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

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FXUS64 KFWD 200034 AAA

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
634 PM CST Mon Feb 19 2018

/00Z TAFs/

Weather challenges abound with only marginal confidence regarding
timing with lower cigs tonight and rounds of SHRA/TSRA later
tonight and through Tuesday.

Currently, scattered very light showers extended from KF00, to
KF46/KHQZ, to KACT and KTPL this hour. Recent PIREP from 10SE of
KDAL reported moderate turbulence around FL100 and another right
over KDAL at FL180, likely from evaporation occurring from dry
pockets aloft on our very recent 00z FWD sounding. Both Bonham and
Cedar Creek cornerposts will be affected by this process and
further turbulence this evening, or until increasing ascent later
tonight can saturate these layers. Otherwise, occasional gusty
south winds will continue through the night in advance of a
shortwave aloft and any SHRA/TSRA, due to a strong surface
pressure gradient and semi-mixing of 40-45 knot 925mb winds that
may increase by another 10 knots by midnight.

VFR cigs will slowly fall into MVFR by mid evening, as gradual
boundary layer cooling occurs and we remain in a lull between mid
level disturbances. High-resolution models are in agreement for
some spotty -SHRA through 08z-09z, before a weakening convective
system approaches from the west. Though models project some weaker
instability toward the I-35 corridor by 12z, there will be surface-
based CINH further east, so it`s hard to say whether wind gusts
will be an issue if convection does truly become elevated. If they
do, then small hail, lightning, and occasional heavier TSRA would
be the main issues. All said, the window of 10z-13z appears to be
the window of greatest certainty of impacts for DFW Metro

As for Waco Regional, the gusty south winds will be generally the
same across Central Texas. Only differences will be convection
possibly starting an hour or two later and lingering a few hours
longer til 15z, as the stronger forcing moves further northeast
away from this area.



.SHORT TERM... /Issued 312 PM CST Mon Feb 19 2018/
/Through Tonight/

Main focus through the short term will be the convective chances
this afternoon and overnight. While the severe potential still
looks limited, a few strong or marginally severe storms can`t be
ruled out, mainly during the overnight hours west of I-35.

A deep trough has taken shape over the western US with a
strengthening surface low located in Colorado. To the east, a
stalled cold front lies in a pressure trough draped through the TX
Panhandle and OK northeastward into KS and MO. To the south a
very warm and humid airmass is in place within a strong southerly
flow regime. Several days of south flow have caused rich Gulf
moisture to overspread the area with near-record PW values of over
1.5" now in place. As shortwaves ripple through the parent
trough, scattered showers and storms will be likely, also aided by
weak ascent from warm advection. Precipitation is starting to
increase slightly as of 3pm primarily along and just east of the
I-35 corridor. This is occurring within the axis of greatest
moisture content where there is also strong warm advection.
Through the evening, would expect a gradual increase in showers
with isolated embedded lightning strike near the I-35 corridor.
South winds will also remain gusty into the evening at 20-30 mph.

Meanwhile, in West Texas, a cluster of showers and thunderstorms
is expected to develop in vicinity of the dryline in the next few
hours which will move northeastward. The exact evolution of this
convection remains uncertain, but indications are that this
cluster of storms will be capable of progressing into our forecast
area counties late this evening. Depending on the organization of
this line, it could potentially pose a threat for some 50+ mph
wind gusts and at least some small hail. Stronger storms will be
more likely if the cluster is able to organize into a broken QLCS
structure, which given the strong shear in the 0-3km layer, seems
plausible. Storms should also have about 1000 J/kg of MUCAPE to
work with. The HRRR remains the most bullish on this overnight
convection, while other high-res guidance keeps it less organized
and weaker. One factor working against these storms will be
increasing CINH through the evening, therefore storms should be
forced to become elevated. This should help lessen the wind
threat, but since synoptic low-level winds are already 50+ knots,
mixing down this momentum to the surface wouldn`t be too
difficult for some strong downdrafts to achieve. Given the strong
shear in place, also wound`t be surprised to see some small hail,
probably remaining below severe limits. Convection should continue
an overall weakening trend as it approaches the I-35 corridor
between midnight and sunrise. The best chance for stronger storms
will be west of I-35. The flood threat with this initial batch
will be low since it will be fast-moving. Rain amounts are
expected to be less than an inch for most areas through Tuesday

Otherwise, the gusty south winds will continue into Tuesday
morning along with a broad area of showers and embedded
thunderstorms. Skies will remain mostly cloudy with rich moisture
continuing to stream in from the south. The additional waves of
rainfall and ensuing potential for heavy rain/flooding are
discussed below.



.LONG TERM... /Issued 312 PM CST Mon Feb 19 2018/
/Tuesday through this weekend/

An active weather pattern is setting up for the week and will be
defined mainly by a deep upper trough over the Rockies and an
upper ridge over Florida. This pattern will leave North and
Central Texas susceptible to periods of strong lift generated by
disturbances in the southwest flow aloft. Scattered showers and
thunderstorms will likely be ongoing Tuesday morning as an initial
shortwave lifts northeast across the Southern Plains. Shear and
instability will be sufficient for a few strong storms early
Tuesday, but the convective mode should gradually shift to a warm
advection showers/isolated thunderstorms regime later in the day as
the shortwave exits to the northeast.

The passing disturbance will help force a cold front south
across the Red River Tuesday night. The front will slow and
possibly begin to stall as it sags south of the I-20 corridor.
Meanwhile, large scale ascent will be on the increase as the upper
low digs south into the Four Corners region. There could be a
lull in precipitation Tuesday night, but convection will ramp up
on Wednesday as ascent increases and the front provides added
focus for development.

An unusually moist airmass will be in place this week,
particularly along and east of the Interstate 35 corridor where
PWATs in the 1.50-1.75 inch range will exist. This could be
problematic regarding flooding potential as it appears that the
front will become quasi-stationary somewhere across the east or
southeast counties.

Model QPF is somewhat variable, but Tuesday through Wednesday
night rainfall totals between 2 and 4 inches are looking
reasonable across a good swath of the central and eastern zones.
Localized higher totals will be possible in the vicinity of the
surface front. Higher totals are not out of the question in the
cold sector as well, especially if there are any training
thunderstorms. For this reason, a Flood Watch will be issued for
areas generally along and east of Interstate 35.

Since convection is expected to increase in coverage and intensity
late this evening and overnight, the Watch will begin Tuesday
morning as locally heavy rainfall may already be in progress.
Another shortwave will lift northeast across the region Tuesday
night through Wednesday. At this time, we have the Watch expiring
at midnight Wednesday night, after-which the shortwave will exit
to the northeast and a lull in precipitation is expected.

Another issue that we will continue to monitor is the possibility
of freezing rain or freezing drizzle across the northwestern-most
counties Tuesday night. This area will be west of the heavier
precipitation, but will also see the coldest temperatures in the
post-frontal airmass. The latest guidance indicates that low
temperatures will be right around freezing Wednesday morning when
light precipitation will still be possible. The current thinking
is that after the recent warm weather, the likelihood of any
accumulations and resulting impacts is low. If later guidance
trends colder, some freezing rain may need to be added for a few
counties during the late tomorrow night/Wednesday morning period.

After a lull in precipitation Wednesday night and Thursday, convection
will ramp up once again Thursday night into Friday as the next
shortwave rounds the base of the Rockies upper low and lifts
across the Southern Plains. The potential for locally heavy rain
will exist as models are indicating PWATs back up around 1.50
inches. Precipitation should finally come to an end Sunday as the
entire upper level system works its way east across the Plains,
with dry and seasonable weather expected Sunday through the early
part of next week.



Dallas-Ft. Worth    67  72  43  45  39 /  80  80  70  70  40
Waco                65  72  49  53  41 /  70  80  80  80  50
Paris               64  71  46  50  43 /  80  80  80  80  60
Denton              66  72  39  43  36 /  80  80  50  70  40
McKinney            66  71  41  46  39 /  80  80  70  80  50
Dallas              67  72  43  46  41 /  80  80  70  80  50
Terrell             67  72  46  51  42 /  70  80  80  80  60
Corsicana           66  71  49  54  45 /  60  80  80  80  70
Temple              65  71  51  55  42 /  70  80  80  80  50
Mineral Wells       63  74  39  42  33 /  80  80  40  60  30


Flood Watch from 6 AM CST Tuesday through Wednesday evening for



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