Forecast Discussion for Dallas/Ft. Worth, Texas

FXUS64 KFWD 200347 AAB

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
947 PM CST Tue Feb 19 2019

Quick update to remove the mention of thunder from the rest of the
overnight grids and to trim PoPs back based on latest radar and
satellite trends.

Mid-evening water vapor loops show the mid-level dryslot has
blasted through our entire CWA which has shunted the reservoir of
deepest moisture off to our east. Recent ACARS soundings show that
we`re still saturated up to about 800 mb, and this is probably
just deep enough to support some episodic bouts of drizzle as the
last shortwave/vort max (seen on water vapor loops impinging on
the I-35 corridor) pivots through the region. With the loss of
deeper moisture, the steeper lapse rate plume above 700 mb should
be out of reach, so the risk for thunder is rapidly diminishing
across our eastern-most counties. Sub-freezing surface
temperatures will lag far enough behind this last shortwave to
preclude any issues with patchy freezing drizzle in our western
and southwestern counties overnight.

While we may see some 1-3 mile visibilities in any heavier
drizzle, don`t envision widespread dense fog issues overnight with
additional high-level moisture streaming overhead after midnight
and decent westerly flow in place just off the surface. We`ll keep
a close eye on observations, however. Updated products have been



.AVIATION... /Issued 555 PM CST Tue Feb 19 2019/
/00z TAFs/

Poor flying conditions will persist this evening, but
improvements are anticipated late tonight and into Wednesday
morning as drier air filters into the region.

Early-evening water vapor loops reveal the mid-level dryslot
associated with this latest storm system pivoting into the I-35
corridor. Associated drying above 750 mb will result in saturated
depths steadily decreasing over the next few hours at our TAF
sites. Lingering lift from a couple straggling shortwaves will
allow for some light precipitation for a few more hours, but the
loss of moisture above 10 kft will result in a transition to
periodic bouts of light drizzle.

As westerly flow deepens overnight, cigs should improve first at
Waco as the initial dry air intrusion sweeps into parts of Central
Texas. The Metroplex TAF sites should take a bit longer to clear
out, but should return to VFR towards the 13-15z timeframe. With
plentiful near-surface moisture in place, the potential for fog
development is non-zero. However, the combination of modest
westerly flow just off the surface and increasing mid-high cloud
cover should temper this possibility. If low clouds clear out
earlier than expected and incoming cirrus is slower to arrive, the
fog potential would increase. We`ll continue to monitor
observations this evening, but the potential for additional
IFR/LIFR vsbys reductions after 09-10z is too low to mention
explicitly in the current TAFs.

VFR conditions will prevail from mid-morning Thursday and onwards.
Surface winds will eventually become southeasterly late Thursday
afternoon and evening.



.SHORT TERM... /Issued 342 PM CST Tue Feb 19 2019/

Showers and isolated thunderstorms will shift steadily eastward
through the evening as the upper trough axis swings through the
region. We will maintain the highest PoPs across the eastern
zones, mainly though mid evening. All precipitation will end
overnight with increasing large scale subsidence on the backside
of the departing system. Although drier air will work in from the
northwest, clouds will be slow to clear due to the depth of the
moisture. Therefore, dense fog formation is unlikely even though
the ground will remain near saturation and wind will be fairly
light. The clouds will also keep temperatures from falling rapidly
so overnight lows should remain in the 30s.



.LONG TERM... /Issued 342 PM CST Tue Feb 19 2019/
/Wednesday through the Weekend/

The main concern in the extended forecast will be the potential
for thunderstorms, mainly across Central and East Texas.
Otherwise, expect generally temperate conditions across the

Dry northwest flow will continue to overspread much of the region
on Wednesday morning which should erode the pesky deck of stratus
that has been in place. NAM forecast soundings do hint at the
potential for some patchy drizzle/fog in the morning hours, but at
this time, the potential looks low with west flow expected.
Wednesday should be rain-free and low clouds will be shunted
eastward. It won`t be nearly as cool with west winds helping to
promote some downsloping. A dense cirrus canopy will stream in
from the southwest which will result in partly sunny skies for
most of the area.

Return flow should establish itself as upper level troughing
induces lee-side cyclogenesis to the northwest late Wednesday and
into Thursday. At this time, most of the pre-dawn hours on
Thursday should remain rain-free, but that will likely change very
quickly after sunrise. Isentropic ascent should ramp up quickly
and with steepening lapse rates (nearly 7-8 C/km), there will be a
risk for some elevated thunderstorms. With the deep layer wind
shear magnitudes on the order of 30 knots, there could be a window
elevated storms capable of hail. We`ll monitor this potential
over the next 12-24 hours.

Warm air advection will increase even more on Friday, which
should yield additional chances for showers and a few
thunderstorms. I`ve undercut blended guidance rain chances a
little as most of the strong forcing for ascent should still
remain west of the area, but given the strong WAA and steep lapse
rates, a 50-60 PoP across at least the eastern 2/3rds of the area
seems appropriate. The severe weather potential isn`t great on
Friday as instability may still remain on the marginal side. If
instability values are a little higher than currently believed,
the risk will increase.

Saturday features the best chances for showers and thunderstorms
as a sharp Pacific frontal feature slides eastward. Model guidance
has converged some on the position of an upper level low and this
yields slightly greater confidence in the positioning of the
aformentioned frontal feature. At this time, it appears that the
upper trough will eject quicker to the east than previous model
iterations and this casts some doubt on the severe weather
potential. I`ll hold on to PoPs across the entire area, but if
these trends continue, we may be able to remove rain chances for
the Big Country all together on Saturday. Low level moisture will
stream northward after a couple of days of good southerly flow.
and this should boost instability values into the 1000-1200 J/kg
range. With a very dynamic upper trough, deep layer shear values
will easily exceed 30 knots and the combination of the
aformentioned instability will promote organized convective modes.
There are a couple of caveats that preclude "sounding the alarm"
right now. 1) A faster frontal timing would mean that convection
would possibly fire at a less than ideal time of day for North and
Central Texas (smaller amounts of surface based instability). 2)
The widespread cloud cover and possibly early day convection (due
to a strong cap) could hamper afternoon storm development and/or
diminish instability necessary for severe storms. While it is a
little too early to nail down the specifics surrounding the
thermodynamic environment currently, it is likely that these
details will be ironed out in later forecasts.

Behind the front, there should be decent west winds and with a
greater degree of vertical mixing, temperatures should easily
climb into the mid to upper 60s with low relative humidity.
Depending on the areal coverage of rainfall over the next several
days, a slightly elevated fire weather threat may materialize.
Sunday will be a little cooler as we lose some of the downsloping,
but conditions look to still remain seasonal. Low rain chances
re-enter the forecast next week as return flow establishes itself
across the region.



Dallas-Ft. Worth    34  56  40  56  45 /  10   0   5  10  20
Waco                33  59  37  56  46 /  60   0   5  20  30
Paris               34  51  36  54  42 / 100   0   5  20  20
Denton              31  55  36  56  43 /  10   0   5  10  10
McKinney            32  53  36  55  43 /  20   0   5  20  20
Dallas              36  56  40  56  45 /  40   0   5  10  20
Terrell             35  54  38  55  44 / 100   0   5  20  20
Corsicana           36  57  41  55  47 /  80   0   5  30  30
Temple              34  60  39  56  47 /  20   0   5  20  30
Mineral Wells       31  57  37  57  44 /  10   0   5   5  10





NWS FWD Office Area Forecast Discussion