Area Forecast Discussion
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FXUS64 KLUB 211704
AFDLUB

Area Forecast Discussion
National Weather Service Lubbock TX
1204 PM CDT SAT MAY 21 2016

.AVIATION...
IFR cigs will continue over the next hour or so at all three
terminals by which time they should become VFR. Thunderstorms are
seeming more likely this afternoon/evening at LBB and PVW. The
timing of the thunderstorms is a bit uncertain but is expected
between 6-9pm. Some thunderstorms may be severe and carry the risk
for strong winds and large hail. A chance for thunderstorms also
exists at CDS, but certainty is not high enough to warrant a TAF
mention. Cigs should become BKN to OVC close to midnight but
should remain in the VFR category. Some fog and sub- VFR cigs are
possible early tomorrow morning, but confidence is to low to
mention in the TAFs at the moment.

&&

.PREV DISCUSSION... /issued 434 AM CDT SAT MAY 21 2016/

SHORT TERM...
Starting today more typical late May weather is in store for the
region with warm temperatures and an active dryline in the area
providing the potential for severe weather beginning late this
afternoon.

Temperatures and dewpoints were running about 10 degrees higher than
yesterday at this same time as southerly/southeasterly winds
continue to transport modified Gulf of Mexico air this direction. We
should see low clouds and patchy fog develop later this morning,
with low clouds blanketing much of the CWA by daybreak. These clouds
will gradually lift and scatter late this morning and early this
afternoon with strong insolation the second half of the day
propelling highs into the middle 80s. Aloft the shortwave ridge that
was occupying the central and southern High Plains will give way to
southwesterly flow as a broad western low shifts slowly eastward
over the coming 24 hours. Falling pressures in the lee of the
Rockies will tighten the pressure gradient locally which will
provide breezy and moist southerly winds through the day while a
dryline sharpens near the TX/NM line by late this afternoon. Forcing
along the dryline should become sufficient to overcome any remaining
convective inhibition leading to isolated to scattered thunderstorm
development across the far western zones by late this afternoon,
while other convection developing within a WAA regime could flirt
with the far southeast zones. Moderate instability on the order of
1500-2000 J/kg of CAPE will support strong updrafts, while 0-6 km
shear around 30 knots would tend to favor multicell organization,
though occasional supercell structures could also manifest
themselves. Modestly high LCLs and progged 0-3 km helicity values of
100-200 m2/s2 are not tremendous, but could support a brief tornado,
but the primary threats will be large hail and damaging winds. The
convection may try to organize into some form of an MCS as it
translates eastward through the evening/night, though it is a bit
unclear how much convection will persist after 06Z. A moist 25-35
knot LLJ feeding into any residual convection/boundaries plus
perhaps a weak disturbance in the southwest flow aloft argue for
keeping a decent chance of storms in the forecast late tonight,
favoring the eastern half of the CWA. Before then, we have
maintained a severe mention in the grids across roughly the western
half of the FA from 20-00Z, then for the entire area in the 00-06Z
time-frame.

Although not everyone will see precipitation late today/tonight,
relatively slow storm motions (especially for any right-moving
supercells or back-building convection) will also mean a risk for
locally heavy rainfall.

The increasingly moist southerly winds will translate to another
mild night tonight with lows mostly in the 60s. Additionally, we
could see another round of low clouds by Sunday morning, though
overnight convection could complicate this a bit.

LONG TERM...
We have a potentially volatile but complicated severe weather
set-up for Sunday. Generally speaking, the ingredients should
fall in place for severe thunderstorms on Sunday for most of the
South Plains area:
1. A dryline near or just east of the TX and NM state line by peak
heating with healthy surface dewpoints in the lower 60s to the
east of the dryline.

2. Modest large-scale ascent from an approaching small shortwave
is expected to overspread the area in the afternoon hours.

3. An approaching upper-level jet will bring increasing mid-level
flow while cyclogenesis over southeast Colo. will back and
increase the low-level winds, resulting in strong shearing and
turning in the wind profile.

Mixed-layer CAPE values are expected to be in excess of 2000 J/kg
east of the dryline, and perhaps closer to 3000 J/kg while 0-6
bulk shear values are progged in the 30-50 kt range. Rotating
updrafts will be readily supported in this environment with the
threat of very large hail and damaging wind gusts. The overall
tornado threat will likely be dampened somewhat by marginal values
of 0-1 km shear. However, in the presence of a supercell with
strong deviant motion or outflow boundary interaction, locally
enhanced low-level helicity may support tornado development.

The complicating factor is the potential for morning convective
activity to interfere with daytime destabilization. Much of the
guidance does show ongoing storms to varying degrees across the
area with cloud cover and cooler temps possibly inhibiting
subsequent development later in the day. For now, we will keep
PoPs in the chance to high chance category to account for this
uncertainty and note that coverage and intensity of storms could
be lower depending on what actually transpires Sunday morning.
Sunday night, we would not be surprised to see a small t-storm
complex or two evolve and move across the Rolling Plains bringing
the threat of severe winds and locally heavy rain.

For next week, the large upper trough will reload several times
while shortwaves eject northeastward across the southern Rockies
into the central plains. Swift southwesterly flow aloft will
likely keep deep moisture confined mainly to the lower elevations
off the Caprock. Shallow moisture may slosh back west on the
Caprock at night, but will likely mix out during the day. The
exception may be across the southeastern South Plains, such as
Garza, southeast Lynn, and perhaps southern Crosby Counties. In
any event, it looks like the best t-storm chances will be focused
on the eastern Rolling Plains through the week. A subtle
shortwave on Tuesday and stronger one on Thursday may boost
chances for any areas that can hold on to the moisture around peak
heating. There is some indications in the med-range guidance that
a deeper upper-low will establish itself out west by next
weekend, which could be the impetus to draw rich moisture back
west and provide more widespread thunder chances but this is still
a ways off.

&&

.LUB Watches/Warnings/Advisories...
None.
&&

$$

51/99/99


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