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Area forecast discussion
National Weather Service Fort Worth Texas
1242 PM CST Mon Feb 27 2017

concerns...thunderstorm chances this afternoon mainly northeast
and east of the metroplex taf sites. Low ceilings and fog
overnight through 18z Tuesday area wide.

At 18z...storms had formed from northwest of dfw to khqz and
were moving northeast away from the taf sites. Large hail will
become likely with these storms.

Ceilings were improving to VFR at metroplex taf sites and VFR
conditions are expected to mainly prevail through mid evening.
Expect MVFR ceilings to move back in around 05z and ceilings
will continue to lower into the IFR by 09z. Although not
mentioned in the tafs...ceilings could go down below 500 feet.
Expect an improvement to MVFR by 18z Tuesday.

At Waco...expect mostly VFR conditions through mid evening with
MVFR ceilings around ovc012 expected by 04z. Ceilings will then
lower into the IFR and areas of fog are also expected. Ceilings
are then expected to fall into the LIFR category 11-16z.



Mesoscale update...
an interesting and challenging weather scenario continues to
unfold across the region this morning regarding the coverage and
placement of strong to severe thunderstorms.

Morning surface analysis revealed a wavy warm front extending
eastward from a diffuse area of low pressure just north of the
Sweetwater area. This warm front, which appears to be nicely
demarcated by the 55 degree f isodrosotherm (as well as locally
reduced visibilities in fog) continues to gradually push northward
near the Red River. To the west, a dryline is sharpening from
near Sweetwater to Iraan and into The Big Bend of Texas. In
between these two features, a moist and increasingly unstable warm
sector is in place. Near-surface mixing ratios on the order of
11-13 g/kg, surmounted by steep mid-level lapse rates on the order
of 7-7.5 c/km are resulting in MUCAPE values inching past 1500
j/kg roughly south of the I-20 corridor late this morning.

Water vapor loops reveal another key to today's forecast--in the
form of an implied east-west oriented jet streak roughly bisecting
Texas and associated shortwave trough axis currently nearing north
and central Texas. Lift associated with these features appears to
be underway based on the presence of cooling cloud tops and the
development of showers and storms across southwestern portions of
our County Warning Area. The presence of the aformentioned moderate instability,
steep mid-level lapse rates, and a kinematic environment
characterized by effective bulk shear values of 40-50 kts will
support supercells at least initially elevated above a capping
inversion near 850 mb per the 12z kfwd sounding with large hail
(perhaps even some very large hail) as the primary threat. High-
resolution guidance suggests convective coverage should remain
generally isolated with the greatest risk area to the north of a
line from near Waco to Palestine and bounded to the west by US
281...this includes the metroplex.

As daytime heating continues, there is some concern that storms
may become surface based. The presence of backed low-level flow in
the vicinity of the aformentioned warm front may locally enhance a
tornado threat mainly east of a Bonham to Terrell to Athens line.



Previous discussion... /issued 358 am CST Mon Feb 27 2017/

No shortage of forecast challenges with the main concern being
the potential for strong/severe thunderstorms capable of producing
very large hail both today and Tuesday, possibly beginning as
early as daybreak today.

Widespread cloudiness overspreads the area this morning with
abundant low-level moisture in place. Where skies temporarily
cleared across our northwestern counties, very low clouds and
patchy dense fog have developed. This is in the vicinity of a
stalled frontal zone roughly near Highway 380 as of 330am. Would
expect low clouds with some patchy drizzle and fog in vicinity of
this front through the next few hours before the boundary lifts
back to the north later this morning.

Broad troughing and southwest flow aloft exists across much of
the southern US with a 100+ kt upper jet streak positioned through
Texas. In addition to the frontal boundary positioned through north
TX, a dryline is draped roughly from Wichita Falls to Abilene.
Dewpoints remain in the upper 50s and 60s east of the dryline
where a potent warm sector airmass is in place. Warm advection
will begin increasing this morning as southerly flow strengthens
through the low-levels. As this occurs, weak isentropic ascent
will overspread much of the warm sector within the capping
inversion layer centered around 900mb. If there is sufficient lift
within this layer, it will be capable of generating at least
isolated elevated thunderstorms given the very steep lapse rates
located through the mid-levels.

As mentioned above, the main concern today will be the potential
for severe thunderstorms capable of producing very large hail,
possibly developing as early as daybreak with additional storms
possible throughout the afternoon. A very favorable environment
for large hail is expected to take shape across most of north and
central Texas as shown by forecast soundings from numerous
sources. This environment is characterized by very steep mid-
level lapse rates (as much as 8.5 c/km), around 2000 j/kg of
MUCAPE, 50-60 kts of deep-layer shear, and wet bulb zero heights
around 9 kft agl. Forecast ship values of 1.5-2 confirm an
environment conducive for storms to produce significant hail. If
storms are able to develop and their buoyancy is able to overcome
the lack of strong large-scale dynamic ascent, they will become
capable of becoming strong to severe rather quickly with the
primary convective Mode expected to be elevated discrete

While the confidence in the severe potential is high, confidence
in the development, timing, and location of storms is rather low.
If storms develop, they are expected to be isolated, hence the
fairly low pops. Latest high-res guidance is pegging various
locations for the primary threat, targeting parts of central Texas
through the morning hours and transitioning northwards towards
I-20 by midday. Trends will need to be closely monitored through
the next several hours, but more than likely the location of
convective initiation won't be known until it a storm has already
begun to develop. If supercells are ongoing this afternoon, it
will be possible for them to become surface based as boundary
layer warming and weak ascent combine to reduce or eliminate the
low-level capping inversion. If this occurs, a low tornado
potential can't be ruled out given the shear and buoyancy
parameters. In addition, if winds remain backed to the southeast
in the vicinity of the frontal boundary as it lifts back to the
north, this would be capable of locally enhancing storm-relative
helicity through the lowest 1 km.

In summary, while storms are not a guarantee to develop today,
any storm that does form will have the potential to become severe
and capable of producing very large hail. If storms become
surface-based this afternoon, a low tornado potential will exist
as well. The timing and location of storms is very uncertain and
any storms that do form should remain fairly isolated.

A similar setup is expected on Tuesday with very steep lapse
rates remaining in place as well as a very juicy warm sector with
dewpoints in the mid to upper 60s expected east of the dryline
once again. The difference will be a strong shortwave trough and
associated pv maximum sweeping through the Southern Plains
Tuesday afternoon. Given the increased dynamic forcing, the
initial thought is that storms would be even more likely on
Tuesday afternoon. However, most guidance, especially short-term
high-res guidance, is showing a dry forecast due to a strong
capping inversion in place on Tuesday afternoon. Given the
convective parameters and the stronger forcing, have essentially
gone completely against most guidance on Tuesday afternoon and
included thunderstorms with chance pops for most areas east of the
dryline which should once again be positioned across the western
portion of the forecast area. The thought is that the stronger
forcing and increased low-level warming should be capable of
breaking the cap in localized areas allowing at least a couple of
isolated strong storms to develop. However, there is uncertainty
as to if storms will be able to form ahead of the dryline Tuesday
afternoon, but if they do, they will likely be capable of becoming
strong to severe. The primary storm Mode would be supercells once
again with the primary threat being large hail for the same
reasons discussed above from monday's setup.

Later Tuesday night and Wednesday morning, the aforementioned
shortwave trough will pull a strong cold front through the area. A
thin line of storms is possible along what should be a fast-moving
cold front as it overtakes the dryline and catches up to the
better moisture. The front will surge south pretty quickly,
entering our northwest counties a little after midnight and
clearing our southeast counties around daybreak Wednesday.
Temperatures will be quite a bit cooler Wednesday through the end
of the week as cooler and drier air works into the region. While
tuesday's highs should be in the upper 70s and 80s, wednesday's
highs are only expected to reach the 60s. Some near- or sub-
freezing low temperatures are possible across rural parts of
North Texas on Thursday morning.

Near-normal temperatures and dry weather are expected through the
end of the week before an upper low digs southward across
northern Mexico by late Friday. This trough should bring
widespread rain to southern TX, but the best moisture supply and
forcing is expected to remain mostly to the south of our forecast
area. Some isolated showers and a couple storms will be possible
over the weekend with chances generally increasing the farther
south your location. Extended guidance is actually in fairly good
agreement on a stronger frontal passage early next week which
could result in additional chances for thunderstorms and much
cooler weather for the second week of March.



Preliminary point temps/pops...
Dallas-ft. Worth 78 64 82 53 63 / 40 20 30 40 5
Waco 76 65 81 53 65 / 40 20 30 30 5
Paris 70 62 76 52 63 / 40 20 40 50 10
Denton 76 62 80 50 62 / 30 20 30 40 5
McKinney 73 64 76 53 62 / 40 20 30 50 5
Dallas 78 65 81 54 63 / 40 20 30 40 5
Terrell 76 65 79 54 64 / 40 20 40 40 5
Corsicana 78 65 81 56 65 / 40 20 40 40 10
Temple 76 64 82 54 64 / 30 20 30 30 5
Mineral Wells 80 58 83 47 64 / 20 10 20 20 5


Forward watches/warnings/advisories...

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