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Area forecast discussion 
National Weather Service Fort Worth Texas
404 am CST sun Dec 28 2014

Discussion...

An upper level trough to our west continues moving east and will
provide lift for additional rain across our south and southeastern
counties today. The radar across central and southeast Texas has
looked active all night but a level of dry air in the lower levels
of the atmosphere has prevented much precipitation from reaching
the ground so far. Even though the radar has given the appearance
that it has been raining almost all night south of a Gatesville to
Athens line...none of the automated weather stations have
reported any rain until an hour ago. Hearne /klhb/ finally reported
rain on their 3 am hourly ob indicating that the lower levels of
the atmosphere are finally starting to saturate down. Expect other
sites in that area to begin reporting light rain before sunrise.
Light to moderate rain will occur southeast of a Lampasas to
Midlothian to Sulphur Springs line today with the best rain
chances in the morning hours. Rainfall amounts will be light and
the rain is expected to end this afternoon. Forecast soundings
indicate that the precipitation will remain liquid with no frozen
precipitation mixing...even as the upper level low approaches
later today. However...will be monitoring for any changes to the
low level temperature profile for possibly some sleet mixing in.

Cloudy skies this morning will clear from west to east this
afternoon as the upper level trough moves over the region. This
will allow for our western counties to warm into the upper 40s
this afternoon but our eastern counties can expect a cool day with
afternoon temperatures in the lower 40s due to the persistent cloud
cover. With the skies clearing tonight and winds becoming
light...temperatures are expected to fall into the upper 20s and
lower 30s across the region. The return of some high cloud cover
on Monday will keep temperatures in the 50s and lower 60s.

Overall...the forecast for the Tuesday through Friday time frame
has not changed much but did make some adjustments to the winter
weather precipitation during this time period. The Arctic front is
still expected to arrive Monday night/Tuesday morning. It will
steadily spread across the region with winds of 10-15 miles per hour behind
it. The air mass will be modified a little during the day on
Tuesday due to south and east 850 mb winds above the front and
west winds above 850 mb. However...Tuesday through Wednesday night
will still be a rather cold period with highs only in the 30s and
40s and lows in the 20s and 30s.

We continue to maintain a dry forecast Monday night through
Tuesday night as there is a lack of lift across the region. We
will be in near zonal flow aloft as a strong upper level low moves
down the West Coast but it will still be too far west at that time
to spread any large scale lift over our area. Isentropic charts
also indicate a lack of lift in our area during this period. We
start to notice some weak isentropic lift between the 800-700 mb
layer beginning Wednesday in our southern zones. Analyzing this
layer in the forecasted soundings...we see moisture in the layer
where the lift is occurring...but the layer below 800 mb...and in
particular below 900 mb...is dry and would likely evaporate any
precipitation before it reaches the ground. So the challenge now
is will the lower layer saturate enough from top-down processes to
allow any precipitation to reach the ground? This appears to be a
low chance as the lift does not look strong enough to generate
enough precipitation to saturate the layer all the way down to the
surface. The radar may show a lot of echoes on Wednesday...but do
not anticipate precipitation will reach the ground. However...will
leave 20 probability of precipitation for drizzle southwest of a Cisco to Hillsboro to
Freestone line in case some light rain does reach the ground.
Depending on the time and temperatures...there could be a brief
period of freezing drizzle west of I-35.

Our main time frame of concern for frozen precipitation is
Wednesday night/early Thursday morning as the isentropic lift
continues but also spreads north over more of the region and
strengthens. The models are not outputting any quantitative precipitation forecast during this 12
hour period from 6 PM Wednesday to 6 am Thursday because they
continue to keep the lower levels of the atmosphere dry /as
indicated on the forecasted soundings/. The lower levels of the
atmosphere will eventually saturate down as the isentropic lift
continues but the question and challenge is when will that occur.
The deterministic models suggest this will not happen until after
6 am Thursday when temperatures are already above freezing
/temperatures are expected to eventually rise Wednesday night under
the influence of warm air advection due to the isentropic
lift/...but it is possible this could occur during the night
Wednesday. So will keep a low chance for light rain and/or light
freezing rain along and south of a line from Breckenridge to dfw
to Emory. The freezing rain will be mentioned along and west of a
line from Weatherford to Lampasas. If freezing rain occurs during
the time from 6 PM Wednesday to 6 am Thursday...we do not expect
significant impacts as temperatures will be warming overnight and
the changeover to liquid rain should melt any minimal ice
accumulations. However...if temperatures that night are colder
than they are currently forecast to be and are realized in the
upper 20s...there could be some minor accumulations and possibly
minor impacts for a short period of time during the night. If
temperatures were to fall into the upper 20s...the most likely
location for this to happen would be west on I-35.

Rain chances are still looking decent across the region on
Thursday and Friday as the upper level low along the West Coast
finally moves into the Southern Plains. Again...this is expected
to remain as liquid rain. There are some differences within the
models regarding the best timing for rain chances on these days
and we will hopefully be able to refine them over the next few
days. One interesting feature to watch will be the possibility of
a shortwave ahead of the upper low on Thursday. The GFS develops
this shortwave giving US better rain chances on Thursday as a
result. The European model (ecmwf) does not have this shortwave and therefore has
better rain chances on Friday as the upper level low moves over
the area. Some thunder will be possible Thursday night and
Friday. Another front is expected behind this system on Saturday
and it is starting to look like the air behind this front could be
pretty cold as well.

82/jld

&&

Aviation...
/issued 1204 am CST sun Dec 28 2014/
VFR will prevail over the metroplex through the period with just
middle-high level cloud cover. Strong upper level shortwave will move
through the region midday Sunday. Ahead of this system...lots of
moisture/lift will result in precipitation originating at an
unusually high level...near 22000ft. This precipitation will be falling
into dry air below it...so mostly virga is expected in the
metroplex. However some saturation will occur which will bring
ceilings down to near 10000ft by sunrise. If precipitation intensity is
greater than expected over the metroplex...wet bulb cooling would
result in thermal profile favorable for sleet or snow. But there
is currently no model showing this much precipitation
available...which would be required to both cool and saturate
before any non-liquid precipitation reaches the ground. We will continue
to watch trends to monitor for this very remote possibility.

For Waco...MVFR ceilings may continue for the next hour or two but VFR
ceilings near 050 should prevail. More low level saturation is
expected there...so -ra is in the taf from 10z to 16z. The thermal
profile looks too warm for any possibility of wintry precipitation at
Waco even after wet bulb cooling. Any rain will also be very
light...and thus no visibility restrictions are forecast.

Otherwise clouds will clear early-middle afternoon with light north
winds turning to the south Sunday evening.

Tr.92



&&

Preliminary point temps/pops...
Dallas-ft. Worth, Texas 46 32 56 34 43 / 10 0 5 5 5
Waco, Texas 44 27 59 33 46 / 20 5 5 5 5
Paris, Texas 43 30 51 33 44 / 10 0 5 5 5
Denton, Texas 46 27 56 28 42 / 5 0 5 5 5
McKinney, Texas 45 26 54 28 43 / 10 0 5 5 5
Dallas, Texas 47 32 56 35 44 / 10 0 5 5 5
Terrell, Texas 43 29 55 34 45 / 20 0 5 5 5
Corsicana, Texas 43 31 56 35 46 / 40 5 5 5 5
Temple, Texas 43 27 61 34 46 / 30 5 5 5 5
Mineral Wells, Texas 47 29 57 28 40 / 5 0 5 5 5

&&

Forward watches/warnings/advisories...
none.

&&

$$

79/82

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