National Weather Service Raw Text Product

620 
FXUS64 KFWD 021152
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
552 AM CST Wed Jan 2 2019


.AVIATION...
/12Z TAFs/

Sub-optimal flying conditions are expected through the 
entirety of this TAF cycle, with low MVFR to IFR CIGs continuing.
All North Texas TAF sites are sitting at MVFR, and will likely 
remain in that category until better lift arrives later this 
morning. Rain is expected to continue, with a decrease in CIGs to
IFR by mid morning. 

For KAFW and KFTW, light freezing rain is occurring as 
temperatures hover between 30-31 degrees. This is expected to 
continue through 16Z as temperatures remain below freezing. After 
16Z, above freezing temperatures end the threat for additional 
freezing precipitation.  

IFR has now invaded Waco, and will continue through this evening
as rain continues to fall. The shortwave responsible for this rain
will move to the east this evening, decreasing precipitation
chances and improving CIGs ever so slightly into the low MVFR
category. 

As one round of rain moves out, the next one will approach the
region by the early morning hours on Thursday. This will lead to
additional rain chances, low ceilings, and the potential for a 
period of rain/snow mix. Given this is out of the current TAF 
cycle, this will be further assessed through the day and addressed 
in subsequent forecasts. 

Hernandez

&&


.SHORT TERM... /Issued 434 AM CST Wed Jan 2 2019/
/Today and Tonight/

Summary…
No changes were made to the existing winter weather advisory. Ice
accumulations within the advisory will range from .01” on the 
eastern extent, to around .05” on the western extent. 
Additionally, rain and isolated thunderstorms will continue across
Central Texas through the morning. Freezing rain threat will 
linger through the day across portions of North Texas before the 
rain/freezing rain transitions to snow across the far western
counties late tonight. 

Freezing Rain Threat…

As the upper level system makes it way closer to North and 
Central Texas, isentropic ascent and moisture content has been 
increasing, leading to the development of showers across the 
region. The shallow layer of cold air continues to be confined to 
the lowest 1500 feet of the atmosphere, with about 8000 feet of 
above freezing temperatures atop the cool air. Given the shallow 
depth of the cold air, forecasting the response of the surface 
temperature as precipitation begins to fall becomes especially 
difficult. Add a densely populated area with many bridges and
overpasses, and forecasting the impacts adds yet another layer 
of complexity.

The freezing line is now draped from Bonham to Denton to Fort 
Worth to Lampasas, and with nothing to push this line further 
east/southeast, the expectation is that temperatures will not 
change much between now and 10AM. Having said that, many locations
are flirting around the freezing mark, and a degree can make a 
difference on whether or not the rain will freeze on contact. For 
that reason, we urge you use caution if you are anywhere within 
the Winter Weather Advisory. 

Locations that are at or below 30 degrees while precipitation is 
falling will likely begin to see a glaze of ice covering exposed 
surfaces. Although ice accumulations will remain light, any ice 
can be problematic to untreated surfaces. The highest ice 
accumulations are likely to remain west of a Stephenville to 
Goldthwaite line, where up to .05” of ice are possible given the
temperatures remaining below 32 degrees for a longer duration.

As the bulk of the precipitation moves overhead, the rain will
help to increase temperatures slightly above freezing by this
afternoon for most locations, leading to a cold rain for most.
After sunset, temperatures will once again fall below freezing
across the Winter Weather Advisory, leading to yet another period
of light freezing rain for areas west of a Gainesville to Comanche
line. 

Heavy Rain Threat…
The appreciable isotropic ascent overspreading the region will 
lead to periods of moderate to heavy rain across Central Texas and
the Brazos Valley. Given the rain is expected to fall over the 
course of the next 18 hours, the decision was made to not issue a 
Flood Watch. If any water issues arise, they would be limited to 
poor drainage, low lying areas, and this can be handled with 
shorter fused flood products.

The batch of precipitation will move to the east as the shortwave
continues on its eastward movement. Then we turn our attention to
the next source of lift associated with the cut-off low barreling
in from the west. As temperatures aloft begin to cool, a 
transition from freezing rain to a freezing rain/snow mix will 
begin early Thursday morning west of a Graham to Comanche line. 
Accumulations prior to 6 AM are unlikely as snowfall rates will 
remain low. 

Hernandez

&&

.LONG TERM... /Issued 434 AM CST Wed Jan 2 2019/
/Thursday Onward/

Confidence continues to increase regarding the potential for some
accumulating snow in parts of North Texas on Thursday. Otherwise,
some persistent rainfall may lead to nuisance flooding or rises on
rivers and streams across our southeastern zones by late 
Thursday.

By daybreak Thursday, the deep upper low should be located across
West Texas, centered near Lubbock. Very strong ascent ahead of 
this feature will see a continued increase in precipitation 
through the morning hours while temperatures hover just below 
freezing across our western zones, and in the mid to upper 30s 
elsewhere. Periods of freezing rain are expected to impact the 
morning commute within the Winter Weather Advisory area 
throughout this time period. Lapse rates will steepen sharply 
through the mid-levels via adiabatic cooling from dynamic ascent, 
thus a more convective presentation of precipitation is likely 
with this Thursday morning wave. In fact, some lightning strikes 
certainly cannot be ruled out, but have not included a mention in 
the forecast just yet. Some ice accumulations of around 0.1" will
be possible. 

The upper low is forecast to dig east/southeast into North Texas
throughout the day on Thursday while intensifying slightly in the
process. The projected trajectory of the low has become 
increasingly favorable for a transition to a rain/snow mix or to 
entirely snow across portions of North Texas, possibly as early as
mid-morning. As strong ascent continues to overspread the area,
the resultant mid-level cooling will act to eliminate the above-
freezing warm nose, and the column should cool entirely below
freezing across our northern and western zones before midday. As a
result, the transition from rain or freezing rain to snow should 
occur throughout this time period. On Thursday afternoon, this 
transition zone should continue to move eastward into the I-35 
corridor, and east of I-35 by Thursday evening. Meanwhile, much 
of Central Texas will begin to become dry-slotted by late morning 
or early afternoon, so warmer temperatures (40s) and mostly dry 
conditions will likely prevail across our southern and 
southwestern zones by mid afternoon. 

This scenario essentially is a textbook snow setup with the only 
caveat being the slightly warm surface temperatures (largely in 
the mid 30s). A well-defined TROWAL wrapping around the western 
side of the mid-level low circulation will supply decent moisture 
content to the subfreezing portion of the storm system. Saturation
should be present to at least -10 to -15 deg C, more than 
sufficient for ice within the dendritic growth zone. In addition,
very strong isentropic ascent and steep lapse rates of as much as
7-8 C/km will contribute to fairly widespread precipitation in 
the low's northwestern quadrant. Also, some weakly negative EPV 
values will exist within an area of strong mid-level frontogenesis
across our far northwestern zones, which may allow for more of a 
banded heavy precipitation structure, with possibly even some 
embedded convective elements. 

Essentially all four of the GFS, NAM, ECMWF, and CMC agree with 
this general setup for Thursday, but remain fairly different 
regarding QPF/precip accumulation fields. While the ECMWF is the 
driest solution, an examination of isentropic charts leads one to 
quickly dismiss its QPF forecast, as it lingers very strong 
ascent, steep lapse rates, and modest but sufficient moisture 
across North Texas throughout the day. The ECMWF QPF forecast 
seems much too low given these considerations. The only real 
difference between this an and the NAM/GFS wetter solutions is 
the presence of greater mid-level moisture. As a result, have 
leaned heavily on the NAM for general trends and accumulations
given what has been a successful performance with handling this
system thus far. It's worth noting that the 06z NAM is even more 
aggressive than the 00z run in terms of amounts and accumulations 
due to its even farther southward projection of the upper low. 
Have continued to make slow refinements to the forecast, but some 
larger adjustments may be necessary in subsequent updates. 

So, while some snowfall is expected in parts of North Texas, the 
potential for significant accumulations remains uncertain. It is 
expected that surface temperatures will warm at least into the 
mid or even upper 30s during the day while snow is falling, which 
would generally act to limit accumulations except on elevated or 
grassy surfaces. As discussed previously, if a band of heavier 
snow or some convective snowfall was to materialize, a burst of 
heavy snow could lead to accumulations rather quickly with >1" per
hour rates. At this time, at least some minor accumulations are 
expected to occur in an area north of I-20 and west of I-35 with 
some totals as high as 1-3 inches possible. The Winter Weather 
Advisory will be maintained in its current configuration to 
account for this potential. If models continue to trend colder 
with temperatures or deeper with the upper low, these amounts may 
need to be increased across our northwest. Areas north of a 
Comanche to Denton to Sherman line remain under the greatest risk 
of accumulating snow through Thursday.  

In addition to the areas discussed above, it's likely that some 
snow will be observed farther south and east as the main core of 
the low tracks overhead later Thursday afternoon through Thursday 
night. At least a rain snow mix is possible in the immediate DFW 
area, but temperatures are still progged to be in the mid to upper
30s while any snow would fall, limiting the potential for 
accumulations and impacts. It's even possible that some snow 
flurries may mix with lingering sprinkles of rain as far south as
Waco before precipitation comes to an end there on Thursday 
afternoon, although no mention has been included in the forecast
there for now. A rain snow mix will likely continue across our 
northeastern zones through Thursday night, where above-freezing 
surface temperatures should continue to limit accumulation 
potential. The upper low will finally depart the region by 
daybreak Friday, bringing precipitation chances to an end in the 
morning. 

In the wake of our winter weather extravaganza, much warmer and
mild weather conditions will return on Friday as skies clear from
southwest to northeast. An anomalously strong upper ridge will
build across the Southern Plains over the weekend, leading to the
return of above-normal temperatures. A fast-moving upper
disturbance will then act to flatten the ridge late in the weekend
and early next week which will pull moisture back northward, 
resulting in increasing clouds and some rain showers late Sunday 
into Monday. 

-Stalley

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    37  34  38  34  55 /  90  60  90  50   5 
Waco                38  35  43  34  58 /  90  70  90  10   5 
Paris               38  35  40  35  53 /  90  80  90  70  10 
Denton              35  32  37  32  55 /  80  60  90  50   5 
McKinney            37  33  38  33  54 /  90  70  90  50   5 
Dallas              38  35  39  34  55 /  90  70  90  50   5 
Terrell             40  36  41  35  55 /  90  80  90  40   5 
Corsicana           40  37  42  34  54 /  90  90  90  20   5 
Temple              38  34  45  34  58 /  90  80  90   5   5 
Mineral Wells       34  31  38  30  55 /  70  70  90  30   5 

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon CST today for TXZ118-133.

Winter Weather Advisory until 6 AM CST Friday for TXZ091>093-
100>103-115>117.

Winter Weather Advisory until noon CST Thursday for TXZ129>132-
141>143.

&&

$$

08/26