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Area forecast discussion 
National Weather Service Fort Worth Texas
553 am CST Sat Dec 27 2014

Aviation...
a cold front had moved through all taf sites as of 530 am. The
passage of the front brought drier air and lifting ceilings. MVFR
conditions will prevail through sunrise with ceilings lifting
above 3000 feet by middle to late morning. Scattered light showers are
possible through the morning but the better rain chances will
arrive this afternoon as large scale lift associated with a
approaching upper trough increases. Any showers that develop this
afternoon will be relatively light but may be accompanied by MVFR
ceilings. A few showers could linger overnight but coverage will
be too limited to include in this forecast. VFR conditions will prevail
tonight with scattered to broken middle and upper level clouds.

A north wind will prevail between 16 and 20 knots today with some
gusts up to about 27 knots. Wind speeds should fall below 15 knots
by middle evening.

79

&&



Previous discussion... /issued 442 am CST Sat Dec 27 2014/
the first of three cold fronts that will affect the region over
the next 7-8 days is currently moving through north central Texas.
At 320 am...the front was roughly along a line from Paris to
Dallas to Goldthwaite. Around middle morning...the front will have
traversed our entire County Warning Area...moving south and southeast. Temperatures
behind the front have fallen into the 40s while temperatures ahead
of the front remain in the 60s. Most locations will experience
their daytime highs near daybreak with slowly falling temperatures
throughout the day. However...afternoon temperatures across our
northwestern counties will likely remain nearly steady...possibly
creeping up a degree or two...before beginning to fall again around
sunset this evening. The region will remain overcast/cloudy today
with some clearing across the west this evening and tonight.
However...high clouds will begin moving in from the west which
creates a challenge for overnight lows. The high cloud cover
should keep overnight lows from fully falling as much as they
would under clearer skies...but still expect a much colder night
with overnight lows in the upper 20s northwest to upper 30s in the
southeast where dense cloud cover will prevail.

Ongoing showers this morning will continue through the day as lift
from an upper level trough to our west continues across the
region. While there could be a few rumbles of thunder at
times...have left the thunder mention out of the forecast for now
as the elevated instability is pretty minimal. Most of the area
will receive less than a quarter of an inch of rain today...with
some isolated higher amounts. Most the precipitation will end this
evening but some additional showers may occur Sunday morning
across our southern counties as the upper level trough swings
through the region. We will see warming temperatures Sunday
through Monday as we await our next system.

The next system continues to be a deepening trough over the West
Coast next week. This trough is currently located off the coast
of British Columbia and will continue a southward track for the
next several days. As this trough develops...it will maintain a
broad latitudinal orientation or positive tilt...which results in
near zonal flow across a good majority of the country. This
orientation will allow cold Arctic air at the surface to rush out
of Canada...into the plains...and down through Texas. The Arctic
front should not meet much resistance moving south and is expected
to arrive here Monday night/Tuesday morning. The front is not
expected to quickly and aggressively move through the region...but
will instead gradually spread across the region with noticeably
colder temperatures behind the front. The 1055 mb plus surface
high behind the front will take a few days to move into the
Southern Plains...meaning the cold weather will be around for a
few days. On Tuesday...highs will range from the middle 30s in the
northwest to lower 50s in the southeast. High temperatures on
Wednesday will only be in the 30s and lower 40s...with the
northwest struggling to reach above freezing. Low temperatures
Tuesday and Wednesday nights will be in the 20s and 30s.
Strengthening warm air advection will actually warm overnight
temperatures Wednesday night/Thursday morning but lows that night
will still be in the 20s and 30s...and highs on Thursday will
remain in the 30s and lower 40s due to cloud cover and rain.

Our main concern and challenge with this system is the potential
for precipitation and especially frozen precipitation. The upper
level trough will cut off into a low over California on Wednesday.
When an upper level low cuts off...it becomes separated from the
main jet stream and will tend to slow down its trek as a result.
This is indicated in the models as the low slows down and remains
over the southwestern Continental U.S. Through Friday. With the upper level
low well to our West...Texas remains in a nearly zonal and/or
slightly ridged pattern aloft. In this pattern...there is generally
a lack of large scale lift across the region so we have to look
closer at smaller scale features for any lift for precipitation
development.

The front is expected to come through dry and the models have
consistently indicated no precipitation along the front.
However...as an h850 high moves into the region Tuesday night and
Wednesday...it turns the 850 mb winds across our southwestern and
western counties to the south which results in some weak warm air
advection over the cold surface layer. Do not think the warm air
advection will be strong enough to generate much precipitation but
there is still a small chance it could develop some light
precipitation and will retain the low probability of precipitation in the southwest
Tuesday night and in the west Wednesday with a mention of light
drizzle/freezing drizzle. Confidence is low this will occur...but
if it does...very light accumulations of ice may occur on elevated
surfaces...windshields...and possibly tree branches. We are
trying to anticipate smaller scale features several days in
advance which is difficult to do...so we want to emphasis that the
chances for frozen precipitation remain low and will continue to
monitor this time period in future forecast runs.

Better precipitation chances are expected Thursday through Friday
as the upper level low begins moving east and increases lift
across the region. Warm air advection and isentropic lift will
start to increase late Wednesday night and Thursday
morning...continuing through the day on Thursday. In this
regime...precipitation will likely develop from south to north
across the region. Even though temperatures will likely be rising
for part of the night...there will still be a period where
temperatures will be slightly below freezing across our western
counties and these areas could see light freezing rain for a few
hours. Impacts should be very minimal during the period of
freezing rain as temperatures are expected to be in the lower 30s
and freezing rain usually does not start to have a significant
impact...especially on roads...unless temperatures are in the
upper 20s. However...we will have to monitor changes to the
temperature forecast that night. But with increasing temperatures
through the night...the warming temperatures and changeover to
liquid rain will act to melt any ice later in the night.

A cold...but all liquid...rain is expected to be more widespread
Thursday through Friday. Have increased probability of precipitation in this time frame
with the model quantitative precipitation forecast indicating some beneficial rain is possible.
There could be two rounds of rain during this time...one on
Thursday and another on Friday. Another cold front is possible
Friday or Saturday.

82/jld

&&




Preliminary point temps/pops...
Dallas-ft. Worth, Texas 45 32 50 33 58 / 40 20 5 5 5
Waco, Texas 49 34 49 32 61 / 40 20 10 5 5
Paris, Texas 50 33 49 32 55 / 40 30 5 5 5
Denton, Texas 42 29 50 32 57 / 50 20 5 5 5
McKinney, Texas 45 31 49 32 57 / 40 20 5 5 5
Dallas, Texas 46 33 50 34 59 / 40 20 5 5 5
Terrell, Texas 52 34 50 32 58 / 40 30 5 5 5
Corsicana, Texas 54 35 49 33 60 / 50 30 10 5 5
Temple, Texas 51 34 49 32 61 / 40 20 20 5 5
Mineral Wells, Texas 42 28 49 31 58 / 50 10 5 5 5

&&

Forward watches/warnings/advisories...
none.

&&

$$

79/82

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