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Area forecast discussion 
National Weather Service Fort Worth Texas
701 am CDT Monday Jul 28 2014

Aviation...
multiple concerns are in play during this forecast period.
However... VFR conditions are still expected to prevail.

Frontal passage has already begun for the dfw metropolitan taf sites and should
push through all metropolitan taf sites within the next hour. Behind the
front...north winds between 5 and 10 kts are expected. Scattered
showers with isolated thunderstorms were present behind the front
and located along the Red River. This activity was drifting
southward around 7 kts. Kept mention of vcsh for the dfw metropolitan for
early this afternoon as instability will be present but not
abundant.

For kact...frontal passage is expected later this morning...around 15z.
Afternoon cape values are forecast to be around 2000 j/kg and
steep middle- level lapse rates are also expected. With the surface
front expected to stall out over central Texas and the added
affect of daytime heating...afternoon convection is possible. Have
put thunderstorms in the vicinity in for a few hours this afternoon for Waco because
coverage should not be widespread. Gusty winds...heavy
rainfall...and frequent lightning are possible with any convection
that develops.

Ajs

&&



Previous discussion... /issued 404 am CDT Monday Jul 28 2014/

08z surface analysis showed a cold front moving slowly south
across the Red River. The front was located near a line from
Lubbock to Bowie to Sherman at the time of analysis. Regional
radars showed scattered showers and thunderstorms north of the
front...moving south at 5 to 10 miles per hour. 00z upper air analysis showed
that upper level ridging once again strengthened over North Texas
over the last 24 hours. Early morning water vapor satellite imagery
indicates that the upper level ridge is likely breaking down early
this morning...and reorganizing west of the County Warning Area. Expect to see 500
mb height falls of 3 to 4 dm on the 12z forward radiosonde observation this morning with
the ridge weakening and moving west of the County Warning Area.

Today...expect that scattered showers and thunderstorms will
continue to develop north of the slow moving cold front through
the middle-morning hours of the day today. Expect that the front will
be located south of the dfw area by this time...so maintained 30
probability of precipitation for this morning for locations generally along and north of a
line from Cisco to Athens.

The front is expected to continue south into central Texas for
this afternoon. From the middle-morning hours to the middle-afternoon
hours...there will probably be a lull in thunderstorm activity as
the best lift for elevated thunderstorms becomes disrupted by
daytime heating. This is expected to occur as heating promotes
better low-level mixing...and dry air entrains into the saturated
layer around 800 to 750 mb where forecast soundings indicate very
little lift is needed for air to reach its level of free
convection early this morning. Once the elevated Reservoir of
thunderstorm initiation is interrupted...new thunderstorm
development will likely have to wait for the peak heating hours of
the day for surface based air parcels to be heated up enough for
air to be lifted above the cap by the weak forcing associated with
the front.

Surface based storms appear to be most likely along and just
north of the surface cold front this afternoon. The frontal
boundary represents a layer of only shallow cool air...and almost
all guidance indicates that deeper moisture is available for
storms north of the front...rather than along or south of the
front. Either way...the shallow frontal inversion will likely mix
out leaving only a wind-shift line defining the leading edge of
the front during the peak heating hours of the day. Models
consistently show the highest precipitable water values north of the front
across central Texas this afternoon...so went ahead and bumped
probability of precipitation up to 40 percent centered along a line from Comanche to Waco
to Palestine this afternoon.

Precipitable waters are forecast to be in the neighborhood of 2 inches...so
brief heavy rainfall and gusty outflow winds are the primary
thunderstorm hazards aside from the lightning threat present with
any thunderstorm activity.

Despite storms more likely to occur north of the front...as long
as the shallow frontal inversion mixes out as expected...forecast
soundings still advertise an inverted-v low-level
profile...indicative of storms capable of producing microbursts.
Severe/damaging winds should not be common due to a somewhat
meager Theta-E difference with height in the atmosphere limiting
the strength of cold pool accelerations. Regardless...steep low
level lapse rates will allow for wind gusts in the 40-50 miles per hour
range...which is strong enough to cause light...localized damage.

Storm motions are prognosticated to be slow this afternoon...however a
widespread flooding threat is not expected as storms will still
produce outflow boundaries that should promote storms to move away
from any one location with propagation likely dominating storm
motions.

Tonight...models are fairly consistent now in advertising
northeasterly flow bringing cool and dry air into the County Warning Area
beginning this afternoon...continuing through the overnight hours.
This should cause any lingering thunderstorm activity east of
Interstate 35 to diminish fairly quickly after sunset. Models are
now also consistently showing an 850 mb Theta-E ridge holding up
over west central Texas...and over the western portion of the County Warning Area
tonight into tomorrow. This Theta-E ridge may become active again
overnight...with a similar coverage of storms to what we are
seeing north of the Red River at the time of this forecast
discussion.

Coverage and placement of storms will be determined by the
orientation of low level flow to the west of this Theta-E ridge.
If winds become southwesterly to the west of this feature...more
storms are likely due to enhanced low-level convergence. If winds
remain southerly...convergence will be shunted more to the
west...leaving the County Warning Area with lower thunderstorm chances and
coverage. Just maintained 20-30 probability of precipitation overnight west of Interstate
35/35w for now without higher confidence in which scenario will
pan out. East of Interstate 35 should be dry...all guidance
supports this...so left the forecast dry for these locations.

Tuesday...model guidance indicates that an 850 mb anti-cyclone
will move southeast from Oklahoma over towards Louisiana from
tonight into Tuesday afternoon. This will cause the transport of
dry cool air in from the northeast to be interrupted as low-level
flow veers around to the east and eventually southeast by Tuesday
night. This will allow the low-level Theta-E ridge to build slowly
eastward from west central Texas back over the County Warning Area Tuesday through
Tuesday night. During the day on Tuesday...most model guidance
indicates that the Theta-E ridge will remain west of Interstate
35/35w so went ahead and kept the highest probability of precipitation over those
locations as surface based storms are most likely within this area
during the peak heating hours of the day. The Theta-E ridge is
expected to advect north overnight and become re-established over
the northeastern County Warning Area by sunrise Wednesday morning. Overnight
thunderstorm activity is most likely within this Theta-E
ridge...so transitioned probability of precipitation to match the model placement of this
feature overnight.

The low-level Theta-E ridge is mentioned a lot here because it is
the easiest way to track our best precipitation chances associated
with the front stalling out...becoming reoriented and changing in
characteristics between a cold to stationary to warm front from
today through Wednesday. Instead of changing the name of the
front several times...the Theta-E ridge can be used as a proxy
for the best precipitation chances associated with the front and
is easy to refer to as one thing throughout the forecast.

By Wednesday...the low-level Theta-E ridge is expected to remain
stationary along the Red River...but dipping southeast over the
northeastern County Warning Area towards Shreveport. Guidance continues to show
that upper level troughing will dominate the eastern Continental U.S. By
Wednesday with upper level ridging over the southwestern Continental U.S..
this leaves the South Plains in northwest flow aloft. Models have
been showing a shortwave trough in this flow regime headed over
the area sometime between Wednesday and Thursday. The consensus of
yesterday evenings models now indicate that this shortwave will
move over the northern portions of North Texas during the day on
Wednesday. This feature is actually track-able on water vapor
satellite imagery now as well...making for a higher confidence
forecast in general. This shortwave was over Baja California California at
the time of this discussion.

With the Theta-E ridge/front in place as mentioned above...the
forcing for ascent induced by the shortwave trough should give US
our best precipitation chances for this week. Went ahead with 40
to 60 probability of precipitation for locations along and northeast of a line from Bowie
to McKinney to Emory on Wednesday and Wednesday night as a result.
Locally heavy rainfall looks much more likely for these areas as
precipitable waters are expected to be in excess of 2 inches during this time
frame...and a nearly stationary Theta-E ridge interacting with
forcing should promote a good coverage of storms. A Flood Watch is
not expected at this time...but we will have to monitor to see if
mesoscale/smaller scale forecast details dictate a more
widespread flooding event in later forecasts. For now it looks
like the threat of any flooding will tend to be localized in
nature.

The Theta-E ridge/front is expected to slide south across North
Texas on Thursday...and kept widespread 30-50 probability of precipitation in place as a
result. With so much going on before this front begins to move
again on Thursday...will not get into the details of this forecast
period as widespread thunderstorm activity over the northeastern
County Warning Area will probably change things quite a bit in later forecasts.

Precipitation chances will continue to diminish behind the front
on Friday...with model consensus showing much drier conditions
over the region by Saturday. Left 20 probability of precipitation in the forecast for
Friday...with a dry forecast for next weekend into early next week
as a result. Temperatures will likely be unseasonably cool across
the area Thursday and Friday...warming back up towards normal by
early next week with drier air and mostly sunny skies in place.

Cavanaugh

&&




Preliminary point temps/pops...
Dallas-ft. Worth, Texas 94 75 93 74 94 / 30 20 20 20 20
Waco, Texas 97 75 92 73 97 / 40 30 20 10 10
Paris, Texas 91 68 87 70 80 / 40 10 10 30 50
Denton, Texas 93 71 90 72 93 / 40 20 20 20 30
McKinney, Texas 95 70 93 72 92 / 40 10 10 20 30
Dallas, Texas 95 76 93 75 94 / 30 20 20 20 20
Terrell, Texas 96 73 95 74 93 / 30 20 10 20 20
Corsicana, Texas 95 74 95 73 95 / 40 20 10 20 10
Temple, Texas 98 73 93 72 97 / 40 30 20 10 10
Mineral Wells, Texas 93 72 91 71 96 / 30 30 30 20 20

&&

Forward watches/warnings/advisories...
none.

&&

$$

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