Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Little Rock Arkansas
646 PM CDT Wed Aug 16 2017
thunderstorms are developing in Kansas and Oklahoma at this time
along a cold front. This cold front will enter northwest Arkansas
early Thursday morning and bring the showers and thunderstorms
into the state. Expect thunderstorms to move through the state
especiallly during the last half of the period. Visibilities and
ceilings will be reduced to MVFR conditions. Otherwise VFR
conditions are expected.
Previous discussion...(issued 301 PM CDT Wed Aug 16 2017
Short term...today through Friday
Goes16 6.9 micron WV imagery continues to show a deep upper level
low situated in the central plain states and moving off to the
northeast. Just underneath (and a bit southward) is a deepening
surface low with an associated cold front which will push through
Arkansas later in this period.
In the near term, insolation continues to warm much of Arkansas to
near the warmest temperatures of August 2017- which isn't saying
much considering the highest temps so far have been in the upper
80s to lower 90s. Southerly winds ahead of the aforementioned low
pressure system has advected a very, very moist plume of air into
the region as dewpoints currently sits in the mid 70s
statewide(with some variation).
With no source of upper level lift and a bit of dry mid level
air, convective initiation has been near absent across most of
the area- as expected. The exception is far northwest Arkansas,
much closer to the approaching shortwave and under the influence
of some upper level / right entrance region divergence.
As the system progresses to the northeast, the associated cold
front is expected to push across Arkansas in the overnight hours
Wednesday into Thursday morning, before stalling out somewhere
across southern Arkansas during the day tomorrow. Quasi-organized
convection will likely form along or just in front of the frontal
boundary this evening. At this time, severe potential looks to be
limited. By the time convection reaches the state, nocturnal
decoupling will result in a near-surface stable boundary layer
which will prevent the strongest of winds from mixing down. On top
of that, deep layer shear becomes more marginal farther
southeast, so any ongoing convection over OK/MO should gradually
weaken as it moves along.
That said, there is still some flash flood potential. Some areas
across the Arkansas River valley into both the Ozarks and
ouachitas have received heavy rain in the last week. With
saturated or near-saturated grounds, it will only take 1 to 2
inches for flash flood potential to be realized. The biggest
concern may be over the southwestern ouachitas in the morning
hours Thursday. Model consensus have the aforementioned frontal
boundary slowing down to a stall, with precipitable waters between 2 and 2.5
inches in the vicinity and 1500+ j/kg ml cape. Combined with the
DPVA of the shortwave and an extended period of isentropic lift
above the stalling front, rainfall may become excessive.
Fortunately, drier mid-level air is expected to filter by later
morning or early afternoon... hopefully preventing flash flood
occurence. Based on uncertain duration and a highly-varied output
from the cams, have held off on a Flash Flood Watch, but it will
be something to watch overnight... especially for
Polk/Scott/Montgomery counties and the near-surrounding area.
With extended cloud cover over the region for much of the
afternoon hours, highs will be below average once again on
Thursday. With lack of upper level support, the front will stall
across southern Arkansas before back north as a warm front ahead
of the next system. Have kept low pops in the region, focused on
where the expected location of the frontal boundary.
As far as Friday highs, all will depend on the movement of the
warm front. If it moves quickly to the north, skies may clear for
a very warm and humid day on Friday. But as we've seen all August
long, isentropic lift and somewhat cloudy skies should keep
temperatures down a bit... though upper 80s to around 90 will
certainly be possible. GFS, per typical as of this Summer, seems
to be having some bigtime planetary boundary layer / land-type / or similar issues and
is heating US way beyond what will be seen, especially in the Pine
region- so have pretty much disregarded this "solution" in the
Long term...Friday night through Wednesday...
Rain chances will be in place at the start of the long term period,
with continued on and off rain chances possible through early next
week as a frontal boundary looks to reside just north of the
forecast area. Flow aloft will be zonal to slightly northwest late
this week, before high pressure builds over much of the southern and
southeastern US and more of a stagnant zonal flow sets up.
With high pressure building in over the region, temperatures will
warm a bit but will remain near seasonal normal(upper 80s to lower
90s). With these temps and southerly flow in place at the surface,
heat index values will begin to creep up. At this time, conditions
will likely be the worst Sunday through Tuesday, and heat advisories
won't be entirely out of the question.