Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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FXUS64 KFWD 200454 AAB

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1154 PM CDT Thu Apr 19 2018

VFR through Friday evening with increasing mid and high clouds
associated with an approaching low pressure system.

An east wind at speeds less than 10 knots overnight will become
more southeast through the day Friday as surface low pressure
deepens over Eastern New Mexico. Wind speeds will increase through
the day Friday as the surface pressure gradient tightens. Wind
speeds Friday afternoon and evening will range from 12 to 15 knots
along with some higher gusts.

Low level moisture will slowly increase through Friday evening,
however, low clouds should not impact any of the TAF sites until
after 12Z Saturday.

Showers and thunderstorms are expected to develop across West
Texas Friday afternoon/night as lift from the approaching low
pressure system interacts with increasing low level moisture. All
storms should remain west of the TAF sites until after 12Z



.UPDATE... /Issued 936 PM CDT Thu Apr 19 2018/
Made a few cosmetic changes to sky grids and thus overnight low
temperatures north of I-20. Otherwise, the remainder of the short
term forecast is in fantastic shape.

High level clouds associated with a compact upper trough will
continue to lift northward, though it looks like it`ll be at a
slightly slower rate than previously thought. As a result, have
delayed the onset of clouds for areas near/north of I-20. With the
dry air, diminishing wind speeds and clear skies, decent
radiational cooling along the Red River appears probable and I`ve
nudged overnight low temperatures down by a few degrees here.
Elsewhere skies will be partly to even mostly cloudy at times.
Otherwise, the rest of the forecast looks to be on track and no
other changes were needed.



.SHORT TERM... /Issued 250 PM CDT Thu Apr 19 2018/
/Tonight and Friday/

Temperatures this afternoon are running about 10 degrees below
where they were 24 hours ago as cold advection continues behind
yesterday`s front. Passing high cloud cover will occasionally
result in partly sunny conditions, but skies should (temporarily)
clear out across all of the area early this evening.

A weak/subtle mid-level impulse will emerge out of Coahuila late
tonight and into Friday morning and proceed to rapidly track into
the Hill Country and Central Texas. Mid-level lapse rates will
steepen (slightly) and the 700-400 mb layer will moisten ahead of
this feature. This combination may just eke out a sliver of
elevated instability Friday morning and into the early afternoon
from I-35 and west. This might be just sufficient to result in
some convective elements within the blossoming mid-level cloud
deck, similar to what has been showing up in recent runs of the
3 km NAM`s reflectivity output. That said, forecast soundings
reveal a surfeit of dry air in the lowest 10,000 feet or so. In
addition, isentropic charts show the more robust upglide/ascent
relegated just to the west of our CWA in association with strong
warm advection ahead of the primary upper-low ejecting into the
Great Plains. As a result, think the signal tomorrow is more for
virga or sprinkles roughly near and west of the I-35 corridor
through midday. By late-afternoon and into the early-evening,
better upglide will begin to leak into our northwestern zones
where we`ve inserted low chances for showers or an isolated
thunderstorm. Severe weather is not anticipated.

With the forecast of thicker cloud cover (at least into the
early-afternoon), sided slightly on the cooler side of guidance
for high temperatures. Winds will gradually return to favor a
southeasterly direction into the afternoon-hours, with some
occasional wind gusts to 20 mph.



.LONG TERM... /Issued 250 PM CDT Thu Apr 19 2018/
/Friday Night through Thursday/

A potent closed upper level low will charge eastward across the
Southern Plains during the day Saturday. While these types of
systems in mid to late April would normally bring us a significant
threat of severe weather (as this mid and upper level pattern
matches a severe weather high risk day in 2007), the low level
airmass is both too dry and cool for such a threat this time
around. Still there will be a small window for severe storms south
of I-20 and west of I-45 in the late afternoon hours along an
advancing front/dryline and ahead of a well organized surface low.

Because the initial low level airmass is so cool, as the warm
conveyor belt ahead of the upper low begins to translate into the
area, it will invoke strong isentropic lift and widespread showers
and a few storms. This activity will begin a few hours before
dawn across the western and northwestern zones, and spread
eastward during the morning hours. Rainfall coverage and amounts
will be highest generally north of I-20 where isentropic lift will
be enhanced along and to the north of an intensifying baroclinic
zone (or organizing cold front). On average rain totals will be
around 1 inch in this area, but isolated higher totals of 2 or 3
inches could result in minor flooding issues. Since dewpoint
depressions will be high when the rain starts, evaporative cooling
will force temperatures to fall or remain in the lower 50s in
rainy areas. This cold pool enhancement will help the cold front
slide southward during the day with the remaining warm sector
getting progressively squeezed and cornered to the southwestern
zones and far southern zones. Prefer the NAM/WRF suite of guidance
which has a slightly faster cold front progression. The SW zones
are the area of the CWA with the best potential for a few severe
storms to develop in the mid to late afternoon hours, but the
limited flow in the low levels and forecast CAPE values only
between 500 and 1500 J/kg suggest just a marginal threat of severe
hail and wind reports. A few of these storms could head northeast
and cross the frontal boundary and pose an elevated hail threat
farther northeast which is why the severe risk area is advertised
as the greater region south of I-20 and west of I-45. Obviously
if morning rain is limited across the southwestern zones and
afternoon sunshine can occur, instability would be higher for more
robust storms, so this is something we will be monitoring. For
now will forecast highs only in the low to mid 70s in the SW zones
ranging down to the mid 50s along the Red River.

Rainfall will end from west to east late Saturday afternoon and
evening as the trough axis clears the region and dry/cold
advection kicks into high gear in the wake of the now accelerating
cold front. Sunday will see gradually clearing clouds,
occasionally breezy northwest winds, and seasonably cool
temperatures. Highs will range from the mid 60s to lower 70s. Lows
Sunday night should fall into the 40s area wide.

Upper level ridging will prevail across the High Plains and
southern Rockies for the early to mid part of next week which will
place our region in northwesterly flow aloft. This will mean
pleasant and tranquil weather for Monday and Tuesday, but as an
upper wave drops through the Midwest on Wednesday it will push
another cold front through the area. Model guidance is not in
good agreement with the speed and position of the upper wave, so
while there is reasonable confidence on the timing of the front
early Wednesday, the rain forecast is very uncertain which also
impacts the high temperatures. For now will stay conservative on
rain chances and keep them at 20 percent and stick with the
median of the model suite for the temperatures.

Late in the week model guidance continues to show a lot of
uncertainty. Some guidance breaks the ridge down and brings
another closed upper low toward the region, while others are much
slower with this system. It is just too early to pick a solution
and thus the official day 7 forecast is also the median from the
model guidance which shows temperatures near normal and slight
chances of rain.



Dallas-Ft. Worth    49  69  54  61  50 /   0  10  20  90  40
Waco                48  70  52  67  51 /   5  10  20  70  50
Paris               43  65  51  62  51 /   0   5  10  90  60
Denton              46  67  53  60  48 /   0  10  30  90  40
McKinney            45  66  51  60  49 /   0   5  20  90  50
Dallas              49  69  54  62  51 /   0  10  20  90  50
Terrell             45  68  51  64  51 /   0   5  10  90  60
Corsicana           47  68  51  65  52 /   0  10  10  80  60
Temple              49  70  54  70  51 /  10  10  20  70  40
Mineral Wells       47  69  53  61  47 /  10  20  50  80  30




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