#2034 Postby Tammie » Sat Dec 16, 2017 5:31 am
Nice long-term write up from NWS-Fort Worth this morning.
545
FXUS64 KFWD 160953
AFDFWD
Area Forecast Discussion
National Weather Service Fort Worth TX
353 AM CST Sat Dec 16 2017
.SHORT TERM...
/Today and Tonight/
The extended dry period should come to an end for much of North
and Central Texas today as an upper low rotates northward across
the Trans-Pecos. Moisture and lift associated with this system
will initially bring increasing mid and high clouds this morning
followed by scattered to numerous showers this afternoon. The best
moisture and lift will reside across the southern/southeastern
zones where we will carry the highest PoPs. Although surface based
instability will be negligible, some weak mid level instability
will be present which may result in a few intermittent lightning
strikes. The favored rain area will shift to the eastern zones
this evening as the upper low lifts into the Eastern Texas
Panhandle. Precipitation will end quickly from west to east
tonight when much drier air moves in from the southwest. Total
rainfall from this system will range from only a few hundredths of
an inch across the west/northwest zones to nearly an inch across
the southeast.
The extensive cloud cover today will offset increasing low level
warm air advection some and keep temperatures near or slightly
below seasonal normals with highs generally in the 50s. Although
the rain will move out tonight, the low levels will remain near
saturation which will result in some low clouds and even some
patchy fog. Overnight lows will be considerably warmer than they
have been and generally in the lower and middle 40s.
We were concerned over the past few days that the fire threat
might become elevated across the northern half of the region this
afternoon. It now appears the clouds and moisture return will be
sufficient to limit fire growth. However, fine dead fuels will
remain abundant and winds will increase this afternoon so some
small fire starts will still be possible.
79
&&
.LONG TERM...
/Sunday and Beyond/
The vigorous shortwave which will be responsible for Saturday
night`s rainfall will be quickly peeling away from the region by
early Sunday morning. This will leave North and Central Texas in a
subsident wake as an incoming shortwave ridge axis fosters rapid
mid-level height rises through the day. While no precipitation is
expected on Sunday, copious amounts of moisture will remain
trapped beneath a strong inversion just about 2000 feet off the
surface resulting in pervasive low cloud cover and fog during the
morning. Subsiding and drying air will, however, eventually erode
the low cloud deck across most of the region during the
afternoon, allowing temperatures to rise into the upper 60s to
near 70 degrees out west, and into the lower to middle 60s
elsewhere.
Another rather vigorous shortwave trough is then slated to
approach the region Monday afternoon, before departing to the east
by Wednesday. This system, which is presently dropping southward
along the California Coast early this morning, was initially
expected to fracture into two to perhaps three separate vorticity
centers, effectively diffusing available forcing for ascent over
the region and keeping precip chances relatively low. Recent
guidance, however, has all trended towards developing a much more
coherent singular upper-low center more or less intact. As a
result, much more lift and moisture appears like it will be
available Tuesday into Wednesday, and PoPs have been raised
significantly across the region. While some initial light warm
advection showers are possible as early as Monday afternoon across
parts of the region, the best chances for rainfall are slated to
arrive on Tuesday afternoon, and linger through the overnight
hours. Isolated thunder has also been added to the forecast given
the increased lift and much cooler mid-levels as the core of the
upper-low drifts overhead.
Not much will be shaking on the weather front on Wednesday as this
latest system departs to the east and lingering moisture is
gradually scoured out. Temperatures should manage to rise into the
mid 60s and perhaps into the 70s across Central Texas under
thinning cloud cover. Thursday also looks like it will feature
rather benign weather outside of some gusty southwesterly winds
ahead of the next set of potentially weather feature(s) into the
Christmas weekend.
First, as we`ve been messaging over the last few days, run-to-run
model consistency regarding the progression, timing, and strength
of an incoming arctic cold front remains quite high during the
Thursday evening-overnight period. And initial sharp north wind
shift and subsequent cool-down is forecast to enter our
northwestern zones as early as early Thursday evening before this
massive density gradient surges into the Gulf of Mexico on Friday
morning. Some light precipitation (non-frozen) is possible at the
surface ahead of the front across our southeastern counties
Thursday evening ahead of the surging wedge of arctic air.
So, piece one of the puzzle can be set down on the table: we`re
fairly confident that much colder air will be present in the
lowest 3-4 thousand feet or so, with wet bulb temperatures
certainly below freezing due to dewpoints in the teens and single
digits.
We can then start to fill in the border pieces somewhat, but
individual details regarding the progression of the all-important
close upper-low (or lack thereof) to our SW remains quite unclear.
We know that exceptional amplification to the +PNA ridge, which
has been a staple of the large-scale weather pattern of late,
should occur as the ridge axis builds through the Arctic Circle,
perhaps coming close to scraping 85 degrees north by next
weekend. We`re also pretty confident that a vort max associated
with energy currently nearing the Bering Sea should crest this
building ridge axis during the middle of next week. What happens
after this is up in the air, and ultimately boils down to where
this shortwave is in relation to the ridge axis as an anticyclonic
wave break occurs. Too far east--like the most recent 00z ECMWF
and last night`s GFS--and this shortwave trough simply slips
eastward, remaining an open wave as it quickly transits Texas sans
much fanfare at all. If this system hangs back just a few hundred
miles farther west as the 2PVU surface pinches off, and you`ve
got a developing closed upper-low near Baja/Sonora.
Roughly speaking, global ensemble and deterministic guidance seems
to be split nearly down the middle into these two camps (a third,
less supported solution is to retrograde the low so far out into
the Pacific it becomes a non-factor in our weather). Since
individual deterministic runs have been swinging back and forth
between these solutions wildly over the last few days and ensemble
guidance remains bifurcated, we can`t say which of these solutions
is the most probable at this juncture.
What we can say, however, is that the upcoming pattern will
support the potential for a Baja-type closed low to usher
moisture into the region up and over the arctic lower-level air,
which would bring a **potential** for impacts due to freezing
rain and sleet (to perhaps the southeastern half of the region)
and snow and sleet (north and west) to the region from Friday
night (December 22nd) through the Christmas weekend (25th). The
alternative being a cold but generally dry scenario through the
weekend.
At some point, ensemble guidance should begin to favor one of
these scenarios enough to begin pinpointing the most likely
upper-level evolution, but until then, nailing down specifics will
be much too premature.
Carlaw
1 likes
Tammie - Sherman TX
The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or STORM2K.
very good discussion out of the FTW NWS office this morning ...bottom line, a few hundred miles could make a difference b/w a winter storm or no storm with this particular setup. Still too far out to make any conclusions and models continue to reflect that uncertainty. I agree with FTW, no need to sound alarms quite yet
