Area Forecast Discussion
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FXUS64 KLUB 060429

Area Forecast Discussion
National Weather Service Lubbock TX
1029 PM CST Tue Dec 5 2017

Light rain or snow bands may continue to generate near or just
south of KLBB through the night before sagging southward after
daybreak Wednesday. Given the very dry sub-cloud layer and lack of
stronger RADAR reflectivity in recent hours, choosing not to
include mention of precipitation in this set of TAFs. Short term
solutions are pointing towards better chance of an MVFR cloud
layer remaining just south of KLBB. Drier air in mid levels will
follow a weak surface pressure surge southward into the area mid
to late Wednesday morning. Indicated a shift to north-northeast
component at that time as well. RMcQueen


.PREV DISCUSSION... /issued 942 PM CST Tue Dec 5 2017/

Per regional radars and recent models, have lowered PoPs across
the northern half of the forecast area while also changing precip
type wording to light rain with light snow. 00z upper air analyses
showed a 100 knot upper jet axis largely atop the region, with an
embedded jet streak nosing eastward across southern NM. The left
exit region of this jetlet may be enough to encourage a resurgence
of a zonal band of light precip across the southern South Plains,
not at all unlike the earlier band this afternoon. Models tend to
agree in this scenario gaining more momentum after midnight, but
at the same time models have been too bullish in moistening a very
stubborn and dry sub-cloud layer of 5-10k feet thick. Recent AMDAR
data from Lubbock show this layer has warmed to 40F at 4100 ft
AGL from earlier values in the upper 30s, so there is concern over
how efficient wet bulbing effects will be in allowing snow to
reach the surface. Considering elevated Fn ascent is departing
south of the region, precip amounts were pulled back to a few
hundredths of an inch through Wed A.M. with only light rain and
snow descriptors for such modest ascent. With such mild temps near
the surface, we`d really need moderate to heavy snow rates to get
any accumulations - particularly on grass. Therefore, we reduced
snow accumulation to just a dusting at most, particularly for any
enhanced bands than can develop and train over the same areas.

PREV DISCUSSION... /issued 538 PM CST Tue Dec 5 2017/

Considerable mid and upper level moisture, as a strong upper
level jet core passes overhead. Region alternately will be in left
exit lift region overnight and we may see a few more bands of
mostly light mixed phase precipitation. Too uncertain at this
point to include mention within mainly the KLBB TAF. Later in the
forecast, the region will transition to right entrance upper jet
region, and areas mainly south of KLBB will be favored for
precipitation. Airmass aloft will remain moist through most of
the 24 hour period. At the surface, solutions agree a light wind
flow will dominate the first 15 hours or so before a weak surface
high pressure ridge sags southward mid to late Wednesday morning
but wind speeds appear mostly 11 knots or less. May address this
wind shift with next package. RMcQueen

PREV DISCUSSION... /issued 313 PM CST Tue Dec 5 2017/

Thickening cirrostratus is also lowering this afternoon as a
persistent fetch of upper level moisture originating from the
eastern Pacific streams overhead. There is a slow increase in
upper level synoptic scale lift ahead of a 120 kt jet streak. This
jet streak will eject across the southern Plains pretty much
overhead this evening, leaving the forecast area under its right
entrance region. This will result in a prolonged period of modest
to moderate upper level lift from far West Texas through the
Permian Basin and then points east with mostly the southern half
of the forecast area on the northern periphery of this area of
lift, and thus, precipitation. For the most part models continue
to advertise a couple bands of mid level (near 650 mb)
frontogenesis, one this evening oriented west to east somewhere
from the middle belt of the forecast area (i.e., Lubbock to
Guthrie) to northern third (i.e., Plainview to Paducah) and a
second that develops late tonight and extends into Wednesday
morning across the extreme southern tier of the forecast area. The
former band will likely have little upper support while the latter
one will coincide with the broader and stronger synoptic scale
lift. The biggest negative to meaningful precipitation is the very
dry subcloud air mass in place across the forecast area. Surface
temperature-dew point depressions are on the order of 30-35
degrees this afternoon, helping point to the magnitude of the dry
air. Thus, a multi-hour period of top-down moistening will be
required to get the precipitation to reach the surface, that is as
long as lift and precipitation rates remain about as expected and
do not exceed that by a significant margin. This top-down
moistening will likely end up favoring snow for the dominant
precipitation type on the Caprock, while off the Cap that extra
depth might keep surface temperatures at or slightly above
freezing and should favor a snow/rain mix, if not entirely rain.
All this boils down to snowfall amounts less than 1/2 inch on the
Caprock and little to no accumulation off. The potential for
banded structure might result in some narrow swaths of heavier

Upper lift shifts southward during the day Wednesday with precip
ending from north to south. Beyond that the forecast looks benign.
Meridional upper flow with a ridge anchored on the West Coast
through Friday points to maintaining cooler than normal
temperatures. The ridge will deamplify and move eastward toward
the Plains through the weekend with temperatures approaching
seasonal normals.




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