Area Forecast Discussion
Issued by NWS Little Rock, AR
FXUS64 KLZK 182324 AAA
Area Forecast Discussion...UPDATED
National Weather Service Little Rock AR
624 PM CDT TUE OCT 18 2016
Updated to include the 00Z aviation discussion below...
Moisture continues to surge north...and will result in some low
clouds overnight into Wed morning for some terminals. Some fog and
DZ will also be possible...with MVFR or briefly lower conditions
possible. Chances for rainfall will be increasing across the NWRN
areas of the state...with some SHRA possible into Wed morning.
Some TSRA will then be possible by Wed afternoon across the NW as
activity develops ahead of an approaching cold front. This front
will drop SE through the state Wed night into Thu morning.
.PREV DISCUSSION...(ISSUED 351 PM CDT TUE OCT 18 2016)
SHORT TERM...Tonight Through Thursday
Early afternoon visible satellite imagery continues to show
scattered to broken stratus/stratocumulus over Arkansas. These
clouds were diminishing in coverage a bit as of 3 PM...but the
clouds are not likely to completely scatter out before sunset.
With the clouds remaining in place longer than expected today...
high temperatures remained anywhere from 5 to 10 degrees cooler
than yesterday despite no significant changes in the large scale
weather pattern. Dew point values were up into the upper 60s to
lower 70s across Arkansas...and another warm overnight is expected
as a result. Water vapor satellite imagery showed that the upper
level ridge that has been responsible for the near record heat
recently has started to move south towards the Gulf of Mexico. An
upper level trough over the northwestern CONUS is responsible for
this large scale pattern change...and is going to be the primary
driver behind our weather changes later this week.
Tonight through Wednesday...As the upper trough moves towards the
Great Plains tonight...a moderate low-level jet will ramp up over
Arkansas. As the low-level jet strengthens...low-level lift will
bring a nearly solid deck of low clouds back over the state.
Overnight lows will remain in the upper 60s for most locations.
The approach of the upper trough will result in some weak
cyclogenesis along a stalled frontal boundary that is draped along
northeast Oklahoma to far northwest Arkansas and southwest
Missouri. This cyclogenesis will cause the front to anchor in
place while sharpening up the gradients of temperature and
moisture along the front through Wednesday morning.
As the frontal boundary sharpens up...mesoscale lift will
strengthen along the warm side of the frontal boundary. Deep layer
shear will also be on the increase in response to the tightening
baroclinic zone and due to the increase in winds aloft associated
with the approaching upper level trough. The mesoscale lift and
abundant moisture located to the southeast of the frontal boundary
will work together to increase the potential energy for storms
The 12Z GFS and NAM now both prog CAPE values to increase to at
least 2000 j/kg over northwest Arkansas by 21Z Wednesday. Deep
layer shear is expected to be on the order of 35 to 40 kts of 0-6
km bulk shear Wednesday afternoon as well. This combination of
CAPE and shear is supportive of organized convective storms. The
strong forcing for ascent near the frontal boundary and the deep
layer shear vectors oriented nearly parallel to the frontal
boundary will primarily promote a multicellular or linear storm
mode due to a high likelihood of cold pool interactions. Assuming
this storm mode is correct...the primary threats from storms
tomorrow afternoon is expected to be large hail and damaging
winds. A localized flash flood threat cannot be ruled out either
if storms train over the same location multiple times. The tornado
threat is somewhat contingent upon the maintenance of a discrete
storm mode. If a discrete storm mode is realized...supercell
thunderstorms are certainly possible...and at least a low-end
tornado threat would accompany these storms.
The frontal boundary is expected to begin moving southeast
Wednesday evening. Once this boundary begins moving...it will
likely be accelerated southeast by the added influence of the
cold pool density current associated with organized storms near
the cold front. This should result in storms becoming elevated
above the shallow frontal inversion or cold pool. As a
result...the severe weather threat is expected to diminish rapidly
once the front begins moving southeast...likely after 9 pm.
Scattered showers and thunderstorms will accompany the front
southeast across the state overnight. Locally heavy rainfall and
localized flooding will be the primary threat with these storms
once they become elevated over the front.
The front is expected to move southeast away from Arkansas by
early Thursday afternoon. Much drier and cooler air should filter
in behind the front...bringing an end to precipitation from
northwest to southeast across the state Thursday afternoon. Highs
on Thursday are only expected to climb into the upper 60s north to
the mid 70s south.
LONG TERM...Thursday Night Through Tuesday
Drier and much cooler airmass wl be settling into the region at the
start of the period following the passage of the CDFNT on Thu. The
assocd high pres system will drop SEWD acrs the region heading into
the start of the weekend...with temps moderating to near normal lvls
by this time.
For the remainder of the weekend and into early next week...the
aforementioned high pres sys wl shift to the SE of AR with S/SWLY
wind flow returning. This wl allow for the warmup to cont with many
locations seeing high temps abt 5 degrees abv normal for this time
of year. Kept the fcst dry as best low lvl moisture return wl rmn
to the W of the FA.