Scientific Forecaster Discussion
fxus64 klzk 201137 aaa
Area forecast discussion...updated
National Weather Service Little Rock Arkansas
637 am CDT sun Oct 20 2019
Updated to include the 12z aviation forecast.
Still seeing occasional IFR/LIFR fg, mainly north and west, and
patchy broken stratus. Vsbys improve to VFR after 20/15z with stratus
scattering out through the morning. Powerful front moves in from
the northwest with MVFR cigs spreading east as the front pushes
through tonight. Thunderstorms and rain likely near/ahead of the front with IFR or
worse in heavy thunderstorms and rain. Frontal passage will be slow with the boundary reaching
central terminals very late in the period. Strong winds aloft may
produce occasional low level wind shear as well, although confidence is too low
to write in tafs attm. Westerly winds will increase behind the
front with gusts over 20 kts possible.
Previous discussion...(issued 451 am CDT sun Oct 20 2019)
short term...today through Monday night
.Severe weather likely across portions of the area tonight
Satellite imagery showing quite a bit of lingering stratus this
morning, which has likely suppressed more widespread fog
development. Clouds are eroding from the west however, and obs
across eastern OK and extreme western Arkansas show patchy dense fog is
developing. Expecting areas of fog, some dense, developing
through around sunrise. After collaboration with tsa/sgf, holding
off on a dense fog advisory for now but monitoring trends closely.
Main concern this period is the well-advertised cold front and
associated severe threat late tonight through at least Monday
afternoon. A powerful upper trough currently over The Rockies
will translate east through the period with vigorous cyclogenesis
favored across the central/northern plains by late this afternoon.
Trailing this cyclone will be a strong cold front that will
approach from the west late tonight.
It now appears a warm front currently across la/Texas will surge
well north into MO through the day today, resulting in a
moderately unstable warm sector (mlcape values on the order of
1000 to 1500 j/kg and dewpoints in the 60s to near 70 degrees)
across most of the area ahead of the cold front. Some timing uncertainty
lingers, but anticipating storms will move into far
western/northwestern areas near midnight. Meanwhile, forcing for
ascent will increase overnight as a 100+ kt upper jet develops to
our northwest. Mid- level flow will be on the order of 80+ kt with
a 60+ kt low level jet. It still appears the phasing of these features will
promote significant divergence aloft atop at least a portion of
the warm sector near the front as it moves through.
Still thinking storm morphology will be linear as convection grows
upscale within the highly dynamic near-front environment. Forecast
soundings indicate potent wind energy aloft and only modest near-
surface inhibition, especially over roughly the western half of
the area, which will support a primary damaging wind threat. Flow
geometry indicates enlarging looped hodographs and increasing esrh
through the night, so rotating updrafts and a few tornadoes will
also be possible. Steep mid-level lapse rates indicate a large
hail threat will exist with stronger cells.
Storms should stretch from southwestern through central and into
northeastern Arkansas closer to daybreak based on current timing. The
degree of destabilization remains uncertain across
Central/Northeast AR, but assuming the boundary layer isn't
significantly stabilized there, all severe hazards will remain in
play. The tornado threat will likely decrease with eastward
extent though based on veering near-surface flow ahead of the
progressing front. Through Monday morning and afternoon, the
front and associated severe threat will push across southeastern
Arkansas. The severe threat appears lowest here as instability wanes and
upper forcing lifts northeast. Still, isolated severe will be
Heavy rain will is likely as well with precipitable water values of 1.5 to 2.0
inches near the front. Flash flood potential doesn't appear to be
widespread, although western Arkansas is most at risk with a more
localized threat elsewhere.
The front clears southeastern Arkansas by Monday evening with the severe
threat ending behind the boundary. Much drier air will filter in
through the end of the period and gusty west winds can be expected
in the wake of this system.
Long term...Tuesday through Saturday
Cooler and drier airmass wl be in place acrs the region into midweek
as sfc high pres settled ovr Arkansas. A new upper trof is fcst to apch
the mid-south later in the week, bringing an incrs in showers and a
few storms. Fcst confidence rmns low later in the period as model
solutions are still in disagreement. The latest GFS is now more
progressive with the main upper trof, swinging thru the region and
not showing a cutoff low affecting the forecast area next weekend.
Meanwhile, the European model (ecmwf) and Canadian solutions are now showing a closed
low forming ovr the srn plains, eventually taking it northeastward along the
I-44 corridor next weekend. If this scenario does materialize, rain
chances wl linger over the region. However, for now kept pops
primarily in the chance Cat, with drier conds returning by Sat.