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

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FXUS64 KFWD 291819 AAB
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
119 PM CDT Sat Apr 29 2017

.AVIATION...
/18Z TAFs/

A special midday sounding shows that the capping inversion has
weakened and lifted, but the relatively flat CU field downstream
of an approaching cold front confirms that this inhibition is
still winning the convective battle. Local numerical guidance
corroborates the weakening trend of this lid, particularly north
of the Metroplex, but additional forcing and/or surface heating
will be needed to break through it. Convective initiation is still
expected to occur this afternoon along the advancing surface
front, which may be in the vicinity of the I-35 corridor when
this occurs. Surface temperatures in the mid 80s and dew points
near 70F, combined with steepening lapse rates aloft, are yielding
MLCAPE values in excess of 3000 J/kg along and east of I-35. This
means that once initiation occurs, it will be rapid with intense
updrafts that will quickly become severe.

The window for thunderstorms at the Metroplex TAF sites will be
small, with the frontal boundary now on the doorstep. A switch to
north flow may be needed well before the storm potential is east
of the Metroplex. Northwest winds in the TEMPO group are designed
to cover both convective winds and FROPA. The scenario for Waco
will be similar, albeit delayed an hour or two on both ends, and
the storm chances will be a bit lower for Central Texas overall.

Although the frontal activity will be confined to East Texas this
evening, with forcing aloft still upstream, some elevated
convection may occur near our TAF sites. However, the chances are
too low to be specifically addressed in the TAFs at this time.
Better chances will accompany a final spoke of energy that will
swing around the base of an upper trough during the early morning
hours Sunday. With our without precipitation, there is the
potential for some MVFR ceilings deep within the cold air.
However, this moisture currently on the high terrain of West
Texas may have thinned considerably by the time it reaches North
and Central Texas. The current TAFs maintain VFR Sunday morning.

In the wake of the departing upper low, deep westerly flow will
ensue. Despite a very stable boundary layer, some of this
momentum will reach the surface, and Sunday will be marked by
gusty west winds. At this time, the crosswind issues appear to be
manageable at DFW and Love Field, but this will need to be
assessed further with subsequent TAF packages.

25

&&

.PREV DISCUSSION... /ISSUED 411 AM CDT Sat Apr 29 2017/
A deep upper level low near the Four Corners appears on satellite
to be making a move eastward along the New Mexico-Colorado border.
Thunderstorms have been erupting across the Texas Panhandle and
Oklahoma overnight along and north of a frontal boundary, due to
strong forcing associated with the upper low. In fact, intense
convection stretches all the way eastward from Oklahoma across
Missouri, Illinois and into the Ohio Valley.

Locally, the regional RADAR mosaic indicates that the front is
now beginning to push southeast, possibly aided by gust fronts
from the numerous storms to the northwest. The expectation is
that the front will move into the far northwestern counties around
sunrise before slowing down as the low level jet begins to mix
out. The front may still generate scattered convection this
morning across the northwestern third of the forecast area, though
it will likely be less intense than the ongoing OK-NW TX
activity. This is advertised well by the majority of the hi-res
model guidance. One fly in the ointment, however, is the 3 KM NAM
which indicates that the storms near the TX Panhandle-OK border
will organized into an MCS. The good news is that the 06Z run
weakens this feature considerably as it moves into the northwest
counties, as opposed to the 00Z run which blasted it all the way
through the Metroplex. Since the latter run is the biggest
outlier, the current forecast will not reflect that scenario.
Nevertheless we will need to keep an eye on the evolution of any
upstream storm clusters / cold pools which may develop.

After slowing a bit following sunrise, the front will pick up
steam again late this morning as the upper low approaches the
Texas-New Mexico border. Rain-cooled air may hamper surface
heating in the northwest, but the rest of the region will have
several hours to destabilize. By early afternoon, several
parameters will be in place for the development of strong to
severe thunderstorms. A moist and highly unstable airmass, good
lift provided by the deep upper level system, and a surface
boundary to provide focus for development.

Where storms initialize this afternoon will depend on the location
of the cold front. Taking a blend of the most recent guidance
places the boundary just west of I-35/35W in that 19-20Z hour
(when convective initiation is most likely), which is similar to
yesterday`s forecast and actually near the western edge of the
latest Enhanced risk area from SPC. Storms that develop will have
a good chance of becoming severe due to the high amounts of
instability and shear present. Since shear is mostly
unidirectional, the primary severe threat will be large hail and
damaging winds. A few tornadoes will still be possible, but if
winds become more backed at the surface, a higher tornado threat
would exist.

Storms should quickly congeal into a squall line early this
evening as they push eastward through the forecast area. The
severe concern would shift to a damaging wind and possible brief
spin-up tornado threat with a gradually lowering large hail
threat. In addition, high PWATs suggest that a brief heavy rain
and localized flooding threat will also exist along the line,
particularly if there are any training segments. The line of
storms should reach the southeastern-most counties around
midnight and exit to the east shortly thereafter. Additional
showers and storms will remain possible area-wide through the
overnight hours as the core of the upper low crosses the Southern
Plains. Forcing will be strong, but limited moisture and
instability will likely preclude any additional severe threat
(though some storms may produce frequent lightning and small
hail).

Cool and dry air can be expected Sunday behind the front with
highs in the 60s followed by lows in the 40s Sunday night.
Abundant sun and the return of south winds will bring a quick end
to the cool snap on Monday and Tuesday. A fast-moving shortwave
trough will provide a chance of showers and storms around the
middle of next week, followed by upper level ridging and pleasant
weather late in the week and into next weekend.

30

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    87  52  68  49  80 /  60  50  10   0   0
Waco                86  53  71  46  81 /  50  50  10   0   0
Paris               80  54  64  46  75 /  80  80  20   0   0
Denton              83  48  65  44  79 /  50  50  10   0   0
McKinney            84  51  65  46  77 /  80  60  10   0   0
Dallas              86  53  68  50  80 /  70  50  10   0   0
Terrell             86  53  68  47  78 /  70  70  10   0   0
Corsicana           87  54  69  49  79 /  70  70  10   0   0
Temple              84  53  72  48  82 /  40  50  10   0   0
Mineral Wells       78  47  67  43  82 /  20  50   5   0   0

&&

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

$$

25/91



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