Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Jackson Kentucky
1232 am EST sun Dec 8 2013
issued at 1140 PM EST Sat Dec 7 2013
Reports of mixed precipitation are bing received from upstream in
middle Tennessee as isentropic lift is beginning and some of the returns to
our southwest are reaching the ground. The 0z model runs generally
support the previous forecast. The 0z GFS may be holding onto the low
level cold air a couple of hours too long while the 0z NAM may be
warming the lower levels a bit too fast across the central and
northern County Warning Area...but both have the same general theme of moistening
up from the top down as the night progresses and into the day on
Sunday. Overall...the previous forecast temperatures are a good compromise
between the too and favored the recent consraw data.
The 0z NAM and 0z GFS would suggest little if any snow...except
perhaps across the far northwest and mostly freezing rain or even mostly rain
in deeper valleys near the Tennessee/VA/KY tri state area. The central part
of the County Warning Area would experience precipitation types similar to the
The 0z GFS would suggests a little heavier quantitative precipitation forecast overnight near the Virginia
border than the current forecast...but that area should also warm the
quickest. The 0z models also suggest heavier quantitative precipitation forecast on Sunday afternoon
in the southeast than currently forecast so heavy rain and flooding
remains a concern. The NAM is near double during this period compared
to the previous run.
Overall...as mentioned...with current upstream reports and new model
data being considered...there was little reason to change
precipitation types at this time.
The Flood Watch and Winter Weather Advisory products were updated
and both remain valid.
Update issued at 709 PM EST Sat Dec 7 2013
Hourly temperatures and dewpoints were updated based on recent
observations. This generally led to colder temperatures initially
on the ridgetops compared to the previous forecast before warm air
advection should lead to those temperatures rising overnight...
especially from 3z to 8z. Drier and colder air gradually advecting in
from the north should lead to a slightly sharper surface thermal gradient
later on tonight somewhere near or to the north of the Interstate 64
corridor as the atmosphere saturates in the lowest levels and
wetbulbing occurs. Hourly temperature gradients were adjusted
slightly for this...but only down a degree or two at most in the far
The 18z NAM continues to support more in the way of mixed
precipitation compared to the colder 18z GFS. This will continue to
be monitored as additional model data arrives. However...at this
time there was no need to change the precipitation types in the
weather grids or snow and ice amounts. The current advisory start and
end times remain valid.
Short term...(this evening through Sunday night)
issued at 257 PM EST Sat Dec 7 2013
Presently...a cold front is stalled over the Tennessee Valley with
much of eastern Kentucky in the upper 20s to lower 30s on the north
side of this boundary. There has been some evidence based on observation that
we are seeing a bit of freezing fog at higher elevations. A middle level
wave over the southern Mississippi River valley will push northeast
tonight into tomorrow morning...bringing the frontal boundary back to
the north as a warm front. Good dynamics will arrive after midnight
over the are with widespread precipitation developing. As it stands
now...we should see a mix of precipitation at the onset as the
moisture runs into the cold air already in place north of the front.
It looks like depending on how far north/south you are will be
dependent on the type of precipitation. Areas in the south will
likely start off as either sleet or freezing rain...with areas
farther north starting off as a period of snow. However...it looks
like any snow should quickly change over to a period of sleet...then
eventually over to freezing rain. As temperatures warm from the
south...we should see the freezing rain threat shift northward and
out of eastern Kentucky during the day Sunday. Deeper moisture will
exit by Sunday afternoon...but low level moisture will remain
supporting a period of drizzle.
Another wave will then push across southeast Kentucky Sunday night
into early Monday...producing the potential for more rain...perhaps
heavy across far southeast Kentucky. This will present the area with
another soaking rain...which will create additional flooding
concerns. Thus...the Flood Watch will continue from late tonight
through Sunday night. This time around the greatest threat for heavy
rain should reside across far southeast Kentucky. Mild weather will
accompany the rain Sunday night with temperatures climbing into the
Long term...(monday through saturday)
issued at 445 PM EST Sat Dec 7 2013
The models are in good agreement aloft as they sweep a large...full
latitude...trough from the plains into the Great Lakes and gradually
south through the rest of the eastern U.S. Into the day Thursday.
Deep layer southwest flow will spread over east Kentucky during this
time with a decent wave passing through at middle levels Monday into
Tuesday. For the latter part of the work week the pattern will
become more split as the eastern trough moves off to the northeast
and a southern stream low crosses into the Southern Plains. This
latter trough will be moving into the Tennessee Valley on Friday...a
bit sharper in the European model (ecmwf) than the GFS. As the core of this trough
moves through east Kentucky another one will drop into the High
Plains from the northwest and help to reestablish one main flow
regime for the nation by Saturday. The latest European model (ecmwf) is most
aggressive with this process and swings a deepening and closing off
trough through the deep south Saturday night. For this time frame...
will favor a blend of the GFS and European model (ecmwf) model and a broader blend of
models over the rest of the forecast where they are even more in
sync with each other.
Sensible weather for the first part of the extended will revolve the
weakening Arctic front just to the southeast of the County Warning Area. A surface wave
or two lifting northeast along this boundary will continue a threat
of light snow or rain on Monday and into Tuesday morning...mainly in
our far southeast. High pressure will then build into the region
keeping most of the weather well to the north of Kentucky through midweek
even as cold air filters in to reinforce the air mass in place over
the County Warning Area. There will be a chance for snow or rain to move back into
the area on Friday and lingering on Saturday when that southern
stream trough and trailing deeper one moves into the region. The
European model (ecmwf) would suggest higher probability of precipitation than currently forecasted for this
period but for now have tempered its forecast with that from the
The CR grid Load came in pretty good with probability of precipitation and sky cover early...
but needed adjustment through middle week for that High Crossing the
region. Also upped the probability of precipitation and sky cover somewhat Friday into
Saturday with that wave represented by the latest European model (ecmwf). Also added
in some small ridge and valley temperature adjustments to lows from
Tuesday night through Thursday night.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1232 am EST sun Dec 8 2013
MVFR ceilings will begin the period at all three taf sites. Conditions
are expected to deteriorate fairly quickly between 6 and 8z...as low
level clouds spill into the area from the south. An approaching
weather system will send another surge of moisture across the area
tonight...and continuing through the end of the period. Ceilings
will lower to and then remain in the IFR category through the end of
the period. In addition...a wintry mix of snow...freezing rain...and
sleet is anticipated to arrive at the taf sites between 7 and 8z...
with the mix changing to freezing rain within 2 to 4 hours after
arrival. Freezing rain should change to rain at the taf sites between
13z and 17z...earliest at loz and sme and later at jkl and sjs.
Winter Weather Advisory from 5 am early this morning to 4 PM EST
this afternoon for kyz044-050>052-059-060-104-106-107-119.
Flood Watch through Monday morning for kyz058>060-068-069-079-
Winter Weather Advisory until 1 PM EST this afternoon for kyz058-
Winter Weather Advisory until 10 am EST this morning for kyz079-