Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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FXUS64 KFWD 060607 AAC

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1207 AM CST Wed Dec 6 2017

Concerns...A light rain/sleet mix between 10z and 18z at the
Metroplex TAF sites but surface temperatures are expected to only
fall into the upper 30s. Light rain may become mixed with some
sleet at the Waco TAF site through 18z and a light rain/light snow
mix will be possible 18z Wednesday through at least 03z Thursday.
Surface temperatures are expected to slowly fall into the mid to
upper 30s by daybreak and remain nearly steady above the freezing
mark during the day Wednesday.

As of 05z, a large band of rain extended south of a KBWD-KTYR
line. Farther north, there are only some very light reflectivity
returns on the radar which may be indicative of some sprinkles or
very light ice pellets (sleet).

For the Metroplex TAF sites, some sprinkles or very light ice
pellets are possible through 10z as light precipitation falling
from mid level clouds around 10Kft mostly evaporates as it falls
through dry air in the lower levels. As the atmosphere gradually
moistens from top to bottom, some light rain is expected after 10z
and some ice pellets may mix in during the morning. Surface
temperatures are expected to slowly fall into the upper 30s after
12z but stay above the critical 37 degrees. The precipitation
should be all rain during the afternoon as temperatures rise into
the upper 40s to lower 50s.

At Waco, light rain may mix with some light sleet after 06z. A
rain/sleet mix is expected 12-18z as the atmospheric column cools
as evaporative cooling occurs as the rain falls through the dry
air. Light rain is expected after 18z with a light rain/snow mix
possible. Ceilings may lower into the MVFR category by 18z.
Temperatures should fall into the upper 30s before 10-12z but are
NOT expected to reach freezing through 06z Thursday.



.UPDATE... /Issued 1003 PM CST Tue Dec 5 2017/
After taking a look at the 00z NAM, what is in of the 00z GFS and
the 01z run of the HRRR, am not planning on making significant
changes to the afternoon forecast. Have lowered PoPs a little
across the north for the overnight period where the layer of
dry air will remain the deepest. Have kept the POPs high across
Central Texas where a band of showers is already moving across the
region. There has even been some lightning east of San Angelo and
will have to keep an eye on that as we do not have thunder in the
forecast at this time. It still looks like some sleet will be
possible with temperatures remaining well above freezing
overnight. Sorry to you snow-lovers but that -SN at Lampasas
(KLZZ) is not actually snow. Forecast soundings when taking into
account evaporative cooling will support some wintry mix Wednesday
mainly south of US281 and along and west of the I-35 corridor but
with temperature expected to be in the upper 30s to mid 40s,
little is any travel issues are expected across Central Texas.



.SHORT TERM... /Issued 320 PM CST Tue Dec 5 2017/

Wind speeds will diminish this evening as the surface high slides
into the region, but isentropic lift above the surface layer will
result in widespread rain across mostly Central Texas and parts of
North Texas. The isentropic lift will occur ahead of a weakening
shortwave disturbance that is currently located in southern
California. Model guidance is in good agreement that precipitation
should start to arrive into our western counties around or after
00Z (6 pm). However, this precipitation will have a layer of dry
air below 800 mb (~6500 feet) to overcome before measurable
precipitation reaches the ground, and it will more likely be
between 03-06Z (9pm-12am) before we start receiving reports of
rain reaching the ground.

Forecast soundings continue to indicate light sleet may occasionally
mix with the rain overnight tonight south and west of a line from
Graham to Rockwall to Hearne. There is a fairly decent difference
between the soundings on the temperature and wet-bulb temperature
profile below about 700 mb, but the wet bulb temperature profile
is close enough to or below zero at times down to just above the
surface to introduce a slight chance of light sleet mixing with
the rain after midnight. Surface temperatures will remain above
freezing due to a lack of cold air advection and overcast skies;
therefore, no impacts are expected due to frozen precipitation.
Very little accumulations of sleet are possible in Central Texas,
mainly west of Interstate 35, but it is more likely any sleet will
quickly melt as it reaches the surface thus resulting in no
accumulations. Overall, this event is expected to be very minimal
in the grand scope of winter weather events.

IF sleet is reported tonight at DFW Airport and/or Waco Airport,
our two official climate sites, it will be the earliest observed
snow/sleet since November 13, 2014 and November 23, 2013



.LONG TERM... /Issued 320 PM CST Tue Dec 5 2017/
/Wednesday through next Tuesday/

A very energetic upper-air pattern will continue on Wednesday (and
really through the weekend and beyond) as an exceptionally
amplified ridge persists across the western CONUS and sharp
longwave troughing prevails to its east. Several shortwave
disturbances embedded within this troughing will be heading east
into our region on Wednesday. These features will facilitate the
persistence of forcing for ascent in the form of isentropic lift
centered right around the 310 K theta surface Wednesday and into
Wednesday evening. Before going any further into the details,
we`ll highlight the main point here that: while a mix of light
wintry precipitation is possible for parts of the area, above-
freezing surface temperatures will greatly limit any impacts to
perhaps some very minor sleet/snow accumulations on grassy
surfaces across parts of Central Texas.

As we start the long term period on Wednesday morning, isentropic
upglide across the I-20 corridor will start to be replaced by
increasingly subsident air, resulting in slowly diminishing
precipitation chances into the early afternoon hours. While the
atmospheric column above 700 mb will be nearly saturated, much
drier air will be present in the lowest 5 kft or so. As a
combination of ice and water fall through this dry layer, diabatic
cooling due to a combination of evaporation and sublimation will
allow the thermodynamic profile to continue to cool towards the
wet bulb temperature. Given wet bulb zero heights perhaps just 1-2
kft off the surface, we`ll continue to advertise a slight chance
for a bit of sleet to mix in with the rain for locations west of
I-45 and south of a Graham to Rockwall line through the morning
hours on Wednesday.

Isentropic downglide looks to gradually take control of the
weather along and north of the I-20 corridor late Wednesday
morning and into the early afternoon, which will shunt the highest
precipitation chances south of I-20 and across Central Texas into
the evening hours. Temperatures across the northern half of the
CWA will likely manage to climb into the lower to middle 50s as
precipitation ends, while temperatures will stay steady or even
slowly fall to the south where precipitation persists. By late
Wednesday morning, top-down moistening should send enough of the
thermal profile (above 5 kft) below freezing to result in a
mixture of rain and snow for locations south of a Breckenridge to
Waxahachie line and west of I-45 through Wednesday evening. Even
the coldest model solutions continue to depict temperatures
remaining above freezing through the duration of precipitation,
with wet bulb temperatures remaining in the mid 30s. As such,
while some VERY MINOR accumulations of snow are possible on grassy
surfaces, roadways are just expected to remain wet. As always,
precautions should be taken while driving on any elevated roads or
bridges when roadways become wet.

Precipitation chances will linger Wednesday night across our far
southern counties, but will be on the downswing as drier and more
subsident air slowly moves in from the north. It`s possible that
future shifts may be able to remove precipitation chances
overnight Wednesday and into Thursday given the degree of
downglide progged to swing through.

Below normal temperatures on Thursday and Friday (in the 40s to
near 50 Thursday, and in the 50s Friday) will moderate into the
60s over the weekend and into next week as the core of the
coldest continental polar airmass heads well east of Texas.
Another cold front is slated to move into the region next Tuesday,
but latest indications are this will come through dry as it moves
into an airmass parched of lower-level moisture.



Dallas-Ft. Worth    41  51  37  50  31 /  20  20  10   0   0
Waco                38  38  36  51  31 /  80  80  30  10   0
Paris               38  53  33  46  26 /  10  10   5   0   0
Denton              38  51  33  48  28 /  20  20   5   0   0
McKinney            37  52  34  48  28 /  20  10   5   0   0
Dallas              41  52  37  50  32 /  20  20  10   0   0
Terrell             39  50  35  51  27 /  20  20  10   5   0
Corsicana           40  46  37  51  30 /  60  50  20  10   0
Temple              39  39  37  51  31 /  90  90  50  20   5
Mineral Wells       38  48  32  48  27 /  20  30  10   0   0




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