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Forecast Discussion for FWD NWS Office
FXUS64 KFWD 201133

Area Forecast Discussion
National Weather Service Fort Worth TX
633 AM CDT Thu Sep 20 2018

/12Z TAFs/

Concerns--Increasing cloud cover and precipitation chances. MVFR
at KACT. South flow continuing.

Moisture is beginning to surge northward across the Texas Hill
Country as a surface low deepens over the Great Plains. This
moisture surge will bring increasing cloud cover and rain chances
through the TAF period and through the next several days. Cloud
cover is already increasing near KACT per imagery from the GOES-16
10.3-3.9 micron "night fog" product...and METAR sites to the
south such as KAUS have shown MVFR to even IFR cigs this morning.
Temperatures should begin to rise after daybreak at
KACT...allowing low-level relative humidity values to fall. This
should preclude a greater risk for prevailing MVFR...but we are
likely to see at least intermittent MVFR cigs through late
morning. Nevertheless...trends will need to be monitored closely
through the morning...and amendments may be necessary. For the
Metroplex TAF sites...low-level temperatures should be warm
enough by the time the moisture surge reaches the area...keeping
cigs in the VFR category.

Concerning precipitation chances...the greatest chances will be
towards the southwest. Opted to go with VCTS at KACT during the
afternoon hours where the better moisture and instability will be.
To the north...coverage should be less...but a few spotty showers
will be likely near/over southern portions of the D10 TRACON.
Precipitation chances will diminish overnight with MVFR cigs
likely to return to KACT by midnight.

Heading into the extended portion of the forecast for KDFW...look
for steadily deteriorating conditions beginning early Friday
morning as moisture continues to surge north...and a cold front
drops south into the region.



.SHORT TERM... /Issued 338 AM CDT Thu Sep 20 2018/
/Today and Tonight/

The much advertised pattern shift is now underway across the
Southern Plains. Water vapor satellite imagery shows that the
upper-level ridge that had been responsible for the recent spell
of hot and dry weather has moved off to the east, and is now
centered over the Tennessee Valley. To the west of us, a deep
trough has begun to amplify across the Rocky Mountains, with the
axis as of this writing along a line from Boise, ID to St. George,
UT to Phoenix, AZ. Cyclonic vorticity advection associated with
this trough has allowed a surface low over northwest Kansas to
deepen, with sea-level pressure values in that area down to around
1002 mb. This lee-side cyclogenesis has allowed a stout low-level
jet to develop across West Texas (as evident on the KMAF WSR-88D
vertical wind profile). This low-level jet will spread east as the
aforementioned surface low lifts northeast, bringing a surge of
moisture into North and Central Texas.

The surge of moisture will first be realized in the form of
increasing cloud cover through the day today. The GOES-16 10.3-3.9
micron "night fog" product shows that stratus clouds associated
with this low-level moisture intrusion just beginning to enter
the far southwestern reaches of our county warning area. These
clouds should overspread the Waco/Killeen/Temple area before or
right around daybreak, with cloud cover increasing gradually
through the day for the Dallas/Fort Worth area. We will also see
an increase in rain chances. The GFS seems to be the most
aggressive with QPF on Thursday, with the EC and NAM both limiting
coverage. The HREF members also seem to be relatively split into
these two camps. Either way, the greatest chances and greatest
coverage will be towards the south and southwest, with the main
debate being whether those PoPs should be in the likely range
(GFS) or the slight chance range (EC/NAM). The difference was
split, with generally chance (30%-40%) PoPs shown in our
southwestern counties during the afternoon hours.

The increase in cloud cover and rain chances will also mean
milder temperatures on Thursday. Afternoon high temperatures will
likely still break the 90F mark north of a Graham to Arlington to
Fairfield line, with 80s forecast to the south. This is still
slightly above normal for late September for DFW, but should be
closer to normal at Waco where highs will likely be in the upper
80s. Heading into Thursday Night, dense stratus should envelop
most of the county warning area, with rain chances increasing
towards the west and northwest as a shortwave trough begins to dig
into the Texas Panhandle.



.LONG TERM... /Issued 338 AM CDT Thu Sep 20 2018/
/Friday through the End of Next Week/

A cold front will extend from a strong surface cyclone near the
Great Lakes southwestward across the Mississippi Valley, through
the Southern Plains and into Northwest Texas at the start of the
period. The front is still expected to push south of the Red
River, but will slow its forward speed late Friday and possibly
become stationary somewhere across or just south of the forecast
area by Saturday. Meanwhile, a shortwave trough over West Texas
will work its way slowly eastward across Central and North Texas
over the weekend. South of the front and ahead of the shortwave,
Gulf moisture will surge northward, priming the region for a
multi-day rain and thunderstorm event Friday through the weekend,
with precipitation possibly lingering into early next week.

The primary concern at this time is the potential for heavy rain
and possible flooding. Precipitable water values will have already
reached the 2 inch mark early Friday. These values (which will
become enhanced by an added influx of mid level Pacific moisture)
will only increase Friday night and Saturday, with a 2.25" spike
in PWAT being advertised along and immediately ahead of the cold
front. Though precipitation should become fairly widespread
throughout the period, the heaviest rain and best potential for
training thunderstorms will occur in the vicinity of the cold
front. The flooding potential then becomes highly reliant on the
speed of the front and if/where the boundary may stall.

The GFS is fairly progressive with the front, pushing it slowly
but surely southeast across the area Friday through Saturday
before stalling the boundary across Louisiana and Southeast Texas
late Saturday. The current feeling is that this may be a bit too
progressive based on the 00Z runs of the NAM and ECMWF, which
develop a more pronounced surface low just south of Wichita Falls
sunrise Saturday. The ECMWF takes the surface low slowly east
along a quasi-stationary boundary, which may simply be meandering
in the vicinity of the I-20 corridor. This scenario would place
the highest precipitation totals along and north of I-20. This
seems more likely based on the agreement between the NAM and
ECMWF at hour 60, and also makes sense being that the shortwave
which gave the front its initial southward push will have long
since exited to the northeast.

The position of the front, the surface low and the slow-moving
shortwave places our highest qpf values from the Red River to
I-20, where 2 to 4 inch totals over 72 hours are looking more
probable (locally higher totals are also possible). Rain chances
would begin Friday with the potential for locally heavy rain
starting by Friday afternoon across the northwest counties and
shifting slowly east across the region through Sunday. A flood or
flash flood watch will probably become necessary, but since it
would likely not begin until Friday evening, we will hold off
issuing one at this time and wait for additional guidance today
and tonight to narrow down the timing and location.

Despite the decent amount of instability present in the warm
sector, the overall severe threat still appears to be fairly low
based on the lack of shear and overall weak flow aloft. That said,
if the surface low strengthens enough, the resulting backed wind
field may generate a few low-topped supercells, which could allow
for a localized winds and brief spin-up tornado threat to exist
along and south of the I-20 corridor. We will certainly keep an
eye on mesoscale parameters ahead of the event and stay on top of
this possibility, but the main focus will be on the heavy rain and
flooding threat.

Precipitation will weaken and move off to the east as the front
lifts north and the shortwave dampens. South winds will return
late Sunday and Monday as a larger-scale trough deepens across
the Plains. This system will force a strong cold front southward
through the region Wednesday. A brief round of convection will be
possible with the front, but the lack of significant moisture will
keep most activity limited to the far southern counties. The most
notable result will be the much cooler weather expected during
the second half of next week, just in time for the first week of



Dallas-Ft. Worth    91  75  86  70  76 /  10  10  60  70  70
Waco                88  75  87  71  81 /  30  20  60  70  70
Paris               92  73  86  70  77 /  10  10  60  70  70
Denton              91  74  84  68  75 /  10  10  70  80  70
McKinney            91  75  87  70  76 /  10  10  60  70  70
Dallas              92  76  87  71  78 /  10  10  60  70  70
Terrell             91  75  88  73  81 /  10  10  50  70  70
Corsicana           90  75  88  72  79 /  10  20  50  60  70
Temple              87  75  86  71  80 /  40  30  60  60  70
Mineral Wells       89  73  83  66  74 /  10  20  70  80  70





NWS FWD Office Area Forecast Discussion

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