Severe Thunderstorm Warning 18, NWS Fort Worth, TX

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
346 PM CDT Sat Mar 17 2018

.SHORT TERM... /Issued 222 PM CDT Sat Mar 17 2018/

Strong to severe thunderstorms are likely this afternoon and
evening. A Severe Thunderstorm Watch has been issued for much of
North and Central Texas through this evening.

A high-quality warm sector airmass is in place across much of
North and Central Texas. A surface low has deepened across West
TX and is located just west of San Angelo as of 2pm. A dryline
extends south from this low and a quasi-stationary front lies to
the ENE roughly along the I-20 corridor. Over the past hour, this
boundary has started a more progressive northward shift as a warm
front and is now draped on the northern fringes of the DFW
Metroplex. Within the warm sector, partial clearing has allowed
temperatures to climb into the upper 70s with dewpoints climbing
in to the mid 60s. These conditions have led to strong surface-
based instability of around 2000 J/kg within the warm sector with
a plume of very steep lapse rates within the 800-600mb layer.

While a few convective attempts have been made within the
forecast area near Stephenville and Comanche, the more robust
convective development will continue to be farther west near the
dryline/front triple point. Lift will be aided by a mid-level
shortwave disturbance that is most identifiable in the 600-700mb
height fields. Lift from this disturbance as well as convergence
along the aforementioned boundaries will continue to result in
convective initiation over the next several hours. We may see 2 or
possibly even 3 waves of convection initiate to the west and move
east throughout the evening as lift continue to act upon the warm
sector airmass.

Storms will have the ability to become strong or severe within
the warm sector with primarily a supercellular storm mode favored
due to large instability and 50-60 kts of deep-layer bulk shear.
Relatively straight hodographs will favor splitting supercells
with both right and left movers capable of surviving. While all
modes of severe weather are possible, hail will probably be the
most common threat through this event. This will be due to the
steep lapse rates located within the hail growth zone, updraft
rotation with primarily a supercellular storm mode, and wet-bulb
zero heights around 9-10 kft. Of course, damaging wind gusts will
also be of concern with more robust convection. The tornado
threat remains somewhat questionable due to the straight
hodographs, weaker low-level flow through about 1 km, and limited
helicity in the 0-3 km layer. In addition, winds within the warm
sector are somewhat veered. If flow was to strengthen at all later
this afternoon or back more to the east, there would be a window
of increased tornado potential. This may end up being most likely
if a supercell storm can become rooted along/near the frontal
boundary where surface winds may be locally backed.

Convection should be on a downward trend later this evening, but
some surface-based instability may last through and even a bit
after sunset. The potential for severe storms, especially severe
hail will linger after sunset and into the overnight hours. The
concern should shift east of I-35 by 9-10pm when convection will
likely have grown upscale and conglomerated into a multicellular
cluster or broken line with a continued hail/wind threat. Am
expecting most convection to have exited the area to the east
around 1-2am with a general lull expected through daybreak Sunday.

-Stalley

&&

.LONG TERM...

/Sunday through Saturday/

A quasi-stationary frontal boundary is expected to extend
along/just north of I-20 at 7 AM Sunday. Some scattered warm
advection showers are possible during the morning. A dryline
is expected to approach the I-35 corridor by early afternoon.
Confidence in how much thunderstorm development we will get during
the afternoon along and east of the I-35 corridor is still not
very high given the expected cloud cover and possible morning
rain. IF storms develop, the shear and instability will be
supportive of some strong to severe thunderstorms with large hail
and damaging winds being the main threats. There would also be a
non-zero potential for a tornado especially north of I-20 where
surface winds should back.

Expect elevated fire weather conditions for areas west of I-35
Sunday afternoon behind the Pacific cold front due to southwest
winds 10 to 20 mph and minimum relative humdities in the teens and
20s. Highs will range from the lower 70s northeast to the mid 80s
southwest.

Rain chances will end by sunset Sunday and a Pacific cold front
will overtake the dry line during the evening and move through the
region overnight. Winds will become westerly 10 to 20 mph area
wide by daybreak Monday. Lows will be the 50s area wide.

As a surface low moves east from northeastern Oklahoma toward the
Ohio River Valley Monday, winds across North and Central Texas
will shift from westerly to northwesterly at 15 to 25 mph with
some gusts over 30 mph likely. The gusty winds combined with
relative humidities in the upper teens southwest to lower 30s
northeast will result in elevated to near-critical fire weather
conditions Monday. Highs will be in the 70s to lower 80s.

Dry weather is expected for the most of the rest of the week as
upper level ridging builds over the Rockies early to midweek and
then moves into the Plains late week. We will have northerly winds
on Tuesday with highs mostly in the 60s. Temperatures will then
warm for the rest of the week with southerly winds returning.
Highs will be in the upper 60s northeast to upper 70s west
Wednesday, 70s to lower 80s Thursday and mid 70s northeast to the
mid 80s west Friday and Saturday.

58

&&

.AVIATION... /Issued 109 PM CDT Sat Mar 17 2018/
/18z TAFs/

Primary concerns through the forecast period are impacts from
convection in the next 6-12 hours. A stationary front currently
resides near I-20 with MVFR cigs positioned to the south. A
surface low and dryline are located to the west and isolated
showers and storms have started developing as of 18z. Additional
storms are expected to develop along and ahead of this boundary
and in vicinity of the surface front. Storms could encroach on
both the DFW Metroplex airports and Waco as early as 4-5pm and
impacts from nearby thunderstorms are expected to last into the
evening. Will continue Tempo groups for TSRA during the most
likely time which appears centered around 00z this evening. Winds
will remain out of the ESE at DFW area airports and out of the
south at Waco throughout this time.

After convection moves east later this evening, the return of some
MVFR cigs is anticipated during the overnight hours. While some
guidance is quite pessimistic with suggesting IFR conditions
through Sunday morning, will continue cigs above 1 kft for now.
Have maintained rain-free TAFs throughout Sunday for now as the
expectation is that most activity will be east of the TAF sites.

-Stalley

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    63  82  55  75  46 /  70  40  20   0   0
Waco                63  83  54  80  46 /  80  40  20   0   0
Paris               57  73  53  73  44 /  40  70  30   5   5
Denton              58  81  53  75  43 /  50  40  20   0   0
McKinney            60  78  54  73  44 /  60  50  30   0   0
Dallas              64  81  57  77  47 /  70  40  20   0   0
Terrell             61  79  54  77  45 /  80  60  30   0   0
Corsicana           64  81  54  79  46 /  90  60  30   0   0
Temple              65  85  54  80  47 /  50  30  20   0   0
Mineral Wells       57  85  52  76  41 /  50  30  10   0   0

&&

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

26/58

Hazardous Weather Outlook
National Weather Service Fort Worth TX
302 PM CDT Sat Mar 17 2018

TXZ091>095-100>107-115>123-129>135-141>148-156>162-174-175-181230-
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-
302 PM CDT Sat Mar 17 2018

This Hazardous Weather Outlook is for North and Central Texas.

.DAY ONE...Tonight.
Severe thunderstorms are expected to affect much of North and
Central Texas through this evening and tonight. The most likely areas
for severe weather and supercell thunderstorms are near and south of
Interstate 20. Large hail will be the primary threat, although
damaging winds and tornadoes are also possible.

Later tonight, storms are expected to grow into a larger cluster or
broken line as they move east of Interstate 35. Damaging winds and
large hail will continue to be possible.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.
Thunderstorm chances will continue for North and Central Texas
Sunday and Sunday night. A few storms could be severe during this
time with mainly a large hail and damaging wind threat. The severe
threat will be highest east of I-35 during this time.

Critical fire weather conditions may occur west of I-35 on Sunday
afternoon and across all of North and Central Texas on Monday
afternoon.

.SPOTTER INFORMATION STATEMENT...
Spotter activation will likely be requested throughout this evening.

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