FXUS64 KFWD 152205

Area Forecast Discussion
National Weather Service Fort Worth TX
405 PM CST Fri Dec 15 2017

Other than a few high clouds across the region this afternoon,
mostly clear skies are in control over much of the state. Surface
high pressure is in control over the Gulf Coast of Texas, while a
lee trough is beginning to strengthen over the Panhandle and West
Texas. Tonight will remain quite dry, allowing for temperatures
to drop into the upper 30s in the urban areas and east, while
dropping to near freezing in the west.



The upper level low in the Baja region will eject northeastward
during the day tomorrow and bring a rapid change in our sensible
weather. The morning will start out mostly clear but cloud cover
will quickly over spread the region from the south as low and mid
level moisture surges northward. The increasing clouds/moisture
and cool temperatures in the 50s to low 60s should inhibit a
critical wildfire threat from developing, but elevated fire danger
still warrants caution. Isentropic lift will be very strong and
should result in a large swath of rain that develops after midday
and encompasses most of the region by sunset. PoPs will be high
due to good confidence in measurable rain, but unfortunately for
most, rain amounts will do little to beat back the developing
drought conditions. Rain should range from a couple hundredths in
the western zones to 1/10 to 1/4 inch in the central and northern
zones. Across the southeast zones however, a half inch to inch
will be possible as moisture and instability will be best in that
area. Any threat of surface based instability will remain well to
the south of the CWA, but a few rumbles of thunder may sound
across the eastern half of the region. This round of rain will be
quick hitting and exit after midnight. Sunday will be a pleasant
day with lots of sunshine and light westerly winds, which will
help boost temps into the low 70s in the southwest and 60s elsewhere.

The various forecast models don`t really know how to handle the
upper level vorticity centers across the Baja area Sunday night
and Monday. The 12z ECMWF threw out a unique solution and combined
all of the centers into a potent upper low that it ejects on
Tuesday and brings a round of widespread rain to the area. The
other guidance shoots this energy across in pieces and thus the
lift and ability to get Gulf moisture back into the area are
unimpressive. In any case will continue with a slight chance of
showers over the southeast half of the area on Monday. Have
extended these pops into Tuesday for the same area out of respect
for the ECMWF solution, but if the ECMWF forecast starts to look
more correct these PoPs will need to be raised dramatically.
Otherwise above normal temperatures look to prevail Monday and
into Wednesday with highs in the mid to upper 60s. Lows will be in
the upper 30s to upper 40s. By Thursday highs should warm to near
70 ahead of our next cold front.

Big changes are in store by the end of the week as an upper level
longwave trough gets carved out across the western US. Upper level
height rises along the Pacific coastline of Canada will foster the
development of a 1040mb surface high across Montana by Thursday.
This will send arctic air southward into the Plains during the day
Thursday...reaching North Central Texas Thursday night. There is
excellent model agreement in timing the arrival of this cold
airmass and therefore confidence in the temperature forecast is
quite high. Furthermore, the positioning of the surface high and
evolution of the upper level pattern is a classic one that brings
cold air into the region and often results in the coldest temps
across our western zones (i.e. not a north-south temp gradient but
west to east). Lows will likely reach freezing across the NW
zones by Friday with highs Friday likely stuck in the low to mid
40s. Northerly winds at 10-20 mph will make it feel even colder.
The high temperature forecast will be contingent on how quickly
cloud cover can develop behind the front, so those temps could end
up colder, but have stayed somewhat conservative.

The following discussion concerns weather beyond the 7 day
forecast period, but there is enormous interest in the weather
ahead of Christmas for travel (and those dreaming of a white
Christmas). Confidence remains low about the eventual fate of the
upper level pattern, particularly on how (or whether) the upper
level low develops at the base of the deepening upper trough. This
isn`t a situation where one model is consistently saying
something and we`re waiting to see if the others jump on board.
This is worse. All of the models keep saying something different
essentially each run and trading places with each other. What
that means is that there are probably 3 reasonable forecast
1) The upper low never develops and the trough remains somewhat
progressive. This would result in a clear and cold forecast
outcome. 2) The upper low develops and sinks into the desert
southwest or Baja region. This would likely result in winter
weather somewhere in our region (not necessarily our CWA). 3) The
upper low develops and drops southwest into the Pacific ocean too
far to provide lift or moisture here. This forecast also would
result in little or no winter weather for our CWA with temps not
quite as cold as either of the first 2 scenarios.

Scenario 2 is the one that most of the guidance is showing today
and the one we`ve got a gut feeling may verify since upper lows
have been developing and dropping into that region for the last
week or so. However, the difference in the strength and exact
location of the upper low determine where, when, what and how much
winter precipitation fall and it`s just too early to get too
involved in those details.

What we can do is use climatology as a guide. This upper level
pattern with a closed low in that region can produce snow in
Texas, but snow is typically favored across the western half of
the state. In general it is too warm aloft here, and our CWA ends
up with freezing rain or sleet. This likely means that ice or
freezing rain around Dec 23rd are a higher probability than snow
and western areas are favored more than eastern areas. If that
low were to eject northeastward it would help cool the atmosphere
aloft and would increase the chances for snow on the 24th and
25th. Again given the wild model variability of late, we can`t say
that will occur with any confidence either. Finally to get some
percentage numbers based on ensemble guidance...half of the GEFS
members and a third of the ECMWF Ensemble members do show a winter
event in our CWA. Ensembles are underdispersive to true
probabilities so those probability chances don`t correlate
exactly. In any case this is going to bear watching.

So in summary:
1) Strong cold front arrives Thursday night.

2) Colder than normal weather for Dec 22nd to 25th is a near

3) Precipitation behind the front is possible with probabilities
of some type of winter event somewhere in our CWA around 25%

4) It is much too soon to get into the details of what, where, and
how much...but Dec 23-25th is the window, ice/sleet may be
favored (at least initially) and western areas have a higher
chance than eastern areas.



.AVIATION... /Issued 1131 AM CST Fri Dec 15 2017/
While flying weather is generally good today...we have received several
PIREPs of moderate to severe turbulence between FL170 and FL250.
This turbulence is associated with a potent jet stream which is
currently passing through northern portions of our forecast area.
This jet should move out of our area by evening...allowing for
this turbulence to diminish.

VFR is forecast to prevail through the TAF period. N`ly flow
conditions are in place across the region...with a transition to
S`ly flow expected late this evening as a trough of low pressure
develops across the Southern High Plains. Cloud cover will begin
to increase after 16/1500Z...but ceilings should remain VFR
through the TAF period. Winds will increase on Saturday as
well...though this is not mentioned explicitly in the TAFs as
winds should remain S`ly.

Just beyond the TAF period...expect chances for SHRA to increase
Saturday afternoon across Central Texas and spread into North
Texas by evening. As the rain overspreads the area...expect
conditions to deteriorate to MVFR...and possibly IFR for a time.
This rain should exit the area Sunday morning with conditions
improving throughout the day.



Dallas-Ft. Worth    39  61  45  68  46 /   0  30  80   5  10
Waco                35  56  43  68  47 /   0  60  90   5  10
Paris               32  59  43  58  45 /   0   5 100  10  10
Denton              32  62  43  66  44 /   0  20  80   5  10
McKinney            33  61  43  63  44 /   0  20  90   5  10
Dallas              39  61  45  67  46 /   0  30  90   5  10
Terrell             33  59  44  66  44 /   0  30 100   5  10
Corsicana           37  57  44  67  47 /   0  40 100   5  10
Temple              36  54  43  67  48 /   0  70  90   5  10
Mineral Wells       32  61  43  70  41 /   0  30  70   0  10




NWS FWD Office Area Forecast Discussion