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

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184
FXUS64 KFWD 181701 AAA
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1101 AM CST Wed Jan 18 2017

.UPDATE...
Inherited forecast is in good shape with only a few changes
through the overnight period necessary.

The morning rain shield continues to slide off towards the east
coincident with the best isentropic ascent along the 290-300K
theta surfaces. The rain has been efficient at generating some
rainfall rates of up to half an inch per hour and some street
flooding was noted across communities in Robertson County.
Overall lightning trends suggest that deep moist convection
capable of prolonged periods of heavy rainfall should remain low
and thus we don`t anticipate the need for any short-fused Flash
Flood Watches or Warnings at this time. We will likely continue to
see instances of urban and small stream flooding across this areas
south and east of an Athens to Rockdale line before the rain
finally tappers off. The conveyor of ascent associated with the
rains across eastern/southeastern zones is expected to slide
eastward as the upper low across the western KS pinwheels towards
the north and even northwest. This feature will be nudged eastward
as another upper trough, now across the west coast of the U.S.
ejects out into the southern plains. As this occurs, mid and upper
level flow will strengthen just a bit and may induce some
additional isentropic ascent across North Texas. Forecast
soundings remain dry, but lapse rates steepen a little bit out
west, so I`ll keep low chance for rain and perhaps an isolated
storm or two here.

Otherwise, the other change was to lower afternoon temperatures
by a few degrees across mainly northwestern zones as widespread
cloud cover should remain in place through the day. In addition,
winds should generally remain out of the north which should foster
some weak CAA. For tonight, clearing should occur from west to
east and with the generally light winds, mostly clear skies and
damp soils, fog seems probable late tonight into Thursday morning.
Dense fog may be possible across far western zones, but for now
will just increase the areal coverage and take a closer look at
this potential with the afternoon package.

The rest of the forecast is in good shape and updated products
have already been transmitted.

24-Bain

&&

.AVIATION...
/ISSUED 539 AM CST Wed Jan 18 2017/
Quite a complex weather scenario remains ahead of us during this
TAF period. Southwesterly flow atop a lingering shallow and cool
surface layer is resulting in continued areas of showers and
embedded thunderstorms early this morning. The primary area of
precipitation is approaching the Waco airfield this hour. Regional
analysis reveals MUCAPE values on the order of 200-400 J/kg
persist in this area, and given ongoing lightning trends, opted to
prevail VCTS through the mid-morning hours as this activity pushes
in from the south. At the Metroplex, elevated instability drops
off precipitously, and as a result, have forgone any mention of
thunder today. There is some anvil lightning associated with
activity heading towards KMKN/KMNZ, but anticipate a continued
weakening trend as this area of showers/storms spreads northward.

At all TAF sites, IFR and even LIFR cigs should be the rule today.
The Metroplex sites are presently a mixture of IFR and MVFR, but
cloud decks will steadily build downwards over the next few hours.
Surface winds should come around to a southwesterly or southerly
direction this evening in response to cyclogenesis across the
Oklahoma Panhandle. As winds swing around, there may be a window
of opportunity for cigs and vsbys to briefly improve towards MVFR.
That said, it`s entirely possible all sites remain socked in
through evening and the current 12Z TAFs are slightly too
optimistic.

Finally, there is some concern tonight that fog may impact the
Metroplex sites. Given a wide variety of possibilities offered up
by guidance this morning, however, have only indicated a 3SM BR
late tonight. This is something that we`ll need to pay attention
to in future TAF issuances, however.

Carlaw


&&



.PREV DISCUSSION... /ISSUED 340 AM CST Wed Jan 18 2017/
Rain continues across roughly the eastern 2/3rds of the CWA this
morning as an upper level low currently over southern Colorado
continues moving northeast. Today will be another cool and damp
day for a large part of the region with overcast skies and
temperatures in the 40s and 50s. Showers with isolated
thunderstorms will continue today, mainly over the southeastern
half of the CWA where we have continued high PoPs. Most of the
rain is expected to shift east this afternoon but isolated to
scattered areas of rain will continue this afternoon, again mainly
in our southeastern counties. Periods of brief heavy rain may
occur on top of already saturated soils which may result in
localized nuisance or minor flooding problems.

Tonight, an extra lobe of energy is expected to pivot around the
upper level low and will swing through North and Central Texas on
Thursday. As this piece of energy swings into West Texas tonight,
some additional isentropic ascent in our southern counties will
likely generate more showers, but the models differ on where the
best moisture will remain. The ECMWF and GFS are quick to turn our
H850-700 winds to the west southwest and west overnight, pushing
the better moisture and rain east and southeast of the region.
However, the NAM and some of the hi-res guidance keep south-
southwest winds in this layer with the better moisture residing
over our eastern counties. Will keep 20-50 PoPs across the eastern
2/3rds of the region with the highest PoPs still confined in our
southeast counties where better isentropic ascent may still be
ongoing. However, depending on model trends and observations
today, these PoPs may have to be reduced or possibly expanded
more. Any precipitation tonight should end early Thursday morning.

Drier and warmer conditions are expected otherwise for Thursday
and Friday. Upper level ridging moves overhead on Thursday, and
then we return to a southwest flow pattern aloft on Friday as
another shortwave disturbance moves northeast from the Desert
Southwest into the Central Plains. Even though we will have some
lift across the region with this passing disturbance, southwest to
west winds in the lower to mid levels of the atmosphere, and
PWATs generally 0.50" or less, are expected to keep us dry.
Temperatures on Friday will likely be in the 70s across most of
the region.

Rain chances start returning to the forecast Friday night ahead of
the next system that will directly affect North and Central Texas.
Moisture will start returning across the region Friday night as
the surface winds back to the south in response to surface
cyclogenesis moving from Colorado into Kansas. The large, elongated
upper level trough will nose into the Southern Plains on Saturday
and then further develop into a closed low as it approaches North
Texas. The development of the closed low is now farther west than
yesterday`s run which causes additional concerns for a severe
weather threat Saturday night into Sunday. The left exit region of
a 130 kt jet comes screaming into North and Central Texas
Saturday afternoon. What remains in question is how much and how
good the moisture return will be which in turn affects how much
instability we will have at that time. Oftentimes, the models
underdo moisture return. Tonight`s model runs have better
moisture over the eastern half of the region with indications of
showers and thunderstorms developing along and ahead of a dryline
that bi-sects the CWA. Forecast soundings are showing the cap
will weaken Saturday afternoon, but they maintain very limited
instability due to dry air in the lower levels of the atmosphere.
This is a pattern we will have to continue to keep a close eye on,
and SPC continues to highlight an area east of us for a severe
weather risk. For now, will keep low PoPs across the region with
slightly higher PoPs across the east Saturday through Sunday. Some
light rain may occur Sunday on the back side of the departing,
powerful upper level low.

On the heels of the weekend system, a Pacific front with very
strong winds will cross the region on Sunday. MOS guidance and raw
model output is already showing 30-35 mph sustained winds are
possible which will easily meet Wind Advisory criteria. We will
see a brief warm up Monday and Tuesday before another upper level
trough crosses the Plains and sends a stronger cold front into our
area around the middle of next week. At this time, no
precipitation is expected with this front.

JLDunn



&&


.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    53  45  72  51  74 /  40  10  10   0  10
Waco                54  46  72  47  74 /  50  10  10   0  10
Paris               52  47  65  48  70 /  60  30  20   5  10
Denton              52  44  70  44  73 /  30  10  10   0  10
McKinney            52  44  68  46  72 /  40  20  10   0  10
Dallas              53  46  71  52  75 /  40  10  10   0  10
Terrell             54  46  70  49  73 /  70  20  10   0  10
Corsicana           54  48  71  50  75 /  90  20  10   0  10
Temple              54  46  73  49  75 /  50  10  10   0  10
Mineral Wells       52  43  73  45  73 /  20  10  10   0  10

&&

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

$$

05/24



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