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

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FXUS64 KFWD 171552

Area Forecast Discussion
National Weather Service Fort Worth TX
952 AM CST Sat Feb 17 2018

The disturbance responsible for this morning`s rainfall is moving
quickly to the east and taking the precipitation with it. The
main change with this update is to end POPs from west to east and
remove the mention of thunder for this afternoon. Models are
indicating some lingering light precipitation this evening over
Central Texas, so will maintain the low POPs across the southern-
most row of counties tonight. No other updates are planned at this



.AVIATION... /Issued 605 AM CST Sat Feb 17 2018/
A line of showers with some isolated thunderstorms extended just
northwest of a KF00-KDTO-KETN line as of 1145z. These showers and
isolated thunderstorm will move through the Metroplex TAF sites
between 12-15z and through KACT between 15-18z. Have just
included SHRA in the TAFS as most of the thunderstorms will be to
the north of the Metroplex. Ceilings in the Metroplex will be MVFR
to lower end VFR through 15z with generally VFR ceilings expected
thereafter. Northwest winds at 5 to 10 knots will back around to
the west late morning. As a surface ridge to our northwest moves
eastward, winds will veer around to the northwest by 20z, and
northeast by 00z Sunday.



.SHORT TERM... /Issued 400 AM CST Sat Feb 17 2018/
/Today and Tonight/

A shortwave currently over eastern New Mexico will move east
across the Southern Plains today. Expect widespread showers and
isolated thunderstorms this morning northeast of a Graham to
Hamilton to Palestine line, with a chance of showers and isolated
thunderstorms farther to the south. Skies will start to clear
across the northwestern zones by late morning. Rain chances should
shift to the southeast of a Goldthwaite to Granbury to Sulphur
Springs line this afternoon. After morning lows in the upper 30s
northwest to lower 50s southeast, temperature will rise into the
mid 50s southeast, where the cloud cover will be the highest, to
the mid and upper 60s across the northwest where there will be the
most sunshine. West to northwest winds of 5 to 10 mph will become
north to northeast this afternoon.

Some showers are expected to linger into tonight southeast of a
Lampasas to Athens line. Some patchy fog and drizzle may develop
across Central Texas toward daybreak Sunday. As high pressure
centered to our north moves east, northeast winds of 5 to 10 mph
will veer around to east at around 5 mph. Overnight lows will be
mostly in the 40s.



.LONG TERM... /Issued 400 AM CST Sat Feb 17 2018/
/Sunday through Friday/

An active pattern will take shape across the Southern Plains
for the upcoming week. This will mean some good opportunities for
widespread rain across a good chunk of North and Central Texas.
The latest model guidance continues to suggest a highly
conditional risk for strong to severe storms on Monday and again
on Tuesday. Temperatures will generally remain near or above
normal with a brief cool down on Wednesday and Thursday. A
moderating temperature trend is possible as we head into the last
weekend of the month.

For Sunday---Rain chances on Sunday look lower compared to 24
hours ago. The 850mb baroclinic zone that has helped to trigger
widespread rainfall on Saturday will slowly lift northward. The
surface front will lag behind this feature, but it`ll also lift
northward. With increasing low level moisture, there could be some
morning fog/drizzle--- especially down across Central TX. Farther
north, wind speeds may keep the PBL overturned enough such that
this potential is largely mitigated. While the magnitude of WAA is
decent, it (the WAA) will more than likely be the only lifting
mechanism. Given that ascent along the warm front is typically
tame (in the absence of strong instability aloft) owing to the
gradual slope of the frontal surface, I`ve lowered PoPs into the
20-30 percent range. This is supported by TTU WRF and NAM NEST hi-
resolution guidance. There may be an isolated storm or two as
well, but with elevated instability being quite meager (only a few
hundred J/kg), I`ll leave this out of the forecast. Despite
widespread cloud cover, there`s a decent 1000-500mb thickness
ridge across the area. This should equate to above normal
temperatures with afternoon readings in the 70s. Some mid 70s are
possible out across the Big Country along the spine of the thermal
axis. Across the I-30 corridor, temperatures may remain 10 to 15
degrees lower than the rest of the area due to limited WAA.

By late Sunday afternoon into Sunday night, a conveyor of rich
low level theta-e will stream in from the southwest and spread
across the northern Big Country, Red River and eventually into the
Ark-La-Tex. This likely indicates an increase in instability and
should be sufficient at generating a bit more in the way of more
widespread and vigorous, but non-severe convection. The best
chance for showers and a few thunderstorms will be northwest of a
Comanche to D/FW to Sulphur Springs line late Sunday afternoon
into Sunday night. Coverage of convection isn`t expected to be
spectacular, however.

For Monday---There will continue to be a chance for isolated to
widely scattered showers and a few thunderstorms roughly across
the northwestern third of the area during the pre-dawn hours on
Monday. Continued strong low level flow will help to keep the PBL
mixed overnight and this should lead to a very mild and breezy
Monday morning.

Into Monday afternoon, models are in good agreement that a dryline
will sharpen some across the Big Country. Strong southwesterly
flow aloft will advect a layer of warm air atop the moist boundary
layer, likely keeping most areas free of rain during the mid-
morning and into the afternoon hours. I`ll maintain a 30 PoP,
however, for northwestern zones where theta-e advection is
expected to persist. The dryline will slowly move eastward through
the day and there should be some convergence along the boundary.
This convergence, however, will be the only meaningful lift and so
this lowers confidence that this will be enough to breach the cap.
What is a little concerning is forecast temperatures vs. the
convective temperature. Minus the NAM, even the coolest guidance
in the remainder of the model suite advertises mid to upper 70s
and low 80s across the area. Convective temperatures are forecast
to be in the mid 70s. If current forecast temperatures are to
verify, it`s possible that surface based convection *could*
develop. Instability is modest with values of up to 1,000 J/kg,
however, and the instability profile is very slender. Updrafts
developing into an environment characterized by 30 to 40 knots of
deep layer shear and this instability profile may not develop
favorably to mature into strong updrafts. IF the updrafts can
survive, they *could* become strong to severe. Unfortunately, this
far out, it`s a little difficult to accurately depict storm scale
processes and as a result, we will continue to highlight only a
conditional risk for severe weather on Monday afternoon.

Late Monday into Tuesday---Convection will more than likely
blossom late Monday into Tuesday. While the lack of diabatic
heating tends to dampen the overall instability and thus the
severe weather potential, it`s important to not let your guard
down when dealing with strong low level flow. Low level winds will
crank upwards of 40 to almost 50 knots across the northwest and
this should transport a healthy batch of low level theta-e. If
convection is ongoing or develops, there is a possibility that it
could grow upscale across northwest zones and move eastward. I`m
not ready to buy this scenario completely given the lack of
higher resolution guidance to support this, but it`s not out of
the realm of possibility. I`ll keep 50 to 60 percent PoPs to the
northwest where this is most likely to happen and for forecast
consistency. Again, a conditional risk for severe storms will
be present with this convection.

Tuesday and Wednesday---Convection will likely be ongoing into
Tuesday morning and I`ll continue to show widespread rain chances
(60-80%). The best rain chances will be along the Red River. The
ECMWF has remained consistent with a slower frontal progression
than most of the other guidance. The last few frames of the 00
UTC NAM and a majority of the 00 UTC GFS ensemble members have
started to shift towards the ECMWF solution as well. In fact, the
deterministic 00 UTC GFS has also slowed a bit (but still remains
the fastest solution). With that in mind, I`ve pushed the
forecast to be more in-line with the ECMWF/NAM/GFS Ensemble
guidance. The bottom line here is that widespread rain chances
will be possible a good portion of Tuesday and into at least the
first half of Wednesday.

The slow moving nature of the front, coupled with ascent
courtesy of jet entrance forcing actually resembles a Maddox Type
I (one) heavy rain pattern. Given that we`ve been quite dry for
sometime, I`m not ready to jump on the flood bandwagon, but
depending on the amount of rainfall received Late Monday into
Tuesday morning, it may not be out of the realm of possibility.
Highest QPF will generally be along and east of I-35 during the
midweek time frame. While that is a broad geographic descriptor,
we will likely refine this as we near the the potential heavy rain
event. It`ll turn cooler on Wednesday as cooler air overspread
the region from the north with brisk northerly winds in the wake
of the front.

Thursday and Friday---Model guidance continues to diverge during
this portion of the forecast and as a result, I`ve leaned heavily
on the blended guidance. The operational GFS, which seems to be
much of an outlier, dries much of the area out fairly quickly on
Thursday and most of Friday. The ECMWF on the other hand develops
quick return flow. Both models have a pretty large trough to the
west and I`d be inclined to believe the ECMWF output which
suggests lee-side cyclogenesis and resultant low level moisture



Dallas-Ft. Worth    62  46  70  60  80 / 100   0  30  30  30
Waco                58  46  73  62  80 /  30  10  30  20  20
Paris               56  42  59  55  75 / 100   0  20  40  30
Denton              62  41  66  59  78 / 100   0  40  30  50
McKinney            58  42  64  58  75 / 100   0  30  40  40
Dallas              60  47  70  60  79 / 100   0  20  30  30
Terrell             57  45  68  61  78 /  60   0  20  30  30
Corsicana           55  47  69  62  79 /  40   5  20  20  20
Temple              59  47  75  62  76 /  30  20  30  10  20
Mineral Wells       65  43  71  61  81 / 100   0  40  30  30




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