Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Fort Worth Texas
643 am CDT Tuesday Jul 29 2014

VFR conditions should prevail for all area taf sites during this
forecast period. Light northeast winds will continue to veer
towards the southeast by late this afternoon...but wind speeds
should remain less than 15 kts.

Despite the radar returns northwest of the metropolitan as of
1130z...showers are not expected to affect the taf sites this
morning as dry air should weaken the showers as they approach the

Isolated showers and thunderstorms are possible today for kact but
chances remain too low to mention in the taf at this time.
Additionally...some model guidance suggests a chance of convection
for the dfw metropolitan taf sites overnight tonight...but like the
previously mentioned kact rain chances...the confidence in
convection affecting the taf sites is too low at this time to
include in the tafs.



Previous discussion... /issued 417 am CDT Tuesday Jul 29 2014/

08z surface observations showed a stalled out frontal boundary
along a line extending from College Station to Austin...arcing
back to the west-northwest towards Midland. Area radar VAD wind
profiles showed that the 850 mb reflection of this front was
located somewhere between kfws and kdyx...extending south near
kgrk radar. Unfortunately the resolution of radar placement is
such that a more refined position is not possible. Regional radars
showed little activity early this morning...but there were some
scattered showers along the Texas Panhandle/Oklahoma border. This
may be close to the 850 mb front...but it is difficult to say for
sure. Water vapor satellite imagery continued to show anticyclonic
flow aloft over much of Texas...however the center of this upper
ridge appeared to be located near The Big Bend area...moving
slowly to the west. Northwest flow was observed from the Texas
Panhandle over North Texas.

Today...thunderstorm chances remain in the forecast today...mainly
for locations along and west of the Interstate 35/35w corridor.
The effects of dry air advection were evident over the dfw
metroplex and points northeast. Dew points were in the upper 50s
to lower 60s in these locations early this morning. East to
northeasterly winds throughout the day today should keep these
locations dry over the next 12-18 hours as a result of this dry air
already in place.

High resolution model guidance continues to advertise the 850 mb
front as a focus for thunderstorm initiation around sunrise this
morning. These models show 850 mb flow over west central Texas
veering around to the southwest...providing some good low-level
convergence along this front early this morning. Left 30 probability of precipitation in
the forecast along and west of a Jacksboro to Temple line this
morning in case this pans out. Most model guidance shows 850 mb
winds coming back around to the south after sunrise in response to
better low-level mixing due to heating...and Lee side cyclogenesis
ramping up later in the day due to a shortwave trough rounding the
broad ridge axis over the Continental U.S. Rockies. This should bring the 850
mb front and Theta-E ridge west a bit...and reorient it north to
south over the western County Warning Area today.

Afternoon showers and thunderstorms are possible along and west
of this boundary...with the best chances remaining west of a line
from Graham to Gatesville this afternoon. Went ahead and left some
20-30 probability of precipitation in place from Waco to Palestine and locations south
because low-level dry air advection never really made it that far
south...and dew points remain in the lower 70s early this morning.
Dry air advection will be shut off for central Texas once the 850
mb front becomes reoriented storms may be possible once
again during the peak heating hours of the day within this low-
level moisture Reservoir. As mentioned above...from the dfw area
northeast...dry air in place will not be helped by any significant
positive moisture advection these locations should
remain dry.

The shortwave trough that is riding along the Continental U.S. Rockies upper
ridge is expected to move out from over The Rockies and over the
southern High Plains by sunrise Wednesday morning. The NAM seems
to have a very good handle on this shortwave trough...which was
fairly well defined over northern Arizona on 08z water vapor
satellite imagery...moving north towards Utah. In response to this
trough...Lee side cyclogenesis is expected to move from
southeastern Colorado out over the Texas Panhandle through Wednesday
morning. The 850 mb front will move east while becoming
reoriented from west to east over Oklahoma Wednesday afternoon.

For North Texas...most model guidance indicates that this
boundary will move from west to east over the County Warning Area from late
tonight through Wednesday morning. Precipitation chances along
this retreating front are not that great because the low-level
atmosphere is really just reorganizing for the northern Arizona
shortwave trough and associated surface low pressure system
developing over the High Plains. Even though the front is
moving over the County Warning Area...the low-level flow regime favors there is really no significant forcing for ascent
associated with this boundary. The change in low-level wind
direction will result in some fairly substantial positive moisture
advection...but this by itself does not guarantee any
precipitation will develop over the County Warning Area. At any rate...left some
small precipitation chances in the forecast along the axis of best
positive moisture moisture convergence may offer just
enough lift to result in some isolated to scattered rain showers
and thunderstorms along and north of the Interstate 20 corridor
from tonight through Wednesday morning.

The primary focus for precipitation over the next 24 to 36 hours
will be over the High Plains of eastern New Mexico and Colorado as
positive moisture advection interacts with orographic lift. By
Wednesday morning...the retreating front will likely be well
defined from the High Plains extending east-southeast over
Oklahoma and should be a secondary focus for very good rainfall
chances and heavy rainfall. The primary forecast challenge after
Wednesday afternoon will be the evolution of the low pressure
system as the northern Arizona shortwave moves east-southeast from
Colorado out over the High Plains through Wednesday evening.

Most guidance shows that as the shortwave trough moves off the
High Plains it will move east-southeast...from the Texas Panhandle
and then over southern Oklahoma Wednesday evening. Models show a
concentration of high quantitative precipitation forecast over the intersection of the path of
this upper trough...and the 850 mb frontal boundary on Wednesday.
This makes sense as this is where low and upper level forcing for
ascent overlap with very deep moisture. However the consensus of
model guidance also shows the surface low moving east-southeast
dragging the front south towards the Red-River Wednesday night.

Models keep a significant amount of quantitative precipitation forecast Wednesday night along and
north of the front...which appears to be largely dominated by
neutral to negative Theta-E advection...and resides in what should
be subsidence northwest of the upper trough. The models fairly
consistently have the 850 mb Theta-E ridge over the northeastern
portions of the County Warning Area extending into northeast Texas. Models also
advertise this area...northeast of the dfw have
precipitable waters in excess of 2 inches with good low-level convergence. If
the model mass fields are would seem like that the
Wednesday night heavy rainfall axis should slide south of where
raw model output is showing its bulls-eye of quantitative precipitation forecast. As a
result...went ahead and bumped probability of precipitation up to 80 percent northeast of
the metroplex Wednesday night and mentioned heavy rainfall in the
worded forecast as model quantitative precipitation forecast seems least in the 12
hour window from Wednesday night to Thursday morning.

There are things the model may be resolving that are not evident
in the mass fields at this point. Convective feedback in model
output is generally regarded as bad...however long-lived latent
heat release can result in the diabatic creation of a middle-level
low pressure system that can displace moisture and convergence
fields. This does not appear to be the case...but it is something
that happens during long-lived precipitation events...and it is
very likely to rain heavily for many hours along the 850 mb front
across Oklahoma during the day on Wednesday. Unfortunately we will
not know if a strong middle-level low pressure system develops from
this precipitation until Wednesday at this time
just sided with the model mass fields which indicate a heavy
rainfall event is possible for the northeastern County Warning Area Wednesday
night. If models trend in the direction of heavy rain shifting
south Wednesday night...a Flash Flood Watch may be necessary
northeast of the dfw area in later forecasts.

Behind the upper trough...the surface low is expected to move
east-southeast over Louisiana by Thursday afternoon...dragging the
now cold front south across the entire County Warning Area during the day on
Thursday. While this would normally seem like a good scenario for
a widespread rainfall event for North Texas...model guidance
strongly suggests that it is not. Unfortunately as the surface low
pulls away to the east-southeast...models are in strong agreement
that strong 700 mb subsidence pushes over the County Warning Area from northwest
to southeast on Thursday. This should result in an abrupt end to
precipitation from northwest to southeast along the cold front on

Unfortunately if this pans out...much of the dfw area may miss
out on any heavy rainfall associated with this storm system
because the middle-level subsidence builds in quickly starting late
Thursday morning. On Thursday...the best chances for thunderstorms
along the cold front likely reside over the far southeastern
County Warning Area...or those locations farthest removed from the middle-level
subsidence. For those following along looking at model data...just
look at 700 mb relative humidity forecasts to track this middle-level was very easy to see on 29/00z model guidance. The
best chance for the dfw area to receive heavy rainfall is if the
surface low and 850 mb front sink much farther south than
currently expected. If this occurs...the dfw area could get into
some of the heavier precipitation amounts Wednesday night. This
does not look likely right now...but wednesdays prolonged rainfall
event over Oklahoma may change the strength of the cold air behind
the front...pushing it farther south than expected for Wednesday

Friday through early next week...assuming the strong middle-level
subsidence is correct in the models...the remainder of the
forecast should be dry. Temperatures will remain below normal
behind thursdays front...but will moderate back up to normal
levels by early next week due to persistent sunshine through the



Preliminary point temps/pops...
Dallas-ft. Worth, Texas 93 75 90 73 81 / 10 20 20 40 30
Waco, Texas 93 73 97 75 89 / 20 10 10 10 30
Paris, Texas 90 70 80 67 76 / 10 30 40 80 50
Denton, Texas 90 72 87 71 81 / 10 30 20 50 40
McKinney, Texas 91 72 84 70 80 / 10 30 30 60 50
Dallas, Texas 93 75 90 73 81 / 10 20 20 40 30
Terrell, Texas 94 73 87 72 79 / 10 20 20 50 40
Corsicana, Texas 91 73 93 74 84 / 10 10 20 20 40
Temple, Texas 93 71 97 74 90 / 30 20 10 10 40
Mineral Wells, Texas 92 72 96 71 83 / 40 20 20 20 20


Forward watches/warnings/advisories...




National Weather Service Glossary of Abbreviations