Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 152335
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
635 PM EST Sun Dec 15 2013
...Updated Forecast and Aviation Discussion...
Issued at 630 PM EST Sun Dec 15 2013
Upper level disturbance now over northwest Minnesota will drop
southeast quickly tonight, bringing a brief chance for light snow
across our northeast zones. Have expanded the area where flurries
are possible to the south of this zone, including our northern
stripe of counties east into Oldham KY and over to northern Fayette.
Given how quick hitting this system will be, totals should be
limited to a few tenths of an inch north of a Scottsburg IN to
Cynthiana KY line. Current forecast handles these details pretty
well, so again this forecast just adds flurry wording farther south
and adjust temps for ongoing trends.
.SHORT TERM (Now through Monday Night)...
Issued at 300 PM EST Sun Dec 15 2013
Upper air pattern currently features broad cyclonic flow centered
around a longwave trough north of Lake Huron. Within this 500mb
northwest flow over the Ohio River Valley, a weak upper air feature
now over the upper midwest will slide southeast across Ohio by early
Monday morning. This disturbance will have no surface reflection,
rather any precipitation would be generated by weak lift aloft.
Forecast soundings over southeastern Indiana and the northern
Bluegrass show a potential for some light snow or flurries beginning
late this evening or around midnight, ending during the pre-dawn
hours Monday. Will only go for a chance of snow, with any light
accumulations staying at or under one half inch.
Any partial clearing this evening will be problematic at best.
Across southern Indiana, a deck of mid-level clouds associated with
our incoming clipper will rapidly overspread any holes developing
with our persistent low strato-cu deck. Farther south, strato-cu
will remain longer into the evening. But, once this erodes, feel
that our best chance for some clearing late tonight will occur
across the south, once the back edge of our low cloud shield
arrives. Lows tonight will range from the mid to the upper 20s. West
winds will diminish this evening and become nearly calm after
Think that low clouds will linger Monday along and north of
Interstate 64 with possibly partly to mostly clear skies to the south.
Weak high pressure over Texas will extend eastwards across the
northern Gulf States, bringing light southwesterly flow.
Temperatures will rise a bit Monday afternoon, especially across the
south. Expect highs to range from the upper 30s north to the mid 40s
across the south. Partly cloudy skies will develop Tuesday night,
with lows varying from the upper 20s to the lower 30s.
.LONG TERM (Tuesday through Sunday)...
Issued at 345 PM EST Sun Dec 15 2013
Tuesday through Thursday...
The medium range forecast period, the multi-model consensus is in
good agreement and shows relatively little spread in both the
deterministic and ensemble datasets. Large scale trough over the
eastern US at the beginning of the forecast period is forecast to
translate eastward with the upper level flow becoming more zonal by
late Wednesday and into Thursday. The latest solutions maintain
good continuity with a larger trough developing over the western US
which will eventually lead to a split flow pattern across the US.
With the trough developing out west, a resultant downstream ridge
across the southeast ridge will become increasingly larger with time.
Sensible weather impacts across our look to be rather minimal here
in terms of overall active weather. A quick moving upper air
disturbance will pass north of the region late Tuesday night and
may bring an increase in upper level cloud cover. Current data
projections suggest that precipitation associated with this feature
will remain to the northeast across northern IN and OH. However,
will need to watch future runs to see if this upper disturbance will
drop a little further south. For Wednesday and Thursday, high
pressure will provide the area with dry conditions. Temperatures
will exhibit a north-south gradient across the region. Highs
Tuesday will likely warm into the upper 30s across the north with
middle to upper 40s across the south. A moderation in temperatures
is expected on Wednesday and Thursday as the high moves off to the
east and we start to see an well established SW develop. Wednesday
afternoon highs look to warm into the lower-middle 40s across
southern IN and far northern KY. Upper 40s to the lower 50s will be
likely across much of KY. By Thursday afternoon, temperatures are
expected to be in the lower 50s across southern IN and northern
KY...with middle to upper 50s down across southern KY. Some
readings near 60 look possible along the KY/TN border. Lows through
the period will start off in the middle to upper 20s Wednesday
morning. By Thursday morning lows will be in the middle to upper
Thursday Night through Sunday...
...STRONG SIGNAL EMERGING FOR HEAVY PRECIPITATION ACROSS THE OHIO
VALLEY FOR THE UPCOMING WEEKEND...
As we move into the extended, the GFS and Euro solutions begin the
period in relatively good agreement. Both models show a fairly
healthy wave within the northern stream pushing through the upper
Midwest and into the Great Lakes. This will result in cyclogenesis
over the northern Plains with a surface low pushing into the Great
Lakes. As this occurs, both models push a cold front from the west
into the region late Thursday night and Friday. A good fetch of
moisture will be entrained northeastward into the Ohio Valley ahead
of this front producing widespread rain shower activity. Some of
this precipitation could be on the heavy side with some isolated
thunder not being out of the question as there will be some
instability available. Given the good continuity and agreement in
the models, we will continue to run high PoP chances for Thursday
night and Friday across the region. Both the GFS and Euro push the
front through the region Friday night and early Saturday which
should lead to decreasing chances of precipitation along with a
return to colder weather.
Significant model differences emerge by Saturday as the models
struggle with the split flow pattern aloft. As mentioned above, the
models develop a split flow pattern aloft with a rather large upper
level low developing over the SW CONUS and a corresponding
downstream ridge across the SE and Caribbean. The operational GFS
then allows this upper low to remain cut off in the southern stream
and strengthens the northern branch of the jet to push a colder
airmass into the northern CONUS. A look at the GFS ensembles and
previous ensembles show minimal support for this solution at this
The operational Euro and its ensembles also develop the split flow
regime along with developing the upper low over the southwest
CONUS. However, those models do allow the upper low to eject
outward while keeping the SE ridge rather strong. The net result
here is that the broad southwesterly flow remains intact across our
region with little in the way of shallow cold air advancing in from
In comparing the last few runs of the Euro and its 40+ member
ensemble, the models have been generally underpredicting the heights
of the 500 hPa fields over the Caribbean. Thus, we feel that the
Euro solution of keeping the SE ridge strong through the period
seems more likely to occur. However, at the same time, there is
concern that the model may be a little too weak with the northern
branch flow which may allow some intrusion of colder air further
south into the CONUS. The strength of the SE ridge and the strength
of the northern jet axis will be the critical players in the overall
evolution of this weekends weather.
Given the rather consistent solutions of the Euro and its ensembles,
we strongly feel that this is the best way to trend the ongoing
forecast. While we see a bit of break on Saturday, we expect the
emergence of the SW CONUS upper to approach the region from the SW
late Saturday bringing a round of heavy precipitation to the
region. This approaching wave combined with an increase in the
low-level jet will allow precipitable water amounts to rise to
around 1.4-1.6 inches across the region. This would be record
setting values by December standards...and this signal has been
present for the last few runs. Thus the potential for significant
heavy precipitation looks increasingly likely for our region. The
exact placement of the heavy precipitation axis is not clear, but a
SW-NE oriented band somewhere 100 miles N or S of the Ohio River
looks likely. A strong temperature gradient will also be noted here
and the possible shallow intrusion of colder air on the northern
side of this precipitation field could lead to precipitation type
issues across portions of the Ohio Valley. At this time, it appears
that our region will lie on the warm side of the system. However,
the possibility of shallow cold air working in from the NW could
result in significant wintry weather for the northern portions of
our area. This will bear watching over the next few days until
stronger model convergence emerges...probably around mid-week or so.
.AVIATION (00Z TAF Update)...
Updated at 630 PM EST Sun Dec 15 2013
Deck of stratus near 2 kft has not changed much the last several
hours, so have gone with more persistent ceilings overnight. Cloud
shield is eroding slowly northeastward over western TN. Should this
trend continue, it would make it to KBWG around 07Z. Model time
height sections are more pessimistic for KSDF and KLEX, bringing in
lower cigs by daybreak, so have tempo groups for IFR conditions
there. These cigs should raise in the afternoon, but only slowly,
with low-level humidity fields staying higher through the day. Winds
will be somewhat variable tonight, with a general southwest to west
flow, and this flow will remain into Monday.