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

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FXUS64 KFWD 200507
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
1207 AM CDT Wed Sep 20 2017


.AVIATION...
06 UTC TAF Cycle

Concerns---Convective cessation at Metroplex TAF sites, wind
shifts and MVFR stratus potential. At Waco...the main concern will
be timing the onset and cessation of IFR and MVFR cigs.

For the Metroplex TAF sites---Convection that developed across
western North TX was able to survive its trek and move
southeastward compliments of sufficient instability and wind
shear. This convection was efficient at producing a large outflow
boundary which has resulted in northerly winds across just about
all of the Metroplex TAF sites. With convection finally decaying,
there shouldn`t be any additional forcing for sustaining the cold
pool and as a result, northerly winds will eventually turn towards
the east and southeast after midnight with a return to south flow
likely for the Wednesday morning push. Coincident with the morning
push, however, will be the appearance of low level moisture and
resultant MVFR stratus. The spatial coverage of MVFR stratus
remains challenging and guidance varies considerably. For now,
will go with simple RH progs which overspreads a large swath of
higher moisture northward. MVFR stratus will likely be
intermittent around FL015-FL018 and so I`ve included temporary
conditions to account for this. Winds should remain elevated and
precludes mention of IFR stratus. VFR will return by mid to late
morning and should continue through the remainder of the TAF
cycle. There may be a low potential for some convection west of
the TAF sites on Wednesday afternoon, but confidence remains too
low for an introduction into the TAFs at this time.


For the Waco TAF site---MVFR stratus will invade before sunrise
on Wednesday with a brief periods of IFR cigs. Low level wind
speeds and subsequent mixing should help cigs to lift through the
morning hours above FL009. MVFR stratus will continue through
around noon with VFR expected thereafter. Convective chances will
remain highest west of Waco, so I`ll maintain a rain-free TAF at
this time.

24-Bain

&&


.UPDATE... /Issued 745 PM CDT Tue Sep 19 2017/
Main update was to increase PoPs up across northwestern zones.
While decreasing daytime heating is expected to limit any other
areas of convection, increasing low level flow at around 925mb may
result in further sustenance of any ongoing complexes. For now,
will leave my minor adjustments in tact and revisit this in a
couple of hours. The main hazards with storms will be strong to
damaging straight line winds and perhaps near quarter hail.

Otherwise, I made some alterations to sky grids to be consistent
with PoP/Wx grids. Updated products have been transmitted.

24-Bain

&&

.SHORT TERM... /Issued 355 PM CDT Tue Sep 19 2017/
/Tonight/

Differential heating to the east of decaying, older elevated
convection south of I-20 and west of U.S. 281 has assisted in some
development of scattered showers and thunderstorms this afternoon.
The likely culprits are max heating with temps in the 90s coinciding
with enhanced ageostrophic/(vertical) motions from the differential
heating for this latest development. High resolution short range
models generally show weak steering currents aloft, but plenty of
mean layer and surface-based instability to result in convection,
eventually being propagation-driven by cold pools and veering
southeast with time this evening. With these new trends, we will
introduce a late afternoon period to go along with our tonight
period and have bulk of low storm chances across our southwest
counties through late evening. With CAPE values ranging between
2000-3000 J/KG below cloud base, some strong downburst winds
between 40-50 mph and frequent lightning will be associated with
any cells that can get any growth and depth on updrafts, before
collapsing. Otherwise, the weak shortwave aloft will continue
progressing east with most activity decaying during the overnight
hours.

Otherwise, modest southerly flow within the boundary layer
overnight will help assist Gulf moisture northward for the re-
development of late night stratus across particularly Central TX
and areas west of I-35. The breezy and humid conditions will
result in lows in the 70s areawide, which will be a good 5-10
degrees above climatological normals for late September.

05/

.LONG TERM...
/Wednesday through Tuesday/

Unseasonably warm, dry weather will persist across North and
Central Texas through the weekend, as upper level ridging lingers
over the region. Hung onto small pops across the far southeastern
zones Wednesday and Thursday where moisture and instability may be
sufficient for a few widely scattered showers and thunderstorms.

A moderate pattern shift will emerge by Monday, as a deep upper
trough evolves across the Intermountain West and places Texas
under a broad southwesterly flow. Large-scale synoptic lift will
be sufficient to trigger scattered showers and thunderstorms
across West Texas by Monday, with scattered PoPs introduced to
much of the area by Tuesday. Believe the majority of the heavier,
more widespread rainfall will remain west of North Texas through
Tuesday, but the ECMWF and GFS both suggest that portions of the
upper trough will break out of the mountains and across the
Southern Plains by Wednesday/Thursday, providing an increased
opportunity for rainfall through the midweek period.

Bradshaw

&&

.LONG TERM... /Issued 745 PM CDT Tue Sep 19 2017/
/Wednesday through Tuesday/

Unseasonably warm, dry weather will persist across North and
Central Texas through the weekend, as upper level ridging lingers
over the region. Hung onto small pops across the far southeastern
zones Wednesday and Thursday where moisture and instability may be
sufficient for a few widely scattered showers and thunderstorms.

A moderate pattern shift will emerge by Monday, as a deep upper
trough evolves across the Intermountain West and places Texas
under a broad southwesterly flow. Large-scale synoptic lift will
be sufficient to trigger scattered showers and thunderstorms
across West Texas by Monday, with scattered PoPs introduced to
much of the area by Tuesday. Believe the majority of the heavier,
more widespread rainfall will remain west of North Texas through
Tuesday, but the ECMWF and GFS both suggest that portions of the
upper trough will break out of the mountains and across the
Southern Plains by Wednesday/Thursday, providing an increased
opportunity for rainfall through the midweek period.

Bradshaw

&&

.LONG TERM... /Issued 355 PM CDT Tue Sep 19 2017/
/Wednesday through Tuesday/

Unseasonably warm, dry weather will persist across North and
Central Texas through the weekend, as upper level ridging lingers
over the region. Hung onto small pops across the far southeastern
zones Wednesday and Thursday where moisture and instability may be
sufficient for a few widely scattered showers and thunderstorms.

A moderate pattern shift will emerge by Monday, as a deep upper
trough evolves across the Intermountain West and places Texas
under a broad southwesterly flow. Large-scale synoptic lift will
be sufficient to trigger scattered showers and thunderstorms
across West Texas by Monday, with scattered PoPs introduced to
much of the area by Tuesday. Believe the majority of the heavier,
more widespread rainfall will remain west of North Texas through
Tuesday, but the ECMWF and GFS both suggest that portions of the
upper trough will break out of the mountains and across the
Southern Plains by Wednesday/Thursday, providing an increased
opportunity for rainfall through the midweek period.

Bradshaw

&&

.SHORT TERM... /Issued 312 AM CDT Tue Sep 19 2017/
Our streak of above normal temperatures will continue today with
highs once again expected to climb into the 90s for most of the
forecast area. Although these temperatures are several degrees
above normal, they won`t come close to reaching record values
which are over 100 degrees at both DFW and Waco. With slightly
higher dewpoints than the past couple of days, heat index values
will climb near or slightly above 100F for many locations this
afternoon. However, an increasing south wind with occasional gusts
of 20-25 mph should keep the heat from feeling too oppressive.

With the aforementioned increase in moisture comes some low rain
chances once again this afternoon and evening. Locally higher PWs
of 1.5-1.7" will be in place across our southeast zones this
afternoon, and we`ll probably see a few convective attempts in
this area during peak heating that may amount to a few showers or
an isolated storm. Have maintained some 20% PoPs and expect
minimal coverage. Later this evening, we`ll need to monitor the
potential for a batch of decaying thunderstorms to enter our
forecast area from the west. This activity should initially
develop in West TX along a vague dryline feature as a subtle upper
disturbance passes overhead in increasingly zonal flow aloft. Any
storms that develop should grow into a convective cluster in
vicinity of San Angelo to Abilene. From there, the chance of any
of this convection making it into our western zones will be
dependent on cell redevelopment along its outflow, since steering
flow alone probably won`t be strong enough to nudge this activity
into our forecast area. Either way, the strength of any convection
should be on the downtrend the farther east it moves, although it
would still be capable of producing some strong winds or small
hail through the early evening west of HWY 281. With fairly low
confidence in this convection making it into our western counties,
have opted for only 20-30% PoPs at this time.

-Stalley

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    77  96  77  93  75 /  40  10  10  10  10
Waco                75  96  76  94  73 /  10  10  10  10  10
Paris               73  94  73  92  71 /   5  10  10  10  10
Denton              76  95  75  94  73 /  50  10  10  10  10
McKinney            76  95  75  93  73 /  30  10  10  10  10
Dallas              77  95  78  93  76 /  30  10  10  10  10
Terrell             75  94  75  93  72 /  10  10  10  10  10
Corsicana           75  95  75  95  73 /   5  10  10  10  10
Temple              73  94  74  91  72 /  10  10  10  20  10
Mineral Wells       74  98  74  93  72 /  40  10  20  10  10

&&

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

$$

24/79


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