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Area forecast discussion...updated 
National Weather Service Fort Worth Texas
1205 PM CST Wednesday Nov 25 2015

/18z tafs/

Ceilings have risen into MVFR at all airportsas of midday with
continued warm air advection and strong southerly winds 15-20 kts through early

Otherwise...warming and mixing through 800mb will allow VFR
ceilings to return by late afternoon and continue through middle-late
evening. Conditions will deteriorate to low MVFR by
the boundary layer cools with 30-40 knots flow at 925mb developing
overnight. Spotty light -dz/-ra should return overnight and assist
ceilings lowering into IFR before sunrise. Possible LIFR ceilings/MVFR
visibilities are possible the remainder of Thanksgiving day morning with
isentropic ascent and showers increasing across the area by early
afternoon. There certainly could be isolated thunder...but have
left thunderstorms in the vicinity out at this time with better confidence likely occurring on
later aviation forecasts.

Otherwise...gusty S/southeast winds sustained around 15 kts will continue
throughout the 18z taf cycle at all airports with pressure falls
continuing to the west in advance of our approaching system. An
Arctic frontal passage still appears on track to arrive in the 12z-15z time
frame Friday with gusty north winds 15-20 kts with higher gusts.



no major changes to the current forecast other than to tweak temperatures
up a couple of degrees along the Red River. Low rain chances
continue through tonight before ramping up through the day on
Thanksgiving. Latest 12z model data is coming in similar to 00z
data with respect to temperatures across our northwest counties.
Temperatures should be in the 33 to 35 range early Saturday morning with
no appreciable chances for frozen precipitation.

Flash flood watches will likely be posted for parts of the region
later today.



Previous discussion... /issued 416 am CST Wednesday Nov 25 2015/
an upper level low is now closing off over northern California
and will become cut off over Nevada through the weekend before
finally moving east into the plains early next week. In advance of
this system...warm and moist southerly flow will prevail at the
surface through Thursday evening before much colder air and
widespread rains arrive with a strong cold front Friday.

Mostly cloudy skies are expected today as rich Gulf moisture
advects into the region. A tiny amount of instability exists below
a Stout capping inversion and therefore a few low topped and fast
moving showers will stream through the area. Rainfall will be
brief and light for areas that see anything. Moisture advection
wanes this evening and instability diminishes so have kept tonights
forecast dry. Despite the clouds...warm advection should help temperatures
warm into the low 70s today for the western and southern zones but
otherwise middle to upper 60s are expected. Another unseasonably
warm night is expected tonight with lows in the low 60s.

Upper level forcing in the form of a shortwave trough moving
across Oklahoma and an incoming jet maximum will begin to spread into
the region from the west Thanksgiving day. As temperatures rise into the
low to middle 70s...some weak instability will become available for
scattered convection to develop in the afternoon. This activity
will continue into Thursday night but will remain largely
disorganized and scattered. Brief heavy rain is possible but both
the flash flood and severe weather threat stay very low.

The main weather story concerns a strong cold front that will
arrive into the County Warning Area early Friday morning. Temperatures will
likely drop 20-30 degrees within a few hours of frontal passage.
Thus the high temperature numbers listed for Friday are
misleading...since they will occur early in the day. Temperatures will
spend most of the day in the 30s and 40s across North Texas...with
the southeastern half of the County Warning Area seeing temperatures drop into the 40s
and 50s after noon. The front will be strong but very shallow...and
we are currently favoring the colder NAM forecast as it is usually
Superior in this weather pattern. The brunt of the cold air and
cold advection will tend to drop south from the Front Range of The
Rockies into West Texas...thus it will be our western zones that
will get the coldest. This process is called Lee side cold air
damming and it occurs when the upper level pattern gets locked
into this configuration and a strong Canadian high heads into the
northern plains. The northeast surface winds along the Front Range
of The Rockies cause the air to pile up along the east side of the
mountain chain because it cant travel through the mountains and
since cold air is stable it really does not want to go up an over
the mountains. This process results in a building of the surface
high southward which causes the cold air to spill south along the
east side of the higher elevation. Since the higher terrain is
well west of this is why West Texas will contend with
wintry precipitation while the air in our area should be a little too
warm and shallow for such. More on that in a bit.

Warm/moist southerly flow will continue in the low levels behind
the front...which means a prolonged period of isentropic lift and
rainfall will occur Friday through early Sunday. Precipitable
water at 1.75 inches will be unseasonably high and at a level
where research shows an increased potential for excessive
rainfall. Instability above the frontal inversion will be low
which should limit rainfall intensity to less than 2 inches per
hour in the heaviest cells...but because this will be a long
duration event...rain totals will steadily add up. Some
thunderstorms are possible Friday behind the front...but the
instability should get used up Friday night with more of a steady
rain over the area. As Hurricane Sandra in the eastern Pacific
moves into the Baja California region will serve to increase
the dynamic lift and divergence aloft which should energize the
rain coverage/intensity Saturday into Saturday evening. Through
the entire event...the best dynamics for lift and strongest
isentropic lift all look like they will occur along and north of
I-20 and I-30 where the highest rain amounts are expected. Totals
near the Red River may be closer to 8 inches but at this time we
will highlight an average of 3-6 inches for all but the southeast
zones where amounts will be closer to 2 inches. Obviously all of
this rain falling on saturated soils and into a cold airmass that
limits evaporation means there will be excessive runoff and
moderate to high flash flooding and river flooding potential. Rain
should end from west to east on Sunday.

So the one other thing to focus on concerns the only significant
change to the forecast...which was to lower temperatures Friday night
and Saturday to favor the colder NAM guidance. This will put
surface temperatures in the ballpark of 32 degrees Friday night and
Saturday morning over the western zones. However...very warm air
above the surface /5000 feet temperatures of 50 degree f and a 10000 feet deep
above freezing layer/ means the precipitation type will be rain.
Furthermore continuous moderate rainfall will help to bring warmer
air down to the surface. Even with surface temperatures near freezing...
rain drops would have to cool 10+ degrees to reach freezing and
then would release even more latent heat to make the phase change
from water to ice. There just is not enough cold air forecast to
make US concerned about a freezing rain event. If the rain were to
stop there is a low potential for some refreezing of water on
elevated surfaces. But to get a true winter weather event we would
need the forecast low level temperatures to be a few degrees colder. One
final note...we already have seen NAM model output floating around
on the web that shows multi-inch snow amounts west of I-35 with
this event. This output is quite simply garbage. Snowfall graphics
are often not an explicit model forecast...they are Post-processed
using very simplistic algorithms and rules of thumb. Although they
look this case there is no way it is going to snow
with that same model showing a massive layer of warm air above.

For the remainder of the forecast the guidance today has shifted
the eventual track of the upper low more northward which means it
will be tough to scour out the low level moisture/clouds over the
region early next week. Thus temperatures will be slow to moderate
with lows in the 40s and highs in the 50s Monday into Wednesday.
The GFS is the only model showing another major rain event Tuesday-Wednesday
of next week. The European model (ecmwf) has some slight rain chances early Tuesday
with a weak cold front while the Canadian is dry. Have kept just
some low probability of precipitation in the southern half Tuesday in favor of the European model (ecmwf).



Preliminary point temps/pops...
Dallas-ft. Worth, Texas 70 62 71 52 60 / 10 10 50 70 90
Waco, Texas 71 63 73 65 67 / 10 10 50 50 90
Paris, Texas 67 60 68 61 63 / 10 10 40 60 90
Denton, Texas 67 62 70 45 51 / 10 10 60 80 90
McKinney, Texas 68 62 69 53 60 / 10 10 50 70 90
Dallas, Texas 69 63 70 57 62 / 10 10 50 60 90
Terrell, Texas 71 62 70 64 66 / 10 10 40 50 90
Corsicana, Texas 72 63 72 65 69 / 10 10 40 40 80
Temple, Texas 70 63 73 65 69 / 10 10 50 50 80
Mineral Wells, Texas 70 62 70 44 50 / 10 10 60 80 90


Forward watches/warnings/advisories...



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