Area Forecast Discussion
Issued by NWS

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000
FXUS64 KLUB 201807
AFDLUB

Area Forecast Discussion
National Weather Service Lubbock TX
1207 PM CST Tue Feb 20 2018

.AVIATION...
Low CIGS around KCDS will quickly dissipate within the next half
hour. A cold front was quickly moving through the region and will
be through the KLBB terminal within the next half hour as well.
Breezy winds will remain at all TAF sites during the overnight
hours after the cold frontal passage. Late Wednesday morning, low
CIGS are expected to move back into the region at all TAF sites,
but MVFR is not anticipated until after 18Z.

&&

PREV DISCUSSION... /issued 345 AM CST Tue Feb 20 2018/

.INCREASING THREAT FOR LIGHT ICY ACCUMULATIONS ON WEDNESDAY AND
THURSDAY...

DISCUSSION...
Early morning satellite showed the west edge of subtropical cirrus
was making steady progress eastward across the region ahead of
broad longwave troughing over the western third of the CONUS.
Despite an overall drying trend aloft behind the subtropical
moisture axis, the low levels will become increasingly moist and
eventually saturated tonight following a sharp cold front. At 3
AM, this front was oozing southward along a line from Clayton to
Canadian. After coordination with neighboring WFOs, we have
played heavily into the NAM and short blend model guidance today
in depicting this front reaching Childress before noon ahead of
plummeting temps. Stout thermal ridging ahead of the front will
probably secure a 30+ degree temp contrast by late day from SW to
NE across our CWA.

Brunt of the effort with this forecast package was dealt to the
potential for frozen precip on Wednesday and again Thursday
morning. Very moist NE post-frontal winds tonight should secure a
shield of stratus by early Wednesday, with saturated depths
increasing to 3000 feet or more by midday Wednesday ahead of the
first upper impulse in SW flow. This wave will bolster isentropic
ascent along the sloped Fn layer aloft and eventually breed light
precip. Despite around 1000 feet or so of sub-cloud moisture
deficits, vertical profiles over time showcase ample saturation
aloft which would garner light sleet or freezing rain over most of
the CWA. Location of richer PWATs off the Caprock still favors
the highest PoPs there, although the bullish ascent farther west
as progged by the ECMWF and 00z NAM would be enough for Winter
Storm Warning accumulations of sleet and/or freezing rain.
Collaboration with WPC Winter`s Desk agreed that profiles over
most of our region would tend to favor more light sleet than
freezing rain, especially given aggressive cold advection and
anywhere from 2000 to 3000 feet worth of sub-freezing temps below
a sharp warm nose. However, we will keep the door open to a thin
glazing from freezing rain as this event is still 24 hours out and
subject to further change.

In the wave of Wednesday`s upper impulse, a lull in isentropic
upglide is expected by the evening before another upper wave
arrives Thursday morning and restores saturated ascent. Models
are in better agreement regarding this second bout of precip in
that they focus the axis of ascent east of I-27. This is due to
much stronger SW flow in the warm nose layer that will advect
lower mixing ratios east with time. As such, PoPs were sharpened
to show lower values on the Caprock and higher values east.
With the deepening and warming warm nose, the preferred precip
phase looks to favor more freezing rain than sleet by Thursday
morning region wide. This poses some concern for heavier icing in
our eastern zones where more than a thin glaze could build up
before temps rise to above freezing in the afternoon. Later
forecasts will dissect this further, but at present we feel both
days would fall within Winter Weather Advisory criteria given
predominantly light precip amounts.

For Friday and Saturday, a steady decrease in moisture is
expected over all but our SE zones where the west edge of Gulf
moisture will linger. Additional ripples in SW flow aloft look to
encourage showers or some storms on Friday and Saturday near this
moist axis, before the upper trough and Pacific front zip through
on Saturday ahead of deep and dry westerly flow. A highly
modified cP airmass may sneak south behind this trough on Sunday,
but such deep drying by this time poses no challenge to the
continued dry and very mild forecast for the close to the weekend.

&&

.LUB WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

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