Area Forecast Discussion
Issued by NWS Little Rock, AR

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729
FXUS64 KLZK 211122 AAA
AFDLZK

Area Forecast Discussion...UPDATED
National Weather Service Little Rock AR
622 AM CDT THU JUL 21 2016

.UPDATE...

Updated to include 12z aviation discussion.

&&

.AVIATION...

Upper level high pressure continues to dominate at this time.
Convection will develop once again this afternoon but remain
isolated in nature and VCTS will suffice. Widespread VFR
conditions are expected through the TAF period, but still could
see brief MVFR conditions this morning due to fog. Light south to
southeast winds will continue with scattered cumulus around 4-5kft
by this afternoon.

&&

.PREV DISCUSSION...(ISSUED 423 AM CDT THU JUL 21 2016
)

SHORT TERM...Today Through Friday Night

Very little change was made to the current forecast during the short
term period. Good agreement in model solutions in regards to
strength and location of upper ridge that has dominated the forecast
lately. Close to a carbon copy temperature wise across the forecast
area today, with some sites in northern Arkansas a degree or two
warmer. Heat Advisory will remain "as-is" for today, with an
expansion to include all counties in the forecast area fro Friday.
The areas added to the advisory for Friday will come close to
advisory level criteria.

Will once again see the chance for shower and thunderstorm activity
across the area once again during the afternoon and evening hours.
Best chance to see one of these isolated storms would likely be over
the eastern half of the state. Slightly better rain chances on
Friday, mainly across southeastern areas during the afternoon and
evening hours as a weak disturbance rounds the eastern edge of the
upper ridge.

LONG TERM...Saturday Through Wednesday

In general upper level ridging is expected to persist over the
region. The good news is that model guidance consistently indicates
that the magnitude of upper level heights will diminish towards
seasonal normals beginning this weekend...and slowly diminishing in
magnitude throughout the week. The North American Ensemble Forecast
System (NAEFS) indicated that 500 mb heights would diminish from
their current height of 598 dm towards 596 dm this weekend and 594
dm for much of the week ahead. These changes in height aloft seem
subtle...but the ensemble situational awareness table shows that
current heights are consistent with a return interval of once every
30 years...whereas a 596 dm ridge occurs once every 2 to 5 years...
and 594 dm ridges are common...occurring yearly over Arkansas.

What this means for our forecast is that the current heat wave has
an end in sight despite the lack of a significant pattern change
aloft. The thickness of the troposphere is directly proportional to
the average temperature of the troposphere...therefore these
seemingly small changes in heights aloft can actually have a fairly
substantial impact on the intensity of heat over the region. The
breakdown in the intensity of the upper level ridge means that large
scale forcing for subsidence will be reduced on average making
afternoon isolated convection more widespread and common.

As a result...have diurnally driven POPs in the forecast increased
from 10 to 20 percent in the short term to 20 to 40 percent the
later we get in the forecast. POPs may need to be increased for
specific days next week...but the timing and placement of shortwave
troughs while an upper level ridge is the dominant feature is of
very low confidence. As a result have high temperatures coming back
down into the lower to mid 90s for much of the state by mid-week.

Heat index values will likely continue to climb above 105 degrees
through the weekend. We may see an end to the near continuous string
of days exceeding a 105 degree heat index though as the upper ridge
breaks down in intensity next week. The current forecast calls for
heat index values holding at or below 105 degrees beginning on
Tuesday.

&&

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


Heat Advisory from 10 AM to 9 PM CDT Friday FOR Boone-Marion-
Newton-Searcy.

&&

$$


Aviation...226



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