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Area forecast discussion...updated 
National Weather Service Fort Worth Texas
548 am CST Thursday Nov 26 2015

/12z tafs/

Concerns...IFR and MVFR ceilings through the period. Showers and
thunderstorms through much of the period. Strong cold front
arrives Friday morning.

Through 12z Friday morning...low MVFR and IFR ceilings will prevail
with strong south to southeast winds of 10-20 kts and higher gusts
occasionally over 25 kts. Showers and thunderstorms will increase
across the region this afternoon with periods of moderate to heavy
rain causing reduced visibilities. Will carry thunderstorms in the vicinity from 21z into
the evening hours but this may need to be changed to thunderstorms and rain in
a future taf issuance. There will likely be a break in the rain
activity at kact this evening and possibly in the dfw
metroplex...but the rain activity will increase again as the front

Between 12-14z Friday morning...the cold front is expected to move
across the dfw metroplex. The front will arrive at kact around
18z. The winds behind the front will be north around 15 kts. Rain
will occur along and behind the front with several rounds of
mostly shower activity continuing through Sunday. The rain will be
moderate to heavy at times.



Previous discussion... /issued 421 am CST Thursday Nov 26 2015/
the overall forecast is in good shape but a few changes have been
made...primarily to today/S forecast which is probably the most
uncertain part of the forecast. This is because we are a little
more concerned about some surface based convection developing
within a strongly sheared environment as a lead impulse and strong
jet streak move across northwest Texas and Oklahoma. Several of
the hi-res models have become more aggressive with their
convective development north of I-20 this afternoon and early
evening and therefore have increased probability of precipitation and also bumped up the
start of the Flash Flood Watch from midnight to 6pm. The question
is whether there will be enough instability for any severe
weather. The raw model output generally says no - that we will be
too cool at the surface...but they are only forecasting upper 60s
for highs today. Models have been too cool with afternoon
temperatures the last couple of days and we currently are already
in the middle to upper 60s. Forecasts from the MOS indicate we should
reach the low 70s and modifying the model soundings for those
temperatures this afternoon gives US enough instability to be
concerned about a marginal wind and tornado threat. Since this
threat is so reliant on daytime instability the window will be
brief from 3 PM to 7 PM.

The only other significant change to the forecast was to shift the
event total rainfall axis about 50 miles to the west as all model
guidance has trended that direction. We are still forecasting
average totals of 3 to 7 inches...with the 7 inch bullseye now
near Gainesville. 3 to 5 inches are expected along the I-20 and
I-30 corridors...with 2 to 4 inches for the southern part of the
Flash Flood Watch. Generally 1 to 3 inches is expected for our
southern and southeastern counties that are not included in the
Flash Flood Watch. Of course given the convective nature of the
activity this afternoon and tonight there will be some locally
higher totals possible.

No changes were made to the timing of the cold front and we
continue to favor the NAM which has displayed excellent run to run
consistency and usually is the best model to use in this weather
pattern. The front should reach the northwest zones around midnight...the
dfw metroplex around sunrise...Waco area by noon and clearing the
southeast zones late afternoon. The NAM temperatures continue to
look very reasonable...with the coldest air knifing south through
the western zones. Temperatures should be 5 to 10 degrees warmer over the
east where the cold airmass will be more shallow and more prone to
moderation from warmer air just above the ground. Temperatures will drop
20 to 30 degrees within an hour or two behind the front so the
numbers listed on Friday/S high temperature forecast is misleading since
temperatures will be dropping into the 30s and 40s for most of north and
central Texas. Temperatures will struggle to warm appreciably
Saturday and Sunday given all of the rain in the area and
continuing weak cold advection.

Temperatures will be near freezing across the western zones
Friday night and Saturday morning as rain is falling. Because the
air just above the surface is very warm...the rain will help
transport some of this heat to the ground. Raindrops will be too
warm to freeze when they reach the no true freezing
rain is expected. However if there is a long enough break in the
rain...some residual rain water may freeze on elevated surfaces.
This is an outside possibility and we do not expect any impacts.

The timing and character of the rain through the event will evolve
as the cause of lift in the atmosphere changes to different
forcing mechanisms. Today and tonight/S rainfall activity will
be convective in nature and be capable of high but very localized
2+ inch per hour rainfall rates. Most of this activity is forecast
to occur along and north of I-20 moving from west to east
gradually during the afternoon and evening hours. Again there
could be a low severe weather threat with this activity. As the
cold front arrives late tonight we should see a solid band of rain
and a few isolated storms right along and behind the front as the
warm moist air is lifted via frontogenesis. This band should
progress southeast through the northwest zones and into the central zones
by sunrise but is expected to weaken during the afternoon hours.
This is because the front will be so shallow as it moves through
the central and southeast zones...its lifting power will become
marginalized. Therefore we expect just a lot of weak showers over
the area Friday afternoon and evening.

Rainfall intensity and coverage will increase again by Saturday
as Pacific Hurricane Sandra begins to influence our area. Sandra
will be moving onshore east of the Baja California region of Mexico on
Saturday. The remnants are forecast to move east and northeast
across Mexico and through Texas on Sunday. While the system will
provide Pacific moisture...the real influence and cause of the
persistent heavy rain over Texas will be from strong dynamics for
lift across the region out ahead of the system. This lift peaks
Saturday and Saturday night and should produce a large band of
moderate rain that streams across the region. This band will
gradually move southeastward across the area. While rain rates
will not be very high with this round of rain activity...the rain
will be persistent resulting in multi-inch rainfall accumulations
on saturated soils. This activity will end from west to east
during the day on Sunday as the remnants of Sandra clear the area.

Have kept the extended forecast dry Monday into Wednesday
primarily due to a lack of model agreement next week. The GFS is
very wet by Tuesday but has been bouncing around with that system
every run. The European model (ecmwf)/Canadian and UKMET show a drier northwest
flow aloft until Thursday. The European model (ecmwf) does show light rain on
Monday as some low level moisture remains trapped in the region.
Again will keep the probability of precipitation at 10 percent...but keep a lot of cloud
cover around with cool and slowly moderating temperatures.



Preliminary point temps/pops...
Dallas-ft. Worth, Texas 73 50 55 38 39 / 70 90 100 90 100
Waco, Texas 75 65 69 39 41 / 60 60 80 70 80
Paris, Texas 71 62 68 39 43 / 40 80 90 90 100
Denton, Texas 71 46 49 36 38 / 80 90 100 90 100
McKinney, Texas 72 53 56 37 40 / 60 90 100 90 100
Dallas, Texas 73 56 59 38 40 / 60 80 90 90 90
Terrell, Texas 74 64 68 39 42 / 50 70 90 80 90
Corsicana, Texas 75 65 71 41 44 / 50 60 70 80 80
Temple, Texas 74 66 70 39 42 / 50 50 70 70 70
Mineral Wells, Texas 72 44 47 33 35 / 80 90 100 90 100


Forward watches/warnings/advisories...
Flash Flood Watch from this evening through late Saturday night
for txz091>095-100>107-115>123-129>135-141>146.




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