Tornado and Flash Flooding in Arkansas, April 26, 2017

Area Forecast Discussion...Update
National Weather Service Little Rock AR
1240 AM Wed Apr 26 2017

.Aviation...

VFR flight conditions with patchy MVFR conditions will be seen
overnight. Isolated IFR conditions may also develop by early
Wednesday morning. To the west, an area of showers and isolated
thunderstorms will slowly move east, and affect KHRO and KBPK early
Wednesday morning. Winds will be southeast to southwest at 10 to 20
mph with some possible gusts. On Wednesday, a cold front is expected
to move through the area, a line of strong to possibly severe
thunderstorms is forecast with gusty winds, low ceilings and a wind
shift to the northwest. The showers and thunderstorms will affect
all Tafs on Wednesday. (59)

&&

.Prev Discussion.../ Issued 931 PM CDT Tue Apr 25 2017/

Convection continues to fire up in NE OK at this time as the upper
trough/cold front continues to advance into that part of the state.
Upper flow remains parallel to the convection keeping the overall
movement of these storms on the slow side as they drift off to the
NE.

All short term models, most noticeably the HRRR, show this area
moving into NW AR during the night and clipping our NW most
counties. Pops raised in this area accordingly but it looks like
the storm movement being what it is will preclude the need to go
much further into the CWA at this time. While HRRR is cranking
out some very impressive rain totals, the heaviest amounts will
likely stay out of our CWA as the precipitation battles mid level
dry air.

&&

PREV DISCUSSION...(ISSUED 247 PM CDT Tue Apr 25 2017)

SHORT TERM...Tonight Through Thursday
Current surface analysis shows a frontal boundary extending from
southeast Kansas into the Texas Panhandle region. Boundary has
stalled slightly, but as the first of two upper troughs associated
with a low over the Rockies kicks out and amplifies the upper
flow. This system will kick off showers and thunderstorms ahead of
the front by late tonight, with a second system behind it
providing the support for showers/storms over Arkansas on
Wednesday.

Parameters are coming together for severe thunderstorm potential
for Wednesday afternoon and evening, with a risk for severe storms
over not only the entire forecast area but the entire state. Any
severe storms will be capable of producing large hail and
damaging winds. Though there will be a bit of a cap initially,
strong low level jet dynamics, along with good moisture transport
into the region should be enough to destabilize things a bit.
Given the amount of low level wind shear present, there will be a
tornado risk.

Expected storm mode would be discrete cells initially, with a line
of storms developing along and ahead of the front. Tornado threat
would be highest with the cells out ahead of the front, and
primarily during the afternoon and early evening hours.

Stronger storms will be capable of producing localized heavy
rainfall. On the whole, an inch or two of rain looks like a good
bet for much of the state from this system, but heavier cells
could allow for higher amounts in localized areas.

As far as timing, the threat for severe storms in western Arkansas
will primarily in the early to mid afternoon hours, while the
central portion will see the highest threat from mid to late
afternoon through early evening, and the eastern portion from
late afternoon through late evening.

Front will clear the state during the overnight hours. Although
there will be a brief shot at precip over northern Arkansas behind
the front, rainfall should largely be over by Thursday morning.
Cooler airmass will follow the front, albeit briefly. Frontal
boundary will stall out well south of the region on Thursday,
before coming back north and starting trouble for the end of the
week and into the weekend.

More on that from the long term forecaster...

LONG TERM...Thursday Night Through Tuesday
Warm front will be moving northward across the area during the day
on Friday, bringing an impressive surge of low level Gulf moisture
across the area.

For the past several days we have been talking about the potential
for severe weather, and possible tornadoes, on Friday. As the warm
air flows northward, low level winds should be predominantly south
to southeast. Right off the deck the 850 winds will be strong out of
the southwest, with a 40-50 kt low level jet. This will set up
strong low level wind shear. Moving further aloft, 500 mb winds will
be continuing from the SW with some speed shear present.

Typically I would be concerned about the 850 mb winds from the SW
coming out of TX introducing drying air. However in this particular
case it appears there will be abundant Gulf moisture over TX, so
this will not be a factor.

At the 500 mb level the models are showing a closed low digging down
into the swrn US, with a swly flow aloft. The models are not really
resolving much at this range, but I think it would be reasonable to
suspect that a few minor shortwaves will kick out across the area
during this period.

So let's talk severe weather potential for Friday. I certainly have
a lot of concerns, especially across the western zones. It looks
like convection will fire in those areas more or less in the daytime
hours, where greater levels of low level instability will be
present. Model soundings showing CAPE values in the range of 2000-
3000 j/kg with just a bit of a cap. With the strong low level shear
in the lowest few thousand feet, I'm am concerned about the
potential for rotating supercells. As things evolve throughout the
day, I strongly suspect this will turn more into a line that moves E-
NE and will become more of a wind and hail event.

By Saturday a cold front will be moving in from the west, although
this front will be moving rather slowly. Once again, I can
envision the potential for discrete cells ahead of a squall line,
although with the trof digging across the swrn US, and the upper
flow becoming more southerly, the low level shear will not be as
great. For the most part I think this will be more of a squall
line event, along a slow-moving front, with cells training N-NE
along the advancing front.

By mid-late Sunday morning most of the convection should be pushing
out of the eastern parts of the forecast area, although some models
do show the front continuing to stall just a bit longer.

Setting aside the severe weather risk, the big issue will be
rainfall accumulations from Friday through Sunday. Deep moisture
will be present from the low levels all the way thru the top of the
troposphere. This will result in thunderstorms producing heavy rains
on Friday, followed by slow moving storms and a training effect on
Saturday and Sunday.

Model QPF guidance has been fluctuating a bit, but overall I would
not be surprised to see widespread 4" storm total amounts across
wide areas, with some areas topping out in the 6 to 8 inch range,
and possibly even higher. If this much rain falls, it will
undoubtedly cause a major flooding problem along area rivers.
Contingency forecasts are showing many rivers and tributaries
swelling into moderate to major flood categories with this much
rainfall.

FIRE WEATHER...
Winds will remain elevated through Wednesday, as a cold front
approaches and moves through the region. Scattered to numerous
showers and storms are expected along and ahead of the front, with
dry weather behind it on Thursday. More storms, as well as heavy
rainfall, are expected for the weekend.

&&

.LZK Watches/Warnings/Advisories...NONE.
&&

56

Hazardous Weather Outlook...updated
National Weather Service Little Rock AR
845 PM CDT Tue Apr 25 2017

ARZ003>007-012>016-021>025-030>034-037>047-052>057-062>069-261000-
Boone-Marion-Baxter-Fulton-Sharp-Newton-Searcy-Stone-Izard-
Independence-Johnson-Pope-Van Buren-Cleburne-Jackson-Logan-Conway-
Faulkner-White-Woodruff-Scott-Yell-Perry-Polk-Montgomery-Garland-
Saline-Pulaski-Lonoke-Prairie-Monroe-Pike-Clark-Hot Spring-Grant-
Jefferson-Arkansas-Dallas-Cleveland-Lincoln-Desha-Ouachita-
Calhoun-Bradley-Drew-
845 PM CDT Tue Apr 25 2017

This Hazardous Weather Outlook is for a Large Part of Arkansas.

.DAY ONE...Tonight

Showers and thunderstorms continue to increase in coverage and
intensity across northeast Oklahoma. A few of the stronger storms
may make it into the far northwest parts of the area overnight as
they progress to the northeast. While widespread severe weather is
not expected, some stronger storms may still produce gusty winds and
hail. Locally heavy rain is also possible.

.DAYS TWO THROUGH SEVEN...Wednesday Through Monday

Chances for thunderstorms return Wednesday and Wednesday night as
the next storm system affects Arkansas. A line of thunderstorms is
expected to accompany the associated cold front that will move
across the state. Damaging winds and large hail will be the main
concerns. However...tornadoes will be possible with any discrete
storms that do develop out ahead of the line of storms...as well
as along the leading edge of the main thunderstorm line.

The potential for severe weather returns again by Friday...as a
warm front lifts northward across Arkansas. Large hail and
damaging winds will once again be the main threats...with
tornadoes also possible with any discrete storms. The potential
for heavy rainfall will also exist...increasing the concern for
flash flooding and river flooding.

The unsettled weather pattern will continue into the weekend. Yet
another storm could produce additional severe weather over the Mid-
South as a new cold front is forecast to move through the region
Saturday into Sunday. Due to forecast uncertainties with this
weekend storm system...please continue to monitor the latest
outlooks from your NWS office.

.Spotter Information Statement...

Spotter activation may be needed Wednesday afternoon and evening.

&&

Visit NWS Little Rock on the web. Go to http://weather.gov/lzk.

62/51/56