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

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000
FXUS64 KFWD 191232
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
632 AM CST Mon Feb 19 2018


.AVIATION...
Outside of precipitation, mostly MVFR ceilings are expected to
prevail through 12z Tuesday at all of the TAF sites. Southerly
winds at 15 to 20 knots will persist through the period with gusts
over 25 knots at times. Although we cannot rule out some scattered
showers/thunderstorms at anytime, the best chances today should
be along and southeast of a KBBD-KFTW-KGLE line. Another band of
showers and thunderstorms is expected to develop mid to late
afternoon across West Texas ahead of a dryline. This activity is
expected to move northeast into the region overnight tonight.

Have indicated a VCSH starting at 16z with a VCTS for the 21z
today through 01z Tuesday period at all of the TAF sites and
placed 6SM -SHRA starting at 07z with a TEMPO 4SM TSRA OVC15CB
for the 08 to 12z Tuesday period.


58

&&


.SHORT TERM... /Issued 358 AM CST Mon Feb 19 2018/
/Today and Tonight/

A northeast-southwest oriented broken line of showers and
isolated thunderstorms that extended from Sulphur Springs to
Killeen at 3 AM was making only slow progress eastward as the
individual cells moved northeastward. Training will continue into
the morning hours, but fortunately, this activity is mainly
producing light to moderate rainfall.

The easterly and southeasterly flow along the south and
southwestern side of a ridge over the Atlantic Ocean is actually
tapping moisture from the Caribbean and the Gulf of Mexico.
Precipitable Water values across the eastern 1/2 to 2/3s of North
and Central Texas will be rising into the 1.6 to 1.7 inch range by
tonight. The next shortwave is currently over Southwest Texas and
the showers and thunderstorms associated with this system are
moving northeast and will move across the forecast area today.
Rainfall amounts today should average 1/4 inch or less.

Am not confident that we will have surface based convection, but
given the shear and modest instability, if it does occur, we could
see some strong thunderstorms late this afternoon into this
evening. Despite the cloud cover, with temperatures starting out
the day in the 60s area wide, highs will reach the 70s across most
of the forecast area with even some sites across the northwest
possibly reaching the 80 degree mark.

Another round or two of showers and thunderstorms is expected to
move northeast across the forecast area tonight. The heaviest rain
should be across the northwestern half of the forecast area where
rainfall amounts will average near 1/2 inch while areas to the
southeast should average less than 1/4 inch. Lows tonight will be
in the 60s.

58

&&

.LONG TERM... /Issued 358 AM CST Mon Feb 19 2018/
/Tuesday through Sunday/

...Overall Event Summary...
The primary concern in the long term forecast period will be the
potential for widespread heavy rain. Despite the below normal
precipitation for the early to mid winter time period, drought
conditions have steadily eased from east to west across the region
over the past several weeks. A few days of prolonged heavy rain
will yields some threat for not only river flooding, but some
instances of flash flooding. With regards to any potential Flood
Watches----although confidence is high in the forecast, after
discussion with surrounding offices, we`ve elected to hold off on
issuing any Flood Watches. It`s likely that one will be issued
either today or early Tuesday morning. Concerning the potential
for winter precipitation---We will keep monitor of the the
possibility for very light freezing rain or freezing drizzle
northwest of an Eastland to Gainesville line on Thursday morning.
At this time, I`ve elected to leave this out of the official
forecast as 1) temperatures are just above 32 degrees and 2)
impacts are likely to be minimal (or possibly non- existent). A
majority of the focus for the long term period was spent on the
Tuesday through Thursday time frame.


...Summary of Impacts...
Multiple rounds of rain, heavy at times, are expected during the
Tuesday through Thursday time frame. Despite the recent dry spell,
extraordinary moisture combined with multiple rounds of
precipitation will necessitate some river and possibly instances
of flash flood concerns during the Tuesday through Thursday time
frame. Areas generally east of the I-35 corridor stand the best
risk for flooding.

...General timing of rain...
The first wave of rain is expected to occur on late Monday night
into Tuesday morning with the highest rain chances (around 90%)
near and west of a Hamilton to D/FW to Bonham line. During this
time period up to 1.5" of rain will be possible. Recent drought
across the area should allow for decent infiltration rates, though
typical low spots and/or areas with seasonally dormant vegetation
or due to recent fire activity may need to be monitored for a
potential flood threat.

Through the day Tuesday and into Tuesday evening, the axis of
showers and thunderstorms will shift eastward to near the I-35
corridor (near and north of the Waco area) by noon. By the evening
hours, the best rain chances will be east of I-35 and north of
the I-20 corridor. Heavy rain and possible training deep moist
convective elements may present a flood threat here, especially
across the larger urban centers where infiltration rates will
generally be lower.

Late Tuesday into into Wednesday, the best rain chances will be
across East and Central TX as the axis of best lift is shunted
eastward. The potential for training showers and thunderstorms may
be greatest during this time period particularly across East TX
and thus the probability of some flood issues increases. Farther
south, heavy precipitation is expected to develop along the slow
moving cold front, across Central TX. Again, recent drought
conditions should help initially help to hinder excessive runoff.
If heavy rain persist long enough, however, some flood issues
could surface.


...Storm total (Tuesday through Thursday) rainfall estimates...
Estimates of storm total rainfall are fluid in nature and will be
dependent on the location of best lift and frontal positioning.
In general, some of the heftier rainfall totals of greater than 4
inches of rain are quite probable across parts of North and
Central TX. The most likely area to see storm total rainfall
amounts in excess of 4 inches is near and east of a Fairfield to
Corsicana to Terrell to Paris line. Areas closer to the I-35
corridor may see rainfall totals in the 2 to 4 inch range with
areas across the Big Country likely seeing rainfall totals in the
1 to 2 inch range.

The heaviest rain will tapper off Wednesday night and into
Thursday. Additional rain is anticipated Thursday through
Saturday, especially north of I-20. Whether or not the ingredients
for another round of heavy rain exist will be determined over the
next several days.


...Meteorological Reasoning...
In terms of the meteorology, this setup seems to have all of the
ingredients coming together for heavy rainfall across parts of the
Ozarks and Southern Great Plains.

The first ingredient will no doubt be the moisture and it`s
already quite humid across the area by late February standards.
The 12 UTC FWD RAOB has already sampled an environment
characterized by a 1.24" PWAT which is approaching the 90th
percentile for PWAT values. Forecast vertically integrated vapor
transport (IVT) suggest that the airmass will become even more
moist as strong southerly flow transports air with source regions
as far away as the western Caribbean northward into our area.

Starting late Monday night into Tuesday, most NWP advertises a
decent 850mb theta-e plume northward out across the Big Country.
This plume of moist and unstable air supports higher resolution
model guidance`s (TTU WRF and NAM NEST) depiction of a convective
complex that travels from West TX into the Big Country after
midnight Tuesday. Widespread severe weather isn`t expected with
this feature, but I wouldn`t be surprised if we saw some gusty
winds. Small hail is possible, but given the very moist regime,
it`s probable that a good deal of melting (since melting is a
function of RH) will occur.

Convection should continue into Tuesday morning as deep
meridional flow continues to pump warm and moist air northward.
Any complex could potentially be fueled by this warm/moist
transport northward. In addition, large scale ascent from the
right entrance region of a 120 knot upper level jet streak will be
in place across the area. With these factors in mind, I think
rain is a sure bet, especially west of I-35 and I`ve bumped rain
chances upward in to the 80-90% category. In addition to pumping
rich warm/moist air northward, the deep meridional flow will
support training of convective cells and when combined with the
anomalously moist atmosphere, this will yield a flood concern.

The risk for heavy rain will increase as a cold front starts to
slide southward through Oklahoma and eventually into Texas late
Tuesday and into Wednesday. The front will help to focus lift for
deeper convection across the area. The area that is of greatest
concern is locations near and east of I-35 and north of I-20 as
outlined by WPC`s Day 2 Moderate Risk for Excessive Rainfall. The
position of the front may ultimately dictate who experiences the
most rainfall. If the front slows down (as advertised by the GFS
and most of its ensemble members), areas west of I-35 may be at
greater risk for heavy rainfall. The ECMWF, NAM and TTU WRF all
push the front a bit swifter through the area. While the consensus
approach to frontal timing may seem appropriate, the deep
southerly flow through the atmosphere may serve to hinder the
front`s progression and as a result, the GFS/GEFS solutions can`t
be completely discounted. With that in mind, I`ve fanned PoPs
outward from the previous forecast to account for a possible
slower frontal progression.

Wednesday into Thursday presents a challenge on two fronts. 1)
The validity of a couple of QPF maxima across Central and East TX
and 2) The potential for a light winter mix across the far
northwest. As it pertains to the QPF max across the Brazos Valley,
there could be some validity to this signal from the coarser model
guidance as the surface frontal boundary stalls and low level
theta-e advection commences atop the cooler surface boundary. In
fact, this feature resembles somewhat of a TROWAL and would be a
very reasonable mechanism for generating another round of very
heavy rainfall. For now, I`ll bump up PoPs across Central and East
TX through much of the day Wednesday and into Thursday.

As it pertains to the potential for winter weather back across
the northwest on Thursday morning---it will need to be monitored
closely over the next several days. There will be a potential for
evaporative cooling below current forecast temperatures as dry air
filters in from the north for locations near and west of an
Eastland to Gainesville line. Freezing drizzle or light freezing
rain will be the main precipitation-type as mid-level drying
should help eliminate any ice crystals necessary for snow or
sleet. The biggest unknown will be the degree of lingering lift as
well as how quickly dry air will rush in from the north. While
currently not in the official forecast, any freezing drizzle or
light freezing rain should have minimal impacts as ground
temperatures should be sufficiently warm due to the recent above
normal temperatures and actual air temperatures will be near or
just below 32 degrees.

Rain should taper off from northwest to southeast through the day
on Thursday. Thereafter, guidance is split (though in better
agreement than previous model cycles) with regards to vigor and
haste of the return flow. Flow atop the cooler surface should
equate to additional rain chances through the remainder of the
week with the best rain chances Thursday afternoon and into
Friday. The most likely area for rain at this time looks to be
north of I-20. Dry conditions are forecast on Sunday and next
Monday.

24-Bain

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    76  65  73  43  47 /  60  80  90  70  80
Waco                76  65  73  50  53 /  40  60  80  80  90
Paris               75  63  73  46  51 /  60  70  90  90  90
Denton              75  63  72  39  44 /  60  80  90  60  60
McKinney            74  64  72  42  46 /  60  80  90  80  80
Dallas              75  65  74  44  47 /  60  80  90  70  90
Terrell             74  63  73  47  51 /  50  70  90  90  90
Corsicana           75  64  74  54  54 /  40  60  80  80  90
Temple              76  64  73  56  56 /  30  60  80  80  90
Mineral Wells       76  62  75  38  42 /  60  80  90  60  60

&&

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

$$

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