Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Jackson Kentucky
622 PM EST Thursday Dec 18 2014
issued at 621 PM EST Thursday Dec 18 2014
Not much to update in the short term other than tweaking temperatures
towards latest observation. Also slightly increased sky for the next few
hours based on the current low level clouds seen in infrared imagery. The only
other thing to note are the snow showers moving through eastern
Kentucky...these showers have not accumulated to much so have left
pop grids alone.
Short term...(this evening through Friday night)
issued at 250 PM EST Thursday Dec 18 2014
19z surface analysis shows high pressure pushing into the state from the
northwest. At the leading edge of this...to the northwest...a band of
light snow and flurries are moving east out of southern Indiana and
along the Kentucky/Ohio border. Elsewhere...some virga and at worst a
few flurries or snow grains...are seen passing through eastern
Kentucky. Although there have been a few breaks in the clouds across
the south today middle and high level clouds dominated for yet another
day. Temperatures this afternoon range from the low 30s in the far
north where low clouds have held in tough for most of the day...to
the lower 40s in the far southeast...taking advantage of The Breaks
in the sky cover. Dewpoints through the area are in the middle to upper
20s as the winds continue to be light out of the northwest.
The models are in good agreement aloft as they all show fairly fast
and flat confluence flow over Kentucky into the day Friday before a
weak and dampening trough slips into the Tennessee Valley Friday
night into Saturday morning. The southern portion of this wave will
be shearing out in response to rising heights and a better defined
cluster of energy moving east through the Ohio Valley. This is a
pitiful result for what at one time was looking like a significant
trough crossing the area this weekend. The trends have been for a
weaker and more dampened system through the past handful of runs.
Accordingly...have favored the weaker solution from the nam12 but did
blend a bit with the 00z European model (ecmwf) and 12z GFS so it isn/T such an
abrupt change to the forecast.
Sensible weather will feature a risk for light snow and flurries to
the north into this evening...but even this should lift off to the
north and east of our part of Kentucky. High pressure will keep
things quiet...but not cloud free...tonight into Friday. The best
chance for clearing will be midday Friday...for a time...ahead of
the weakening southern system. This next system then brushes into
southern parts of the County Warning Area with a potential for light snow...but more
likely just flurries given the model trends. Chilly temperatures
continue through the period with dewpoints low enough to make
wetbulbing an issue for any precipitation that moves in that is not very
light. This would support more frozen rather than liquid precipitation for
this disappointing non event starting late Friday night.
Again used the bcconsshort as a starting point for the T/TD/wind
grids for the first 20 hours or so before populating with the
superblend thereafter. Only minor adjustments were made for terrain
effects both tonight and Friday night given the clouds. As for
probability of precipitation...ended up a bit above the met and mav numbers this evening
north and then closer to the drier mav ones for Friday night...as a
step toward the even drier met values should current trends persist.
Long term...(saturday through thursday)
issued at 322 PM EST Thursday Dec 18 2014
Active pattern is expected through the long term period with several
systems expected to impact eastern Kentucky. First system will be ongoing
into the long term on Saturday. Synoptically there is a shortwave
moving into eastern Texas tonight and a area of developing low pressure
across southern Texas. The area of low pressure will move into the Gulf
of Mexico Friday night into Saturday. Right now models are in decent
agreement with this...however the moisture looks more limited in the
latest runs. Given this did opt to bring probability of precipitation down and this may
needed to continue to be trended in that direction. Temperatures will
be cold enough early Saturday for snow but as we move through the day
expect that to transition to all rain.
After this we see more zonal flow aloft and surface high creeping
back across the region briefly ahead of a amplified trough developing
across the central US. This will aid in developing another surface
low pressure across the Gulf of Mexico Sunday night. Moisture will
stream north across the region Sunday night into Monday. Models are
differing on timing and will lean toward the blend at this point.
Based on the profiles this precipitation will be in the form of rain across
eastern Kentucky. This system will progress NE across the Atlantic coast
On the heels of this is yet another developing surface low pressure
across the upper Midwest. This amplified upper level trough will
eventually cutoff as we move into Tuesday night and this will
continue to deepen low pressure system across the Great Lakes and
Ohio Valley. Right now the models continue to differ in the
development and movement of this system...however the models are in
agreement that a frontal boundary will cross the region sometime
Tuesday night into early Wednesday. The issue becomes just how much
quantitative precipitation forecast and when do we see the cold air filter into the region. Given
this setup we do run the risk of some impacts to travel Christmas evening
day to the County Warning Area...however given that this is in the longer range
portion of the forecast this will need to be monitored as we get
closer to that forecast period. The upper level trough is expected to
continue to strengthen and negatively tilt Wednesday into Thursday.
This will give way to northwest flow and potential for continued chances of
snow Wednesday into Wednesday night particularly across the southeast
portions of the County Warning Area. This system will also have strong jet
support and some of this will be transferred to the surface. So expect
windy conditions across the region Wednesday into Thursday. High
pressure tries to build east briefly as we move into later Thursday.
Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 125 PM EST Thursday Dec 18 2014
Surface high pressure has basically won out over the area...
evaporating all the light radar returns before they reach the ground.
The lower clouds across the north are expected to break up during the
afternoon while high and middle ones remain. Cannot rule out an odd
flurry or sprinkle through the evening...but most locations will
remain precipitation free. The VFR ceilings will lower tonight into Friday morning
but there shouldn/T be any restrictions. Even these clouds will
likely break up during the day Friday keeping conditions VFR. Winds
through the period will be light and mainly from the northwest.