Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 131114
AFDLZK

Area Forecast Discussion
National Weather Service Little Rock AR
614 AM CDT Fri Apr 13 2018

...Severe weather likely this afternoon and tonight with a flash
flood threat potential increasing this evening into tonight...

.SHORT TERM...Today Through Saturday

11Z objective analysis reveals a significant upper trough
stretching from MT to the Four Corners region. Broad upper
divergence continues overspreading the area this morning as
increasingly amplified southwesterly flow migrates east with the
parent trough. Rounding the base of this feature are belts of
flow in excess of 80 kts, with embedded jet streaks on the order
of 90 to 100 kts, at the H300 and H500 levels. Nearer the
surface, 50+ kt flow at H700 continues slowly translating eastward
across portions of TX, OK, and KS. A stout 50 kt LLJ is anchored
from near the TX Panhandle eastward across AR. This is
contributing to increasing low-level moisture advection, which
will continue through much of today, and is clearly evidenced by
IR/WV imagery.

At the surface, a compact sub-990 mb low was analyzed across
northern KS, with an increasing pressure gradient oriented NW to
SE across the state, keeping winds elevated. Southerly surface
flow continues supporting poleward transport of rich Gulf
moisture, with low 60s F dewpoints already pushing into southern
AR. Warm air advection will continue as well, and broad ascent
within this WAA regime has already generated showers across OK
into west- central AR. Slightly more intense convection in
southern OK is evident from mosaic radar imagery, coincident with
a tongue of elevated instability (MLCAPE around 1000 J/kg) nosing
northward ahead of the primary surface trough, which stretches
southward from KS into West TX.

As the parent upper trough continues pushing east today,
indications are the surface cyclone will begin migrating
northeast across the Central Plains. An attendant dryline is
forecast to push into eastern OK by this afternoon as the main
surface wave matures. By late evening into tonight, a cold front
will sweep east, overtaking the dryline and crossing the area by
Saturday morning. Moisture advection will continue ahead of these
features, with surface dewpoint temperatures expected to climb
into the mid- 60s F across much of the area by this afternoon.
Diurnal heating will contribute to increasing instability, with
peak values in excess of 1500 to 2000+ J/kg MLCAPE by early
afternoon. The initially capped warm sector is expected to become
quite volatile before the cap erodes in the early-to-mid afternoon
timeframe.

Depending on the extent of cloud cover and resultant surface-based
heating/capping, as well as the timing and degree of mechanical
forcing from an approaching pre- frontal surface trough and
approaching mid/upper level dynamic support, convective
initiation may occur in a more isolated fashion early on (between
18Z and 21Z) across eastern OK into western AR. Mixed convective
modes appear likely as convection spreads west-to-east into
central and eastern AR by late afternoon and evening.

CAM guidance remains quite ambiguous with storm mode and
evolution, however, and this continues to be the main point of
contention with the forecast, along with system timing (this
issuance slowed the overall timing by a couple of hours compared
to the previous forecast). That said, impressive dynamics
certainly support discrete supercell structures across any portion
of the area with clusters and line segments also likely. The
going thinking has initial discrete or semi-discrete convection gradually
exhibiting more upscale growth into clusters and line segments
with time, before transitioning to a large squall line overnight
into Saturday morning, affecting mainly central and eastern AR.

All severe hazards will be possible if not likely today,
including damaging winds, tornadoes, and large hail. Of these, the
tornado and hail threats are the most conditional given their
dependence on storm mode. A higher number of discrete supercells
will support a higher tornado and hail threat given the
overwhelming shear and helicity profiles anticipated (0-1 and 0-3
km SRH values exceeding 300 to 400 m2/s2). Should this pan out, a
significant tornado or two as well as significant hail cannot be
ruled out, especially in western and portions of central AR.
Embedded rotating cells within line segments and/or clusters will
also support a tornado threat through the duration of the event.
Forecast soundings from all available operational models continue
highlighting large looping hodographs, although some guidance
(namely the NAM/NAMNEST) exhibit varying degrees of veer- back-
veer profiles below 3 km. This could potentially mitigate a
higher-end tornado and hail threat if mesocyclones are not able to
become as robust as the overall atmosphere would otherwise
support.

Any line segments that develop will carry an enhanced wind
threat, particularly if the take on bowing characteristics. The
anticipated transition to a more linear system late tonight into
the pre- dawn hours Saturday suggesting the threat for damaging
winds will become the predominant hazard, especially across
portions of central and eastern AR. The potential for tornadoes
will remain, however -- particularly with any embedded
mesocyclones within the main squall line.

Finally, given significant tropospheric moisture, the threat for
flash flooding is expected to increase later this evening and into
the overnight hours. Models continue to struggle with the max QPF
axis, but indications are S and SE AR will see the highest overall
flash flood threat. That said, widespread PWAT values of 1.5 to
1.75 inches (99.5 percentile based on NAEFS climatology) suggest
torrential rain will be possible with any convection today, and
isolated flash flooding cannot be ruled out anywhere in the
southern half of AR. Deferring to subsequent shifts to consider a
watch given the uncertainty thus far this morning, and the
precedence the severe threat carries.

One final note: strong southerly flow in the 25 to 30 mph range,
with gusts to near 40 mph, has prompted the issuance of a Wind
Advisory for portions of eastern AR for this afternoon and
evening. Caution should be exercised for all motorists and those
on area water ways.

Expect initially warm high temperatures in the 70s and 80s F to
cool into the 50s and 60s by Saturday afternoon. Lows Saturday
morning will range from the 60s F in the Delta, to the low and mid
40s F across N/NW AR.

Cooper

&&

.LONG TERM...Saturday Night Through Thursday

The extended forecast will start with the strong upper and surface
systems over the eastern Plains and pushing east through the region.
The secondary area of lighter convection will be pushing east of AR
associated with the strong upper low pressure system as it gradually
moves east. Sunday will start chilly over parts of northern to
northwest AR, and a few locations will approach the freezing mark.
Winds should stay up a bit to limit extent. The rest of Sunday, will
be mostly dry with cooler highs only in the 50s over most spots,
with breezy north winds. Sunday night, an area of around freezing
temperatures will be possible over mainly northern AR, as the cold
surface high pressure filters in over the region, and winds become
light and variable.

Monday will see dry and mostly sunny conditions with the surface
high pressure the main weather feature over AR. Warmer highs will
reach the 50s in the east, to the 60s central and west. Dry with a
warming trend into Tuesday, with highs from the mid 60s to main
locations in the 70s. Tuesday night to Wednesday, models show a
quick moving cold front to go through AR, with a low chance of any
light convection. Forecast only has a slight chance in spots over
AR, as well as uncertainty is present on the amount of moisture. Dry
weather is forecast Thursday with the warming trend with highs again
from the mid 60s to upper 70s west.

&&

.PRELIMINARY POINT TEMPS/POPS...
Batesville AR     73  57  67  37 /  60 100  20  10
Camden AR         81  54  66  37 /  90  90  20  10
Harrison AR       75  45  57  33 /  50  70   0  10
Hot Springs AR    78  53  63  36 / 100  90  20  10
Little Rock   AR  78  56  66  38 /  80 100  20  10
Monticello AR     80  60  67  40 /  60 100  40  20
Mount Ida AR      77  50  61  34 /  90  80  10  10
Mountain Home AR  75  50  61  35 /  60  80  10  10
Newport AR        74  59  67  39 /  40 100  20  20
Pine Bluff AR     80  57  64  39 /  60 100  30  20
Russellville AR   77  51  62  36 /  90  90  10  10
Searcy AR         74  57  66  37 /  60 100  20  20
Stuttgart AR      78  59  66  40 /  50  90  30  20
&&

.LZK Watches/Warnings/Advisories...
Wind Advisory from noon today to 10 PM CDT this evening FOR
Arkansas-Desha-Drew-Independence-Jackson-Jefferson-Lincoln-
Lonoke-Monroe-Prairie-White-Woodruff.

&&

$$

Short Term...COOPER / Long Term...59



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