Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 281141
AFDLZK

Area Forecast Discussion
National Weather Service Little Rock AR
641 AM CDT Wed Jun 28 2017

.AVIATION...28/12Z TAF CYCLE

VFR conds are expected to prevail thru the PD. Just some patchy
fog noted over SE AR, with brief MVFR vsbys early in the PD. High
clouds wl cont to spill SWD, mainly acrs N AR today, with some
SCTD CU forming toward midday. Light winds this mrng wl bcm S/SELY
this aftn at 10 to 15 kts. Could see some low clouds advect into
SE AR late in the PD, but confidence with timing is low and opted
to hold off for now. /44/
&&

.PREV DISCUSSION...(ISSUED 420 AM CDT Wed Jun 28 2017)

SHORT TERM...Today through Thursday Night

Rather benign wx pattern in place acrs AR early this mrng, with a
weak ridge of sfc high pres noted over the FA. A light E/SE wind
flow was observed, with temps ranging from the mid 60s to the lower
70s.

No sig changes were made to the going fcst this mrng. The sfc ridge
wl cont to weaken, allowing SLY winds to return, along with incrsg
low lvl moisture. Daytime highs wl also trend upward a degree or two
in the coming days, with most areas rmng just below seasonal norms.

Dry conds wl persist thru today, with small rain chcs returning to
the fcst for parts of AR on Thu. Low pres is fcst to lift NEWD along
the Gulf coast region on Thu, with a SLGT CHC of convection already
mentioned acrs the SERN part of the FA. On Thu ngt, convection is
fcst to form to the NW of AR, ahead of slow moving CDFNT. This
activity is not expected to affect the area until Fri.

LONG TERM...Friday through Wednesday

A strong upper level trough is expected to move southeast over
Nebraska and Iowa by Friday evening. The upper trough axis is
expected to extend to the southwest over Kansas and Oklahoma
spreading some weak large scale forcing for ascent over Missouri and
Arkansas. This lift is expected to be sufficient to cause
thunderstorms to develop along a cold front located northwest of
Arkansas early Friday afternoon. Assuming that thunderstorms develop
along the front as expected...thunderstorms will likely grow upscale
into a line effectively aiding the front southward into Arkansas
from late Friday afternoon through the overnight hours.

There will be some threat of severe weather Friday afternoon and
overnight...however the farther storms move into the state from the
northwest the weaker the shear that they will encounter. Models
indicate that CAPE will be on the order of 3000 - 4000
J/kg...maximized over northwestern Arkansas. Deep layer shear is
expected to be at 30 to 35 kts with the shear vector oriented nearly
parallel to the front. Strong forcing for ascent along the front
combined with the orientation of the deep layer shear vector should
promote cold pool mergers favoring a linear or QLCS storm mode. The
primary threat from this storm mode is damaging straight line winds
with hail and isolated tornadoes possible...but less likely to
occur. This threat should be maximized over the northwestern
portions of Arkansas Friday afternoon and evening. Assuming storms
organize into a line and accelerate southeast...the lack of stronger
low-level wind shear should allow the outflow boundary to outpace
updrafts causing them to become tilted and reducing the overall
strength and organization of the line of storms. This will reduce
the threat of severe weather for the remainder of the state Friday
night.

Storms are expected to dissipate along the front Saturday morning.
This should leave a frontal boundary or strong outflow boundary
stalled out somewhere across the state. Wherever the front stalls
out should provide a focus for new thunderstorm development across
Arkansas on Saturday. This front may remain baroclinic enough to
provide lift for additional storms on Sunday as well. Confidence in
thunderstorm location is inherently low this weekend as it seems to
depend on how organized storms are on Friday and where the front or
effective frontal boundary stalls out on Saturday. With lower
confidence in these details...went ahead with a 60 POP along the
front on Friday...but went with more broad brushed 30-50 POPs on
Saturday and Sunday afternoons. Thunderstorms are not expected to be
severe this weekend as deep layer shear is expected to be even
weaker than on Friday.

Monday through Wednesday...The large scale flow aloft is expected to
transition to northwest flow as upper level ridging amplifies over
the western CONUS. The GFS and ECMWF both advertise the passage of
some shortwave troughs in this pattern providing forcing for ascent
over Arkansas. The timing and intensity of these troughs differ from
model to model and from model run to model run. As a result
confidence in thunderstorms during this period is high...but
confidence in their coverage and intensity is low. Went ahead with
20 to 30 POPs during this period...with slightly higher rain chances
located over the northern portion of the state. Temperatures are
expected to remain near seasonal normals...but humidity will remain
high keeping heat index values up in the mid to upper 90s.
&&

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

$$



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