Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Fort Worth Texas
1137 PM CST Friday Nov 27 2015
IFR to occasional MVFR ceilings and mostly MVFR visibilities will
continue at all North Texas taf sites through Saturday evening.
Periods of light to moderate rain will also continue through
Saturday. However...there should be a decrease in the
precipitation intensity at all taf sites beginning around 08z as
per current radar trends.
Northerly winds will continue through Saturday in the 9 to 14
knot range along with some higher gusts.
no major updates planned with the existing forecast. Good
850-500 mb frontogenetical forcing persists in a narrow axis from
around San Angelo to McAlister OK this evening...roughly
coincident with the band of steady precipitation seen on regional
radars. 00z soundings depict some small amounts of elevated
instability (200-350 j/kg)...thus explaining the isolated
lightning strikes observed over North Texas this evening.
Given the generous isentropic lift across the area...very
confident about widespread synoptically-driven rainfall
persisting across all of north and central Texas through the
night. Thus...100% probability of precipitation look good everywhere overnight. Not as
confident about the evolution/maintenance of locally heavier
mesoscale forcing...though it cannot be totally ruled out.
Rainfall rates have remained below 1"/hour this evening...and in the
absence of additional convective instability/mesoscale forcing...am
inclined to believe this trend of light/moderate rain will
continue overnight. Much of the area along an axis from
Eastland/Comanche NE through dfw to Paris will see additional
rainfall totals of around 1.5-2" through daybreak...with lighter
amounts farther to the southeast (and in the extreme northwest counties as well).
These rainfall rates should preclude serious flash flooding
concerns...but small stream flooding and general ponding will
remain a hazard across much of North Texas through the night.
On the flipside...winter weather concerns should remain confined
to the west of our forecast area through the night. We do think
surface temperatures will slip down to around 31-32 degrees over
parts of Young and Stephens counties prior to dawn. However...with
such a major warm nose aloft...the rain reaching the surface
should remain warm enough to preclude any icing problems on roads.
Some minor icing on trees and other exposed objects is not out of
the questions in these counties...however. Elsewhere...no icing
issues are expected.
Previous discussion... /issued 410 PM
no major changes to the current forecast as a cold rain event
continues across North Texas. The cold front has cleared all of
north and central Texas at this hour with temperatures in the middle
30s across the northwest and middle 40s to the south. Most of the
rainfall is currently falling generally along and north of
Interstate 20. Rainfall rates have diminished significantly behind
the cold front but we are still seeing heaviest rates around 3/4
inch per hour. With grounds saturated across the area...even light
additional amounts may cause more flooding problems.
Water vapor imagery shows an impressive pattern across North Texas
with a large upper low across the southwest U.S. And a landfalling
tropical cyclone in central Mexico. While the tropical system
doesnt directly impact Texas...its associated upper level
anticyclone helps create a large diffluent upper wind field across
the Southern Plains. This is clearly evident in the water vapor
imagery. A 150kt upper jet will slowly impinge on the area later
this evening with large scale forcing for ascent increasing
through the overnight hours.
Our greatest area of concern through tonight is across the
northwest half of the County Warning Area...in areas that received copious amounts
of rain last night. Short term guidance indicates that there will
be an area of enhanced frontogenetic forcing that sets up across
the northwest counties...likely as colder air aloft tries to push
a littler farther east. Strong isentropic ascent continues up the
frontal slope with moisture content remaining very high. Think
there will be another round of moderate to heavy rain overnight in
these areas with an additional 1 to 2 inches of rain expected.
Concerning temperatures tonight...many areas across the northwest
counties have fallen into the middle 30s with dewpoints in the 33 to
35 degree range. There will be no room for wet bulb cooling as the
lower atmosphere is saturated...so the only way they will cool
further is by cold advection. That appears to become neutral by
later this evening so think temperatures across the area will stay
just above freezing. No wintry weather impacts are expected in our
By Saturday morning...the strong isentropic ascent expected
tonight will be weakening with the bulk of the precipitation
moving to the northeast. This wont be the end of the rainfall
though...but rather just another brief break. Moderate warm
advection will resume by midday and again be maximized during the
overnight hours Saturday night. This is also when it appears that
a piece of shortwave energy swings across the area. The upper
dynamics are quite strong and will continue to support deep ascent
across the region. All of the rainfall should start to taper off
during the day on Sunday as the upper trough to the west starts to
lift off to the northeast.
Concerning additional rainfall amounts...
We are expecting another 2 to 4 inches of rain across parts of
the area through Sunday. Highest amounts will again be along and
north of Interstate 20.
Concerning flooding impacts...
Rain rates will generally be 1/10 to 3/4 of an inch per hour
through the remainder of this event. While these rates are not
terribly high...the grounds are completely saturated and cannot
take any more rainfall. Even light rainfall can cause additional
flooding with little advance warning and can be especially
dangerous at night. We have already had four reported fatalities
with this flood event...so it must be stressed...do not drive into
areas where water covers the Road!!
Remainder of the forecast...
as the main upper trough ejects northeast there will be another
strong shortwave that moves through quickly late Tuesday into
Wednesday. There will be some available moisture to work with...so
will keep some low chance probability of precipitation in south of Interstate 20. No major
changes in the extended.
Preliminary point temps/pops...
Dallas-ft. Worth, Texas 37 42 39 46 42 / 100 100 100 70 20
Waco, Texas 39 43 41 46 42 / 100 100 100 80 20
Paris, Texas 41 45 42 48 44 / 100 100 100 80 40
Denton, Texas 35 40 38 45 40 / 100 100 100 70 10
McKinney, Texas 38 42 39 47 42 / 100 100 100 70 20
Dallas, Texas 38 42 40 47 43 / 100 100 100 70 20
Terrell, Texas 40 43 41 48 44 / 100 100 100 80 30
Corsicana, Texas 41 45 42 48 44 / 100 100 90 80 30
Temple, Texas 39 43 41 46 43 / 100 100 100 80 20
Mineral Wells, Texas 34 37 36 43 38 / 100 100 100 60 5
Flash Flood Watch through late Saturday night for txz091>095-