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

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FXUS64 KFWD 171922

Area Forecast Discussion
National Weather Service Fort Worth TX
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.



.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.



.LONG TERM... /Issued 357 AM CDT Sat Mar 17 2018/
/Sunday through next Friday/

Convection may be ongoing Sunday morning, most likely north of
I-20 and up to the Red River and along and east of the I-35
corridor within a region of very strong isentropic lift. Rather
than a continuation of Saturday afternoon and evening`s activity,
this may be another round of elevated convection altogether forced
by the aformentioned warm/moist advection atop a northward-moving
warm front. Forecast soundings in this part of our CWA seem to be
contaminated to a degree by Saturday`s thunderstorms, but steep
lapse rates in the 600-850 mb layer and about 30-40 kts of
effective deep layer shear may support some elevated supercells
with a hail potential into the mid-morning hours. Given the lack
of more robust lapse rates, it looks like nickel to perhaps
quarter-sized hail (in the strongest storms) would be the main
threat with these storms. There may also be a brief window for a
bit of backbuilding/training of storms immediately north of the
warm front as upwind Corfidi Vectors orient parallel to the
surface boundary, but all hi-res guidance indicates this activity
will eventually dislodge and move out of the area ahead of the
next wave of ascent. Some localized flooding will be possible here
into the mid-morning hours.

There may be a bit of a lull in convective activity late Sunday
morning, although some showers may persist as warm advection
continues. The combination of a higher coverage of storms
presently anticipated for today (Saturday), combined with a
potential for pervasive low cloud cover and showers through the
day on Sunday have actually **diminished** forecast confidence
regarding convective chances late Sunday afternoon and evening.
Forecast soundings look pretty worked-over from Saturday`s
activity along with fairly limited moisture quality/depth in the
lowest 1 km, in addition to plentiful dry air in the 700-850 mb
layer. All of this would work to diminish the overall severe
threat during the afternoon.

Model guidance does advect a renewed EML plume overhead during
the mid-afternoon, with 700-500 mb lapse rates steepening to
around 7-7.5 C/km as a surface dryline begins to mix eastward
towards the I-35 corridor. That said, whether storms can even
initiate (and subsequently sustain themselves) in an environment
characterized by /1/ Increasing capping as the aformentioned
renewed EML plume arrives, /2/ dwindling moisture in the 700-850
mb layer, and /3/ a delayed onset of the more robust height falls
until very late Sunday evening cast a great deal of uncertainty
onto the Sunday afternoon-evening forecast.

Since a handful of the extended hi-res guidance still convect
during the afternoon (most notably the HRRRx), we`ll keep likely
PoPs painted east of I-35/35E, tapering down to chance (30-50%)
along and west of I-35 during this timeframe. Depending on the
thermodynamic situation during this period, the kinematic
parameter space would obviously be supportive of supercells with
very strong mid- and upper-level flow present overhead. Thus, a
highly conditional risk for strong to severe storms still exists,
primarily along and east of I-35 with a large hail and damaging
wind threat. The tornado potential, while low, is nonzero,
especially the farther north of I-20 and east of I-35 you go where
surface winds will be a bit more backed.

The Pacific front will quickly overtake the dryline late Sunday
evening and overnight as an occluding surface low barrels right
along the Oklahoma/Kansas border. While none of the coarse-
resolution model guidance is convecting along the Pacific front,
I`m wondering if a narrow line of additional showers and
thunderstorms may develop during this period as the best forcing
for ascent of the day overspreads the area, coincident with
steepening 600-850 mb lapse rates. I`ve introduced some low
(20-30%) PoPs across parts of the forecast area to account for
this potential. Northwesterly winds and building high pressure
will bring an end to any precipitation chances by early Monday

...Fire Weather Potential Sunday afternoon and Monday...
As the aformentioned dryline mixes eastward Sunday afternoon,
locations to the west of a roughly Bowie to Mineral Wells to
Lampasas line will see their relative humdities drop below 30
percent, and even into the teens across parts of Eastland,
Stephens, and Young counties. As this occurs, west to southwest
winds of around 15 to 20 mph are expected to develop. With little
in the way of rainfall here, an elevated or critical fire weather
threat could materialize. We`ve elected to hold off on a Fire
Weather Watch for the time being, but one may be warranted today
or tonight if model trends continue.

Very dry air along with strong and gusty northwest winds will
envelop all of the region on Monday in the post-frontal airmass.
Even with some cold advection, temperatures should still manage to
warm into the 70s to near 80 degrees across Central Texas. The one
fly in the ointment here may be the potential for some post-
frontal stratus mainly near the Red River, which could impact
temperatures and humidity values. However, with afternoon
relative humidity values expected to fall well below 30 percent
(teens across our western counties), another Critical Fire Weather
potential exists. The eastward extent of this threat will depend
on where the heaviest rainfall falls this weekend, but another
Fire Weather Watch may be necessary, especially west of I-35 where
the lowest rainfall amounts are anticipated.

Dry and tranquil weather is expected Tuesday and Wednesday. Gusty
south winds will return on Thursday ahead of the next approaching
low pressure system. Some showers may be possible under a
strengthening capping inversion Thursday and Friday, but this
potential is too low to warrant mentionable PoPs at this time.
Shower and storm chances could return next weekend, however, as
another dryline/cold front approach the area.



Dallas-Ft. Worth    59  82  55  75  46 /  70  40  20   0   0
Waco                63  83  54  80  46 /  60  40  20   0   0
Paris               58  73  53  73  44 /  70  70  30   5   5
Denton              56  81  53  75  43 /  40  40  20   0   0
McKinney            58  78  54  73  44 /  60  50  30   0   0
Dallas              61  81  57  77  47 /  70  40  20   0   0
Terrell             60  79  54  77  45 /  80  60  30   0   0
Corsicana           63  81  54  79  46 /  70  60  30   0   0
Temple              64  85  54  80  47 /  30  30  20   0   0
Mineral Wells       56  85  52  76  41 /  30  30  10   0   0




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