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

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138
FXUS64 KFWD 101211
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
611 AM CST Tue Dec 10 2019

.UPDATE...

Quick update concerning snow potential through the next few hours.
The 12Z FWD sounding sampled +5.8C at 900 mb which is considerably
warmer than previous runs of the HRRR/RAP guidance for this time.
The latest RAP guidance appears to have caught on to the depth of
the warm nose and continues a warmer trend over the next few hours
despite strong dynamic forcing for ascent. While there will likely
be some cooling of this layer, it is appearing less likely that
we`ll see all snow. There is some dry air beneath 850 mb and this
will support some wet-bulb cooling and will result in thermal
profiles conducive for sleet mixing with rain mainly from
Comanche to Fort Worth to Paris through mid morning. In the
meantime, we`ll lower snow probabilities and slightly nudge up
sleet probabilities through mid morning and continue to monitor
any further changes over the next few hours.

Dunn

&&

.AVIATION...
/12Z TAFS/

Despite light to moderate rain for much of the night, VFR cigs
generally prevail from the Metroplex northward. Cigs deteriorate
rapidly to the south where low level moisture is better. IFR
conditions will continue south and east of the Metroplex through
mid morning before improving. We`ll continue to advertise a few
more hours of rainfall at the Metroplex airports, before drier air
filters in from the north and conditions improve. We`ve removed
the mention of RASN for now, although there could be a few sleet
pellets mixing in with rain through mid morning. This shouldn`t be
a significant issue with no accumulation expected.

At Waco, the MVFR/IFR conditions will persist through much of the
morning with conditions improving early this afternoon as
precipitation comes to an end. VFR is expected overnight into
Wednesday.

Dunn

&&

.SHORT TERM... /Issued 336 AM CST Tue Dec 10 2019/
/Today and Tonight/

Water vapor imagery this morning shows a large upper trough
digging through the Central U.S. with a compact trailing shortwave
centered over the northern Mexican state of Chihuahua, southwest
of El Paso. To the east of this shortwave is a large area of
enhanced mid and upper level clouds streaming northeast,
indicative of strong forcing for ascent. Across North Texas, a
cold airmass has settled into the region with temperatures in the
mid and upper 30s with brisk north winds. A large area of light
to moderate rain extends from the Big Bend northeast across the
region and into the Arklatex.

The main concern over the last 24 hours has been the potential for
rain to transition to a wintry mix across parts of North Texas,
and there is still some concern for this to happen through the
morning hours, although low level temperatures have trended a
touch warmer over the last several hours. Dual-pol analysis from
the Dyess AFB radar shows a fairly extensive bright band signal in
reflectivity from western Tarrant County southwest through Erath
and into Comanche County. Correlation coefficient data show a well
defined area of mixed phased hydrometeors between 3000-8000 ft
AGL. This is essentially snow that is falling and melting in a
warm layer above the surface, becoming a mix of snow and rain.
Further melting results in a transition to all rain around 3000
ft AGL and a corresponding increase in correlation coefficient.
This data helps support RAP and HRRR model analysis of the low
level thermal profile, which both indicate a warm nose between
925-850 mb. This warm nose is about a degree warmer in the latest
few runs as opposed to RAP/HRRR runs from late yesterday evening.

So is it going to snow? As is usually the case around here, it`s
difficult to say for sure, but it is likely we`ll see at least
some wintry mix through the morning hours across parts of the
region. The bright banding on Dyess radar has expanded in coverage
over the last hour and lowered some in height, which indicates
that the mix of rain/snow is making it a little closer to the
ground. In addition, the near surface layer (just above the ground
to around 2000 ft AGL) is getting colder and is below freezing
from Eastland to Mineral Wells to Sherman. This would certainly
support some re-freezing of melted snow into sleet pellets through
the morning. We`re fighting an intrusion of dry air from the north
as indicated by an abrupt ending of the precipitation northwest of
Decatur and cloud ceiling heights still in the 6000-7000 ft AGL
range. Despite the rain we`ve seen, we still haven`t fully
saturated the lowest levels of the atmosphere. It appears that
given the warmer low level thermal profile, we`ll mainly see rain
through the morning hours.

The one caveat that still remains will be the very strong forcing
for ascent from the shortwave to the west. This broad lift will
peak across North Texas over the next 6 hours with moisture still
in place. Strong dynamic ascent can oftentimes cool the thermal
profile very quickly and by a greater amount than indicated by the
model guidance. If this does indeed occur, then bursts of snow
mixed with rain or a quick transition to snow could occur right
before the precipitation ends later this morning. Unfortunately,
this is often only observed in real time or forecast with very
little lead time based on radar/satellite trends. Given that the
model guidance appears to be sampling the shortwave well and the
thermal profile well, confidence in this happening is generally
low.

All that being said, rain will continue through the morning hours
for most of the region with a steady southward push into the
afternoon. We will continue to advertise a chance of snow/sleet
from Comanche to Fort Worth to Paris and areas north through the
morning, with minimal accumulations and impacts expected.
Precipitation will come to an end across our far southern counties
by early evening with skies clearing from north to south this
afternoon into the evening. It will be a cold night across the
region with lows below freezing across the entire area.

Dunn

&&

.LONG TERM... /Issued 336 AM CST Tue Dec 10 2019/
/Wednesday and Beyond/

Return flow will begin on Wednesday as the axis of a surface
ridge shifts to our east and a rapidly-approaching shortwave
trough strengthens a surface low to our northwest. The shortwave
will swing relatively uneventfully across the Southern Plains, as
it will not be able to tap into any significant amount of Gulf
moisture until it gets east of the Mississippi Valley. A weak
front will accompany the shortwave, but will likely stall before
reaching North Texas. This should allow for a gradual warming
trend during he second half of the work week.

A pair of disturbances will then rotate through the Midwest over
the weekend, giving the front enough of a push to make it through
the region. The current plan is to follow the faster side of
guidance with the front at this time, which brings the boundary
through the area on Saturday. Little to no precipitation is
expected with the initial passage of the front due to the absence
of moisture and lack of any appreciable lift.

The next upper level trough will drop southeast from the Pacific
Northwest to the Four-Corners region on Sunday, then work its way
into the Southern Plains on Monday. Isentropic lift will increase
as the system approaches, which will begin our next decent
chances for our rain late in the day Sunday. The surface boundary
will retreat northward as a warm front Sunday-Sunday night, and
may reach the Central Texas counties before pushing south again.
The Sunday-Monday temperature forecast hence contains a fairly
large spread from north to south. This forecast also includes
isolated thunder, as elevated instability looks sufficient for the
occasional lightning strike. Pops will be mainly along and east of
Interstate 35, with the western third of the region likely
remaining dry.

The front will surge southward on Monday as the upper trough moves
east of the region, ending rain chances from west to east Monday
night. After Monday, a cooler than normal and dry period looks
likely through most of next week.

30

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    47  31  53  34  54 /  80   0   0   0   0
Waco                45  29  55  31  56 / 100   0   5   0   0
Paris               44  28  51  31  51 /  60   0   0   0   0
Denton              46  28  52  32  54 /  40   0   0   0   0
McKinney            46  29  52  32  53 /  70   0   0   0   0
Dallas              47  30  53  34  54 /  90   0   0   0   0
Terrell             44  29  53  32  54 / 100   0   5   0   0
Corsicana           45  30  54  33  54 / 100   5   5   0   0
Temple              44  30  54  32  56 / 100   0   5   0   0
Mineral Wells       46  25  53  31  54 /  50   0   0   0   0

&&

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

$$

91



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