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

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FXUS64 KFWD 162337

Area Forecast Discussion
National Weather Service Fort Worth TX
537 PM CST Sat Dec 16 2017

Tricky forecast for the next 24 to 30 hours for the airports as
rain, ceilings, and visibilities all changing. The short version
of the forecast is this: Rain will continue this evening before
shutting off around Midnight. Once the rain moves northeast,
ceilings will begin to lower from VFR to MVFR and IFR in the
matter of a few hours. Visibilities may be reduced to around 2
miles due to patchy fog as well, although the confidence in this
is lower than the rainfall and ceilings. By Sunday morning,
conditions will improve rapidly during the late morning and early
afternoon hours, with sunshine returning.

For a deeper look, there is a rain shield and extensive cloud
cover across much of the region at 23Z. High resolution models all
consistent in that large scale and isentropic lift shuts off
drastically around 9 PM, at which time the cool temperatures and
high humidity will allow for the ceilings, currently around
040-060 to begin to drop to around 010. Dry air advecting near the
surface may allow patchy fog to develop and drop visibilities to
around 2 miles. It is possible that visibilities may drop a bit
more than this, for this forecast, the confidence is not nearly
high enough to go lower than 2 miles.

By Sunday morning, we will start out with cloudy skies and low
ceilings. Between 9 and 10 AM, these clouds should begin to
scatter, allowing for a few hours of MVFR ceilings before VFR
conditions return after 18Z.



.SHORT TERM... /Issued 341 PM CST Sat Dec 16 2017/

Extensive cloud cover has shrouded most of North and Central
Texas now, and rain has begun to fall across portions of Central
Texas. The continued influx of moisture across the region,
combined with appreciable lift from the main upper level low
across West Texas will allow for an expanding area of rain for
most of the region this evening into tonight.

No severe weather is expected overnight, but can`t rule out a
rumble of thunder with any thunderstorm that manages to take
advantage of the meager elevated instability south of I-20 and
east of I-35. Rain amounts should remain below half an inch across
most of the region, with the exception of areas southeast of an
Athens to Groesbeck line where strong frontogenetic forcing may
lead to a beneficial rainfall between 1-1.5 inches.

A strong push of dry mid to upper level air will move in behind
the system as it continues its northeasterly trek overnight,
leading to an end in precipitation from west to east early Sunday
morning. Although the mid and upper levels will begin to dry out,
a decent amount of moisture will remain at the low levels. This
moisture will need to be monitored overnight given it could lead
to either a low stratus deck, or patchy fog. Current thinking is
that winds between the surface and 925mb should stay strong
enough the allow enough mixing and keep fog from developing. This,
however, will need to be monitored this evening in case new model
guidance output depicts weaker low-level wind speeds.



.LONG TERM... /Issued 341 PM CST Sat Dec 16 2017/
/Sunday Through Friday Night/

The weekend shortwave disturbance will be quickly exiting the
northeast, away from the area with shortwave ridging in it`s wake
on Sunday. Modest southerly winds near 10 mph will return at the
surface, while mid levels dry out in response to subsidence
occurring in this system`s wake. However, a shallow layer of
stratus, along with a lingering cirrus canopy will remain through
much of the day. Had worried about patchy dense fog developing
Sunday morning with wet soils and light winds, but it appears the
sub-boundary moisture remaining will be relatively deep with winds
speeds in the boundary layer high enough to keep hydro-lapse
rates steady with no fog mentioned in the forecast. Despite modest
low level warm advection later on Sunday, temperatures will only
warm into the 60s, as insolation is minimized with any lingering
cloud cover and soils remain moist from the first round of rain
we received Saturday night.

In a fujiwara fashion, a second shortwave will arrive Monday into
Tuesday and induce low level warm advection and isentropic ascent
once again across particularly our southwest counties by Monday
morning. As this system approaches later Monday and Monday night,
rainfall will spread east-northeastward across mainly our Central
Texas counties and those south of I-30 in the east. Moistening
mid levels and slightly cooler 500mb temperatures will steepen
lapse rates to between 6-7 deg C/km later Monday afternoon into
Monday night. Skinny MUCAPE 500 J/kg or greater is forecast above
an elevated subsidence inversion anchored around 800mb. This
elevated instability may be enough to allow charge distribution to
occur aloft with isolated elevated tstorms expected with the
increasing band of showers or rain activity. I did raise POPs
across the southeast half of the region where low level warm
advection and isentropic ascent will be maximized with this second
shortwave. No severe weather is expected, but don`t be surprised
to hear a random crackle of thunder, especially as we move into
Monday night and Tuesday. These first two shortwave disturbances
will likely give some drought relief. The rainfall will be more of
an off/on, gradual soaking rainfall, especially across Central
Texas where 1-3 inch totals are possible. This second shortwave
disturbance will lift out after midnight Wednesday morning.
Otherwise, the light winds, cloud cover, periods of rain/isolated
tstorms will keep diurnal temperature trends small with mild
conditions for late December expected. Highs in the 60s and lows
in the mid 40s-mid 50s will be the rule across the area.

Medium range models continue to show good consistency with a cold
front arriving Thursday night and early Friday morning. Ahead of
the cold front on Thursday, partial sunshine combined with
veering, warm, low level flow will likely assist highs back into
the lower 70s, except possibly the immediate Red River area.
Scattered showers may occur within the better moisture across
Central Texas and areas mainly east of I-35 late Thursday night
and Friday with the well-advertised, shallow but strong arctic
cold front moving through. With the arctic cold front well east of
the better upper support and with the shallow nature of a very
dense and chilly airmass, we do not expect much in the way of any
tstorms at all. Now for the Friday period and into the holiday
weekend, which everyone is curious about. Model run-to-run
consistency both amongst the deterministic ECMWF/GFS/and Canadian
models continues to remain very high and differing quite a bit
from their ensemble means. Will we have very shallow cold air in
place?? The resounding answer is YES! However, these models and
their ensembles are struggling with position, strength, and track
(to an extent) of the Western U.S. upper trough. These parameters
can make all the difference in the world on whether we get all
rain, a wintry mix, ice, or nothing at all, especially areas west
of I-35/35W and north of Highway 380 to the Red River where colder
surface temperatures are expected to arise.

After all that verbiage, the conclusion is...we just DO NOT have
enough agreement and consistency with ANY of the medium range
models to say which scenario pans out with a very low confidence
on evolution. My gut feeling is somewhere winter impacts may occur
Friday into next weekend. However, I just do not have the
confidence to say to what extent north-south or east-west across
the area at this time with so much variability with the deep, mid
level longwave trough. As such, persistence was carried through
on the forecast with this period now entering into the end of our
7-day extended forecast. Be sure to stay informed from trusted
weather sources through the week and beware of what you read on
social media forums. We should see models coming more in line on
this system as we push through this week and the system becomes
better sampled by our upper-air network.



Dallas-Ft. Worth    45  64  47  65  52 /  80   0   5  20  30
Waco                43  65  49  64  55 /  90   0  10  30  50
Paris               44  59  44  64  52 /  90   5   5  10  30
Denton              43  63  43  66  50 /  80   0   5  10  20
McKinney            43  61  44  65  51 /  90   0   5  10  30
Dallas              45  63  47  65  53 /  90   0   5  20  30
Terrell             45  63  45  66  52 /  90   5   5  20  30
Corsicana           45  63  47  65  54 / 100   0  10  30  50
Temple              43  65  49  63  55 /  90   0  10  30  50
Mineral Wells       43  67  42  66  49 /  60   0   5  20  20




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