Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Jackson Kentucky
424 am EDT sun Sep 21 2014

Short term...(today through monday)
issued at 410 am EDT sun Sep 21 2014

Early this upper level ridge was centered over the
southern rockies and West Texas region with ridging northwest into The
Rockies and east into the southern appalachian region. To our
southeast a trough of low pressure extended from the Carolina
coastline south into Florida and the eastern Gulf of Mexico. An initial
shortwave is moving into the eastern Great Lakes region with the main
shortwave of interest locally working through the upper Midwest and
western Great Lakes region at this time. At the surface...high pressure was
centered across the southeastern Continental U.S. With a surface low pressure
working across the Great Lakes region. A cold front extended south
and southwest from the low into the middle MS valley region with the
boundary beginning to approach the lower Ohio Valley region.

Through the period...the ridging into the southeastern Continental U.S. Will be
shunted to the south with the southeastern trough...the initial
trough and the trough approaching from the northwest carving out a
trough from Ontario and Quebec into northeastern US region down into
the Ohio Valley region by this evening. This trough axis is expected to
shift east and northeast pushing east and southeast of east Kentucky by middle
morning on Monday and extend from Quebec into the Middle Atlantic
States and into the southeastern states at the end of the period.
This trough will push the cold front south and east of the Ohio River
this morning into early afternoon and across central and eastern Kentucky
this afternoon into the early evening hours. Surface high pressure
currently over the plains states will build into the Ohio Valley region
behind the boundary and is expected to be centered from the middle Ohio
Valley region west to the middle MS valley region by the end of the

Some valley fog is present this morning along with some passing
clouds in the 6 to 8kft range. 1a6 and i35 recently reported visible down
below 1 mile with dense fog at i35. Patchy fog has been included in
the grids for the remainder of the overnight with a couple more hours
of patchy dense fog in the grids for the deeper eastern and southeastern
valley locations in portions of the Big Sandy...Kentucky...liking and
upper Cumberland River basins. In more open terrain locations...the
pressure gradient has been sufficient to dissipate the fog over the
past couple of hours or prevent it from forming all together.

A couple bands showers and some thunderstorms are expected to move
into or develop across eastern Kentucky by late morning into the early
afternoon hours and depart most of the area by the cold
front approaches and works across the region. The model consensus and
recent short term model runs of the hrrr suggest that the best
chances for measurable precipitation and thunder will be across the
southern half of the County Warning Area. Probability of precipitation in the 50 to 60 degree range were used
in this area. Pending the degree of instability from heating and low
level moisture increase ahead of the front...model forecast middle level
drying...and decent speed shear could combine for the strongest
storms to produce some hail or strong winds gusts during the
afternoon to early evening. The Storm Prediction Center day 1 outlook is also consistent
with this thinking. This potential will be added to the severe weather potential statement. With a
moderate pressure gradient across the region and some stronger winds
aloft that could mix down...sustained winds of 10 miles per hour or so will be
possible late this morning into early this afternoon...especially
across the more open terrain locations across the western and
northwestern parts of the County Warning Area and the higher terrain. Some gusts
outside of any convection into the 20 to 25 miles per hour also appear possible.

The wraparound moisture late tonight into early on Monday may be deep
enough as the 500 mb trough axis works across the region with the low
level flow being deep enough for touch of drizzle or sprinkles
generally across the eastern part of the County Warning Area and the Virginia border
counties. The 0z GFS generates some light quantitative precipitation forecast in some of this area
late tonight into early on Monday. The other operational models have
any measurable precipitation further northeast into WV. Either way there
should be a period of low stratus or stratocu overnight into early on
Monday across the eastern half or so of the County Warning Area. Overnight lows
tonight will be more cold air advection driven with some of the far
Eastern Valley locations that warmest on average.

The lingering clouds and cold air advection will bring temperatures
at least 10 degrees below normal for most locations on Monday. The
morning stratocu should gradually lift into some flat cumulus just below
the subsidence inversion with these gradually mixing out or largely
mixing out by the end of the period...except possibly the far
southeast part of the County Warning Area.

Long term...(monday night through saturday)
issued at 348 am EDT sun Sep 21 2014

A strong upper level ridge will move from the central Continental U.S. At 0z the Mississippi River valley by 12z.
Meanwhile...surface high pressure will also remain strong over the
region...with winds both at the surface and aloft pulling dry and
colder air in from Canada. Some of the strongest winds aloft will be
moving off to our east with the departing upper level trough...but
cold air will linger through the day Tuesday. Morning lows will only
be in the low to middle 40s...with afternoon highs topping out around
or just above the 70 degree mark. Over the course of the extended
period...the surface high will shift slightly to our NE...but the
upper level ridge will remain blocked over the region. As such...dry
air will persist across much of the Ohio River valley...though the
loss of strongest nearly wind flow at both the surface and aloft will
lead to airmass modification and rising temperatures throughout the
extended. Temperatures will reach the 80 degree mark in some locations once
more by Saturday.

The latest European model (ecmwf) model is pointing at a weak low developing near the
region late in the period...namely Saturday and Sunday...allowing
moisture flow and precipitation chances to move into portions of the County Warning Area.
While a couple of our neighboring offices were on board with
including slight chance probability of precipitation...the uncertainty favored leaving them
out at the jkl County Warning Area at this time. So far...the model agreement past
the middle term is quite low. The European model (ecmwf) seems to be the outlier with
this precipitation...and very low quantitative precipitation forecast at that. More than likely...the super
blend is being influenced by this latest European model (ecmwf) run...which led to
the mention of a few 15 probability of precipitation in the Saturday night forecast.
Meanwhile...the GFS shows no sign of this low near the region /it
has it developing much farther S and much weaker/ or any precipitation
moving into Kentucky. Given the uncertainty at this point...lowered
any 15 probability of precipitation for Saturday night back down to 14. Will wait until
models come into better agreement or show more consistency between
runs before deciding to include probability of precipitation in the forecast package.


Aviation...(for the 06z tafs through 06z Sunday night)
issued at 145 am EDT sun Sep 21 2014

Outside of patchy fog...affecting loz and sme with mainly MVFR fog
and some IFR or lower fog in some of the deeper valley locations...
mainly VFR should prevail through around 14z. A cold front will drop
south into the area around sunrise with clouds and precipitation
chances on the increase. At this appears ceilings and visible with
rain showers and some thunderstorms and rain should generally remain in the VFR range...though some
brief MVFR is possible in some of the stronger showers or storms. As
the front moves into the region and the nocturnal inversion mixes out...
light winds should become southwest to west winds and increase to
around 10kt for the taf sites by around 15z...with some gusts up to
20kt possible...especially 16z to 22z. Winds will become more
northwesterly behind the front generally 21z through 0z and as the
pressure gradient relaxes winds should decrease to below 10kt late in
the period.


Jkl watches/warnings/advisories...


long term...jmw

National Weather Service Glossary of Abbreviations