Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Fort Worth Texas
328 am CDT Thursday Apr 2 2015

Discussion...

Early morning water vapor satellite imagery showed more or less
zonal (flat west to east) flow aloft over the Southern Plains. The
shortwaves that brought thunderstorm activity across the County Warning Area
yesterday had moved well off to the east...and another shortwave
trough was located over the California/Nevada border...moving
east. 00z upper air analysis showed a plume of steep middle-level
lapse rates upstream (west) of the County Warning Area...with a 700 to 500 mb
temperature difference of 25 degree c observed over kama and kabq.
These dry adiabatic middle-level lapse rates are expected to advect
east in the zonal flow aloft and build over North Texas by this
afternoon. Early morning surface observations and infrared
satellite imagery observed a moist low-level air mass surging
north across the County Warning Area with middle-60s dew points and low-level stratus
moving over the area from the Gulf Coast.

By this afternoon...the juxtaposition of the low-level Gulf
moisture surging north and the steep middle-level lapse rates is
expected to result in potential energy for convection on the order
of 3000 j/kg. The zonal flow aloft compared to the southerly flow
at the surface will result in a nice clockwise looping hodograph
with speed and directional shear resulting in 4o to 50 kts of bulk
shear over the County Warning Area after 21z. The simple overlap of this
cape/shear parameter space has been shown to be extremely
favorable for supercell thunderstorms...and if any storms develop
this afternoon...this is the anticipated storm Mode. The primary
problem is that the energy for thunderstorm initiation is expected
to be capped by warm temperatures located at the base of the middle-
level airmass characterized by the steep lapse rates. This cap
looks sufficiently strong to prevent thunderstorm initiation
today...unless some strong lifting mechanism can overcome the cap.

Looking upstream at water vapor satellite imagery...there are no
obvious vorticity maxes or shortwave troughs that will move over the
region today and provide the lift needed to weaken the cap. The
only significant lifting mechanism that models show will be
present across the County Warning Area today is the dryline. Will not get into a
discussion of the dryline lifting dynamics again today...however
the basic problem stands...the strong lift associated with a
dryline typically occurs on the western/dry side of the dryline.

The consensus of guidance indicates that a strong dryline
circulation will set up over the County Warning Area today...west of Interstate
35. Models show this strong lift in their low to middle-level Omega
fields...and most models also show a fairly rapid increase in
middle-level relative humidity over the dryline between 21 and 00z
late this afternoon. Despite these indications of lift...models
also consistently advertise that storms will not initiate along
the dryline this afternoon...likely due to the strength of the cap
over the very moist but somewhat cooler air to the east of the
dryline. To get storms...a middle-level updraft would have to
develop over the dryline...move east over the low-level moist
air...and somehow ingest the near surface air that is potentially
very unstable. Most model guidance indicates that while middle-level
updraft initiation is likely...these updrafts will probably not be
strong enough to lift the really moist/unstable air near the
surface up through the cap this afternoon.

For the forecast...only kept a 20 pop mentioned along and east of
the dryline this afternoon and evening. The consensus of model
guidance is probably right...we will see middle-level updrafts go up
over the dryline...move east and dissipate resulting in an
increase of middle-level moisture and cloudiness without any
significant thunderstorm activity developing. Did not want to
simply go with a dry forecast however because there will likely be
numerous middle-level updraft initiations...and it only takes one to
tap into the low-level moist/unstable airmass to get one very
large supercell initiated east of the dryline today. In the
absence of an upstream shortwave trough...the cap will probably
simply be too strong for storms today.

It will be important to watch the strength of middle-level updraft
initiations along the dryline today. If those updrafts are strong
and persistent enough to produce some light to moderate
rainfall...this rain could actually cool the capping inversion
enough to allow a middle-level updraft to interact with the low-level
moist/unstable environment. It seems like a long shot...but as
mentioned above...it really will only take one middle-level updraft
to connect with the l0w-level air to result in a well organized
supercell thunderstorm this afternoon. If a supercell does manage
to develop...very large hail and damaging winds would be likely.

Tonight into Saturday...if storms do not develop along the dryline
by sunset...it is not going to happen. The dryline will start to
slosh back to the west after sunset and the only significant
lifting mechanism for storms will move away from the region.

A cold front is expected to move south across the plains Friday
morning as a result of the California/Nevada shortwave trough
moving over the Central Plains tonight. This front is expected to
be south of the Interstate 20 corridor after noon on Friday...so
chances are that the front will move through the northern portions
of the County Warning Area without any precipitation. Wherever the front is
Friday afternoon...there is a better chance that isolated to
scattered thunderstorms will be able to develop as the air to the
southeast will have more time to heat up and become more
potential unstable. Went ahead and left some 20 to 30 probability of precipitation in the
forecast for locations generally along and south of the Interstate
20/30 corridors Friday afternoon to account for this possibility.

The slower the front moves through the area...the better the
chance of storms and severe weather for north and central Texas.
At this time the cold front is expected to be across most of the
County Warning Area by 21z...so think the chances for severe/organized
thunderstorms are best along and southeast of a Palestine to
Temple line. Large hail and locally damaging winds would be the
most likely severe weather hazards if organized storms develop
Friday afternoon.

The front will continue southeast bringing in a much cooler and
drier low-level air mass to start off the weekend on Saturday.

Saturday is expected to be dry with a much drier air mass in place
behind the cold front. Highs are expected to top out in the upper
60s to lower 70s...so even though the temperatures are expected to
be below normal...it's looking like a really nice day to get
outdoors on Saturday.

By Easter Sunday...a shortwave from the Pacific Ocean is expected
to move over north and central Texas. Model guidance is all in
good agreement that this shortwave will spread rapid moisture
return and large scale forcing for ascent over the region Sunday
morning. Think that widespread rain shower activity is likely
across the region...with some isolated thunderstorms possible as
well. Bumped probability of precipitation up into the 50 to 60 percent range for most
locations as a result of this model to model consistency. Due to
the rain and overcast skies...temperatures on Sunday will likely
hold several degrees cooler than Saturday...despite the strong
warm air advection regime active over the cold front.

Next week...models continue to show the Southern Plains getting
back into the warm sector by Monday. Model guidance advertises at
least a low chance of thunderstorms each day Monday through
Wednesday. It will likely not storm each day...but the details
that will determine precipitation chances each day are fairly
small scale in nature...and will not be resolved by model guidance
for a couple of days still. Some severe thunderstorm activity
looks likely across the region during this period. 00z model
guidance was indicating that Tuesday might be the most likely day
to see severe storms across north and central Texas...but this is
likely to change over the next few days. Went ahead with 20 to 30
probability of precipitation each day...increasing probability of precipitation a bit on Wednesday where the European model (ecmwf)
and GFS advertise a cold front moving into the Southern Plains.
Temperatures are expected to be slightly above normal in a fairly
typical Spring-Time pattern for the region.

Cavanaugh

&&

Aviation...
/issued 1200 am CDT Thursday Apr 2 2015/
/06z tafs/

Concerns...nocturnal stratus...thunderstorm potential Thursday.

Subsidence on the back side of a departing shortwave will prevent
any additional showers or thunderstorms overnight. Fws vwp showing
50-60kts at 2-4kft above ground level. Vwp data from grk is closer to 40kts at
this hour. Although MVFR stratus is still south of Temple/Killeen
at this time...the speed of the wind within the low level jet should assure
a rapid northward surge by advection alone. Extrapolation puts the
arrival of stratus into Waco around 06z or soon thereafter...and
into the metroplex around 09z (4 am cdt). The stratus will
steadily lift Thursday morning...but with a strong inversion in
place...it may take much of the day to scatter out.

Although a dryline will approach the I-35 corridor Thursday
afternoon...the cap will likely prevent thunderstorm development
or otherwise limit coverage.

Stratus will return late Thursday night into Friday morning in
advance of a cold front. The front will arrive around middle-morning
Friday...which is beyond the scope of the 06z dfw taf.

25



&&

Preliminary point temps/pops...
Dallas-ft. Worth, Texas 86 67 73 47 69 / 20 20 20 5 5
Waco, Texas 83 66 76 48 68 / 10 10 20 5 5
Paris, Texas 80 65 72 43 66 / 20 20 20 5 5
Denton, Texas 86 66 72 43 68 / 20 20 10 5 5
McKinney, Texas 82 66 72 44 68 / 20 20 20 5 5
Dallas, Texas 85 67 74 47 68 / 20 20 20 5 5
Terrell, Texas 82 66 74 46 68 / 20 20 20 5 5
Corsicana, Texas 82 65 76 48 69 / 10 10 20 5 5
Temple, Texas 83 66 78 49 68 / 10 10 20 5 10
Mineral Wells, Texas 92 66 72 44 70 / 10 10 5 5 5

&&

Forward watches/warnings/advisories...
none.

&&

$$

/

National Weather Service Glossary of Abbreviations