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

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FXUS64 KFWD 260449 AAA

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1149 PM CDT Sat Mar 25 2017

Light mostly easterly winds will become southeasterly at 5 to 8
knots overnight. Low clouds over South Texas will spread northward
overnight and may move into the Waco area toward 12z and into the
Metroplex TAF sites after daybreak. Thus have indicated a TEMPO
BKN015 at KACT from 12 to 16z and a TEMPO BKN020 at the Metroplex
TAF sites for the 13 to 16z period.

Southerly winds will be on the increase Sunday as a lee side
trough deepens. Expect south winds 10-12 knots at 13z to increase
to 15-17 knots by 16z with some gusts of 25 knots possible through
the afternoon. Although a capping inversion will be in place,
isolated to scattered thunderstorms may form along the dryline by
mid afternoon mainly along and north of the I-20 corridor. Thus
have placed VCTS in the Metroplex TAF sites for the 21z through
00z period. Some MVFR ceilings should spread northward again
Sunday night, thus have indicated BKN020 at 04z at Waco and 06z
at KDFW.



Main update was to include a mention of patchy fog for southern
zones as well as adjust temperature and dewpoint trends during
the overnight hours. Also, made a few slight changes to PoPs based
on the latest arriving model output.

Surface analysis this evening revealed that low level moisture
was beginning to lift northward along the Upper TX Coast with the
00 UTC RAOBs indicating the 1" PWAT isopleth was slowly lifting
northward. Closer examination of these RAOBs indicate that the
moisture depth is about 1km or so suggesting that perhaps a
slightly richer quality of moisture does exist. The 4-Corner`s
upper trough will continue to pivot eastward and with subsequent
lee-side pressure falls ongoing and forecast to continue, this
plume of low level moisture should have little in the way to
impede its progress through the overnight hours. With the
increasing low level moisture, there`s a potential for some light
and patchy fog mainly across Central zones. Farther north,
northerly winds, due to the influence of the weak front/surface
low across the Missouri Valley, will eventually become more
easterly and southeasterly, but not before temperatures fall into
the low 50s. If southeasterly winds do not occur before sunrise,
it`s possible that temperatures along the Red River are not low
enough. This front will have ramifications for PoPs on Monday
morning and this is discussed below.

With regards to Sunday`s severe weather potential, the threat
still remains contingent on several factors. The first will be the
degree of low level mixing that occurs as low level moisture lifts
northward. RAP output indicates that low to mid 60 degree
dewpoints will be possible along the Red River. This is somewhat
interesting as the RAP typically mixes quite deeply and one would
not expect such high dewpoints as far north as the Red River.
Other model guidance, however, remains tightly clustered around a
solution that places the 60 degree isodrosotherm a little farther
south with mainly 50s along the Red River and I`ve trended
dewpoints closer to this solution. This is important as even a
minor change in surface dewpoints can result in the amount of
instability and low level buoyancy to perhaps breach the cap. If
the RAP is correct, there could be a bit more instability in place
and a slightly weaker cap than is advertised at this time.

With regards to the cap---a majority of hi-resolution and coarser
guidance do weaken the cap sufficiently in the 21-00 UTC time
frame near the Highway 281 corridor. With sufficient convergence
along the dryline, we should see a few storms develop.
Environmental parameters do support a supercell storm mode. With
an unseasonably warm and well mixed boundary layer, LCLs will
likely remain high which would foster more of a damaging wind and
large hail potential. If the quality of moisture is a bit better
and can resist being scoured out completely, LCLs would in theory
be lower. This combined with an environment characterized by
favorable turning of the low level winds (large hodographs) does
mean that a slightly greater tornado threat would materialize
should the low level moisture quality be a bit higher than
currently thought. This is something that we will closely monitor
with additional model output and a new suite of upper air data
Sunday morning. With that said, I nudged PoPs upward by a hair and
extended them a bit farther south and west along the axis of
greatest instability and weakest inhibition for Sunday afternoon.
As has been stressed the past few days, the probability for
numerous severe storms is low (likely just a handful at best), but
should storms get going, they will more than likely become severe.
The most likely area for strong to severe storms will be
generally along and north of I-20.

The only other change in the forecast was to add a low chance for
showers and storms Monday morning across eastern/southeastern
zones. As the upper forcing slides eastward, it appears that a
cold front will overtake the dryline. Various NAM solutions continue
to produce QPF. With the forcing and expected low level moisture
in place, I went ahead and added a sliver of 20 PoPs out across
eastern zones. The remainder of the forecast looks to be in good



.PREV DISCUSSION... /ISSUED 400 PM CDT Sat Mar 25 2017/
Mostly sunny skies and pleasant temperatures prevail across North
Texas behind last night`s cold front. West-northwest winds have
ushered in some drier air making for a nice day across the region.
Temperatures are generally in the 60s and 70s with dewpoints in
the 40s.

Springtime usually means rapidly changing weather as the upper
pattern is usually fairly progressive. Tomorrow will be no
different. Shortwave ridging today and tonight will give way to a
fast moving shortwave trough by tomorrow and increasing severe
weather chances. As the upper disturbance approaches, rapid
surface cyclogenesis will commence during the morning hours. This
will result in strengthening southerly flow and a rapid northward
transport of moisture. By midday, surface dewpoints will likely
be near 60 degrees along the I-35 corridor toward to the Red
River. In addition, a dryline will advance eastward across West-
Central Texas.

By late afternoon, an elevated mixed layer will be in place across
North Texas resulting in a capping inversion around 800mb with
very steep lapse rates through about 500mb. Even as the main
shortwave tracks across northern Oklahoma, stronger forcing for
ascent will spread into North Texas by late afternoon. This should
help to lift the capping, which is not expected to be overly
strong. We`ll likely sample this with supplemental balloon data
tomorrow afternoon. The bigger concern regarding convective
development will be the quality and depth of moisture return into
the area given the northerly track of the shortwave. Surface based
instability is expected to be around 2000 J/kg with dewpoints in
the lower 60s, although this moisture is fairly shallow. Mean
layer instability (around 1500 J/kg) will likely be more
representative of the environment tomorrow afternoon from the
Metroplex and areas southward. Most of the high resolution
guidance develops convection to the west-northwest of the
Metroplex by 22Z, although these initial convective attempts
quickly diminish by 00Z. This suggests that the initial
development along the dryline may not have enough support to allow
them to move off the dryline into a more unstable environment.
Forcing will be stronger the farther north, so areas generally
north of I-20 should have a better chance for thunderstorms
through the early evening hours.

Our latest thinking is that the best chance for severe
thunderstorms in our area will be in the Red River counties,
particularly Cooke, Grayson, and Fannin counties, from about 22Z
to 00Z. However, it should be noted that if any storms farther
south can be sustained, then the environment is more than suitable
to support severe weather. Forecast soundings for tomorrow
afternoon feature lapse rates in excess of 8 C/km through a deep
layer along with large clockwise curved hodographs through the
lowest 2 km. While all modes of severe weather will be possible,
the quality/depth of moisture remains the biggest limiting factor
for a more appreciable tornado threat. Very large hail (baseball
size or larger) will be the main severe weather threat through the
evening hours. We`ll maintain 20-30% PoPs across mainly the
northern half of the CWA with the best chances along the Red
River. Any convection that develops will move east through the
evening with a cold front moving into the area during the
overnight hours.

Monday looks like it will be a rather nice day behind the cold
front but it won`t last long. Another strong upper low will
move into the southwest U.S. by Tuesday. This will again allow
moisture to spread northward and rain/storm chances increasing
late Tuesday through Wednesday. There may be an initial severe
weather threat late Tuesday afternoon/evening as convection
develops across West Texas along a dryline and moves eastward.
Rain/storm chances increase significantly across North Texas
Tuesday night into Wednesday where one or more rounds of
thunderstorms may move through the region. At this time, the
severe weather threat does not look overly impressive, although
there will be sufficient instability/shear to support rotating
storms during this time. We`ll monitor this closely over the next
48 hours. Heavy rainfall will be possible through Wednesday with
current forecast rainfall amounts between 2 and 4 inches across
parts of North Texas. While the system looks like it will remain
progressive enough to preclude a more significant flash flood
threat, localized training of storms would support flash flooding.

This system moves east Wednesday night into Thursday with quiet
weather through the end of the week.



Dallas-Ft. Worth    55  84  60  80  56 /   0  30  20   5   0
Waco                54  83  62  83  59 /   0  20  20   5   5
Paris               50  80  60  77  53 /   0  10  50   5   0
Denton              51  82  58  78  53 /   0  30  20   5   0
McKinney            49  81  59  78  54 /   0  30  30   5   0
Dallas              55  84  62  81  57 /   0  30  30   5   0
Terrell             50  82  62  79  56 /   0  20  40   5   0
Corsicana           54  84  63  82  60 /   0  20  30  10   0
Temple              57  84  60  84  60 /   0  20  10   5   5
Mineral Wells       52  86  54  80  53 /   0  20  10   5   0




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