Severe Thunderstorm Warning 27, NWS Fort Worth, TX

Area Forecast Discussion...CORRECTED
National Weather Service Fort Worth TX
333 PM CDT Sun Mar 18 2018

..SHORT TERM...
/Through Tonight/

Main concern through the short term is the potential for isolated
strong/severe thunderstorms this afternoon and evening.

Much uncertainty still surrounds the next ~8 hours of the
forecast, largely due to the stubborn bank of low clouds that have
been slow to scatter through midday. Ingredients are in place for
some thunderstorms to become severe this afternoon, should they be
able to develop. The uncertainties lie largely on the coverage of
storms and the most favorable location for initiation. A pair of
shortwave troughs are currently moving through the Southern
Plains. A deep surface low resides in southeast Colorado while a
more ill-defined low exists across the San Angelo/Abilene area. A
surface dryline is positioned southward through the surface trough
axis, and a stalled frontal zone is draped from east to west
through North Texas. The warm sector bounded by these features has
begun to destabilize with gradual clearing starting to take place
across our western zones where temperatures have climbed into the
low 80s. When combined with dewpoints in the 60s and very steep
lapse rates aloft (as illustrated by the FWD 18z sounding), these
conditions are yielding a narrow tongue of large SBCAPE values
immediately ahead of a westward-bulging dryline.

As lift increases with height falls from the nearby shortwaves
along with low-level convergence near the boundaries, we should
start seeing attempts at convection. Areas that have cleared
immediately along the dryline may destabilize sufficiently to
break the cap or eliminate it entirely, whereas areas to the east
have remained capped due to the presence of widespread low clouds
and substantially cooler surface temperatures. As a result, am
thinking the greatest potential for convective initiation will
probably be along the dryline and near the triple point
intersection with the stalled front. With strong instability and
50-60 kts of deep layer shear, a supercellular storm mode is
likely. If supercells can develop in this area and move eastward
into the warm sector, they will be capable of surviving as
elevated supercells within a capped environment with primarily a
large hail threat. If some clearing/heating occurs through late
afternoon farther east in the warm sector, a wind/tornado threat
would also accompany storms if surface based instability exists.
The weakly capped environment may actually act to enhance the
severe potential if stronger isolated storms can get going. A weak
cap would suppress weaker convective attempts, allowing stronger
isolated supercells to progress eastward unimpeded.

The above scenario is supported by the most recent runs of the
HRRR and a couple other CAMs. However, on the whole, high-
resolution models remain wildly inconsistent with much
disagreement regarding convective initiation locations and
coverage. Some models are developing practically no convection at
all, which I also have to admit is a realistic scenario if enough
lift/instability is not realized. The most likely scenario is
probably a couple of discrete storms developing across
west/northwest portions of the forecast area which will move
eastward with a potential for large hail. Any convection will move
east fairly quickly and exit the area by midnight or so at the
latest.

Overnight, the dryline/Pacific front boundary will surge eastward
bringing much drier low-level air through the entire region. This
will scour the low level moisture leading to a cooler Monday
morning with some breezy west winds.

-Stalley

&&

.LONG TERM...

/Monday through Sunday/

A strong surface low will move east from northeastern Oklahoma
into Tennessee during the day Monday. The resulting pressure
gradient will lead to gusty westerly winds which will become
northwesterly during the afternoon as another cold front moves
through the region. Skies will be mostly sunny and highs will
range from the upper 60s northwest to around 80 degrees southeast.
The combination of 20 to 25 mph winds with gusts over 35 mph at
times, and minimum relative humidities in the upper teens and 20s
will mean critical fire weather conditions across the western
parts of the forecast area with elevated fire weather conditions
elsewhere.

Wind speeds will be slow to decrease Monday night with northwest
winds 15 to 25 mph prevailing with some gusts over 30 mph. Lows
will be mostly in the 40s.

An upper level ridge will build across the Rockies early to mid
week and then move into the Plains late in the work week as an
upper level low moves east to off the East Coast by late week. This
will result in dry weather across North and Central Texas for the
upcoming work week. It will be cooler Tuesday with highs in the
60s. As a surface high moves east, moderating temperatures will
occur Wednesday through Friday. Highs will be in the upper 60s to
mid 70s Wednesday, 70s to lower 80s Thursday, and mid 70s
northeast to upper 80s west Friday. Southerly winds will increase
Thursday and Friday and despite increasing humidities, elevated
fire weather conditions may occur across the western counties.

With the continued southerly flow and a weakening upper level
ridge next weekend, there will be low chances of showers and
thunderstorms along and east of I-35 Saturday and most of the
region on Sunday. Warmer than normal weather will continue with
highs mostly in the 80s and lows in the 60s.

58

&&

.AVIATION... /Issued 1247 PM CDT Sun Mar 18 2018/
/18z TAFs/

Low clouds have been slow to clear this morning, but are finally
showing some signs of improvement as of 18z. The deck of IFR
clouds is beginning to lift/erode from the west as a shortwave
trough approaches while some warmer/drier air arrives from the
southwest. Will continue IFR conditions through 19-20z before
improving to VFR at all TAF sites through the late afternoon.

The next concern is the potential for convection to impact
airports later this afternoon or early this evening. Coverage of
thunderstorms remains in question, as does the most favorable
location for initial development. Stubborn cloud cover has
prevented strong destabilization from occurring thus far, and this
may inhibit the initiation of more widespread convection
throughout the afternoon. Given the presence of the strong
shortwave to our west, still think isolated thunderstorms are in
the cards especially if lift from the shortwave can be aided by
other boundaries present, such as the stationary front draped
through North Texas or the dryline positioned to the west. Will
continue a brief VCTS mention at DFW Metroplex airports centered
around 00z, but will keep rain-free TAFs at Waco for now.
Southerly winds will prevail through tonight.

Overnight, predominantly VFR conditions are expected as a Pacific
front/dryline moves through the area, although a brief period of
MVFR is possible around midnight just prior to the front's
arrival. Have not included MVFR in the TAFs for now as the
potential looks low. Drier air behind the front will scour all
low-level moisture from the area and VFR conditions will prevail
into Monday.

A crosswind event is likely to occur through the day on Monday as
breezy west winds of 20G30KT are expected. Later in the day, winds
will turn more to the northwest as a cold front arrives, but will
remain breezy through Monday evening.

-Stalley

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    56  78  46  65  44 /  20   0   0   0   0
Waco                52  79  45  67  42 /  10   0   0   0   0
Paris               53  73  45  62  41 /  40   0  10   5   0
Denton              51  74  43  65  42 /  20   0   0   0   0
McKinney            51  74  44  63  42 /  20   0   0   0   0
Dallas              56  76  46  66  45 /  20   0   0   0   0
Terrell             53  75  45  65  41 /  30   0   0   0   0
Corsicana           55  77  46  65  43 /  20   0   0   0   0
Temple              51  78  45  69  43 /  10   0   0   0   0
Mineral Wells       50  75  41  65  40 /   5   0   0   0   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Monday morning through Monday evening
for TXZ100-115-116-129-130-141>143-156-157.

&&

26/58

Hazardous Weather Outlook
National Weather Service Fort Worth TX
254 PM CDT Sun Mar 18 2018

TXZ091>095-100>107-115>123-129>135-141>148-156>162-174-175-191315-
Montague-Cooke-Grayson-Fannin-Lamar-Young-Jack-Wise-Denton-Collin-
Hunt-Delta-Hopkins-Stephens-Palo Pinto-Parker-Tarrant-Dallas-
Rockwall-Kaufman-Van Zandt-Rains-Eastland-Erath-Hood-Somervell-
Johnson-Ellis-Henderson-Comanche-Mills-Hamilton-Bosque-Hill-Navarro-
Freestone-Anderson-Lampasas-Coryell-Bell-McLennan-Falls-Limestone-
Leon-Milam-Robertson-
254 PM CDT Sun Mar 18 2018

This Hazardous Weather Outlook is for North and Central Texas.

.DAY ONE...Tonight.
Isolated thunderstorms are expected to develop this afternoon and
evening, mainly across parts of North Texas. Any storms that develop
will be capable of producing large hail as well as damaging wind
gusts. A tornado cannot be ruled out near or north of I-20 towards
the Red River where surface winds remain more easterly.

Elevated fire weather conditions are expected through sunset west of
a Graham to Mineral Wells to Lampasas line.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.
Critical fire weather conditions will occur west of I-35 on Monday
where a Fire Weather Watch is in effect. Elevated fire weather
conditions will occur elsewhere.

Isolated thunderstorms are possible along and east of I-35 on
Saturday. Cloud to ground lightning would be the main threat with any
storms.

.SPOTTER INFORMATION STATEMENT...
Spotter activation may be requested late this afternoon and evening.

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