Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Memphis Tennessee
300 PM CDT Thursday may 28 2015
Main forecast concerns this afternoon revolve around the evolution
of the mesoscale convective vortex now across central Arkansas...transitioning to the
prolonged period of precipitation chances through the weekend.
Currently...weak steering flow is in place across the middle-south as
weak shortwave ridging is progressing east. A remnant circulation
was moving through northwest Arkansas with a convergent axis
extending south-southeast where a line of thunderstorms was
located. These storms are largely driven by internal cold pool
dynamics as very weak upper level flow is limiting storm
As of now...pure extrapolation of the leading edge of this complex
would bring thunderstorms to western portions of the County Warning Area after
00z. The airmass ahead of this complex is uncapped and strongly
unstable...SBCAPE values at or above 2500 j/kg...so outflow moving into
this environment should continue to initiate new convective
development. The question is how far east this continues. Given
the very weak wind fields/forcing...believe that this activity
will struggle to make it to the Mississippi River. A few
showers/isolated thunderstorms are certainly possible...but until
better storm organization occurs...chances will be kept low.
On Friday...a better large scale signal exists for more widespread
thunderstorm activity. A shortwave ejects into the Southern Plains
during peak heating which should allow for much greater coverage
of thunderstorm activity. Wind fields continue to be weak but
instability is sufficient for a few stronger thunderstorms
potentially producing isolated damaging wind events across central
and eastern Arkansas. Thunderstorms will again be slow moving
with heavy rain potential maximized in nearly stationary
Through the weekend...a frontal boundary will sag very slowly
south attached to a departing low pressure system in the
northeast. Upper level flow will become parallel to this front
allowing it to stall for several days. Precipitable waters both days will hover
between 1.5 and 2 inches...and given the slow moving nature to the
thunderstorms...isolated flash flooding appears possible.
In the long term...the synoptic frontal boundary begins to wash
out across the mid-south. Thunderstorms will become tied more to
diurnal heating during this period with coverage decreasing
through mid-week. It also appears that high temperatures will warm
slightly into the middle and upper 80s. In summary...warmer than
average temperatures with isolated thunderstorms will be the theme
of the long-term period of the forecast.
18z taf cycle
Challenging short term forecast this afternoon regarding
convective potential for western sites early this evening. A line
of storms over western Arkansas early this afternoon continues to
advance eastward into moderately unstable airmass represented by
2500 j/kg of SBCAPE. An associated mesoscale convective vortex continues to lift to the
north and brings into question whether the convection will be able
to generate a new cold pool to keep storms progressing east or
whether the lack of better support (ie. Mcv) will orphan existing
storms resulting in their demise. At this time...have added thunderstorms in the vicinity
into kjbr and kmem this evening to account for potential storms
associated with this activity.
Otherwise...MVFR ceilings will shortly lift to VFR and persist outside
of thunderstorms through the afternoon and evening hours. Winds
will remain southeasterly below any critical thresholds through
the period...outside of any storms.
Low stratus should again develop after 08/09z tonight with IFR to
LIFR ceilings likely which will persist into early Friday morning
before gradually lifting to MVFR by the end of the period.
Scattered convection should also redevelop beyond 29/16z.