Area forecast discussion
National Weather Service Fort Worth Texas
324 PM CDT Sat may 18 2013
Discussion...
a slow moving upper level trough over the plains will put the
region in an active weather pattern through Tuesday with severe
weather possible each day. Primarily the northwest/northern parts
of the County Warning Area will be impacted by convection tonight through
Monday...but the central and southern County Warning Area will be under the gun
for a widespread rain event Tuesday afternoon and evening. The
main change with this forecast concerns tonight...and the addition
of probability of precipitation for areas north of I-20...not necessarily for supercell
type convection...but for very high based weak convection that
could cause strong gusty winds via heatbursts late this evening.
This afternoon the dry line remains well to the west of the region
and short-term guidance suggests it will stay as far west as a
Childress to San Angelo line. This results in some concerns about
whether surface based convection will initiate close enough to
enter the County Warning Area. Middle level westerly flow is stronger today...and
therefore storm motions would be more easterly. Surface based
convection would have enough shear and instability to become
supercells and produce significant severe weather. Local objective
analysis indicates the best axis of instability and diminished cin
is to the west of the County Warning Area...with the airmass over our western
counties not quite as unstable with cape at 1500-2500 j/kg and
higher cin due to dewpoints dropping into the low 60s. Water vapor
loop and model guidance does indicate a shortwave is approaching
from the southwest and the lift provided by this feature should
allow surface based convection to initiate and organize west of
the County Warning Area. Some of this activity should enter the northwest zones...but
believe the cells may weaken a bit and become elevated due to the
slightly less favorable environment. In summary...a severe threat
exist for primarily hail/wind...and probability of precipitation are near 20-30 percent in
the northwest zones early this evening. The risk for giant hail or
tornadoes does not look as high as it did yesterday in our County Warning Area.
Currently...weak elevated convection based at around 14000ft
continues to bubble up over the western zones. Most of the precipitation
is evaporating before reaching the ground...but this elevated
moisture plume looks like it will have implications on the forecast
over the northern half of the area tonight. Essentially there are
a few models now indicating a moisture plume from 500-600mb
entering the northern half of the area...likely associated with
anvils from storms to the west. The issue is that this moisture
will be within an area of dry adiabatic lapse rates...and
coincides with upper level forcing from the shortwave. The stage
is set for scattered elevated convection with bases above 12000ft.
Most of the rain will evaporate due to very dry air below 600mb...
but sporadic lightning strikes will be possible tonight.
Meanwhile...a 50kt southerly low level jet will be developing and
the environment will be favorable for this high based convection
to Transfer strong winds to the surface in what is technically a
type of heatburst. This is tough to communicate to the public in
simple terms...but the probability of precipitation are for measurable rain and the chance
for that is low. However gusty winds and light rain may affect a
large portion of the northern County Warning Area tonight...and therefore have
bumped up south winds to 20-25 miles per hour and mentioned gusts to 40 miles per hour
in the zones. This forecast is not of high confidence...partly
because it may be The First National Weather Service forecast that has ever tried to
include a risk of heatbursts in a zone forecast. However looking
at all of the data...the setup tonight looks very favorable for
period of gusty south winds beneath elevated radar echoes. We
shall see.
A strong shortwave trough will rotate through the Southern Plains
tomorrow and will push the dryline farther to the east into the northwest
zones. With daytime heating...cin should be low enough over the northwest
zones for isolated severe storms to develop in the afternoon.
Wind fields will strengthen a bit and supercell storm motion will
be easterly. West to east storm tracks on Sunday evening means
probability of precipitation will cover the areas north of I-20. Low level helicity ramps
up around sunset Sunday evening and if supercells can overcome the
weak cin and tap into surface parcels...a tornado threat will
exist. Monday looks like a Carbon copy of Sunday/S pattern with
the main features in the same place...just a little weaker cin so
probability of precipitation are slightly higher/farther south. Temperatures will remain near
persistence and winds will be breezy Sunday and Monday.
On Tuesday the upper trough will begin to finally move east and
this will push a cold front into the northwest zones Tuesday afternoon.
Scattered to numerous storms should fire along the cold front
Tuesday afternoon and work east and southeast across much of the
area into Tuesday night. Rainfall may be widespread and average
1/2 to 1 inch. Severe weather will be a concern given high
instability and adequate deep layer shear.
The front will push through the region and drier air will end rain
chances from north to south Wednesday. Low temperatures will cool a bit
and into the 60s due to the lower dewpoints...but highs will
continue in the middle 80s to near 90 through the remainder of the
week. Will keep the forecast dry as an upper ridge settles into
the area...but we may have to watch for northwest flow mesoscale convective system activity on
Friday night in the northwest zones. Too far out to rely on the model
forecasts for this possibility at this time.
Tr.92
&&
Preliminary point temps/pops...
Dallas-ft. Worth, Texas 73 90 72 89 72 / 20 10 20 10 20
Waco, Texas 72 90 73 90 73 / 10 5 5 5 10
Paris, Texas 70 88 70 87 70 / 20 10 20 10 40
Denton, Texas 73 90 71 89 70 / 20 10 20 20 30
McKinney, Texas 73 89 71 88 71 / 20 10 20 10 30
Dallas, Texas 75 91 74 90 74 / 20 10 20 10 20
Terrell, Texas 72 88 71 89 73 / 10 5 20 10 20
Corsicana, Texas 71 89 72 89 73 / 10 5 5 5 10
Temple, Texas 71 89 73 90 73 / 5 5 5 5 10
Mineral Wells, Texas 71 93 70 92 71 / 20 20 20 20 20
&&
Forward watches/warnings/advisories...
none.
&&
$$
