Area Forecast Discussion
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FXUS64 KLUB 122140

Area Forecast Discussion
National Weather Service Lubbock TX
340 PM CST Thu Jan 12 2017

East-northeast winds have spread across the forecast area today and
temperatures have remained fairly flat since late morning. The cold
air near the surface will deepen tonight while isentropic lift will
be on the upswing. Forecast cross-sections show that a layer around
850 mb will begin to saturate this evening with moist advection over
the cold air. Low stratus will spread northwestward eventually
covering most if not all of the forecast area with overcast skies by
early Friday morning.

The coldest guidance has performed better so far today with this
shallow polar airmass, so we have followed it closely for
temperatures tonight and Friday. Lows tonight are expected to be
around freezing or a few degrees below expect for the southwest
South Plains, where temperatures may remain a few degrees warmer.
The airmass below 850 mb will initially be very dry, but will begin
to saturate due to cooling and moist advection. Around midnight or
so, we expect to see pockets of drizzle or light rain showers
develop first in the southern Rolling Plains then spread north and
west through the night and early Friday morning. The warm layer
centered around 850 mb looks to be pretty stout, with temperatures
as warm as 10 degrees c. However, the airmass between about 800 mb
and 400 mb is very dry. The shallow saturation should keep precip
amounts through the morning on the light side, while the warm nose
will have precip initially as liquid in that layer. However, with
surface temps near or slightly below freezing and some sub-cloud
evaporative cooling to consider, there is the potential for some
light icing Friday morning. Whether or not the ice will cause travel
issues will depend on just how cold the surface is and how much
precipitation falls (since it will be very light overall, ranging
from just a trace to maybe 2 or 3 hundredths). In general, the areas
of most concern will be from about Lubbock County to the north and
east - favoring the colder temperatures. At this time, we will hold
off on any advisories or other highlights as the confidence in
widespread ice developing on roads just isn`t there - however, this
could certainly change later on tonight if confidence increases in
accumulating ice.

Weak to moderate isentropic lift will continue across the forecast
area during the day Friday, continuing the chance of precipitation.
However, dry air about 800 mb will limit precipitation production.
Temperatures will remain cold but should warm above freezing across
at least western and southern areas in the afternoon. Freezing rain
or drizzle will remain a threat in the far southern Panhandle and
adjacent portions of the South Plains and Rolling Plains where we
anticipate temperatures will remain at or just under the freezing

Summary of changes: The greatest PoPs of 90-100 percent have
been confined to Sunday to reflect a continued slowing of the upper
cyclone`s track. We have also re-introduced some freezing rain on
Saturday morning over all but our far western zones, but otherwise
the forecast theme from recent days is still on track with a
continued preference for the colder NAM given its performance with
today`s cold front and temps in its wake.

Friday night opens with a steady uptick in moist isentropic ascent
as a 35-40 knot southerly LLJ transports much deeper Gulf moisture
poleward. Current feeling is that this advection component will be
most productive in generating precip near the remnant baroclinic
zone poised in a SW-NE fashion somewhere on the Caprock. Models also
keying on a 700 mb shortwave trough reaching this boundary toward
daybreak Saturday, so a window of stronger ascent should result with
some light-moderate rainfall. Temp-wise, we used a blend of our
prior forecasts with the 12Z 18Z NAM runs to establish freezing
temperatures over all but the western 2/3rds of our CWA late
Fri through Sat morning, but the overall marginality of these
values so close to or just below freezing could offer some relief
from steadier ice accumulations in moderate rainfall. If these temps
still look reasonable in later forecasts, then a freezing rain
advisory will be issued as both the coverage and rate of precip will
be more pronounced than Fri morning.

Temperatures are likely to warm to AOA freezing midday Saturday from
SW-NE as the shallow cold layer further modifies from precip and a
subtle deepening of the warm nose aloft, so further icing should
draw to a close as temps remain just warm enough Sat night and early
Sunday. As the much anticipated upper cyclone pivots northeast from
near El Paso on Sunday, stout 500 mb height falls on the order of 120
meters in 6 hours will spur the absolute best chances of rain CWA-
wide. Unseasonably rich PWATs AOA one inch will have no problem
being wrung out as the trough trends negatively tilted and centers
strong upper difluence squarely over the South Plains. Heavy
rainfall can be difficult to justify here in January, but with such
exceptional moisture and lift, the seasonal climatology simply does
not apply. On a similar note, SPC`s slight risk on Sunday is not
necessarily far fetched given the magnitude of forcing and
sufficient elevated CAPE of 500-700 J/kg which could yield some hail
with storms. However, this severe threat looks more focused
southeast of our FA near a warm front and comparably better
instability. Greatest threat on Sunday for us will certainly be
widespread heavy rains and an associated risk for some flooding.

Dry slotting then invades most of the region Sun night with some
signal for light deformation snow to catch our far W-NW counties
behind the departing upper low. This will also release a Cp airmass
through the CWA overnight on brisk northerly winds. Pattern
thereafter features additional trough energy reloading the broader
trough axis, but this focuses well to our SW with remoistening of
the dry slot looking unlikely. Else, seasonal temps should resume




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