western OK into northwest TX as low-level
hodographs become more favorable due to the strengthening low-level
jet, and before the boundary layer stabilizes. Otherwise, primary
severe threat is expected to be large hail. Additional storms may
develop later Monday night along evolving warm conveyor belt but
within a strongly sheared environment. Extent of severe threat
overnight is somewhat uncertain given a stable to marginally
unstable near-surface layer. Nevertheless, some of the updrafts
could become near the surface.
Have introduced a marginal risk this update, given the conditional
nature and uncertainties regarding extent of thunderstorm initiation
during the more favorable time of day and evening. An upgrade to
higher probabilities could be needed in later updates over a portion
of this area if these uncertainties can be mitigated.
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