Area Forecast Discussion
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673
FXUS64 KLUB 140537
AFDLUB

Area Forecast Discussion
National Weather Service Lubbock TX
1137 PM CST Fri Jan 13 2017

.AVIATION...
Very challenging flying conditions expected throughout this TAF
cycle as airmass continues to slowly saturate and showers become
more widespread. Freezing line has returned south to KPVW so
started precip there as freezing drizzle. With broad isentropic
lift across the area and upper level support bringing periods of
measurable precip see no reason to raise ceilings above LIFR
during the day. Instability remains sufficient for a moderate
shower or a thunderstorm mainly at KLBB or KPVW through late
morning.

&&

.PREV DISCUSSION... /issued 706 PM CST Fri Jan 13 2017/

UPDATE...
Quick update for temperatures running just a degree or two above
forecast and to lower PoPs through the evening hours. The latest
observations and short-term guidance suggest that precip will
mainly hold off until around midnight. Before then, we can expect
some scattered light showers to develop in the next few hours and
spread northwest across the forecast area. At the same time,
another area of light showers - mostly virga at this time - may
spread into the far southwest Texas Panhandle.

PREV DISCUSSION... /issued 546 PM CST Fri Jan 13 2017/

AVIATION...
Cloud top cooling to our south increases confidence of
rain/freezing rain spreading north overnight into terminals.
Until then kept previous forecast pretty much in tact with drizzle
slightly delayed at KLBB with larger dewpoint depressions than
further north. Wet-bulb zero line has shifted north of KLBB so
precip remained all liquid there. Along with arrival of
showers/broken shield of rain will come IFR ceilings which are
expected to remain through end of valid TAF period. Adding to
complicated forecast will be thunder threat overnight and during
the day on Saturday but forewent any attempt at timing these.

PREV DISCUSSION... /issued 350 PM CST Fri Jan 13 2017/

DISCUSSION...

Surface observations early this afternoon revealed temps above
freezing everywhere except generally north of the Highway 70
corridor. Of greater significance is the collapse of what was
previously respectable dewpoint depressions of 10 to 18 degrees to
what is now generally 5 degrees or less over all but our SE zones.
Upshot of this is that the wet bulb cooling potential is
diminishing. At 2 PM, the wet bulb zero line was near a Plains to
Snyder line and is progged to retreat rapidly NNE in the coming
hours, likely sitting northeast of Highway 84 by 6 PM. No
additional CAA or drying of the sub-cloud layer is foreseen, so
chances of freezing rain falling too far outside our advisory
appear very slim. This wet bulb zero line will be critical as the
first real wave of deep saturated ascent arrives sometime from 3
to 6 AM Saturday. Water vapor imagery clearly shows this lift in
the form of enhanced cloud top cooling SW of the Big Bend at 230
PM, so available forcing does not appear to be in short supply at
all.

Until early Saturday, overall ascent will remain largely subdued
as the main axis of isentropic ascent resides just east of our
CWA. A strong SE low level jet tonight with deep moisture will
help thicken our current shallow stratus layer to several thousand
feet, likely resulting in some light rain showers at times, with
spotty ice accumulations finally gaining traction in our freezing
rain advisory counties. As stronger ascent and deep saturation
overspread the region early Saturday with the aforementioned wave,
there is a definite opportunity for meridional training of showers
and some thunderstorms. NAM is more defined with an elevated
baroclinic zone arcing clockwise across the western half of our
CWA which would only serve to boost rainfall amounts. Should this
weak Fn layer verify, there is concern for precip to become
displaced west of the freezing surface temps likely to reside in
the far southeast Panhandle and nearby counties. This concern is
evident on some high resolution models, but is too conditional to
bother addressing in our PoP grids at this time. As lift subsides
Saturday afternoon from south to north behind the upper wave,
precip coverage and rates will scale back. Surface temps will
probably modify with any heavier rainfall, so the likelihood of
additional icing after late Sat morning remains very slim.

By Saturday night, only weak height falls are progged to advance
out of NM well in advance of a vigorous cyclone reaching the El
Paso vicinity by midday Sunday. This slowing theme of the upper
low has been a consistent theme with most model runs, so
widespread precip chances Sat night could be in jeopardy as we
will be dependent on LLJ mechanics enhancing isentropic lift,
generally near the NM state line. Sunday will still be the most
dynamic of days as mid-level heights collapse 90+ meters in 6
hours or less in response to the cyclone lifting NE across
southeast NM in a negatively tilted fashion. Strong UL diffluence
ahead of this low will only help sharpen a trowal structure
across our CWA already characterized by exceptionally rich PWATs
AOA one inch. Setup for heavy rain and the usual minor flooding
is solid despite what the calendar may say. Also on that note,
prospects for some low-end severe storms in the form of elevated
hailers is also real given sufficient elevated CAPE of 500-800
J/kg and such potent forcing.

As the cyclone treks into the TX Panhandle Sunday night, dry
slotting will erode much of the remnant moisture with only some
deformation zone snowfall possible generally in our NW zones.
Models are at heavy odds over the prospects for snow, but such a
compact low and a cold front wrapping south in its wake fit the
pattern for at least light snow accumulations in our usual
snowbelt counties. No other major wx impacts are foreseen
beginning MLK Day as additional PVA reloads an elongated trough
axis, mostly well to our SW. However, moisture deficits from the
prior dry slotting look too much to overcome within this broad
trough axis, despite some higher elevation snow in NM. Seasonal
high temps early in the week give way to steady warming with
breezier W-SW winds by midweek.

&&

.LUB WATCHES/WARNINGS/ADVISORIES...
Freezing Rain Advisory until noon CST Saturday for TXZ023>026-
029>032.

&&

$$

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