Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Nashville Tennessee
1132 PM CST Monday Dec 9 2013
Aviation 06z discussion...
wintry precipitation to overspread the middle state through 09z. The
precipitation should end in southwest areas of middle Tennessee around 10z and
then northeast areas around 14z. Conditions will improve markedly
by 18z with clearing skies toward 00z.
The bulk of the precipitation type will be -sn at bna and ckv.
Csv...however...will contend with -pl and freezing rain with a transition
to some snow around 09z.
Previous discussion... /issued 1044 PM CST Monday Dec 9 2013/
radar indicating that the precipitation approaching bna is all
snow above 1200 feet. Melting below that level looks unlikely and
thus...snow should begin shortly across northern areas. Mostly
sleet is falling central...and freezing rain South. Lawrence and
Marshall counties are both reporting ice coating most elevated
Cold air advection to continue. Therefore...current forecast is on track in terms of
transitioning. We have recently upped our snow
amounts...particularly NC and NE areas where 1 to 2 inched of
snow is now possible. Less than 1 inch across the non advisory northwest
For southern areas...up to one quarter inch of ice will be
All precipitation should end shortly after sunrise across the plateau.
Updates have been sent. Still concerned with latest RUC which
paints a little extra quantitative precipitation forecast from Nashville NE through the eastern
Highland Rim area through 09z. Will await the whites of its eyes
before upping snow amts again. Not sold on RUC.
Previous discussion... /issued 712 PM CST Monday Dec 9 2013/
surface freezing line now extends generally along I-65...with sub
freezing temperatures to the west except 32-34 southwest. Temperatures across
the plateau are in the low to middle 30s. Radar trends from radars to
our southwest show a definitive increase in precipitation coverage across
MS and eastern Arkansas. Models are in agreement with spreading this
moisture northeastward rather rapidly. By 06z...much of the
southeastern two thirds of the middle state will be receiving
precipitation. It still looks as though the southern and plateau
areas could receive total liq equivalent amounts of between one
quarter and one half inch. If the bulk of this falls with sub
freezing temperatures in place...we will need to consider upgrading to a
Winter Storm Warning for that eastern and southern area. For now
though...will keep the advisory going but will increase ice
amounts across the south and plateau to two tenths of an inch.
Will also adjust graphics so that snow amounts support the
advisory criteria on the northwestern edge of the advisory.
Otw...latest sounding is rather dry above 800 mb and looks like a
snow sounding considering the upcoming wet bulb cooling that will
take place. Further south...freezing rain will be the bigger
impact across the south and plateau. Again...this area will need
to be watched. Updates out shortly.
Previous discussion... /issued 552 PM CST Monday Dec 9 2013/
Aviation 00z discussion...
another storm system will move northeastward from the northern
Gulf and spread wintry precipitation back across the middle state
overnight. Some significant icing will be possible near csv after
06z. A gradual shift to all snow can be expected between 9 and
12z. Overall precipitation amounts will be greatest across csv with much
lighter amounts expected at ckv.
The IFR ceilings will continue through 18z. After 18z...partial clearing
Previous discussion... /issued 420 PM CST Monday Dec 9 2013/
Another day another round of wintry precipitation is expected yet
again tonight across the middle state as precipitation developing to
the southwest spreads northeastward atop the cold Arctic airmass
entrenched over the region. Showers and thunderstorms are now
developing across Texas/Louisiana/Mississippi...and this activity
will spread northeastward very rapidly and reach the middle state
this evening before spreading quickly across the area tonight and
exiting by Tuesday morning. Models are finally in decent agreement
on evolution of system tonight...and after an extensive analysis
of all 12z model data a Winter Weather Advisory was issued for the
southeast two-thirds of the middle state...roughly southeast of a
line from Linden to Gallatin.
An extensive look at 12z GFS/NAM forecast soundings indicate areas
along and south of a line from Waynesboro to Crossville will
initially see precipitation fall as rain with temperatures in the middle
30s...but as temperatures fall below freezing later this evening
and tonight a transition to all freezing rain is anticipated. Quantitative precipitation forecast
amounts suggest around one tenth of ice accumulation is most
likely in this area...with main impacts being potentially icy
bridges and overpasses. Precipitation may changeover to all snow towards
the end of the event but little or no snow accumulation is likely
in this area.
North of the line from Waynesboro to Crossville in the advisory
area...a mix of precipitation types is expected initially with soundings
depicting a deep surface cold layer with only a relatively weak
warm nose aloft...suggesting predominately sleet or snow further
north and freezing rain further South. Strong lift and saturating/cooling
of the entire column should change any sleet/freezing rain over to snow
near/after midnight in this part of the advisory area. Quantitative precipitation forecast
amounts indicate a dusting to 1 inch of snow would be most common
with highest amounts east of I-65 and north of I-40...with travel
seeing the most impacts due to potentially snowy bridges and
overpasses. Longer duration of precipitation and cooler temperature profiles
suggest northern Cumberland Plateau could end up with higher snow
accums...and this will have to be closely monitored as the event
Precipitation will be encountering a very dry airmass aloft to the
northwest of the middle state...and a sharp northwest edge to the precipitation
shield is anticipated likely somewhere along a line from Linden to
Lafayette...with little or no precipitation at all anticipated in
northwest zones. This back edge to the precipitation gives the forecast
considerable bust potential for the northwest counties within the
advisory area. Unfortunately...this back edge to the precipitation will
also likely bisect the Nashville metropolitan...so northwest portions of
the metropolitan area may see little or no precipitation while southeast
sections are more impacted. Even more uncertain are convective
elements with the precipitation as soundings depict some elevated
instability...so if stronger convection can form freezing rain/ice/snow
amounts could be higher in some locations.
Main forecast focus was on winter weather tonight...so forecast
for rest of the week relied heavily on the 12z GFS. Appears clouds
will finally clear across the County Warning Area by Tuesday afternoon as strong
northwest flow develops in the low levels...but this will result
in several cold days for the middle of the week with lows in the
teens northwest to 20 elsewhere and highs only in the 30s and 40s.
Warming trend anticipated by Friday as surface ridge moves east
and southerly flow develops. Could see a bit of a rain/snow mix on
the plateau early Friday night...but eventually all rain is
expected everywhere as temperatures warm considerably into Saturday.
Strong cold air advection in the wake of the upcoming weekend system will bring
much colder temperatures and our next chance for light snow
Saturday night...although main deformation band looks to stay well
north of the middle state.
Winter Weather Advisory until 9 am CST Tuesday for tnz008>011-