Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 221637
AFDLZK

Area Forecast Discussion
National Weather Service Little Rock AR
1137 AM CDT FRI JUL 22 2016

.UPDATE...
Latest surface observations show that most of central and eastern
arkansas well on their way to heat indices exceeding 110 degrees
this afternoon and an excessive heat warning has been issued.

Cloud cover and convection not as extensive as yesterday at this
time allowing temperatures to rise a degree or two higher. Grids
updated to increase dew points over these areas these area as
well. All products out.

&&

.PREV DISCUSSION...(ISSUED 611 AM CDT FRI JUL 22 2016)

DISCUSSION...

Updated to include the 12Z aviation discussion below...

AVIATION...

While VFR conditions will be dominant through this TAF
period...some isolated afternoon SHRA/TSRA will be possible. As a
result...have mentioned VCTS for now. Some patchy fog may again be
seen during the early morning hrs on Sat. Otherwise...winds should
be less than 10 kts...with only afternoon CU with bases around
4-6 kft.

PREV DISCUSSION...(ISSUED 402 AM CDT FRI JUL 22 2016)

SHORT TERM...Today Through Saturday Night

Same story...different day...a statement that pretty much sums up
the forecast again this Fri. The center of the upper ridge remains
just west of the state...and while hot and humid conditions are
expected as a result of this upper ridge...there will be some
potential for precip this afternoon. Just like in the past few
days...some isolated afternoon convection will be possible. Some
gusty winds and locally heavy rainfall could be seen with this
convection.

The bigger story is the ongoing excessive heat. Highs will again be
in the 90s to around 100. Heat index values should peak above 105
for most if not all locations this afternoon...and a heat adv is now
in effect for all counties within the CWA. Cannot completely rule out
a few sites exceeding 110 deg heat index this afternoon...but
potential seems a bit too isolated to upgrade to an excessive heat
warning at this time.

Hot and humid conditions will again be the story leading into Sat.
Overnight lows will be in the 70s to low 80s...with little relief
from the heat expected after dark. Highs on Sat will be similar to
what is expected this Fri afternoon. However...the influence of the
upper ridge will be a bit less due to lowering heights aloft.
So...temps may end up being a deg or two cooler than forecast this
Fri. Have also increased pops slightly to low end chance as a bit
more scattered coverage of diurnally driven convection may be seen
Sat afternoon with less influence of the upper ridge expected.

LONG TERM...Sunday Through Thursday

Upper level ridging is expected to persist over the region through
the weekend...but the ridge is expected to weaken which may allow
for some break from the persistent heat by early next week.

The 00Z LZK RAOB showed the upper level ridge building west of
Arkansas towards the high plains. 500 mb heights increased over the
Southern Plains to 599 dm while falling slightly to 597 dm over
central Arkansas. Early morning water vapor satellite imagery shows
the center of anticyclonic circulation aloft moving slowly to the
south and west with weak northerly flow aloft building over
Arkansas.

For this weekend...the subtle re-centering of the upper ridge to the
west of the state will allow the upper level ridge axis to continue
to weaken over the state. This will not immediately provide a
respite from the persistent heat over the region... but it should
result in less large scale forcing for subsidence over the state
which will allow for more convection during the peak heating hours
of the day. Given equal sunshine/insolation...Saturday and Sunday
will be every bit as hot as the past few days have been. However...
with less subsidence in place convection initiation may occur
earlier in the day and become more widespread than it has been
Wednesday and Thursday. A slight uptick in coverage and earlier
initiation will allow for more cloud cover if nothing else...
resulting in highs only climbing into the mid 90s instead of the
upper 90s. A heat advisory may be needed through the entire
weekend...but afternoon heat index values should be trending more
towards 105 versus 110 degrees by Sunday.

Early next week...Model guidance is consistent in indicating that an
upper level low southeast of the Carolina coasts will cut off and
retrograde /relative to how we are used to seeing weather systems
move across the CONUS/ over the southeastern CONUS Sunday through
Tuesday. This feature appears to simply cut off from a traditional
northerly trough and get brought back to the west in the easterly
flow aloft south of the upper ridge axis in place over the central
United States.

The important thing to note is that dynamically it is a cold core
upper low. It can be a bit scary to see model guidance advertising
what appears to be a closed upper low originating from the tropics
over the southeastern United States during the summer months...but
there is no doubt that this is NOT a tropical low pressure system in
the model data. It is just a shortwave trough moving in the opposite
direction that we are used to seeing it move. To verify that this is
not tropical in nature...simply look at the 500 mb temperatures in
the middle of the upper low. The temps at that level are cooler than
the air all around it which is dynamically opposite of what you
would see in a developing tropical low. There are other ways to
verify this...but that is probably the easiest and quickest.

At any rate...the westward moving shortwave trough will likely
remain south of Arkansas as it moves over the Gulf coast and towards
Texas on Tuesday. This will keep the bulk of the large scale forcing
for ascent associated with this trough to our south or southwest. At
the same time...a traditionally moving /east or southeast/ shortwave
trough is expected to move over the Great Lakes region squeezing the
ridge axis over Arkansas and Tennessee into a deformation zone
aloft. This transition will essentially eliminate the large scale
forcing for subsidence over the region and may even allow for some
weak frontogenesis/focused lift over Arkansas during the day on
Tuesday. As a result...went ahead with 40 to 50 POPs across the
state expecting a decent increase in the coverage of convection as a
result of the shortwave troughs putting the squeeze on the upper
ridge. If nothing else...the increase in cloud cover should keep
highs in the lower 90s across Arkansas...all but ensuring an end to
the persistent heat advisories that have been in place for the past
several days.

Wednesday through Friday...The consensus of model guidance is that
the Tuesday squeeze play will result in the upper level ridge
breaking off into distinct pieces...one off the Carolina
coasts...and one over the southwestern CONUS. This would leave
Arkansas under weak northwest flow aloft which should keep
temperatures right around seasonal normals and keep rain chances
higher than normal...or around 20 to 30 percent each day. Heat index
values are expected to remain below 105 degrees through the end of
the week if this pattern aloft is realized.

&&

.LZK Watches/Warnings/Advisories...
Excessive Heat Warning until 8 PM CDT this evening FOR Arkansas-
Bradley-Cleburne-Cleveland-Desha-Drew-Faulkner-Grant-
Independence-Jackson-Jefferson-Lincoln-Lonoke-Monroe-Prairie-
Pulaski-Saline-White-Woodruff.


Heat Advisory until 9 PM CDT Saturday FOR Arkansas-Baxter-Boone-
Bradley-Calhoun-Clark-Cleburne-Cleveland-Conway-Dallas-Desha-
Drew-Faulkner-Fulton-Garland-Grant-Hot Spring-Independence-Izard-
Jackson-Jefferson-Johnson-Lincoln-Logan-Lonoke-Marion-Monroe-
Montgomery-Newton-Ouachita-Perry-Pike-Polk-Pope-Prairie-Pulaski-
Saline-Scott-Searcy-Sharp-Stone-Van Buren-White-Woodruff-Yell.

&&

$$

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