Area Forecast Discussion
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438
FXUS64 KLUB 241728
AFDLUB

Area Forecast Discussion
National Weather Service Lubbock TX
1228 PM CDT TUE MAY 24 2016

.AVIATION...
There is certainly a threat for TS within a 30 mile radius of KCDS
in a window from about 23z-05z, but actual areal extent of the
coverage and more precise timing leads to a lack of enough
confidence to insert a TEMPO group at this time. Otherwise,
generally expecting VFR conditions through the period. Veering low
level flow later tonight will likely keep stratus shunted to the
south and east of the terminals Wednesday morning.

&&

.PREV DISCUSSION... /issued 1124 AM CDT TUE MAY 24 2016/

UPDATE...
Will be on convective watch again this afternoon as areas east of
the dryline will have MLCAPE on the order of 3500-4000 J/kg.
Models indicate a weak cap in place with MLCIN generally single
digits and convective temps near or a degree or so above forecast
highs. Modest low level convergence or even confluence in addition
to any topographic effects should winds east of the dryline back a
bit from late morning direction should be enough for convection to
initiate. Models not in terribly good agreement with either
chances of initation or placement of convection, so confidence not
too high at the moment. However, upper support in the form of a
modest mid level speed max is expected to move out of southern New
Mexico to the South Plains late afternoon to early evening. This
would support the most convective converage from the northeastern
quadrant of the forecast area and points to northward. Progged
bulk shear values easily support supercellular storm structure
given the amount of instability expected. It may take a bit of
time for the storms to organize enough to support that structure,
but there is certainly a risk again of large hail and damaging
winds. There is also a tornadic risk across the northeastern
zones, especially if the low level flow is able to back later this
afternoon in response to pressure falls west of the dryline. In
summary, previous forecast handles this scenario well. Grids are
in good shape, with no updated needed late this morning.

PREV DISCUSSION... /issued 630 AM CDT TUE MAY 24 2016/

AVIATION...
Patchy low stratus will continue in the vicinity of all terminals
through 14 or 15 UTC. Ceilings will be mainly VFR with shorter
periods of MVFR or IFR. Light fog may also reduce surface
visibility through about the same time. Then, VFR conditions
should prevail at all terminals, although there is a chance of
TSRA in the vicinity of KCDS late this afternoon and evening,
which is represented by a TEMPO group.

PREV DISCUSSION... /issued 344 AM CDT TUE MAY 24 2016/

SHORT TERM...
Same song, different verse. We will continue to have southwest flow
overhead today downstream of broad troughing across the western U.S.
It is difficult to pick out any significant embedded impulses in the
water vapor imagery and model guidance seems to confirm by showing
vertical motion remaining largely neutral over West Texas through the
day. At the surface, a small t-storm complex moving from the
southeast TX Panhandle through the Red River Valley early this
morning has laid down an outflow boundary which surged westward as
far as Dimmitt, but which has appeared to stall out recently.
In the near-term, the main forecast issue is to what extent low
stratus will cover the forecast area this morning. The HRRR appears
to have the best handle on the current coverage, and we`ve have
followed it`s forecast closely, showing stratus filling in across
most of the area from now through about 7 am, then eroding from
the edges in pretty quickly during the morning, with ceilings
clearing out by 10-11 am or so. The dryline is forecast to mix a
little farther to the east today, perhaps to around the Caprock edge
by peak heating, 3-4 pm.

We suspect that the current outflow boundary, while gradually washing
out, may hold the dryline`s progress back a little bit to the west -
especially across the northern half of the forecast area, so we have
held PoPs a little farther back to the west than the latest guidance
would support. As for convective initiation, guidance is again
focusing in on our northeast quadrant, generally from eastern Floyd
and Briscoe Counties eastward, with lower chances farther south on
the dryline. This appears reasonable and we have trended PoPs in that
fashion. The overall convective environment has changed little from
previous days, impressive MLCAPES of 3000-4000 J/kg will build up
east of the dryline. Deep-layer shear, while not as impressive,
should be adequate for rotating updrafts and possibly support a long-
lived supercell t-storm or two. The tornado threat appears to hinge
on any outflow boundary interaction and will likely be limited to the
same area as the past two days, namely eastern Briscoe, Hall and
Childress Counties.

Any t-storm activity should track eastward out of our area by 10 or
11 pm. Overnight, the dryline will retreat westward across the
south much more quickly than the north /absent any outflows/. Some
guidance suggests that approaching shortwave energy from southwest
Texas could interact with the returning moisture to bring a
small chance of some early morning t-storm activity across the
southern South Plains and Rolling Plains.

LONG TERM...
The semi-permanent western trough will eject an upper low via the
Desert Southwest later this week. This low is progged to track
from southern California midday Wednesday through the Four Corners
Thursday and then into the Front Range of Colorado on Friday. This
synoptic evolution will maintain southwesterly flow aloft over
West Texas while the dryline continues to roam the region too. It
still does appear there will be a sufficient westerly component to
the flow on Wednesday to push the late day dryline and best storm
chances just east of the CWA. However, the respite from the storms
will be short-lived as backing flow aloft in advance of the
approaching upper low should help maintain the dryline in the
Rolling Plains on Thursday afternoon. Strong instability and
sufficient deep layer shear will mean the risk for additional
severe thunderstorms along and ahead of the dryline late Thursday.
In fact, the retreating dryline could also become active up onto
the Caprock Thursday evening/night as lift from the upper low
glances the area and the moist front meets up with an approaching
Pacific front. Given the magnitude of the instability and the
prospects for increased forcing overnight we did decide to include
a severe mention in the grids where PoPs were carried (roughly
along and east of a Silverton to Lubbock to Brownfield line)
Thursday night. Breezy and dry westerly winds will spread across
the CWA on Friday taking the best opportunity for more robust late
day convection with it.

Low amplitude shortwave ridging will follow on Saturday providing
relatively quiet and warm weather to kick off the holiday weekend.
Moisture should then quickly return on Sunday as southwesterly flow
aloft redevelops overhead downstream of the reloading western
trough. This will set the stage for more rounds of stormy and
potentially severe weather for the remainder of the Memorial Day
weekend. Deep layer wind shear may initially be on the weaker side
Sunday, though instability may overcome that to some degree. This
stormy pattern may persist right into the middle part of next week,
right on schedule for late May and early June which typically
coincides with the peak of severe weather on the South Plains.

Temperatures will remain on the warm side through the extended with
only minor cooling associated with the trough passage late week.
Overall, the blended guidance had a good handle on the forecast
though wind speeds were increased during the day west of the dryline
Wednesday through Friday.

&&

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

$$

07



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