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Forecast Discussion for FWD NWS Office
FXUS64 KFWD 130340 AAB

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
940 PM CST Wed Dec 12 2018

Near term forecast is in good shape. Just made some minor cosmetic
alterations to the PoP/Sky/Weather grids based on current trends
and hires guidance. Mid and high-level radar returns are
beginning to expand across the Low Rolling Plains associated with
a lead shortwave/PV anomaly which is currently swinging towards
Del Rio. Our evening sounding revealed a pretty unimpressive low-
level moisture profile, with a notable layer of dry air in place
between 500 and 850 mb. As a result, most of the radar echoes are
not reaching the surface at this point. Still, as lift increases
ahead of this first system, showers should continue to expand
through the overnight hours. The highest PoPs will remain along
and east of I-35 where the best low-level moisture is in place. As
this ascent reaches our far east/southeastern counties late
tonight, some thunderstorms may begin to develop as updrafts start
tapping into some marginal instability. Deep layer wind shear
remains fairly weak, so organized strong-severe convection is not
anticipated, but some brief gusty winds will be possible south and
east of an Athens to Fairfield to Centerville line as storms head

Regarding the "main event" for discussed in this
afternoon`s AFD, even small wobbles in the track of the mid-level
low center will have dramatic implications regarding who sees
snow or a 40 degree F wind-blown rain due to the marginal thermal
profiles. The 00z NAM highlights the remarkable sensitivities
that our sensible weather will have on these rather minute
alterations to the synoptic-scale set up. The 850 mb low in this
case jogged about 40 miles to the northeast compared to the 18z
and 12z runs--a bit closer to this morning`s ECMWF solution.
While this doesn`t really change the prospects for snow across the
immediate I-35 corridor (with temperatures still too warm for
much in the way of snow), it increases the residence time of sub-
freezing 925-800 mb air west of US-281 and thus raises the
potential snowfall amounts. For now, it looks like our message of
basically 1-4" of snow in the Winter Storm Watch still seems
reasonable, but it`s possible these amounts may need to be
increased should the rest of the overnight guidance follow suit.

It also appears there will be a brief window for some strong to
perhaps marginally/briefly severe storms Thursday afternoon east
and north of a Gainesville to Dallas to Athens line as moisture
and instability wrap westward into the occluding surface low. Deep
layer shear values never really get that high before instability
is cut off (generally under 35-40kts), so widespread organized
severe convection doesn`t seem too likely. However, some of the
stronger storms could produce some 40-50 mph wind gusts and small
hail as they head northward.

Finally, the early evening guidance continues to indicate very
strong winds/wind gusts will occur with this system, most notably
along and west of I-35 Thursday afternoon and evening. With 925 mb
winds increasing towards 50-60 kts, it`s looking plausible that
some areas across our north and west may reach High Wind Warning
criteria for a time Thursday evening (sustained winds above 40 mph
and/or gusts over 58 mph). Based on the latest guidance, the main
area of concern for the very highest winds would be west and north
of a Gainesville to Mineral Wells to Cisco/Eastland line. We`ll
hold off on hoisting anything at this point, but I think some 60
mph gusts in this area aren`t out of the question.



.AVIATION... /Issued 557 PM CST Wed Dec 12 2018/
/00z TAFs/

Pesky MVFR stratus is hanging tough just east of the Metroplex
terminals early this evening where modest low-level warm advection
is encountering better 925-850 mb moisture. Based on the
orientation of the near-surface wind fields per area VWPs and our
currently-ascending evening balloon, feel that the brunt of these
FL015-025 cigs will remain mostly east of the Metro through the
mid-late evening hours. The one exception will be KDAL where we`ll
show these cigs building back a bit more quickly after 13/02z.

An initial mid-level disturbance which is just now swinging into
southwest Texas will foster additional warm advection across the
region later this evening. While moisture won`t be all that deep
across the region, there should be enough ascent to squeeze out
some showers around or just after midnight and into early Thursday
morning. Cigs will once again pose a challenge as a notable
gradient between predominantly VFR and low-MVFR will likely exist
right along the I-35 corridor. For the sake of brevity, opted to
continue to carry a mention of BKN025 conditions overnight
everywhere, but it`s possible that the western Metro sites remain
mostly VFR. We`ll continue to monitor observational trends this
evening, however.

This initial batch of light precipitation will move east of Waco
during the mid-morning hours, but the Metroplex sites will be
close enough to an approaching low to carry VCSH through the day.
Contemplated adding a mention of VCTS at all of the Metroplex
locations Thursday afternoon given the intensity of approaching
large-scale lift and the presence of at least some instability
with anticipated breaks in the low cloud cover. However, owing to
lingering uncertainties with the upper-low`s track, decided to
leave this mention out, but one may be needed in future TAFs,
potentially during the 13/20-24z time frame.

Finally, slowed down the timing of the north wind shift with the
strong cold front by a few hours based on latest guidance trends.
Either way, winds will gradually increase through Thursday
afternoon. The strongest winds will occur Thursday evening and
overnight, and an AWW for gusts well in excess of 35 kts looks
probable. Rain showers will expand in coverage during this time
frame as well. At present, it appears as if temperatures just off
the surface will remain a few degrees above freezing through
Thursday night, limiting the wintry weather potential at the
immediate Metroplex sites. That said, we won`t rule out some wet
snowflakes mixing in, but this potential wouldn`t occur until
after 14/06z, or thereabouts.



.SHORT TERM... /Issued 330 PM CST Wed Dec 12 2018/
/Severe Weather Potential Tonight into Thursday/
Main Takeaway:
Any severe storm potential will likely be realized Thursday
afternoon into the evening east of I-45/US-75. Marginally severe
hail is the primary hazard.

With the approach of a powerful upper-level jet and PV anomaly
towards the southern Plains over the next 12-24 hours, surface
cyclogenesis will initially occur over the Texas Panhandle
tonight. Prior to the mid/upper system reaching a tighter low-
level theta-e gradient / baroclinic environment, this low should
be forced southeastward across Northwest Texas without undergoing
much deepening. Nonetheless, a return flow regime ahead of the
cyclone will draw increasing moisture northward, with mid/upper
50s to even lower 60s dew points expected to reach portions of
Central and East Texas towards daybreak Thursday.

Prior to the arrival of the primary system, an initial southern-
stream impulse (pushing across the Gulf of California as of 21Z
Wednesday) will eject across southern/eastern Texas through 12Z
Thursday. Ahead of this southern system, a strengthening warm-
advection regime should promote at least isolated/scattered shower
activity spreading northward from Central Texas to the Arklatex
later this evening into tonight. Convection should initially be
shallow, with no thunder expected. However, weak surface-based
buoyancy developing towards the late night/early morning hours
may promote isolated thunderstorms, primarily towards Palestine
and Athens. While deep-layer wind fields will be strengthening
ahead of the primary shortwave trough, questionable moisture
quality (e.g., only modest mean mixing ratios from 12Z Wednesday
soundings at BRO/CRP) should keep any storms below severe limits.

Some marginal severe potential may then be realized during the
afternoon and evening hours Thursday, generally east of the
I-45/US-75 corridor. While considerable spatial uncertainty
remains, strong mid/upper forcing for ascent will foster renewed
surface cyclogenesis over parts of North and/or Central Texas
through the afternoon. Coupled with enhanced convergence and warm
advection ahead of the low-level cyclone, scattered thunderstorms
are expected to blossom from near the I-35 corridor eastward
during the afternoon. Despite aforementioned moisture concerns,
around 500-1000 J/kg of MLCAPE and robust mid-level wind fields
may yield a few organized/rotating updrafts (primarily capable of
marginally severe hail) generally along/east of a Bonham to
Palestine line. Convective mixing/transport may yield isolated
stronger wind gusts as well. Lastly, low-level wind fields /
storm-relative helicity may be hampered considerably by the
passage of the initial southern-stream system (and its effect on
the surface pressure gradient); in turn, tornado potential should
be fairly low, especially considering questionable moisture

Severe potential should end quickly by mid/late evening, following
the eastward shift of any substantive buoyancy towards the
Mississippi Valley. Still, thunderstorm potential may persist
within the warm conveyor/isentropic ascent corridor associated
with the rapidly deepening mid/upper cyclone. For more details,
keep reading below!



.LONG TERM... /Issued 330 PM CST Wed Dec 12 2018/
Obviously the primary concern is the high wind and winter
weather potential associated with the dynamic upper level system
that will deepen over the region. Collectively none of us can
remember an event with such rapid deepening of a mid level cyclone
over our forecast area in the last 15 years, so this is an
unusual occurrence and therefore lowers our confidence on some
parts of the forecast. In some ways this rapid deepening is
perplexing considering the lack of any real low level
baroclinicity, but clearly extremely favorable upper level jet
dynamics are in play to create this system. Either way this system
will bring some of the strongest gradient winds we`ve seen behind
a front in years, a good chance of rain, and a potential for
winter weather.

Considerable wrap-around precipitation will develop as a trough
of warm air aloft (TROWAL) wraps into and is lifted on the back
side of the cyclone. Strong frontogenesis and isentropic lift
suggests that the western and northern CWA will see the heaviest
precipitation that will begin Thursday afternoon and last into
Friday morning. Liquid equivalent precipitation amounts of 0.75 to
1.5 inches will be possible. There will be a risk of isolated
thunderstorms in the wrap around precip as instability should be
high enough for a few lightning strikes in the stronger convective
elements. Eventually this wrap around precipitation will move
southeast across the remainder of the CWA with the upper low, but
it may not be until Friday morning that the central and
southeastern zones see post-frontal precipitation and amounts
there will be less than a half inch due to more limited duration.

Strong cyclogenesis will result in a 50kt 925mb jet across the
western CWA Thursday evening. Cooling temperatures above the
surface will steepen the low level lapse rates and as
precipitation begins to fall into this layer a transfer of these
higher winds aloft down to the surface will occur. The highest
winds with sustained speeds of 30 to 40 mph and gusts to 55 mph
will occur west of the I-35 corridor. Winds will decrease about 5
to 10 mph from those values along the I-35 corridor and will be
below wind advisory criteria across the southeast zones. We have
issued the wind advisory for the impacted areas now. It is still
possible the far northwestern zones (Eastland to Montague county)
may need to be bumped up to a high wind warning (for gusts to 60
mph) but we would like to hold off on this headline until
confidence increases. MOS guidance still isn`t as high as I`d like
to see for high wind warning, so we`ll await more of the high-res
guidance overnight. Downed trees and power outages will be
possible in this area. Winds will increase behind the front which
will move into the northwestern zones in the morning and track
eastward across the region through the day. The strongest winds
will occur from 3 pm west of I-35 (6 pm east) through early Friday
morning before gradually diminishing during the day Friday.

This is probably about as difficult as they come for the sole
reason that no arctic airmass is in place across the Plains.
Usually we can look at and track all of the main ingredients that
go into winter weather (cold air, lift, moisture) in a current
analysis. In this case those ingredients are forecast to "appear"
over us. Virtually all of the cooling of the atmosphere we see is
in response to dynamic forcing and lift. In essence, the
deepening of this cyclone is creating its own environment
favorable for winter precipitation. And in that respect we are
totally at the mercy of numerical weather prediction correctly
forecasting the amount of lift and the location of precise
temperature gradients from the impacts of mesoscale lifting and
diabatic cooling effects. The one bit of good news is that model
guidance is in good agreement on the main points of the
forecast...that the western and southwestern zones appear to be
the favored areas where forcing is maximized and strong enough to
cool the atmospheric column below freezing. The bad news of course
is that there`s no guarantee that the upper low will develop in
that spot and take the track its forecast to.

The winter storm watch will continue as is for the western zones,
with the exception of adding Lampasas and Hamilton counties on
the southern edge of it. Rain should begin to change over to snow
once the dynamic lifting can cool the column below freezing. Then
surface temperatures will need to be cooled effectively from the
reservoir of cold air aloft as warmer surface air is vented into
the lower atmosphere due to super-adiabatic lapse rates and
through diabatic heat loss via precipitation either
evaporating or melting in the low levels. The result is that the
warm surface temperatures will cause most of the precipitation to
fall as rain, but eventually a transition of snow should occur as
early as late Thursday afternoon in the western zones. This
transition from rain to snow will also likely occur at the higher
elevations (> 1500 ft) of our western zones before they do in the
elevations that are a few hundred feet lower. Thus high points in
the western CWA (in the winter storm watch area) are most at risk
for multi-inch snow while valleys may see little or no
accumulating snow. In other words, the range for snowfall in the
winter storm watch is from a Trace to 4 inches due to these
uncertainties (and not because we don`t want to try to narrow it
down). We estimate that snow will struggle to accumulate on roads
until about 2 inches falls. Even then we believe that the warm
ground temperatures means that roads will become slushy instead of
icy or snow packed. The combination of high winds when and where
snow is falling could create visibilities of less than a half mile
and also cause travel issues.

Odds are that locations farther to the east (the I-35 corridor)
will not be able to cool the 925mb layer below freezing and precip
will remain all rain. However this is still highly dependent on
the track of the upper low. A track that is shifted about 50 to
100 miles farther north would increase the snow potential for all
of the I-35 corridor (and western zones too). This is because a
more northerly position of the low would be able to grab slightly
cooler air over Oklahoma to wrap into the cyclone. As it stands
now guidance is showing warmer air from northeastern Texas
wrapping into the low which means the northern part of the cyclone
would have difficulty staying cold enough - thus focusing the
snow threat near the track of the core of the upper low. There is
one final caveat, and that is mesoscale frontogenesis can and
often does provide strong enough lift to cool localized layers of
air below freezing. One such band of frontogenesis is forecast to
develop across the I-35 corridor from Hillsboro to the DFW
metroplex into Sherman Friday morning. This would be the time we
could see rain mix with or change over to snow, but I worry we`ll
not know if this will or will not occur until Friday morning.
Right now our message to people for this part of the CWA is that
they need to be aware that localized bands of heavy precipitation
will have the potential to change to snow and thus result in some
accumulations. So plan on rain, but make contingency plans for an
inch or two of snow too. By midday Friday it should be warm
enough everywhere that only rain is possible.

The remainder of the forecast is pretty straightforward. Generally
seasonable temperatures and clear skies through midweek. We`ll see
the next slight chance of rain with a weak system Wednesday.



Dallas-Ft. Worth    55  60  39  48  38 /  30  80  90  60   5
Waco                55  61  39  49  36 /  40  50  80  50   0
Paris               55  62  42  46  37 /  70  80  90  60  10
Denton              53  58  39  48  36 /  30  80  90  60   5
McKinney            56  61  40  46  37 /  40  80  90  60  10
Dallas              56  61  40  47  39 /  40  80  90  60   5
Terrell             57  63  40  48  37 /  50  80  80  60  10
Corsicana           58  63  40  48  37 /  50  50  80  60   5
Temple              53  59  39  49  37 /  40  30  80  30   0
Mineral Wells       50  54  35  49  33 /  30  90  90  40   0


Wind Advisory from 3 PM Thursday to noon CST Friday for

Wind Advisory from noon Thursday to noon CST Friday for TXZ091-

Winter Storm Watch from Thursday afternoon through Friday
morning for TXZ100-101-115-116-129-130-141>143-156.




NWS FWD Office Area Forecast Discussion

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