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

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000
FXUS64 KFWD 270958
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
358 AM CST Mon Feb 27 2017

.DISCUSSION...

No shortage of forecast challenges with the main concern being
the potential for strong/severe thunderstorms capable of producing
very large hail both today and Tuesday, possibly beginning as
early as daybreak today.

Widespread cloudiness overspreads the area this morning with
abundant low-level moisture in place. Where skies temporarily
cleared across our northwestern counties, very low clouds and
patchy dense fog have developed. This is in the vicinity of a
stalled frontal zone roughly near Highway 380 as of 330am. Would
expect low clouds with some patchy drizzle and fog in vicinity of
this front through the next few hours before the boundary lifts
back to the north later this morning.

Broad troughing and southwest flow aloft exists across much of
the southern US with a 100+ kt upper jet streak positioned through
TX. In addition to the frontal boundary positioned through North
TX, a dryline is draped roughly from Wichita Falls to Abilene.
Dewpoints remain in the upper 50s and 60s east of the dryline
where a potent warm sector airmass is in place. Warm advection
will begin increasing this morning as southerly flow strengthens
through the low-levels. As this occurs, weak isentropic ascent
will overspread much of the warm sector within the capping
inversion layer centered around 900mb. If there is sufficient lift
within this layer, it will be capable of generating at least
isolated elevated thunderstorms given the very steep lapse rates
located through the mid-levels.

As mentioned above, the main concern today will be the potential
for severe thunderstorms capable of producing very large hail,
possibly developing as early as daybreak with additional storms
possible throughout the afternoon. A very favorable environment
for large hail is expected to take shape across most of North and
Central Texas as shown by forecast soundings from numerous
sources. This environment is characterized by very steep mid-
level lapse rates (as much as 8.5 C/km), around 2000 J/kg of
MUCAPE, 50-60 kts of deep-layer shear, and wet bulb zero heights
around 9 kft agl. Forecast SHIP values of 1.5-2 confirm an
environment conducive for storms to produce significant hail. If
storms are able to develop and their buoyancy is able to overcome
the lack of strong large-scale dynamic ascent, they will become
capable of becoming strong to severe rather quickly with the
primary convective mode expected to be elevated discrete
supercells.

While the confidence in the severe potential is high, confidence
in the development, timing, and location of storms is rather low.
If storms develop, they are expected to be isolated, hence the
fairly low PoPs. Latest high-res guidance is pegging various
locations for the primary threat, targeting parts of Central TX
through the morning hours and transitioning northwards towards
I-20 by midday. Trends will need to be closely monitored through
the next several hours, but more than likely the location of
convective initiation won`t be known until it a storm has already
begun to develop. If supercells are ongoing this afternoon, it
will be possible for them to become surface based as boundary
layer warming and weak ascent combine to reduce or eliminate the
low-level capping inversion. If this occurs, a low tornado
potential can`t be ruled out given the shear and buoyancy
parameters. In addition, if winds remain backed to the southeast
in the vicinity of the frontal boundary as it lifts back to the
north, this would be capable of locally enhancing storm-relative
helicity through the lowest 1 km.

In summary, while storms are not a guarantee to develop today,
any storm that does form will have the potential to become severe
and capable of producing very large hail. If storms become
surface-based this afternoon, a low tornado potential will exist
as well. The timing and location of storms is very uncertain and
any storms that do form should remain fairly isolated.

A similar setup is expected on Tuesday with very steep lapse
rates remaining in place as well as a very juicy warm sector with
dewpoints in the mid to upper 60s expected east of the dryline
once again. The difference will be a strong shortwave trough and
associated PV maximum sweeping through the Southern Plains
Tuesday afternoon. Given the increased dynamic forcing, the
initial thought is that storms would be even more likely on
Tuesday afternoon. However, most guidance, especially short-term
high-res guidance, is showing a dry forecast due to a strong
capping inversion in place on Tuesday afternoon. Given the
convective parameters and the stronger forcing, have essentially
gone completely against most guidance on Tuesday afternoon and
included thunderstorms with chance PoPs for most areas east of the
dryline which should once again be positioned across the western
portion of the forecast area. The thought is that the stronger
forcing and increased low-level warming should be capable of
breaking the cap in localized areas allowing at least a couple of
isolated strong storms to develop. However, there is uncertainty
as to if storms will be able to form ahead of the dryline Tuesday
afternoon, but if they do, they will likely be capable of becoming
strong to severe. The primary storm mode would be supercells once
again with the primary threat being large hail for the same
reasons discussed above from Monday`s setup.

Later Tuesday night and Wednesday morning, the aforementioned
shortwave trough will pull a strong cold front through the area. A
thin line of storms is possible along what should be a fast-moving
cold front as it overtakes the dryline and catches up to the
better moisture. The front will surge south pretty quickly,
entering our northwest counties a little after midnight and
clearing our southeast counties around daybreak Wednesday.
Temperatures will be quite a bit cooler Wednesday through the end
of the week as cooler and drier air works into the region. While
Tuesday`s highs should be in the upper 70s and 80s, Wednesday`s
highs are only expected to reach the 60s. Some near- or sub-
freezing low temperatures are possible across rural parts of
North TX on Thursday morning.

Near-normal temperatures and dry weather are expected through the
end of the week before an upper low digs southward across
northern Mexico by late Friday. This trough should bring
widespread rain to southern TX, but the best moisture supply and
forcing is expected to remain mostly to the south of our forecast
area. Some isolated showers and a couple storms will be possible
over the weekend with chances generally increasing the farther
south your location. Extended guidance is actually in fairly good
agreement on a stronger frontal passage early next week which
could result in additional chances for thunderstorms and much
cooler weather for the second week of March.

-Stalley

&&

.AVIATION...
/ISSUED 1141 PM CST Sun Feb 26 2017/
Low level moisture will continue to surge northward overnight and
result in lowering ceilings and falling visibilities. MVFR
ceilings at 06Z will fall to IFR overnight with some patchy
drizzle and fog. Visibilities overnight through sunrise Monday
should range between 2 and 5 miles. IFR conditions will prevail
through mid morning Monday and slowly improve through early
afternoon. VFR conditions are expected late Monday afternoon
through Monday evening.

A south to southeast wind will prevail between 7 and 14 knots.


&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    78  64  82  53  63 /  40  20  30  40   5
Waco                76  65  81  53  65 /  40  20  30  30   5
Paris               70  62  76  52  63 /  40  20  40  50  10
Denton              76  62  80  50  62 /  30  20  30  40   5
McKinney            73  64  76  53  62 /  40  20  30  50   5
Dallas              78  65  81  54  63 /  40  20  30  40   5
Terrell             76  65  79  54  64 /  40  20  40  40   5
Corsicana           78  65  81  56  65 /  40  20  40  40  10
Temple              76  64  82  54  64 /  30  20  30  30   5
Mineral Wells       80  58  83  47  64 /  20  10  20  20   5

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

91/26



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