Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
547 PM EST Thu Jan 29 2015
..updated aviation discussion...
Short term (now through Friday night)...
issued at 315 PM EST Thu Jan 29 2015
..a few slick spots Friday morning?...
Cold front is almost through the eastern County Warning Area with temperatures
already in the upper 30s over some of our southern Indiana counties.
In contrast, southeast Kentucky is still in the upper 50s just ahead of the
front. Temperatures will be falling the rest of the afternoon and
evening. Data still supports some isolated to scattered showers
developing mainly along and east of I-65 late this afternoon into
the evening and overnight. This would be ahead of a pv anamoly
dropping out of the mid Mississippi River valley and correspondent
with a northwest surface winds aiding in upslope flow across our far east.
With lapse rates steepening, p-type is expected to transition to
snow this evening. Previous forecast had mentioned a small chance
for some freezing drizzle, and can't totally rule that out at this
time, however latest data supports saturation up to -7 or -8 c
before temperatures fall below freezing. Will not directly mention
this p-type in the forecast although may allude to the possibility
in other products like the severe weather potential statement. Also, not expecting snow fall to be
very intense as there isn't very much (if any) lift through the
saturated layer. Will continue to mention a dusting of snow possible
through the overnight and early morning hours across our east, where
best coverage of snow is likely to be. A few slick spots aren't out
of the question for the Friday morning commute and will consider a
Special Weather Statement to highlight threats.
Otherwise, lows will drop into the upper 20s tonight, with low
clouds lingering through the first half of Friday under the
inversion. We'll see some sun by the afternoon hours with temps
staying in the 30s.
Surfac high pressure settles over the by Friday evening, with good
radiational cooling conditions Friday night. This should allow for
lows in the mid teens NE to around 20 most other places.
Long term (saturday through thursday)...
issued at 329 PM EST Thu Jan 29 2015
..accumulating snows increasingly likely late this weekend across
southern Indiana and portions of northern Kentucky...
synoptic scale overview
A split flow pattern aloft will be found at the beginning of the
forecast period. By Sunday, a moderately strong mid-level wave will
drop out of the northern plains and amplify as it approaches the
mid-Mississippi Valley. As this occurs, surface cyclogenesis will
take place over Arkansas with the newly formed surface low moving
from SW Tennessee into the mid-Atlantic region by Monday morning.
With cold air in place across the northern half of the forecast
area, a swath of accumulating snows looks increasingly likely.
Further south, near the surface low track, a wintry mix of rain and
snow appears likely...eventually transitioning to all snow by Sunday
night. The upper air pattern is then forecast to relax a bit,
however, significant amplification is expected to occur by midweek
as waves in the northern and southern stream attempt to phase and
carve out a large/deep trough over the eastern third of the United
Over the last 24 hours, we have started to see a more definitive
case of model convergence on the evolution of the late weekend storm
system. All of the 29/12z guidance has trended colder with regards
to the lower boundary layer temperatures. The GFS and Euro have led
the trend in the colder solutions, while the NAM and Canadian Gem
have now started trending colder as well. The NAM is still the
warmest of the models, while the GFS/Gem/Euro are generally the same
as their previous runs. Noticeably, the Euro and GFS solutions have
continued to show remarkable continuity in their runs, which results
in increasing forecaster confidence on the evolution of this
system. Given the current datasets and trends, it appears
increasingly likely that accumulating snows will be a threat to
southern Indiana and portions of northern Kentucky.
The model data continues to exhibit some spread regarding the
mid-late week system as the upper level flow and timing of features
remain uncertain. However, there appears to be at least some hint
of agreement that another weather system will affect the region
Wednesday into Thursday.
For this forecast cycle we will be utilizing a blend of the previous
forecast while incorporating the colder solutions from the new
datasets. The forecast itself remains largely unchanged, but given
the relatively good model agreement, we will be increasing pops
for Saturday night through Monday morning. As for qpf, a blend of
the GFS/Euro/wpc guidance will be incorporated.
From midweek Onward, have generally stuck closer to the blend of the
29/12z GFS ensemble and the 29/00z Euro ensembles along with
incorporating our previous forecast.
Forecast confidence on the occurrence of precipitation from Saturday
night through Monday morning is high given the good model
agreement. Forecast confidence on the precipitation type is much
lower given the uncertainty in the forecast of lower atmospheric
temperature profiles. This is especially true across much of
southern Kentucky where low-level warm air will likely keep
precipitation mixed or in plain liquid. Forecast confidence is
increasing however, that much of southern Indiana and portions far
northern Kentucky will see mostly snow with the late weekend system.
Forecast confidence from midweek Onward can be described as
low-moderate in both temperature and precipitation departments.
Sensible weather impacts
A quiet day of weather is expected on Saturday as we wait for the
approach of the late weekend weather system. The day will likely
start off partly sunny, but clouds will be on the increase late in
the day. Highs look to warm into the upper 30s in the north with
lower 40s across the south.
By Saturday night, we expect a band of precipitation to develop out
to our northwest within a large baroclinic zone from the Central
Plains into the Midwest. Moisture is expected to return to the
region during the overnight period with precipitation breaking out
across southern Indiana and central Kentucky very late Saturday
night. Thermal profiles generally look cold enough for mostly snow,
but a mix of snow and rain may occur down across southern Kentucky.
Precipitation is expected to expand and increase in intensity during
the day on Sunday across the region. Model proximity soundings and
thermal cross-sections suggest that mainly snow will occur across
southern Indiana and across far northern Kentucky. Just exactly how far
south the rain/snow line will be is not definitive at this time.
However, incorporating the multi-model consensus, it does appear
that the previous forecast of rain and snow across much of
south-central Kentucky still looks to be likely at this time.
However, should thermal profiles end up being slightly cooler, the
lower boundary layer may be sufficiently cool enough to have snow as
the pre-dominant precipitation type despite the fact that surface
temperatures may be several degrees above freezing.
As the surface low passes through the far southern portion of our
forecast area, colder air will be pulled in from the north resulting
in all areas seeing snow Sunday evening. The GFS and Euro continue
to show a good deformation band signal within their data sets this
afternoon. This deformation band looks to start off in central in
Sunday afternoon and then many pivot southeastward into Kentucky
late Sunday night and early Sunday morning. Based on the current
datasets, the best chances for accumulating snows looks to be from
Sunday afternoon through early Monday morning.
Considerable uncertainty remains in the evolution of this system.
It should be noted that the weather features forcing this system are
located quite a ways from the North American upper air network and
will not be properly sampled until late tonight or Friday morning.
Because of the numerical models sensitivity to initial datasets,
oscillations in the model solutions are still possible. We
encourage people with travel plans this weekend to keep a close eye
on subsequent forecasts as travel may be negatively impacted late
Quieter weather is expected for Monday and Tuesday as we will be in
between weather systems. High and low temperature forecasts are
particularly hard to nail down at this point as they may be greatly
impacted if we have snow cover. For now, have stuck close to the
superblend guidance with highs in the mid-upper 20s to the lower 30s.
From midweek on, we will need to keep an eye on another weather
system that will affect our region in the Wednesday/Thursday time
frame. The track and evolution of this system is highly unknown and
may be modulated to an extent by the late weekend system. For now,
have kept a rain/snow mix chance in for Wednesday with some light
snow possible Wednesday night. Much colder conditions look likely
as we head to towards the end of the week. Current guidance
generally gives highs in the 20s, but this could be too optimistic
if a snow pack exists. Overnight lows in the upper single digits to
lower teens are expected. However, sub-zero readings will be on The
Table if a significant snow pack is realized. Stay tuned.
Aviation (00z taf update)...
issued at 545 PM EST Thu Jan 29 2015
Northwesterly winds will continue tonight in the wake of the cold
front. Gusts will diminish through the evening hours, but wind
speeds will stay up in the 8-12 knot range overnight. Very light
snow showers will be possible at Lex and perhaps sdf overnight.
Ceilings are expected to remain MVFR with fuel alternate at times. They
should begin to lift overnight and become VFR by late morning to
early afternoon tomorrow. Winds will remain out of the northwest to
north tomorrow and be lighter than today.