Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
1217 am EST Sat Dec 20 2014
Issued at 1215 am EST sate Dec 20 2014
Light radar returns have begun to spread across the area from the
west. While much of this is not reaching the ground, a few
flurries/light snow will be possible across the area overnight. Also
have seen a few reports of sleet from just south of the Kentucky border in
TN, so went ahead and added sleet into the forecast over south
central Kentucky. Neither the snow or possible sleet should have any
negative impacts overnight.
Issued at 854 PM EST Fri Dec 19 2014
Forecast is on track this evening although did make a few minor
tweaks. A southern stream storm system will largely pass by to our
south and east through the overnight, with just enough moisture to
squeeze out some very light snow across our southern counties.
Additionally, a narrow batch of light snow/flurries could fall along
the Ohio River as just enough deep moisture pools ahead of a weak pv
anomaly. Was reluctant to add this mention as it will mainly be
falling out of mid deck. Nevertheless, upstream obs in the MO
bootheel are picking up precipitation so will mention a slight
chance after midnight. Perhaps a sprinkling of very light snow could
dust vehicles and elevated surfaces, but no impacts are expected
Temperatures are running a couple degrees cooler than anticipated so
updated grids to reflect latest trends. The column does look cold
enough for all snow so will not mention a rain/snow mix. Updated
products already out.
Short term (now through Saturday night)...
issued at 300 PM EST Fri Dec 19 2014
Southern stream disturbance remains quite suppressed as it pushes
east across Texas, with minimal surface reflection along the
Louisiana Gulf Coast and a precip shield still struggling to spread
north of Interstate 40. After a decent window of sunshine over parts
of central Kentucky today, clouds will fill back in this evening.
However, any precip tonight will be minimal and limited to south
Soundings appear just cold enough for snow, but precip chances will
be limited to 20-30% and quantitative precipitation forecast will be quite light. Overnight timing
and air temps near freezing will further mitigate the impact of snow
accumulations that will be less than a half inch. Best chance of any
accumulation is near Lake Cumberland, and that's still not much.
This system will scoot off to the east Saturday morning, with zonal
flow aloft and a NE-SW oriented surface ridge remaining across the
Ohio Valley. Model guidance seems quick to scour out the low clouds,
but given recent trends this seems a bit optimistic. Max temps
Saturday afternoon could be tricky depending on how much clearing
occurs, so will go just under the cooler NAM MOS out of respect for
the seemingly endless cloud cover.
Saturday night will be a few degrees cooler than tonight, and if
skies do manage to clear mid 20s certainly aren't out of the
Long term (sunday through friday)...
issued at 300 PM EST Fri Dec 19 2014
An active, changing weather pattern next week looks to bring a host
of forecast challenges, including precipitation chances/timing/type,
winds and temperatures.
Sunday - tuesday:
The 19.00z deterministic guidance is in good agreement that Sunday
the upper level pattern will feature a broad longwave trough across
the Central Plains with a strengthening 300 mb jet coming into the
Pacific northwest. The results in some weak downstream ridging
across the Ohio Valley and with surface high pressure across the
mid-Atlantic, seasonable temperatures and dry conditions are
expected. Highs 42-48, coolest across southern Indiana. Guidance has
slowed and shifted further south the system for Sunday night
considerably, so pops were adjusted down and confined mainly to the
southeast forecast area. Lows 35-40.
By Monday, as the main northern stream energy comes into northern
plains, an upper level shortwave pinches off and deepens. Ridging
ahead of this trough builds across the area and weak ripples in the
flow are expected to ride through the periphery of this trough,
which will keep rain chances in the forecast. Southerly flow in the
low levels will pull up Gulf moisture with dewpoints climbing into
the 40s. This allows the air mass to become increasingly more moist
through Tuesday as the trough to the west deepens further. In fact,
pwats increase to 1.15 to 1.25 inches Tuesday afternoon, which is
roughly +2 Standard deviations above normal. Additionally, very warm
air surges into the area ahead of the cold front and highs Tuesday
could reach 60 in the south with mid 50s elsewhere on Tuesday.
The 19.12z guidance trended westward with the surface low track, but
a time analysis shows the models really are struggling where exactly
the surface low begins to deepen. Once it lifts toward the Great
Lakes Tuesday evening, colder air pushes into the region behind the
cold front. However, soundings show that the surface to 850 mb
temperatures remain sufficiently warm enough to keep most of the
precipitation in liquid form. It's not until late Tuesday night that
profiles would support snow, but by then a lot of the moisture/lift
would have exited the area. At this point, continued the rain/snow
mix across the area Tuesday night.
Christmas eve - Christmas day:
As the weather system deepens across the Great Lakes region, the
main story for southern Indiana and central Kentucky will be the
blustery winds, noticeably colder conditions and threat for snow
showers Christmas eve. As the pressure gradient tightens, sustained
west/northwest winds could approach 20 mph with gusts over 30 mph
possible. Plan on highs in the mid 30s to low 40s and combined with
the wind, it'll feel much colder.
There is likely to be a good fetch of wrap around moisture combined
with a few shortwaves in the cyclonic flow aloft. This environment
would result in wrap around snow showers Wednesday afternoon and
night. Soundings show steep lapse rates, good saturation through the
atmosphere and plenty of low-level lift. Boundary layer temperature
profiles suggest precipitation would fall as snow. A model consensus
of high chance to low likely pops looks good at this point. It
remains too early to speculate on snow amounts, but there is a
chance for minor/light accumulations Christmas eve across southern
Indiana and central Kentucky.
For Christmas day, the upper level flow may become more zonal which
would give the area a seasonable, dry day with a fair amount of
sunshine. However, this depends on how quickly the upper low over
the Great Lakes weakens or departs. For now, kept slight chances of
snow showers across the northeast forecast area with highs ranging
from the upper 30s to mid 40s, or slightly below normal.
Aviation (06z taf update)...
issued at 1145 PM EST Fri Dec 19 2014
The main challenge for this taf period will be overnight with the
potential for MVFR ceilings. An upper level disturbance will
continue to cross the area tonight. The atmosphere is beginning to
saturate from the west and light returns can be seen on radar.
Ceilings should lower at all sites overnight. However, whether they
will drop below 3000 ft or remain scattered at that level remains a
bit in question. Have kept things optimistic and VFR at this time.
With this system, bwg and sdf may see some light snow/flurries, but
it is not expected to cause visibility restrictions. Winds will be
light through this taf period. Clouds are expected to start to clear
late in the afternoon into the evening.