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Area forecast discussion 
National Weather Service Fort Worth Texas
651 am CDT Sat Aug 30 2014

Aviation...
VFR conditions will prevail for much of the day today...with only
the occasional middle level clouds this morning. There is also a
possibility of a few areas of fog...but visibilities should stay
at 5 miles or greater through the day. Next issue will be
scattered convection this afternoon. While confidence is high in
the occurrence of storms...confidence is quite low that any one
storm will impact the terminals. Have therefore kept the forecast
without mention of thunder...but it will be something to keep a
close eye on through the day.

&&



Previous discussion... /issued 408 am CDT Sat Aug 30 2014/

Early morning water vapor satellite imagery showed a shortwave
trough over northern Missouri moving off to the northeast with a
cyclonic shear axis extending south across Oklahoma and over
northwest Texas. A second...but weaker...shortwave trough was
observed over southeast Colorado...moving southeast towards the Texas
Panhandle as of 08z/3am. It was difficult to pick out this second
trough on 00z upper air analysis as this feature was masked by
height rises following the stronger trough exiting the region to
the northeast. However...there were some subtle height falls and a
cyclonic wind shift observed over eastern Colorado and northern
New Mexico at the 250 mb level which are likely associated with
this secondary trough. Otherwise...upper air analysis indicated
upper level ridging over the southwest and southeast Continental U.S....which
are expected to bridge over the Southern Plains by early next week
becoming the dominant weather feature over the region for much of
the week ahead.

Early morning infrared satellite imagery showed a band of middle-
level clouds developing along a line from Gainesville to Mineral
Wells to Brownwood. This is likely associated with some weak
forcing for ascent associated with the trailing cyclonic shear
axis aloft that is moving east over the area right now. The forward
00z radiosonde observation indicated a fairly significant subsidence inversion at
the 525 mb level yesterday evening...so chances are that this
developing middle-level accus will not result in thunderstorms this
morning. However...with no commercial airline activity to grab
radiosonde observation data from recently...it is difficult to know the strength of
this elevated cap early this morning over the region. Went ahead
and mentioned isolated thunderstorms along and southeast of this
line this morning with a 10 pop to account for the possibility
that lift has cooled/removed this elevated cap this morning. Once
the 12z radiosonde observation is in...or after we get some morning air traffic
processed through dfw and Dal...we will have a better idea of
morning elevated thunderstorm chances.

Otherwise...the primary feature we will be watching for today is
the secondary shortwave trough moving southeast towards North
Texas this afternoon. The large scale forcing for ascent
associated with this trough is expected to be weak...and mainly
focused over the northwestern County Warning Area this afternoon. The best way to
track the lift associated with this trough might be to look for
500 mb cold air advection. Outside of looking at a dynamic
tropopause map...it's just not that easy to find the lift
associated with this trough. 500 mb cold air advection seems to
line up well with the primary area of lift associated with the
trough this afternoon.

00z radiosonde observations and early morning GOES sounder imagery show the deepest
moisture in place over East Texas and the Texas Gulf Coast...with the
driest air observed over the Texas Panhandle. As a result...we
will be getting the best lift this afternoon where the moisture is
most lacking...and we will have more or less neutral forcing where
moisture is deepest. As a result...went ahead and just expanded
the 20 probability of precipitation we had over the eastern County Warning Area to cover the entire area
for this afternoon into the early evening hours of the day.

Because lift and moisture do not line up well...think that
wherever storms develop...they will be limited in coverage to 10
to 20 percent across the region. High resolution models are
favoring thunderstorm development along and west of the Interstate
35 corridor...closer to the lift associated with the shortwave
trough. Thunderstorms to the east will most likely be dependent
upon peak heating/buoyancy as the primary lifting mechanism...
leading to a more chaotic development of storms this afternoon.
Forecast soundings indicate little cap for surface based free
convection this afternoon across the region...so confidence in
storms developing is high...despite the expectations that coverage
will remain isolated in nature. Deep layer shear is expected to
hold in the 15 to 20 knots range...so no organized severe weather is
expected at this time. Localized strong wind gusts associated with
downbursts...and heavy rainfall are the expected thunderstorm
hazards...aside from cloud to ground lightning.

Expect thunderstorm activity to dissipate quickly after sunset as
the forcing for ascent associated with the secondary shortwave
trough is not expected to be strong enough to keep storms going
with the loss of daytime heating.

Sunday...the shortwave trough is expected to continue off to the
east overnight...and is expected to be east of the County Warning Area by Sunday
morning. This will leave the County Warning Area under weak subsidence aloft with
no obvious upstream shortwave troughs. The consensus of guidance
is for zonal flow to build over the northern and central Continental U.S.
While upper level ridging builds over the southern third of the
Continental U.S.. the only exception to this is along the Texas Gulf Coast where
models hold on to a weak upper level disturbance at least through
Sunday afternoon. With this feature holding in place...but the
rest of the County Warning Area seemingly dominated by weak subsidence
aloft...went ahead and maintained 20/30 probability of precipitation for locations along
and southeast of a line from Athens to Killeen Sunday afternoon.
These locations are closest to the weak upper level disturbance
and hold on to the deepest moisture...so some unorganized
afternoon convection cannot be ruled out. For the remainder of the
County Warning Area however...dry conditions should prevail as subsidence builds
weak capping over the region.

Monday through Friday...the consensus of guidance is to build
upper level ridging over the County Warning Area for the week ahead. The GFS
continues to hold on to the weakness aloft over the Texas/la Gulf
Coast...but the Canadian and European model (ecmwf) favor ridging dominating the
region...so went ahead and removed the probability of precipitation the previous forecast
had over the southeastern County Warning Area each afternoon from Tuesday through
Thursday. The only way the upper level weakness along the Texas/la
Gulf Coast is going to be maintained is through daily persistent
and somewhat vigorous thunderstorm activity. If that activity is
observed Sunday through Tuesday...we may have to adjust the
forecast to place these probability of precipitation back in. Because the GFS maintains
this feature in the absence of focused thunderstorm
activity...sided with the European model (ecmwf) and Canadian which made more
conceptual sense at this point.

Ridging aloft should keep US dry and seasonably hot for the week
ahead. Kept temperatures below 100 degrees as the ridge aloft is
not super strong...and we maintain southerly winds through the
entire week due to Lee side troughing associated with zonal flow
over the northern and central Continental U.S.. persistent southerly winds
will keep some shallow moisture in place resulting in some morning
stratus intrusions...and an afternoon fair weather cumulus field
each day...both of which should prevent a big heat wave from
setting up over the region.

Cavanaugh



&&




Preliminary point temps/pops...
Dallas-ft. Worth, Texas 95 76 95 77 97 / 20 10 5 0 0
Waco, Texas 95 74 95 76 97 / 20 10 10 5 0
Paris, Texas 88 71 92 73 94 / 20 10 10 0 5
Denton, Texas 95 74 95 76 96 / 20 5 5 0 0
McKinney, Texas 92 73 94 75 95 / 20 10 5 0 5
Dallas, Texas 95 76 95 77 96 / 20 10 5 0 0
Terrell, Texas 93 74 94 75 95 / 20 10 5 0 5
Corsicana, Texas 93 74 94 75 96 / 20 10 20 5 5
Temple, Texas 94 73 95 75 97 / 20 10 20 5 0
Mineral Wells, Texas 96 73 96 75 97 / 20 5 5 0 0

&&

Forward watches/warnings/advisories...
none.

&&

$$

14/69

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