Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
1146 am EST Mon Dec 29 2014
..updated aviation discussion...
issued at 625 am EST Mon Dec 29 2014
Have updated the forecast to leave mention of chances for measurable
rain in our southeast counties a bit longer (~8 am cst). Area
webcams and a few light radar returns still show the precipitation
ocurring. Cannot rule out a brief mix of wintry precipitation with
any light returns, however surface temps will largely remain above
freezing, so not expecting any problems.
Short term (now through tuesday)...
issued at 305 am EST Mon Dec 29 2014
Some light rain/drizzle is lingering around the Lake Cumberland area
early this morning, however expect this to be temporary as deeper
moisture pushes off to the east behind the front. Will have to worry
about some fog in this area toward dawn, so will continue to mention
in the forecast.
Otherwise, high pressure will slowly begin to gain control of the
area as it builds in from the northwest later today. Expect gradual
clearing (other than lingering mid and high clouds) from northwest to southeast as
this occurs. Have undercut guidance on today's high temperatures a
bit for a couple of reasons. 1.) Steady NE flow at the surface is a
consistently cooler setup. 2.) Uncertainty in how quickly skies will
By tonight, expect mostly clear skies although some mid and high
clouds will continue to be possible. Dew points will drop into the
low and mid 20s across the area with high pressure building in,
however will not completely tank lows due to a steady NE wind
continuing and some clouds. As a result, most areas should expect
lows in the 25 to 30 range. A few decoupled north valleys could possibly
see lower 20s.
Tuesday brings high pressure more firmly in control. Dry weather
will continue, although at the price of cooler air. Highs will be a
couple/few degrees lower than today. Look for upper 30s north to low
Long term (tuesday night through sunday)...
issued at 300 am EST Mon Dec 29 2014
Middle of the week will be dominated by Canadian high pressure, with
cold and dry weather to ring in 2015. Latest GFS is quite bullish
with cold temps on Tuesday night, but expect just enough north-northwest wind to
keep temps from bottoming out. After highs struggle to around
freezing on Wednesday afternoon, Wednesday night looks like the
colder night. However, confidence there is even a bit limited as west-southwest
return flow will be trying to set up. If that is not felt at the
surface we could drop well into the teens, with some of our
traditional cold spots down around 10 above. Current forecast is for
upper teens and lower 20s most places, but either way expect it to
feel more like 10-15 degrees by daybreak Thursday.
Limited confidence regarding the late week southern stream system
even in the face of surprisingly good model agreement. In terms of
the overall pattern, the GFS and European model (ecmwf) both show the same slower
trend with the main upper low trying to lift out of the Desert
Southwest. Main discrepancy is the track of the upper low late in
the weekend, as the European model (ecmwf) runs it up the Ohio Valley on Sunday while
the GFS lifts it well up into the Great Lakes. Main change to the
forecast will be to continue to trim back Thursday night pops as we
could go well into Friday or even Friday night before precip spreads
across central Kentucky. Will also introduce chance pops for Sunday,
as the trend now points to a substantially slower system.
Precip type could be an issue at onset, but even that is not looking
as Dicey as in previous runs. The cold mid-week surface high settles
mainly to our south and west, which lends itself to a quick erosion
of any cold air we may have in place. The slower onset of precip
also puts US into a warmer part of the day. That said, will still
carry a vague mention of a rain/snow mix for at least one more
forecast cycle. Probably more noteworthy will be the soaking rains
this system will bring over the weekend. Long duration event with
the Gulf wide open supports widespread 1-2 inch rainfall totals,
which could go even higher depending on timing and possible embedded
thunderstorms. Potential Hydro impacts will be mitigated by the
slow, soaking nature of the rainfall and the recent rainfall
deficits in south central Kentucky where the quantitative precipitation forecast is the highest.
Temps near climo Friday, then solidly above normal for the rest of
the weekend, albeit with much smaller diurnal ranges than advertised
Aviation (18z taf update)...
issued at 1145 PM EST Mon Dec 29 2014
Surface high pressure will slowly build in from the west through the
taf period. Drier air near the surface will slowly filter in through
the afternoon. The persistent low clouds will slowly lift through the
afternoon and scatter out. Sdf should see clouds scatter out over
the next couple of hours. Bwg and Lex should become VFR by early
this evening with VFR conditions persisting through tomorrow. Winds
will be out of the NE to north-northeast through the period and generally in the
5-8 knot range.