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

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FXUS64 KFWD 061705

Area Forecast Discussion
National Weather Service Fort Worth TX
1105 AM CST Tue Dec 6 2016

Main update this morning was to remove mention of dense fog for
all of North and Central TX as well as to cancel the advisory
that was in effect. Added a mention of patchy fog/drizzle during
the overnight hours into Wednesday morning for areas south of
I-20. We also will monitor the potential for some light wintry
precipitation mainly south of I-20 and west of I-35 on Thursday
morning. More details on that will come with the full afternoon
forecast package.

Surface analysis at this hour showed a precipitation-free cold
front slicing southward through much of the southern plains this
morning. The strong mixing in the wake of the front coupled with
some dry air advection limited fog production this morning and
conditions improved drastically. As a result, I went ahead and
cleared all of our counties from the dense fog advisory a little
earlier. Low level clouds will remain this afternoon across a bulk
of the region with good CAA continuing. There may be some partial
clearing along northern zones this afternoon as a deeper batch of
drier air slides southward. While partial clearing may occur, a
bulk of the strongest CAA should occur here where the deepest cold
air is present. I won`t make any adjustments to temperatures right
now as they look to be fairly reasonable. Zonal flow across the
Rockies should help north winds become easterly and perhaps even
southeasterly. With the gradual upglide and weak moisture
advection, we could see another round of patchy fog/low
clouds/drizzle, mainly across southern zones tonight. With this
potential, I`ve included a mention of patchy fog down across this

The strong cold front that we`ve been advertising still appears
to be on track with some challenges still revolving around FROPA
timing. The front will be very shallow in nature, bringing the
coldest air of the year southward into North and Central TX. As
mentioned previously, models typically struggle with the timing of
these shallow and cold air masses. As a result, this introduces the
concern for light wintry precipitation, mainly south of I-20 and
west of I-35 early Thursday morning and we examine this potential
very closely with the afternoon package.




/ISSUED 556 AM CST Tue Dec 6 2016/
For the Metroplex TAF sites:
Visibilities have been slow to fall this morning, but the last
push of cooling seems to have kicked the vis reductions into high
gear. A short period of LIFR or even VLIFR visibilities/cigs will
be possible through about 14Z (perhaps a touch beyond) before a
cold front works its way through the Metroplex. A prefrontal
windshift has already occurred, but wind speeds will increase as
the actual front sweeps through in a few hours. The drier air
behind the cold front will be fighting an uphill battle trying to
dislodge the cool and moist airmass, and present indications are
that it will not succeed today. While cigs should gradually lift
above IFR, MVFR cigs are expected to hold fast into this evening.

While surface winds will stay northerly, winds just off the
surface will quickly veer around to the south tonight,
transporting additional moisture above the shallow frontal
inversion. As a result, opted to introduce MVFR cigs at the
eastern Metroplex sites (DFW, DAL, and GKY) tonight. It`s possible
these MVFR cigs build into the western Metroplex sites, but
confidence in this was not quite high enough to introduce into the
TAFs at this time. Surface winds will return to a southerly
direction late Wednesday morning.

For the Waco TAF site:
A period of fog is also anticipated at Waco before the surface
cold front sweeps through around 15-16Z. With similar forecast
reasoning to the Metroplex sites playing out at Waco, MVFR cigs
should remain entrenched through much of the day before lifting to
VFR for a short period later this evening. Moisture transport
above the frontal inversion will likewise result in another round
of MVFR (potentially even IFR) developing overnight.



The cold front this morning continues to move into far west and
northwest parts of North-Central Texas. Any dense fog along and
north of the I-20/I-30 corridors has dissipated with patchy areas
of light fog and drizzle expected the remainder of the morning.
This was the Dense Fog Advisory portion that was going to expire
at 8 am CST anyways. As for the portion of the Dense Fog Advisory
for areas south of I-20/I-30 corridors, will only continue that
area along and east of a Granbury, Meridian, Killeen line, as
westerly 925mb flow of around 20-25 kts has brought in some drier
sub-boundary layer air into the southwest counties and have
removed any of those counties from the southern advisory that`s in
effect until 11 am CST.

Otherwise, it will be brisk and cool today with periodic areas of
light drizzle, as some buoyancy and low level lapse rates will
continue even well behind the passing cold front. Highs will
struggle to warm much past current temperatures this morning with
the expected low level cold advection and clouds lingering through
much of the day.



.PREV DISCUSSION... /ISSUED 252 AM CST Tue Dec 6 2016/
A challenging forecast expected today, especially this morning as
areas of fog, some dense have formed well out ahead of a cold
front that is now moving into Northwest TX and Southwest OK
currently. A Dense Fog Advisory currently exists until 8 a.m. CST
for all but some of our far western and northeast counties. Per
BUFKIT and high-resolution model guidance, along with trends on
fog infrared imagery of moisture building southwestward ahead of
the cold front, I have included all BUT Mills and Lampasas
counties in the southwest, and Lamar, Delta, and Hopkins county in
the northeast. Tracking the front, I have extended the Dense Fog
Advisory across our Central Texas counties through late morning.

One caveat here with the fog is the strong westerly 925mb winds
via WSR-88D VWPs just to the west and south of the CWA. The strong
westerly 925mb component could likely result in any dense fog
lasting only an hour or two across my far west/southwest counties.
We will have to continuously monitor the fog through the morning
hours and adjust as needed. Once the cold front passes through
much of the area by midday with help from a shortwave transversing
east across the Central Plains, expect some patchy lingering
drizzle through the afternoon hours. BUFKIT RAP/NAM forecast
sounding analysis indicates some buoyancy and low level laspe
rates continuing through much of the afternoon within the stratus
layer. Any drizzle will be light and patchy and not expected to affect
visibility behind the cold front. The cold front will stall across
South-Central and Southeast TX, before lifting slowly north as a
weak warm front into our CWA late tonight into Wednesday. As such,
moisture will not get scoured completely out and the airmass will
not cool down too much. A shortwave will transition eastward
across the state late Wednesday into Wednesday evening and have
continued low rain chances across mostly the eastern half of the
CWA. Moisture isn`t overly rich, so do not expect rainfall to
total more than a tenth of an inch, if even that.

The larger scale Central CONUS upper trough deepens over the
Plains for the latter half of the week, which support our first
arctic cold front and airmass intrusion of the season Thursday and
Friday. As has been noted recently, this will be the coldest
airmass our area has seen in two years. It is highly recommended
you cover outdoor faucets and pipes, as well as make sure you
check on people who may not have adequate heating. Outdoor pets
should also be prepped for the frigid weather expected the latter
half of the week. We will not reach our official hard freeze
criteria of 10 degrees or less for any highlights. That said, with
the warm growing season having lingered longer than expected this
year, it`s a good idea to take necessary precautions now. Highs on
Thursday will be hard pressed to get out of the 30s, with lows
Friday morning ranging from the mid teens northwest to the mid 20s
across parts of Central Texas. Some gradual modification of the
airmass will occur Friday, as the broad surface high and colder
air shift gradually east. However, I still undercut guidance
temperatures by several degrees and may still be too warm on
Friday. Models do have a tendency to scour out dense arctic
air masses too quickly and the better low level warm advection
looks to hold off until late in the day Friday and Friday night.

We will see a rapid modification to the arctic airmass this
weekend, as the main Central U.S. longwave trough lifts northeast
into eastern Canada and heights rise over the Southern Plains with
a strong zonal flow in place. This pattern usually leads to strong
lee surface cyclogenesis to our northwest with a return breezy
southerly winds. After a chilly start, highs Saturday should
rebound into the 50s most places. A shortwave will move east
across the Plains with best moisture flux occurring from E TX into
the Ark-LA-TX and Lower Mississippi Valley moving into the latter
half of the weekend. I see enough instability aloft and low level
warm advection above the shallow frontal inversion to maintain a
low chance of showers and thunderstorms across areas east of I-35,
with a few lingering showers across the east moving into early
next week. Highs will briefly modify into the 60s Sunday, before
another cold front, not of arctic nature, drops high temperatures
slightly to between 55 and 65 degrees.



Dallas-Ft. Worth    53  41  55  29  37 /  10   5   5  20   5
Waco                54  41  59  32  39 /  10   5   5  20  10
Paris               51  39  50  27  36 /  10   5  20  20   5
Denton              51  37  53  26  36 /  10   5   5  10   0
McKinney            51  39  53  28  36 /  10   5  10  20   0
Dallas              52  42  55  30  37 /  10   5   5  20   5
Terrell             54  41  55  30  37 /  10   5  10  20   5
Corsicana           57  44  57  33  38 /  10   5   5  20  10
Temple              56  43  59  33  39 /  10   5  10  20  10
Mineral Wells       49  35  54  27  36 /  10   5   5  10   5




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