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afdlzk

Area forecast discussion
National Weather Service Little Rock Arkansas
653 PM CDT Thu Apr 27 2017

Aviation...

Moisture will continue to increase through the night tonight as a
warm front lifts north through the state. As a result there will
be some small chances for precipitation at khro and kbpk tonight.
With the increase in moisture...ceilings will lower overnight to
MVFR conditions. Wind shear will also be a concern overnight as
winds aloft will strengthen. MVFR conditions will persist all day
on Friday.

&&

Previous discussion...(issued 234 PM CDT Thu Apr 27 2017
)

Short term...tonight through Sunday
well, today has been nice. That's probably the last day you'll be
able to say that for awhile.

Front which caused all the meteorological mayhem yesterday has
begun to hang up over the Gulf of Mexico, as it hits an upper
ridge centered south of there. Longwave pattern will amplify over
the western conus during the next 24 hours, with an upper low in
The Rockies deepening as it moves into the plains. Southwest
upper flow will develop, which will send the front back to the
north, and into the region.

At the same time, a deepening surface low moving off The Rockies
will send Gulf moisture up over the front and into the region.
The warm front will hang up over the Ozarks into the weekend. This
will set the stage for a rather wet and unsettled few days, with
heavy rainfall a serious concern, and severe weather possible.

I'll address the primary concerns in order.

Models continue to focus on an axis of heaviest rainfall from the
ouachitas into the Missouri bootheel, though their amounts have
varied and even backed off a little. Problem is, with antecedent
conditions (heavy rain over the last day or two, nearly-saturated
ground, and High River stages in places), it will not take much to
cause flash flooding and river flooding issues. The combination of
Gulf and Pacific moisture will yield anomalously high precipitable
water values. The over all slow propagation of the system will
allow for storms to develop and move over the same areas
(training). All this considered, think the Flash Flood Watch
issued by the midnight shift was a good call.

For the current forecast package, total rainfall amounts in the
heaviest axis will likely reach five to seven inches, with
localized areas exceeding eight inches. The axis of heaviest
rainfall will all depend on where the frontal boundary stalls, and
at this point, the most likely area will be from around Pocahontas
to Fort Smith.

Severe weather concerns...
forecast soundings show quite a capping inversion in much of the
area as the warm sector expands into the region on Friday. That
being said, strong warm advection will drive temperatures up into
the 80s in much of the central and south, with 70s dewpoints
moving in as well. Aside from that capping inversion, the forecast
soundings also show quite a significant amount of low level shear,
as well as cape values in excess of 2500 j/kg and decent mid-level
lapse rates. Biggest window for severe storms will be during the
evening and into the overnight hours, with large hail and damaging
winds being the primary threat. If storms can become surface-
based, there will be a tornado threat as well, and this will be
highest in the vicinity of the frontal boundary. Given the amount
of capping, there's not a lot of confidence on what could happen.
But, there's confidence that if things can get going, something
will happen.

Surface low pressure will move into the area from Oklahoma on
Saturday, with more storms along and ahead of the attendant front.
Again, severe thunderstorms will be possible, more in a linear
Mode versus discrete cells, though there could be embedded
supercells in the line. Wind will be the primary threat, though if
anything can get rooted in the boundary layer, there would be a
tornado threat as well.

As the now closed upper low drops through the plains, it will push
the surface system eastward, with rainfall gradually starting to
wind down by Sunday afternoon.

So, to reiterate...

-Flash Flood Watch in effect for much of the area Friday night
through Sunday.

-Rainfall amounts of five to seven inches are likely from
northeast Arkansas into western Arkansas. Some localized amounts
could exceed eight inches. Even into central Arkansas, two to
five inch rainfall amounts are possible.

-Heavy rainfall placement will depend on where the warm front
stalls.

-The heaviest rain is expected Friday night through Saturday
night.

-Severe weather threat on Friday night through Saturday night.

-Highest severe weather concerns will be hail in excess of one
inch in diameter, and damaging wind gusts in excess of 60 miles per hour.

-Tornadoes will be possible, though this will depend on if storms
become surface-based.

-Optimum severe weather threat on Friday and Friday night will be
over western into northeast Arkansas and primarily during the
afternoon and into the late night hours.

-Optimum severe weather threat on Saturday will generally be west
of Little Rock to El Dorado, centered on the Ouachita Mountains,
and during the afternoon and into the overnight hours.

Long term...Sunday night through Thursday
by Sunday evening and Sunday night a cold front should be east of
the Mississippi River, bringing an end to an otherwise very rainy
weekend. Cooler and drier air will follow, with lows Sunday night
expected to drop into the 40s to low 50s. Monday will be a dry day
with highs in the 60s to low 70s.

For Tuesday Onward there is considerable uncertainty as to how the
pattern will evolve. A weak boundary will sag into Arkansas late
Monday in the wake of the departing upper low. The question is how
much of a focus this will be for rainfall late Tue night through the
end of the period. The GFS is by far the wettest of available
models, with another similar wet pattern as what is expected this
weekend, setting up from late Tue night/Wed morning through Thu. In
contrast, other models show a similar pattern but shift it further
southeast and have less rain overall. The moral of the story is,
expect to see more rainfall during the mid week period next week,
but will play things conservatively for the time being as there is a
lot of uncertainty. Will carry only some 20-30 percent rain chances
during that period as a result.

Fire weather...
fire weather concerns are minimal during the next seven days.

&&

Lzk watches/warnings/advisories...
Flash Flood Watch from Friday evening through Sunday evening for
Arkansas-Baxter-Boone-Clark-Cleburne-Conway-Dallas-Faulkner-
Fulton-Garland-Grant-Hot Spring-Independence-Izard-Jackson-
Jefferson-Johnson-Logan-Lonoke-Marion-Monroe-Montgomery-Newton-
Perry-Pike-Polk-Pope-prairie-Pulaski-Saline-Scott-Searcy-sharp-
stone-Van Buren-white-Woodruff-Yell.

&&

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