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

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FXUS64 KFWD 171809

Area Forecast Discussion
National Weather Service Fort Worth TX
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.



.SHORT TERM... /Issued 357 AM CDT Sat Mar 17 2018/
/Today and Tonight/

Morning satellite imagery shows an extensive area of low clouds
across southeast Texas spreading northward into the region
signaling a substantial increase in low level moisture. The latest
surface analysis across the region shows quite the range in
dewpoints from the low 20s across our western and northern
counties to the mid 60s across our southeast counties. In the wake
of a strong shortwave trough that moved through the Plains
yesterday, a weak cold front is sliding southward and is currently
positioned just south of a Bowie to Sherman line. This front will
continue to slowly make southward progress and may become a
significant player in convective chances later this afternoon.

Later today, a strong trough over the western U.S. will continue
to dig southward through the Great Basin. Southwesterly flow
across northern Mexico into Texas will begin to strengthen as a
result. A mid level disturbance embedded in this strengthening
flow aloft is expected to move across North Texas this afternoon.
As it does, a weak surface low currently over west Texas will
spread eastward and should be located in the vicinity of Abilene.
A dryline will be draped south of the surface low with the
aforementioned cold front still draped across North Texas in the
vicinity of the I-20 corridor. Rapid boundary layer moistening
will result in much of North Texas being located within a well
defined warm moist sector this afternoon with dewpoints in the
low to mid 60s.

Most of the area will remain capped initially, but as stronger
isentropic ascent spreads northeast through the afternoon, we
should see this capping lift or erode completely by 2-4 pm. With
low level convergence maximized near the triple point of the weak
surface low, dryline, and cold front, thunderstorms should rapidly
develop to the southwest of the Metroplex around 3-4 pm.
Thermodynamic parameters will be favorable for severe weather with
very steep lapse rates in the 800-600 mb layer and surface
dewpoints in the mid 60s yielding surface based CAPE around
2000 J/kg late this afternoon. Wind profiles are generally weak in
the lowest 2 km although they strengthen considerably above that
with 50-60 kt of total deep layer shear. The long straight
hodographs suggest that splitting supercells will be possible with
mainly a large hail threat as convection spreads east into the
evening hours. We`ll show PoPs increasing from around 40-50% to
the southwest of the Metroplex by mid afternoon to 70-80% by the
evening hours and continuing into tonight.

Concerning a tornado potential...All of the high resolution
guidance is indicating a narrow corridor of strong low level
instability that develops from near the I-20 corridor in the
Metroplex southward to between Waco and Hillsboro late this
afternoon after convection has already developed. Low level
instability can sometimes compensate for a lack of stronger low
level wind shear, especially when shear aloft is strong. There
appears to be a small window late this afternoon and evening, from
around 5-8 pm where low level instability becomes maximized, in
conjunction with an increase in 925-850 mb winds, as the low
level jet begins to strengthen. Surface winds may also be locally
backed near the frontal boundary resulting in substantially more
curved hodographs and an attendant increase in tornado threat.
This threat should diminish after dark with boundary layer cooling
and an increase in surface based inhibition. We will continue to
monitor this potential through the afternoon.

Ongoing showers and thunderstorms will continue to push eastward
tonight with rain chances diminishing from west to east after
midnight. The retreating surface frontal boundary should be near
the Red River prior to daybreak, and we`ll hang on to some PoPs
mainly across the northeast into the early morning hours.



.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    82  59  82  55  75 /  30  70  40  20   0
Waco                83  63  83  54  80 /  80  60  40  20   0
Paris               77  58  73  53  73 /  10  70  70  30   5
Denton              78  56  81  53  75 /  20  40  40  20   0
McKinney            77  58  78  54  73 /  20  60  50  30   0
Dallas              80  61  81  57  77 /  30  70  40  20   0
Terrell             80  60  79  54  77 /  20  80  60  30   0
Corsicana           83  63  81  54  79 /  40  70  60  30   0
Temple              84  64  85  54  80 /  60  30  30  20   0
Mineral Wells       79  56  85  52  76 /  40  30  30  10   0




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