Drought in Texas, December 1 to 19, 2017

Area Forecast Discussion
National Weather Service Fort Worth TX
1126 PM CST Thu Nov 30 2017

.AVIATION...
Pacific moisture associated with an upper low off the coast of
Baja will result in scattered to broken high clouds across the
entire region through Friday evening. Prevailing VFR conditions
are expected at all TAF sites. However, Waco may see some
temporary visibility restrictions between 3 and 5 miles around
sunrise due to light winds and a minimal temp/dewpoint spread.

A light north to northeast overnight will become light easterly
around sunrise and south/southeast by mid morning Friday once
surface high pressure moves east and a weak lee trough develops
across the Central High Plains. Wind speeds will increase during
the day Friday but speeds should remain below 12 knots.

79

&&

.SHORT TERM... /Issued 344 PM CST Thu Nov 30 2017/
/Tonight and Friday/

Winds will gradually slacken and become light and variable overnight
as high pressure crests overhead. Increasing southerly 925-850 mb
winds will transport a slug of moisture and associated low cloud
cover northward late tonight, but this looks to stay mostly west
of our forecast area across the Concho Valley. While periodic
bouts of high cloud cover will stream overhead, these shouldn't be
prolonged or dense enough to substantially mute the radiational
cooling potential. As a result, we'll probably see some lows fall
into the upper 30s in the most sheltered and low-lying areas, and
into the lower to mid 40s elsewhere. In addition, forecast
soundings show that there is some potential for shallow and
splotchy fog development where the greatest cooling occurs across
our Central Texas counties. Given the anticipated localized nature
of any visibility reductions, opted to forgo any inclusion in this
forecast package and let the evening shift assess trends.

Southerly surface flow will gradually re-establish itself across
the region on Friday, with continued high cloud cover muddying up
the skies a bit. Temperatures, however, will still manage to
climb into the upper 60s near the Red River and lower to mid 70s
elsewhere for the first day of meteorological winter.

Carlaw

&&

.LONG TERM... /Issued 344 PM CST Thu Nov 30 2017/
/Friday Night through next week/

As a compact upper level low off of the southern California
coastline translates eastward across Baja and the Sonora desert,
low level winds will become southerly late Friday and low
temperatures will moderate and only fall into the 50s. Pleasant
but unseasonably warm weather will prevail on Saturday with highs
reaching the lower 70s.

While a surface low and trough will develop along the southern
Texas coastline during the day Saturday and help to keep the rich
surface moisture to our south, the increasing southerly flow at
850 mb will transport a plume of moisture and low clouds northward
into the CWA by Saturday night. As the aforementioned upper level
low moves across Central Texas Sunday, the increased lift and weak
amounts of elevated convective instability should result in
scattered showers across the southeastern half of the CWA.
Temperatures will continue to remain unseasonably warm with highs
in the 70s despite mostly cloudy skies. Lows will only fall to
near 60 degrees Sunday night. In the wake of the upper level
system, lift will be absent, but the low level moisture will
remain so will keep a slight chance of a few showers over the
eastern zones into Monday.

The extend part of the forecast continues to advertise a pattern
change that will end the warm spell and bring us a prolonged
period of near normal temperatures. However, the various model
guidance continues to display an unusually large amount of
variance from run to run and each other. The differences are a
result of how the models handle the PV anomaly that drops into the
western part of the trough Tuesday. Some solutions keep a sharp
positive tilt upper trough that moves from northwest to southeast
across the country while others break off the energy into a closed
low and either move it out into the Pacific and leave it there or
eventually bring it eastward. In any case, the evolution of the
upper trough is critical to both just how much cold air makes it
into the region and our rain chances (especially behind the
front).

The reason why the evolution of the trough is so critical to the
temperature forecast in this pattern, is because the center of the
trough will be positioned to our east, meaning we and the Plains
are on the backside where subsidence and warming off of the Rocky
Mountains will be tremendous. Shift that trough a few hundred
miles farther west, and the subsidence warming will not impact the
source region of our air and it will be colder. On today's set of
model runs, it is the GFS that is the coldest, while the ECMWF
and Canadian are much warmer. These models have all been trading
places the last few days.

So what we DO know is that the front looks like it will arrive
Tuesday morning with temperatures dropping into the 50s during the
day behind it. There will be a good chance of rain along and just
behind the front as moisture and instability look sufficient for
some scattered convection to develop. There could even be an
isolated thunderstorm too. Lows will likely fall into the 30s or
40s by Tuesday night, but this will depend on whether we clear out
or not.

The GFS, with more troughing farther west than all of the other
models today, does show a second wave of isentropic lift behind
the front Tuesday night and Wednesday which would result in some
precipitation well behind front. Because this solution is
plausible will show some low chances of rain, but since this lift
occurs at a fairly high level of the atmosphere where moisture
quantity is limited, it would keep QPF very low and less than
0.10". The GFS also does indicate rain changing to snow where
precipitation is heavy enough because the airmass it shows is very
cold aloft. It is the warm air in the lowest 5000 feet of the
atmosphere that is the missing ingredient to get worried about a
snow event. That's not to say that snow flakes would not be
possible, but we would have to rely upon the cooling effects of
latent heat absorption from melting/evaporating precipitation to
get surface temperatures cold enough for snow to reach the ground.
In other words, the chance of seeing snow is a long shot that
relies on precipitation intensity since the low levels just don't
look cold enough in a traditional sense. Furthermore the chance
of this being a snow event with impacts to society are just barely
above zero, so there is really no reason to get worried about it.

Chances of rain will certainly end by Wednesday night as all model
solutions by this time show the lift ending and/or dry air in
place. Temperatures should be cool, but essentially near normal
for the time of year. Again model forecasts are showing a lot of
spread but will split the difference with lows in the 30 and highs
in the 50s to end next week.

TR.92

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    46  70  52  73  55 /   0   0   0   0   5
Waco                43  73  50  74  55 /   0   0   0   0  10
Paris               43  66  47  69  50 /   0   5   0   5   5
Denton              39  69  50  73  52 /   0   5   0   0   5
McKinney            42  69  50  71  52 /   0   5   0   0   5
Dallas              48  71  52  73  55 /   0   0   0   0   5
Terrell             43  71  49  73  52 /   0   0   0   5  10
Corsicana           46  72  50  73  53 /   0   0   0   5  10
Temple              44  74  49  75  55 /   0   0   0   5  10
Mineral Wells       41  72  49  74  52 /   0   5   0   0   5

&&

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

79/24

Hazardous Weather Outlook
National Weather Service Fort Worth TX
345 PM CST Thu Nov 30 2017

TXZ091>095-100>107-115>123-129>135-141>148-156>162-174-175-011415-
Montague-Cooke-Grayson-Fannin-Lamar-Young-Jack-Wise-Denton-Collin-
Hunt-Delta-Hopkins-Stephens-Palo Pinto-Parker-Tarrant-Dallas-
Rockwall-Kaufman-Van Zandt-Rains-Eastland-Erath-Hood-Somervell-
Johnson-Ellis-Henderson-Comanche-Mills-Hamilton-Bosque-Hill-Navarro-
Freestone-Anderson-Lampasas-Coryell-Bell-McLennan-Falls-Limestone-
Leon-Milam-Robertson-
345 PM CST Thu Nov 30 2017

This Hazardous Weather Outlook is for North and Central Texas.

.DAY ONE...Tonight.
No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.
Isolated thunderstorms are possible Monday through Tuesday. Severe
weather is not expected.

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected at this time.