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

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560
FXUS64 KFWD 151832 AAC
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1232 PM CST Sun Jan 15 2017

.AVIATION...
Fog has reduced acceptance rates throughout the region this
morning. But a northwestward advancing warm front, just east of
our TAF sites at midday, should soon improve visibility above the
LIFR category. The window to clear the backlog will be small as
scattered thunderstorms approach from the south later this
afternoon. In addition, widespread convection across West Texas
will begin disrupting traffic to/from the west. The north-south
extent of this activity will likely require NW/SW arrivals to be
redirected through the Bonham and Cedar Creek cornerposts,
respectively. A strong/severe storm could be among the initial
cells, but the stronger activity will arrive with the main line
from the west, still on schedule for 06-09Z (midnight-3am CST)
along the I-35 corridor.

Southwesterly surface flow and VFR conditions will ensue in time
for the morning push Monday. Westerly winds on Monday should be of
insufficient speed to create any crosswind issues. A cold front
will arrive toward the end of DFW`s TAF, and the wind shift has
been introduced with this TAF package.

25

&&

.UPDATE...
No major changes to the current forecast other than to extend the
Dense Fog Advisory through 3pm for areas southwest of the
Metroplex. Visibilities are still around 1/4 mile in these areas
although improvement is expected through early afternoon. Surface
observations indicate that the warm front is now retreating
northward and a warm sector is expanding into North Texas with
dewpoints in the low/mid 60s across our southeast counties. This
trend should continue into the late afternoon/early evening hours.
Water vapor imagery shows the strong upper trough approaching from
the west with a line of convection now moving across West Texas.
The areal extent of showers and a few thunderstorms across
central and southeast Texas has been increasing over the last few
hours indicative of increasing large scale forcing for ascent. We
should see scattered showers and thunderstorms increase in
coverage across North Texas through the early evening ahead of the
main line which will come through tonight.

Our short term concern with the leading activity ahead of the main
line will be a low end severe threat. Granger radar VAD wind
profile shows a moderately sheared 0-3km layer which will only
strengthen through the early evening. A few cells south of College
Station have already shown signs of rotation which will become
more concerning in our area later today as some surface based
instability develops. At this time...no major changes to the
forecast or severe weather threat are needed...although we did
increase PoPs through the early evening across much of North and
Central Texas.

Dunn

&&

.SYNOPSIS...
Several forecast challenges will present themselves over the next
24 hours, the primary one being the chance for strong to severe
storms this afternoon through tonight. This potential is
contingent on the behavior of the frontal bounday to our south
which will surge northward as a warm front later today. The finer
details of the severe threat are discussed below. Otherwise, fog
will continue through the morning hours and a Dense Fog Advisory
remains in effect for much of the area; many sites continue to
report visibilities of 1/4 mile or less. Forecast confidence for
the middle and end of the week generally remains low.

&&

.SHORT TERM...
A stationary front remains draped roughly from San Angelo to
Austin to Texarkana with widespread fog/mist/drizzle in the cooler
airmass to the north. Most locations will continue to see
visibilities around 1/4 mile or less and the Dense Fog Advisory
has been expanded eastward to include Paris, Canton, Athens, and
surrounding areas. To the south of the front,
temperatures/dewpoints are as much as 20 degrees higher and
visibilities generally are quite a bit better. Drizzle and mist
will continue through the morning hours north of the boundary until
the stationary front lifts northward as a warm front later today.

Morning water vapor imagery depicts a strong upper low over
northwest Mexico that is responsible for the ongoing widespread
showers and storms across NM and West TX. This system will
continue to move east/northeast today, overspreading the region
with strong height falls. An attendant surface low will deepen
across West TX by midday, and strong southerly flow will ensue in
the vicinity of the stalled frontal boundary. As a result, the
stalled front should finally lift back to the north beginning
later this morning and into the afternoon. Surface winds will veer
to the east and eventually southeast and the warm sector airmass
will begin to overspread North and Central TX. Have stuck with the
slowest guidance regarding the front`s movement which includes a
blend of the NAM and RAP which have done better at resolving the
very shallow arctic airmass north of the front. Needless to say,
the movement of the front will have significant implications for
the next stage of the forecast.

By mid afternoon, expect that the warm front will be entering our
northwestern counties with dewpoints climbing into the 50s and
low 60s to the south. Skies should remain mostly cloudy or
overcast through the day which will limit the amount of diabatic
heating, but temperatures are still expected to warm into the
upper 50s and 60s due to warm advection with low 70s likely in our
southeast counties. By late afternoon, the upper low will be
entering West Texas and strong dynamic forcing coupled with
intense low-level warm advection should result in a marked
increase in convection across much of the state. We`ll be keeping
an eye on 2 waves of activity beginning late this afternoon. The
first will be scattered storms generated by warm advection within
the warm sector airmass which will occur in the 4pm-10pm time
frame. The second concern will be a line of storms that will take
shape in West TX along a Pacific cold front which will move into
our area from the west later this evening and overnight.

The first wave of activity will consist of scattered showers and
storms developing and streaming northward within the warm sector
late this afternoon and evening. The strong/severe potential with
this activity is highly dependent on the warm front`s behavior
and how much warming/moistening has taken place at the surface.
Most of this activity will be along/west of I-35 during this time
where the strongest warm advection will be situated. A low level
jet of 40-50 kts will have overspread the area by this time with a
mid-level speed max of 50+ kts nosing into the region as well.
With 0-1 km shear of 30 kts and 0-6 km shear of over 50 kts, the
resultant hodographs will certainly support rotating showers and
storms. As has been the case for the past few days, instability
remains to be the big question mark. If the warm front does lift
far enough north, 500-1000 J/kg of CAPE will exit in areas west of
I-35 and south of I-20 by late afternoon and evening. While much
of this buoyancy will be confined to the mid levels where cooling
due to ascent from the approaching upper low will steepen lapse
rates, the warm/moist air at the surface may result in surface
based instability. If an isolated storm is able to become surface
based during this time, there will be a severe potential
including tornadoes given the ample shear. Strong winds and some
hail will also be of concern. In addition to the severe threat,
there will also be a localized flood threat during this time. PW
values will climb to around +3SD above normal and thunderstorms
should be very efficient rain producers. If cell training occurs
(oriented S-N with the mean flow), a localized flash flood threat
will exist where some multi-inch rain totals could exist in a
short time.

The second wave of activity will occur later in the evening and
overnight. The line of storms that will have taken shape in West
TX along the Pacific cold front will be approaching our western
counties between 6-9pm. Some portions of this line could be strong
to severe as well given that most of the wind shear (30-40 kts)
will be concentrated in the lowest 3 km and MUCAPEs will be
500-1000 J/kg as lapse rates through the mid levels continue to
steepen. At this time, would anticipate wind being the biggest
threat with this activity as the high-momentum air already in
place through the column could be transferred to the surface
rather easily, especially within any bowing segments in the line.
There may also be a low potential for a brief embedded tornado,
but this again is highly conditional on the warm front position
and warm sector airmass properties ahead of the line. Latest
guidance has the line approaching the I-35 corridor around or
shortly after midnight when it should begin losing some vigor as
it outruns the better forcing and more favorable instability. At
this time, it seems like the line should begin slowing down with
the weakening trend continuing as the line gets east of I-35; the
strongest forcing will be lifting off to the northeast with the
upper low during this time. We`ll also need to keep an eye on
hydrology concerns during this time as heavy rain and localized
flooding will be a threat with all activity overnight.

&&

.LONG TERM...
Monday`s forecast will weigh heavily on the position of the
slowed/stalled Pacific front associated with the overnight line
of storms, thus will not delve too deep into the mesoscale
details just yet. If the Pacific front remains stalled across our
southeastern counties on Monday afternoon as some guidance is
indicating, it could provide a focus for additional convective
initiation, including some strong to marginally severe storms.
The lacking feature will be the strong ascent that was
responsible for Sunday night`s activity which will be lifting way
off to the northeast by this time. However, there will still be
sufficient shear and decent instability in place just ahead of the
Pacific front. Behind the front, much drier and slightly cooler
air will be in place and have shown a dry forecast for the
northwestern half of the area.

A surface cold front associated with the northeastward-departing
low pressure system will be pulled through the area Monday
evening/night bringing a slight temperature drop for Tuesday.
This front should stall somewhere across Central TX late Tuesday
where some showers and storms will be possible as a positively-
tilted upper trough lingers to our west. A warming trend should
begin through the second half of the week as this stalled front
becomes washed out and southerly flow returns to the region. Given
the strong dynamics associated with the newly formed upper low to
our west, it looks like at least some low shower/storm chances
should continue through the later portion of the week. Have
generally left low PoPs with low confidence at this time.

-Stalley

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    54  67  43  59  45 / 100  10   5  20  20
Waco                57  68  45  58  46 /  90  20  10  30  30
Paris               59  67  44  57  43 /  90  50  10  10  20
Denton              50  65  38  57  42 / 100   5   5  10  20
McKinney            55  66  41  57  43 / 100  20   5  20  20
Dallas              57  67  44  60  46 / 100  10  10  20  20
Terrell             59  67  45  58  46 / 100  30  10  20  20
Corsicana           60  69  49  57  47 / 100  40  20  30  30
Temple              58  68  47  58  47 /  90  30  20  40  30
Mineral Wells       47  64  38  57  42 / 100   5   5  10  20

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 3 PM CST this afternoon for TXZ129>134-
141>145-156>159.

&&

$$

25/91



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