Area Forecast Discussion
Issued by NWS Amarillo, TX

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000
FXUS64 KAMA 171800
AFDAMA

Area Forecast Discussion
National Weather Service Amarillo TX
100 PM CDT Sun Sep 17 2017

.SHORT TERM...

Bottom line up top:
-Severe weather expected
-Main hazards will be large hail(up to 1.75in) and damaging winds (up to 70 mph)
-Additional hazards are for an isolated tornado (see below)
-Greatest area to see severe: along and south of a Dalhart to Perryton line
-Timing of Hazards: 4PM through Midnight
-Storm motions: East at 25 to 35 mph


Details: Hi-res models continue to show the potential for severe
weather this afternoon and into the overnight hours primarily
across the southern and eastern Texas Panhandle. In fact we are
already seeing discrete storms forming along the eastern range of
the New Mexico Rockies. These storms are anticipated to move
across the eastern New Mexico Plains and into the Texas and
Oklahoma Panhandles around 4 PM to 5 PM this afternoon. Hodographs
show that while there may still be a few discrete storms as
convection moves into the southern portion of the area that they
will merge into a cluster of storms as they continue to progress
eastward. As storms move into the area we will see CAPE in the
1500 to 2500 J/kg range with deep layer shear of 40 kt. This will
be supportive of organized convection of the supercell variety
while discrete. Large hail up to golf balls will be possible with
damaging wind gusts up to 70 mph.

Storms which develop north of the above noted area, may be more of
the elevated variety given that those areas have been socked in
with low clouds throughout the day. Despite some portions of the
cloud back dissipating, they will still lag behind the rest of the
area on warming sufficiently to overcome the cap. This won`t
prevent severe storms from moving over them, but it will limit the
northern extent of the isolated tornado risk.

Speaking of the tornado risk. Our risk for an isolated tornado
resides along the edge of our low clouds this morning. IR
satellite imagery showed a low cloud bank roughly from Dumas to
Borger to Lipscomb this morning. These low clouds have greatly
hindered warming throughout the day as evident of the 59 degree
temperature in Dumas as of noon. This will set up a thermal
gradient along where the clouds have been which could provide
enough additional low level lift and low level helicity this
afternoon for a brief spin up. This is especially true of any
storm which has a storm motion parallel to the gradient. The
orientation of the thermal gradient would also provide a
horizontal roll already in place for an updraft to ingest. This
presents our tornado threat. Hi-res models have struggled to
resolve this thermal gradient and persistent northerly winds north
of the boundary.

Storm are expected to move across the Panhandle through the
overnight hours before exiting around 3am. We expect to see the
severe risk diminish after midnight.

Monday through Tuesday...
Mostly dry conditions are expected to
prevail through the first half of the work week; minus an isolated
storm across the eastern zones Monday afternoon. The most notable
weather impact will be the return of near record high
temperatures on Tuesday. Current forecast runs about 5 degrees
below record highs. These warm and windy conditions (sustained 25
to 30 mph) could also bring the risk for elevated fire weather
conditions across the northern zones. This risk could be mitigated
if the northern zones get a fair amount of precip tonight.

&&

.PREV DISCUSSION... /Issued 1254 PM CDT Sun Sep 17 2017/

AVIATION...
For the 18Z TAFs, a low confidence forecast with potential for
amendments later today. IFR to MVFR cigs at KGUY and KDHT will
slowly lift and erode at KDHT this afternoon while improving to
MVFR at KGUY during this same time period. The greatest threat
for thunderstorms late this afternoon and evening will be at KAMA
followed by KDHT. Have incorporated mention of this element at
both sites, and omitted at KGUY due to low confidence in its
occurrence. Low confidence in whether IFR cigs will return at the
terminal sites late tonight so have only included a sct deck for
this issuance.

Andrade

PREV DISCUSSION... /Issued 1057 AM CDT Sun Sep 17 2017/

UPDATE...
Adjusted max T grids down some across northern zones this
afternoon due to extensive low clouds in post frontal atmosphere.
Remaining forecast grids remain plausible at this time. Updated
products have been sent.

Andrade

PREV DISCUSSION... /Issued 643 AM CDT Sun Sep 17 2017/

AVIATION...
For the 12z TAFs...
Short broken line of thunderstorms currently north of AMA moving
east at 10 to 15 kt.  These will move east of AMA shortly.
Remaining terminals expected to not be threatened by thunder this
morning.

IFR and MVFR ceilings moving into northern terminals at this time
expected to persist through much, if not all, of the morning as
moist northeast low-level winds continue.

Scattered thunderstorms expected to redevelop this afternoon and
evening, with a good chance for all terminals to receive a direct
impact.

Another round of low clouds could materialize late tonight and
early Monday morning, but certainty of extent and timing is too
low to include in forecasts at this time.

Cockrell

PREV DISCUSSION... /Issued 535 AM CDT Sun Sep 17 2017/

DISCUSSION...
With another day of southwest flow aloft, another shortwave is
progged to cross the area this afternoon. This shortwave will
bring added instability to the already moisture-rich profile
thanks to Hurricane Norma off the coast of the Baja Peninsula.
Short term models and CAMs are not in full agreement on just how
the day will evolve; however, all models are indicating storms
will likely form just to our south or west between 3pm and 6pm
and move through the Panhandles. Storm mode will likely be
supercells due to 35 to 50kts of bulk shear. CAPE profiles vary
between the models with the NAM being the most aggressive with
between 2000 and 3000 J/kg. With large amounts of CAPE in the hail
growth zone, large hail, up to golf ball size, will be possible.
Decent amounts of DCAPE and fast storm motions will mean damaging
winds could be possible for collapsing storms. Low level helicity
and hodographs show that a brief tornado could not be ruled out.
Storms should generally clear out of the area between 3am and 6am.

Meanwhile, the storms firing right now have been pulse
thunderstorms and should die off later this morning. Monday will
see a transition in the upper levels to zonal flow which will cut
off moisture from Norma as well as limit shortwaves from coming
across. This will also allow for downsloping winds on Tuesday
which will bring warm temps and strong winds, leading to elevated
fire weather conditions. Wednesday will see a frontal passage that
will bring temps back to seasonal norms. The end of the work week
and beginning of the weekend will see a return of southwest flow
as a positively tilted longwave trough pushes across the Rockies.
This will allow the return of shortwaves and thus thunderstorm
chances.

Beat

FIRE WEATHER...
Moisture-rich air will keep humidity values above 20 percent
through Monday. With RH values dropping into the teens and strong
winds on Tuesday, short-lived elevated fire weather conditions
for the northwest half of the combined Panhandles will be likely.
RH values will return above 20 percent by Wednesday along with
lighter winds.

Beat

&&

.AMA Watches/Warnings/Advisories...
TX...None.
OK...None.

&&

$$

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