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Area forecast discussion 
National Weather Service Fort Worth Texas
1200 am CDT Sat may 30 2015

thunderstorm complex over West Texas is expected to move
southeast across north and central Texas overnight into Saturday
morning. In the 850mb front that lies pretty close
to the I-20 corridor according to the 00z analysis... will be a
focus for additional development. These storms will move east
along this 850mb front...and training of echoes will result in a
flash flood hazard overnight.

A cold front is forecast to move through the metroplex middle to
late morning with a wind shift to the northwest and then north at
around 10 knots. Some rain showers will continue through the
morning. An improvement to lower end MVFR is forecast for the
afternoon and continue through Saturday evening.



minor update to the afternoon forecast package to reduce probability of precipitation for
the evening hours...7 PM to 10 PM...and 20 to 30 probability of precipitation for the
10 PM to 1 am time period. Also reduced cloud coverage over the
southern and southwestern counties to mostly clear skies for the
evening. 75


Previous discussion... /issued 354 PM CDT Friday may 29 2015/

Early afternoon water vapor satellite imagery showed a compact
shortwave trough in place over central Texas. This trough was
spreading lift/ascent over the eastern half of the County Warning Area as of
20z/3pm. Regional radars showed scattered storms over areas
just northeast of the dfw metroplex at 3 PM, out ahead of this
shortwave trough. Locally calculated convective parameters
indicate that the thermodynamic environment in which these storms
are developing is characterized by cape values of around 1,000
j/kg. Deep layer shear magnitudes were calculated at 35 to 40
kts. The combination of cape and shear supports a multi-cellular
storm Mode that may allow for storms producing hail around 1 inch
in diameter...and microburst winds of 50 to 60 miles per hour. A localized
heavy rainfall threat is possible as well. Do not expect wide
spread severe storms or widespread flooding with this thunderstorm
activity this afternoon...however we will keep a very close eye on
both possibilities...especially it will not take
much rain to result in flooding today or tonight.

Expect that the ongoing early afternoon thunderstorm activity will
tend to move east of the County Warning Area through this evening as the shortwave
trough continues east...bringing some temporary subsidence in over
north and central Texas. This shortwave trough is expected to
continue moving east of the County Warning Area this evening...with subsidence
moving east out of the area after midnight tonight. Once the
subsidence is no longer in place over the County Warning Area...another shortwave
trough is expected to approach...bringing some weak lift back over
the region early tomorrow morning.

Once subsidence has exited the region...there are two areas of
concern we'll be watching closely for new thunderstorm development
after midnight. The first is thunderstorm activity that is
expected to develop over the southern High Plains...immediately
downstream of this second shortwave trough...which was located
over southwestern Colorado at 20z/3pm. This complex of storms
appears very likely to develop due to the fact that relatively
strong lift associated with the Colorado shortwave trough seems
well handled by model guidance which seems to have the position
and strength of the shortwave trough well resolved at this time.
This complex of storms is expected to congeal into a line and move
southeast along a low-level instability axis which would send the
linear mesoscale convective system over our southwestern County Warning Area well after midnight tonight.

The second area of thunderstorms that are expected to develop are
expected to occur along an east-west oriented line...likely
associated with an 850 mb frontal zone that is expected to be in
place anywhere from the Red River to the Interstate 20 corridor
tonight. The position of this elevated frontal boundary is
important, because high resolution model guidance indicates that
new thunderstorm activity will develop on this boundary by 08z/3am
and then train east along the boundary through sunrise Saturday
morning. If the boundary is located near the Red River, the
greatest threat for this zone of training storms would exist south
of the the I-20 corridor...placing another band of
heavy rainfall over areas that have experienced high end flash
flooding impacts over the past few days. If the elevated frontal
zone ends up over the I-20 corridor, the biggest threat for
training storms and heavy rainfall would be concentrated along and
south of I-20 to the Waco/Palestine areas. These areas have
experienced high impact flooding as well over the past week,
however some of these locations haven't had as much rain and
flooding over the past day or two. If we get additional heavy
rainfall anywhere across north or central Texas at this
point...flooding will probably occur. We are simply too saturated
to store any additional rainfall anywhere across the
the Flash Flood Watch will remain in effect until Saturday

A couple of things to watch for tonight: most high resolution
model guidance indicates that this east-west band of heavy
rainfall/thunderstorms will develop after midnight through 3 am.
The duration of this rainfall does appear to be tied to the timing
of the line of storms building in from the southern High Plains.

If the southern High Plains line of storms moves across the
southwestern portions of the County Warning Area faster than expected, it will
likely change the orientation of low-level wind vectors away from
the 850 mb frontal zone... disrupting the primary mechanism that's
expected to cause the storms to train along this boundary
overnight. In short...faster High Plains mesoscale convective system seems to equal a
lower chance of additional high-impact flooding along and south of
the 850 frontal zone tonight.

On the other hand, if the Southern Plains mesoscale convective system is slower...or
simply does not materialize...convergence and low-level flow into
the 850 mb frontal zone will be left undisturbed...allowing storms
to train along this frontal zone from 3 am until 8 or 9 am. With
rainfall rates of around 1 inch per hour expected...this is
obviously not the solution that would benefit the region as we
would likely receive another round of high impact flooding with
many areas receiving 1-2 inches of rain...with a focused zone of
3-4 inches or greater likely near the 850 front.

The current forecast calls for the most likely scenario where
storms develop along the 850 front between midnight and 3 am...and
train for a few hours before the southern High Plains mesoscale convective system sweeps
in and disrupts the low-level flow towards the frontal boundary
from west to east through 6 am. This would limit the potential
training along the frontal boundary to a few hours...resulting in
about half of the "worst case scenario" rainfall totals mentioned

We will simply have to watch the mesoscale environment closely
this evening into the overnight hours to get a better idea of
which scenario is most likely to pan out. It would be nice if no
additional thunderstorms develop tonight across the
region...however that seems very unlikely due to the approach of
the southwest Colorado shortwave trough.

The Colorado shortwave trough is not expected to move over the County Warning Area
until late tomorrow it will continue to spread lift
over the region through that time on Saturday. Do not expect
additional rounds of heavy rainfall to develop on Saturday because
a cold front is expected to push southeast across the County Warning Area from
northwest to southeast Saturday late morning through early
afternoon. While additional shower and elevated thunderstorms are
certainly possible behind the cold front...most of the low-level
moisture fueling our recent episodes of heavy rain and flooding
are expected to be advected southeast away from the region behind
this front. Therefore...any additional shower or thunderstorm
activity on Saturday is expected to remain unorganized and
generally light in nature and should not pose anymore threat for
widespread flooding.

Sunday through next week...the general pattern seems to shift to a
somewhat stagnant one where the Colorado upper trough/low stalls
out over Louisiana or Mississippi on Sunday...keeping the Southern
Plains in weak ridging and subsidence from Sunday afternoon
through the first half of next week. If this large scale pattern
holds true...this should enter the region into at least a short
term period of dry weather. Most models agree on generally dry
weather from Sunday afternoon through the first half of next hopefully we'll get a chance to dry out as models
currently advertise.



Preliminary point temps/pops...
Dallas-ft. Worth, Texas 68 80 63 81 63 / 70 70 20 5 5
Waco, Texas 69 81 64 81 61 / 50 60 40 20 5
Paris, Texas 65 78 61 78 61 / 80 80 30 5 10
Denton, Texas 66 79 61 80 59 / 80 80 10 5 5
McKinney, Texas 66 79 61 79 60 / 80 80 20 5 5
Dallas, Texas 68 81 63 81 64 / 70 70 30 10 5
Terrell, Texas 69 81 63 80 62 / 40 60 30 10 5
Corsicana, Texas 69 81 64 80 63 / 30 60 40 20 5
Temple, Texas 68 81 64 81 62 / 40 60 40 20 5
Mineral Wells, Texas 66 78 61 79 59 / 60 60 10 5 5


Forward watches/warnings/advisories...
Flash Flood Watch through Saturday morning for txz091>095-




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