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Area forecast discussion 
National Weather Service Fort Worth Texas
344 am CDT Wednesday Apr 1 2015

Discussion...

Early morning water vapor satellite imagery showed a compact
shortwave trough over northern Mexico near the Texas border at
Presidio. This trough was moving east-northeast towards central
Texas...and is expected to maintain that track throughout the day
today. Over the northern Texas Panhandle...there was another
subtle trough moving east over Oklahoma. These two features are
expected to provide some weak lift for showers and thunderstorms
across the County Warning Area...primarily from early this afternoon through this
evening. 00z upper air analysis showed a plume of relatively steep
middle-level lapse rates over north and central Texas. An area of
nearly dry-adiabatic lapse rates was observed over the Texas
Panhandle northward across central Colorado and Wyoming.

Today...the two shortwave troughs mentioned above will provide
some weak lift over parts of the County Warning Area today...however the lift
associated with these features is forecast to be fairly weak in
most model guidance. The southern shortwave trough certainly
appears to be better organized on water vapor satellite imagery
this morning...so went ahead with higher probability of precipitation across the
southeastern County Warning Area versus the northern County Warning Area expecting some stronger
lift across eastern portions of central Texas this afternoon.
Expect that the dryline will sharpen up and move east into west
central Texas again this afternoon...however the dryline is
expected to remain west of the County Warning Area. Even if storms develop along
this feature...they may not make it into North Texas this
afternoon or evening with some weak subsidence expected behind the
shortwave troughs.

With a plume of somewhat steep middle-level lapse rates still in
place across the region...and ample low-level moisture to support
thunderstorms if the cap can be overcome...any precipitation that
falls across the area today will likely fall in the form of
isolated to scattered thunderstorms. Although the best lift if
expected to exist over the area east of I-35 and south of
I-20...this area is also the most likely to remain under upper
level cloud cover all day...limiting insolation and surface based
potential instability. This should limit the severe thunderstorm
potential across central Texas this afternoon. If storms
develop...the middle-level lapse rates would likely still support
storms capable of producing some hail and strong wind
gusts...however the storm Mode would likely be unorganized...
limiting the potential for higher impacts.

Upper level cloud cover is more likely to clear out over the
western/northwestern portions of the County Warning Area this afternoon...which
should drive cape values up to around 1500 j/kg during the peak
heating hours of the day. Deep layer shear is actually expected to
be stronger today than yesterday...so if any storms were able to
develop...they would have a better chance at becoming
organized/severe. Large hail and damaging winds are still the
primary expected hazards...but the overall threat of large hail
and damaging winds would simply be greater for any storms that
develop west of I-35 and north of I-20 this afternoon. The primary
ingredient missing for severe thunderstorms over this area this
afternoon is coherent/focused lift. This area should be
experiencing weak subsidence aloft behind the shortwaves mentioned
above. There is the dryline which could provide lift...but the
dryline is expected to be well west of the County Warning Area this afternoon. As
a result...only went with 20 probability of precipitation for these areas...and it is
possible that storms will not be able to develop...depending on
how strong the subsidence is behind the shortwaves.

Thursday...nearly all model guidance indicates that the dryline
will surge east towards the I-35 corridor Thursday afternoon. The
consensus of guidance continues to advertise MLCAPE values of
around 2500 j/kg right out ahead of the dryline...overlaid with
45-50 kts of deep layer shear. The shear vectors and the flow
aloft are oriented perpendicular to the dryline...so if any
thunderstorms were able to develop along this boundary...the above
mentioned parameters strongly suggest a discrete supercellular
storm Mode. The main limiting factor for thunderstorm chances
along this dryline Thursday afternoon is large scale lift/support.
There is no guidance that indicates any significant lift will
exist along the dryline...aside from the dryline circulation
itself. Granted...there is no strong signal for subsidence
either...but relying on the dryline circulation alone for
thunderstorm initiation is difficult.

A discussion regarding the dynamics associated with the dryline
circulation follows...to continue on with just the normal
discussion...skip the next 3 paragraphs...

The problem with relying on drylines alone for lift and
thunderstorm initiation is that the lift along a dryline in the
afternoon typically works like a backwards front. With any strong
front...the tight thermal gradient along the front forces a
vertical circulation in an attempt to balance out the temperature
gradient. That circulation results in lift on the warm side of the
front...and sinking motion on the cool side. Lift in the
atmosphere provides cooling...so lifting the warm air is the
atmosphere's attempt to cool off the warm air. The opposite is
true for sinking motion on the cool side of the front. While this
is pretty straight forward in cold fronts...you get lift in the
warm moist air ahead of the front...resulting in thunderstorms/
rain near the front...this circulation does not always cause
storms to develop along drylines.

With drylines...the lift occurs on the back side of the
boundary...in the very hot...but very dry air that typically
exists west of the boundary. In the warm...moist... potentially
unstable air to the east...a downward motion is typically present.
So how thunderstorms typically develop along a dryline is that
some middle-level moisture exists over the surface dryline
circulation. The lift interacts with this moisture resulting in
very high based updrafts that move east off the dryline and
attempt to ingest the really moist and unstable low-level air to
the east.

The problem is...how strong is the downward moving air on the
moist...somewhat cooler side of the dryline? If the downward air
is too strong...a strong inversion sets up preventing the moist
low-level air from being ingested into the middle-level based
updraft. Unfortunately there's no real way to know if dryline
initiations will be successful or not. Typically the deciding
factor between these middle-level thunderstorm initiation attempts
failing (typically referred to as Turkey towers...based on the
classic anvil and skinny dissipating updraft remnants that
visually mark failed thunderstorm attempts for storm spotters) or
not (resulting in full fledged thunderstorms) is the presence of
ambient large scale lift which serves to weaken the capping
inversion to the east of the dryline. Because that lift does not
appear to be present on Thursday...we are siding with the dryline
not initiating storms Thursday afternoon at this time.

Back to the "normal" discussion...

While thunderstorms along the dryline are unlikely Thursday
afternoon...did place a 20 pop in the forecast right along and
east of the dryline. While no models explicitly initiate storms
along the dryline...all guidance shows a rapid moistening of the
700 mb level which is indicative of those persistent middle-level
thunderstorm initiation attempts mentioned above. Even though the
chances are small...it only takes one of the attempts to come to
fruition for one very large thunderstorm to develop. If even one
supercell organizes along the dryline across the County Warning Area Thursday
afternoon/early evening...it is very likely to produce large hail
and damaging wind gusts. At this time...it simply appears that the
cap will be too strong to overcome...resulting in a dry-dryline
across north and central Texas.

Friday...a cold front is expected to move out of the Central
Plains and across the County Warning Area Friday late morning through the early
afternoon hours of the day. Forecast soundings indicate that the
lift associated with the front will be too weak/transient to
overcome the low-level capping inversion during the morning
hours...so wherever the front is Friday afternoon is the most
likely time that storms will develop. At this time...it looks most
like that the front will be along an Athens to Temple line by
Friday afternoon...so kept the highest probability of precipitation along and southeast of
this line as a result. Forecast cape/shear values are nowhere near
as impressive along the cold front on Friday as they were along
the dryline Thursday...but expect that a few strong to marginally
severe storms are possible Friday afternoon. The front is expected
to move fairly quickly though...so the window of opportunity for
strong or severe storms across our County Warning Area seems fairly small at this
time.

This weekend...much cooler air is expected to build into the
region behind the cold front...keeping temperatures in the 60s for
highs on Saturday and Sunday. Rain shower and thunderstorm chances
increase on Sunday as the consensus of model guidance is that a
shortwave trough will move over the Southern Plains on Sunday.
This will send a surge of Gulf moisture up over the frontal
inversion...resulting in widespread rain showers with some
isolated elevated thunderstorms mixed in.

Early next week...persistent southerly flow is expected to bring
North Texas back into the "warm sector" by Monday afternoon as a
weak warm front like feature moves north across the County Warning Area early
Monday morning. This will bring a good surge of Gulf moisture over
the region while southwest flow aloft prevails. This will once
again set the stage for an elevated mixed layer to advect over a
moist Gulf airmass in the lower atmosphere across North
Texas...resulting in healthy...but capped...potential instability
for thunderstorms early next week. Kept a low chance of storms in
the forecast each day as thunderstorm chances will depend on sub-
synoptic features which cannot be resolved by global models this
far out. The climate forecast system continues to pick up on a
nice overlap of model generated convective precipitation and the
supercell composite index early next week...so there will be an
attendant severe weather threat for any thunderstorms that
develop. Our best chance of storms looks to occur on Wednesday
when models indicate that a cold front will move across the
region. The timing of this front is likely to change in future
forecasts however...so kept probability of precipitation below the 60 percent that MOS
guidance was advertising.

Cavanaugh



&&

Aviation...
/issued 1203 am CDT Wednesday Apr 1 2015/
/06z tafs/

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

Storms overnight will be primarily confined to Oklahoma...aided
by a transiting middle-level disturbance. The activity skirting the
Red River will be primarily elevated and non-severe...and will
shift well east of the north departure corridor by morning
operations.

Latest guidance is projecting a low level jet weaker than the fws vwp output
of 40-45kts at 2kft above ground level and up. The Oklahoma storms should sustain
a potent low level jet during the early morning hours...helping to pull MVFR
stratus into all taf sites before dawn. There should be less of a
westerly component than Tuesday morning...enhancing the chances
that this deck is able to blanket the western metroplex airports.
Still expect Dallas/Arlington/Waco will be the last to scatter
Wednesday...in fact...these sites may simply transition from
morning stratus to a broken afternoon stratocu deck.

An impulse with a more southerly track is currently moving through
Chihuahua and The Trans-Pecos. This feature will help to spread
scattered showers/storms across north and central Texas on
Wednesday. For Waco...appears most of the activity will be earlier
in the day...primarily showers. Afternoon thunderstorm chances are
still too low for categorical mention...but will maintain a late
afternoon window of thunderstorms in the vicinity for metroplex sites.

Stratus will return earlier Wednesday night. Have introduced this
into all tafs.

25



&&

Preliminary point temps/pops...
Dallas-ft. Worth, Texas 80 66 85 67 75 / 30 20 20 20 10
Waco, Texas 79 65 84 66 78 / 40 30 10 10 20
Paris, Texas 77 63 79 66 71 / 30 30 20 20 20
Denton, Texas 80 65 86 66 72 / 30 10 20 20 10
McKinney, Texas 79 64 82 66 73 / 30 20 20 20 10
Dallas, Texas 80 65 84 66 75 / 30 20 20 20 10
Terrell, Texas 79 64 82 66 74 / 30 30 20 20 20
Corsicana, Texas 79 64 82 67 77 / 40 30 10 10 20
Temple, Texas 78 65 84 66 79 / 40 30 10 10 20
Mineral Wells, Texas 83 64 91 66 73 / 30 10 10 10 10

&&

Forward watches/warnings/advisories...
none.

&&

$$

/

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