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

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FXUS64 KFWD 221751 AAB

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1251 PM CDT Mon May 22 2017

/18Z TAFs/

A very challenging day as the old surface front remains well
south of the area. In addition, moist soils from overnight rains
are combining with late May heating and a shallow frontal
inversion to result in cigs at various levels. This is making the
cig forecast very challenging with potential for IFR at Waco and
MVFR at DFW sites possible through mid-late afternoon. Timing the
next northern branch trough and embedded shortwave disturbances
also presents a challenging 9-30 hour cig forecast.

Currently, we continue to watch the previous southern stream
shortwave that resulted in widespread rains from I-20 and points
south exit the area. A respite in convective activity will
continue through mid-late evening, as we remain more under an area
of subsidence from the first departing southern stream shortwave.
The next disturbance can be seen on satellite moving southeast
across the OK/TX Panhandles and northern OK. The first shortwave
will give us increasing convective chances across the airports
during the morning hours on Tuesday. Another reprieve is likely,
until a cold front and second shortwave arrive Tuesday afternoon
with yet another round of convection

For DFW airports...
We`ll continue to advertise spotty MVFR cigs this afternoon, then
see VFR through late evening, before MVFR cigs return. Will
introduce VCSH for the late evening hours with the approach of
the first shortwave, with VCTS in the 08Z-13Z period. Afterward,
there is a high degree of uncertainty, but will slowly bring DFW
airports back to VFR beyond the 24 hour period. For now, left
VCTS out of the 24-30 hour forecast and will let future issuances
address that time period with better confidence.

For Waco...
Similar trends to DFW area airports except IFR cigs early this
afternoon and perhaps a slight delay on convective timing
overnight and into Tuesday. In addition, they may not break out to
VFR at all Tuesday if the cold front gets in there relatively
early in the day as expected.

Look for light ENE-ESE winds less than 10 kts, going variable
ahead of the cold front later tonight. All sites will likely see
FROPA either side of 12Z Tuesday with winds NW 5-10 kts,
increasing to 10-15 kts Tuesday afternoon.



Main update to the forecast was to reconfigure PoPs/Wx and Sky
grids for the afternoon and evening hours based on latest

Manual surface analysis this morning revealed a stationary
baroclinic zone remained parked along the upper TX coast. The
isentropic upglide along the 305K theta surface above the cooler
surface air likely explains the widespread precipitation shield
that overspread much of Central and East TX this morning. Trends
in mosaic radar imagery would suggest (and some hi-resolution
guidance confirms) that this isentropic upglide is gradually
weakening. Weakening ascent should result in a continued
reduction of the rain shield through the afternoon hours. As a
result, I`ve trended PoPs downward across eastern and southeastern
zones. In addition, much of the convection is nearly void of any
lightning and while a stray rumble of thunder or two cannot be
ruled out, a majority of the activity should be lightning-free.

Farther north across Oklahoma, a decaying complex of convection
will continue to slowly slide southeastward. Overall, the
probability of expansive re-invigoration of this convection during
the afternoon appears low, given the extensive cloudiness. Cloud
cover may thin somewhat with the late May sun and so I`ll carry
just a 20 PoP across northwestern zones just in case we see a slow
uptick in activity. I won`t completely rule out a strong storm
this afternoon, with perhaps some near-severe hail or gusty
outflow winds.

The zonal flow aloft this morning will gradually amplify as a
shortwave trough across the Great Basin swings east and
southeastward. As this feature aloft approaches, lapse rates will
steepen upstream and an additional complex of storms (likely with
more vigor than the current one across OK) is expected to develop
and push towards the southeast. Forward propagating Corfidi
vectors of around 20 to 30 knots per NAM and GFS output would
favor this potential complex racing eastward across North and
Central TX. Given current observations and latest input from hi-
res guidance, I`ve elected to delay the onset of this possible
convective complex into our CWA. With that in mind, the highest
rain chances will be closer to the 03 UTC time frame with likely
PoPs across far western zones. There is some uncertainty to the
maintenance of the MCS farther to the east, so for now will leave
inherited PoPs in tact. There will be a low end severe wx risk,
west of I-35 with the main hazards being strong winds and perhaps
near-severe hail.

The rest of the forecast remains unchanged and updated products
have been transmitted.



.PREV DISCUSSION... /ISSUED 358 AM CDT Mon May 22 2017/
A widespread area of showers and a couple embedded thunderstorms
prevail across much of North and Central Texas early this Monday
morning. While the frontal feature responsible for the generally
east-northeasterly surface flow sits idle across the Mid and
Upper Texas Coast, broad and gradual ascent atop this quasi-
stationary boundary--evidenced by northward surging theta-e values
in the 850-700 mb layer--are helping to supply the region with
some beneficial light to moderate stratiform precipitation.

Based on the recent uptick in convection to our south, it appears
another wave of ascent is presently impinging upon the
aforementioned stalled front. While a vast majority of this
additional lift looks to remain confined to the Hill Country/Upper
Texas Coast, enough of an additional impetus exists to warrant
likely PoPs across our southeastern counties this morning where
convection should continue to steadily fill in. Given an overall
paucity of elevated instability across the northwestern half of
the CWA, elected to limit thunder chances to just "isolated" this
morning, but show likely chances roughly south and east of a
Killeen to Canton line where instability will be maximized. While
we can`t rule out some small hail in any of the more robust cells
across this region, the primary hazard from storms will be locally
heavy rainfall given PWATs hanging out near the climatological
maximum for this time of year, and deep warm cloud layers in
excess of 10,000 feet. As a result, some instances of mainly
nuisance flooding of low-lying areas appears possible through the
morning hours.

As this disturbance shifts east of the region later this
morning/afternoon, we may actually see a downtrend in
precipitation coverage as a brief pocket of large-scale subsidence
slides in overhead. As a result, we`ve trimmed PoPs to reflect
mainly slight chances across the northwestern 2/3rds of the
region, and low-end chance values across our far south and eastern
zones this afternoon--and even these may be somewhat overdone.
Will, however, let the dayshift assess trends later today. While
effective bulk shear values will be on the increase through this
period, it seems probable that this morning`s mass of convection
near the Gulf Coast will significantly retard the return of more
appreciable instability, limiting the threat with any additional
convection to small hail.

Later this afternoon and evening, we`ll turn our attention to the
north and west as several rivulets of energy embedded within the
split flow aloft migrate across Texas. The signal for an upscale-
growing MCS towards this evening to our west was actually a bit
stronger in last night`s model runs. As a result, several levels
of uncertainty plague the evening and overnight portion of this
forecast. At this time, it appears that moisture will have
returned in sufficient quantities across the western half of Texas
to support convective development from the Trans-Pecos region into
the Panhandle as a vort lobe and associated 100+ kt jet streak
overspread the area late this afternoon. Should this activity
organize and grow upscale into an MCS, forward-propagating
Corfidi Vectors would support an east/east-southeastward movement
towards our western/southwestern counties during the late evening
hours. As this potential complex heads east, however, it would
likely start encountering an increasingly hostile environment for
convective sustenance given (1) more limited moisture/instability
return across our western counties, (2) less impressive low-level
wind field with sub-700 mb flow less than 20 kts, and (3) the less
favorable arrival time during the late-evening hours. All of this
taken together casts considerable doubt on the severe wind/hail
potential accompanying this system. Given these uncertainties,
we`ll mention the potential for some severe-caliber wind gusts and
hail entering our west/southwest counties this evening, but
latest indications are that this complex will be on a weakening
trends as it approaches our CWA.

The actual surface cold front will then blast through the region
on Tuesday, bringing a renewed thrust of showers and storms to the
area. Given a pretty limited supply of buoyancy across the
northwestern half of the CWA and time of day, the severe threat
here looks pretty low. A low risk for some strong to perhaps
marginally severe storms may arise across our south and eastern
counties during the late-morning and afternoon given diurnal gains
in instability. The main hazards would be marginally severe hail
and perhaps some gusty winds. Strong cold advection behind the
front will result in blustery northwest winds Tuesday across much
of the region--perhaps close to Wind Advisory criteria--and
cooler temperatures.

Any lingering precipitation should be out of the picture by
Wednesday morning, leaving us with a nice dry stretch of weather
through the rest of the week. Moisture will vigorously return
Thursday night into Friday, but stout capping supplied by a plume
of +25 C air at 850 mb should keep convective chances at bay
until Saturday. In the meantime, high temperatures will likely
rocket into the middle 90s out west Thursday-Saturday as a dryline
oscillates diurnally to and fro. The Saturday-Tuesday period
could start to get active again as several impulses slide in on
northwest flow aloft. While it`s far too early to get into
specifics, the thermodynamic/kinematic profiles would support at
least a threat for some strong-severe thunderstorms during this
time frame.



Dallas-Ft. Worth    76  63  75  56  82 /  10  40  40  10   5
Waco                73  61  76  54  83 /  20  50  40  20   5
Paris               76  59  71  52  75 /  10  40  40  10   5
Denton              76  60  73  50  79 /  10  50  40   5   5
McKinney            75  59  74  53  79 /  10  40  40  10   5
Dallas              76  63  75  57  81 /  10  40  40  10   5
Terrell             73  61  76  54  79 /  10  40  40  10   5
Corsicana           72  63  76  55  82 /  30  50  40  20   5
Temple              73  62  76  54  83 /  20  50  40  20   5
Mineral Wells       75  59  73  50  82 /  20  50  40  10   5




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