Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Fort Worth Texas
700 am CDT sun Mar 9 2014
Concerns...none significant. MVFR ceilings gradually lifting.
As the cold air mass in place modifies today...cloud bases will
lift into the VFR category. Dry westerly flow above the boundary
layer will help shunt the remaining cloud bank off to the east by
late in the day. The surface ridge will transit the area
tonight...and southerly flow will return on Monday.
Previous discussion... /issued 409 am CDT sun Mar 9 2014/
Early morning water vapor satellite imagery showed a strong
shortwave trough moving south-southeast over northwestern Mexico.
There was an amplified upper level ridge extending from central California
northeast over the northern plains with a large upper trough
extending from the southern Great Lakes back southwest to the
northwest Mexico shortwave trough. The upper trough was
significantly elongated and sheared compared to this time
yesterday...and the southwestern portion of this trough looks like
it will become cut off from the jet stream/westerlies aloft over
the next 24 hours. The primary evidence suggesting this /outside of
looking at forecast models/ is that a shortwave trough over the
in/Ohio border was moving east while another shortwave trough
located near the Texas/OK panhandles was nearly stationary early this
morning. Assuming this trend continues...the OK/Texas part of the
trough will become separated from the southern Great Lakes trough
resulting in the cut-off trough over Texas that has been advertised
by models over the past several days.
Today through Monday...in reference to yesterdays forecast
discussion...there were two scenarios mentioned with regards to
the evolution of the upper trough for Sunday /today/ and Monday.
The first was a wetter solution for the southeastern counties of
the County Warning Area where the cut-off upper low more or less consolidates into
one large circulation that spreads large scale forcing for ascent
and middle-level moisture return to locations east of Interstate 35
and south of Interstate 20 on Monday. The second solution
indicated that the upper trough would maintain some sort of closed
circulation...but would remain more open than the first solution
with a piece of energy to the north of the closed low maintaining
some attachment to westerly flow aloft over the Central Plains.
This second solution was drier because the upper low to the south
of the County Warning Area would be weaker on average...and middle-level winds would
tend to be veered...preventing significant moisture return to the
southeastern County Warning Area.
Based on water vapor satellite imagery early this morning...it
looks as though the second solution is much more likely to
verify...so in general...bumped down probability of precipitation for Monday and Monday
night in this forecast. With the northwestern Mexico shortwave
trough still digging south early this morning...it should remain
well south of the County Warning Area when it eventually moves east over the next
24-36 hours. The northern piece of energy/shortwave trough is easy
to see on water vapor satellite imagery...and was not moving early
this morning. Therefore it seems unlikely to become absorbed into
the southern piece of energy...limiting its strength and ability
to deepen significantly as it moves east over south Texas.
Left some 20 probability of precipitation in the forecast for now for locations along and
southeast of a Killeen to Athens line for Monday and Monday night
due to uncertainty of the exact track of the upper low once it
begins moving east tonight. The Canadian model indicated that the
upper low will have more of a northeasterly trajectory compared to
the GFS as it moved east Monday afternoon. If the upper low does
pull northeast at all...there would be at least a small chance for
rain showers over the southeastern County Warning Area sometime Monday or Monday
night. We will probably remain dry...but kept the 20 probability of precipitation in place
just in case the Canadian solution verifies. The forecast is dry
for the remainder of north and central Texas.
Otherwise...temperatures will be on the increase from today into
Monday as surface winds veer around to the south despite the upper
low moving south of the area. Strong Lee side cyclogenesis in
response to the approach of a strong northern stream upper trough
will dominate the pressure field over the South Plains by Monday.
The return to south winds should help bring temperatures up into
the upper 70s where we have sunshine on Monday...holding closer to
70 where clouds hang in place much of the day. Clouds are more
likely over central Texas than North Texas...so North Texas will
likely be several degrees warmer than central Texas as a result
Monday afternoon. The upper low to our south will push east of the
region for Tuesday...which should be a very warm day across the
County Warning Area.
Tuesday...the strong northern stream upper trough is expected to
push southeast over the Central Plains sending the strong High
Plains surface cyclone across the County Warning Area during the day on Tuesday.
This will eventually send a strong cold front across the region
Tuesday evening and overnight...however during the day on
Tuesday...the surface cyclone may be accompanied by a strong
dryline which could ramp up fire weather danger concerns.
Temperatures are expected to be well above normal on Tuesday with
most guidance showing an h850 thermal ridge of 16 to 18 degree c out
ahead of the cold front Tuesday afternoon. This should easily
bring temperatures up into the upper 70s to middle 80s across the
County Warning Area...or about 15 degrees above normal.
If this very warm air is accompanied by a strong dryline
push...relative humidity values could tank into the single digits
while winds shift out of the west at 20 miles per hour or so. These
conditions would support critical fire weather danger...mainly
west of Interstate 35/35w Tuesday afternoon until the cold front
arrives after sunset. In general...the dryline is a mesoscale
feature and as such is not well resolved in the GFS and European model (ecmwf). The
NAM output is confusing in that it does indicate a dryline-like
sudden decrease in dew point across the County Warning Area Tuesday
afternoon...but it does not shift the winds out of the west. The
point here is that there is the potential for critical fire
weather danger conditions Tuesday...however confidence remains too
low in the strength and position of the dryline to know if these
conditions are likely to exist in the County Warning Area. As a result...will not
issue a Fire Weather Watch with this forecast package...but will
continue to monitor for the potential for a Fire Weather Watch or
red flag warning in future forecasts.
Tuesday night and Wednesday...whatever happens with the dryline
and critical fire weather conditions on Tuesday will be very short
lived...because a strong cold front will move south across the
region from Tuesday evening through Tuesday night. A strong
northerly low-level jet of 50 kts is expected to accompany the
front south across North Texas Tuesday night. This may necessitate a
Wind Advisory for much of the area if some of these strong winds
aloft can mix down behind the front overnight. Vertical mixing in
the atmosphere overnight behind a front is notoriously weak due to
relatively stable air retarding vertical motions. However if cold
air advection near the h850 low level jet is strong enough...the
lower atmosphere could be somewhat unstable allowing for more
efficient downward transport of the high momentum air aloft. This
is just another thing to look out for moving forward...however it
is Worth mentioning that the NAM and Canadian are the strongest
with winds behind the front for our County Warning Area...showing northerly winds
of 25 to 30 miles per hour with gusts to 40 miles per hour Tuesday night. Temperatures
will fall quite a bit behind the front...with highs holding 30
degrees cooler on Wednesday than on Tuesday across the region. No
precipitation is expected along the front at this time.
Friday...another compact shortwave trough is expected to move east
over the County Warning Area. At this time there was no model guidance indicating
that there would be sufficient moisture in place ahead of this
trough to result in precipitation chances Friday afternoon. This
makes sense as Tuesday nights front is forecast to plow south all
the way across the western Gulf of Mexico...scouring out low-level
water vapor...preventing any potential return to North Texas as
early as Friday. Maintained a dry forecast as a result.
Next weekend...the GFS and European model (ecmwf) both indicate that a large upper
trough will get close to North Texas. In general...if it moves
directly over US...or south of US...we will need to add
precipitation chances to the forecast. If it moves to our
north...we will likely remain dry...which is what the current
forecast indicates. Will continue to watch the evolution of this
strong upper trough in later forecasts...however the consensus of
09/00z guidance was for a more northerly track of the upper
trough. Either way...a front will likely move across the region
Sunday or Sunday night...resulting in a cooler start to next week.
Preliminary point temps/pops...
Dallas-ft. Worth, Texas 60 41 76 48 81 / 5 5 10 5 0
Waco, Texas 59 40 71 45 76 / 20 10 10 10 5
Paris, Texas 55 38 73 46 80 / 10 5 10 5 5
Denton, Texas 59 36 75 45 82 / 5 5 5 5 0
McKinney, Texas 59 37 74 45 81 / 5 5 10 5 5
Dallas, Texas 59 42 75 50 81 / 10 5 10 10 0
Terrell, Texas 58 40 74 47 77 / 10 5 10 10 5
Corsicana, Texas 60 42 73 49 77 / 20 10 10 10 5
Temple, Texas 58 41 69 46 76 / 20 10 20 20 0
Mineral Wells, Texas 61 37 76 44 83 / 5 5 5 5 0