Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 181656
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1156 AM EST Thu Dec 18 2014
...Updated Forecast and Aviation Discussions...
Issued at 1155 AM EST Thu Dec 18 2014
The issues caused by this morning`s wintry mix have improved as any
steadier precipitation has dissipated. The better moisture to the
southwest is running into a very deep, dry airmass just above
925mb. Therefore, expect nothing more than perhaps a stray flurry
or sprinkle for the remainder of this afternoon. Given the
abundance of clouds, did go ahead and lower high temperatures a
degree or two in most locations.
Issued at 810 AM EST Thu Dec 18 2014
Went ahead and upgraded the SPS to a Traveler`s Advisory, as we have
gotten several reports of accidents and slide-offs across portions
of southern Kentucky, one of which involved multiple vehicles and
shut down both the north/south lanes of I-65 in Simpson County.
Much of the precipitation has ended, but a few patches of freezing
drizzle or even some light sleet will still be possible over the
next couple of hours. Elsewhere, conditions will likely remain dry
given the very dry mid-level airmass, which has caused the earlier
precipitation shield to largely dissipate.
.SHORT TERM (Now through Friday)...
Issued at 300 AM EST Thu Dec 18 2014
...Light wintry precip possible this morning across our southern counties...
Widespread isentropic lift at mid-levels ahead of a weakening 500mb
wave now over Missouri is spreading mixed wintry precipitation
across southern Missouri and western Tennessee. This precip is
however running into dry air at lower levels across the Commonwealth
and will weaken as it moves northeast into central Kentucky later
Think that best chances for light mixed precipitation or snow
accumulations later this morning will lie along and south of a line
from Ohio County through Adair County, including the Bowling Green
area. Forecast soundings for this area shows a few hours of
saturation that may support snow accumulations of less than one
inch. With light snow possible during the morning rush, will
continue our ongoing Special Weather Statement highlighting slick roads
across our southwestern counties.
Think that flurries or sprinkles at worst are likely for areas along
or north of Interstate 64 towards mid to late morning, again, due to
dry air at lower levels. Winds will stay light from the north today
at around 5 to 7 mph.
Mostly cloudy skies expected this afternoon with lingering flurries,
mainly south of Interstate 64. Highs will rise into the mid to upper
30s once any precipitation ends.
High pressure will nose south across Kentucky tonight bringing light
winds and eventually partly cloudy skies later tonight and Friday.
Temperatures will stay seasonable with lows tonight in the mid to
upper 20s and highs Friday near 40.
.LONG TERM (Friday Night through Wednesday)...
Issued at 332 AM EST Thu Dec 18 2014
Long Term Synopsis
A moderate progressive flow aloft will be in place at the start of
the forecast period. Flow is being driven by a fairly fast Pacific
jetstream which is being driven by two height anomalies out over the
Pacific. A well-advertised mid-level wave will shear out as it
passes through the region Friday night and Saturday. The upper flow
then becomes more zonal again by the late weekend and into early
next week before amplifying significantly toward the Christmas
holidays. The amplification is driven by an emerging -AO pattern
combined with a swing back to a -EPO out over the Pacific. This
result in a negative height anomaly over the gulf of AK while we see
ridging out over the western US. At the same time, signals also
point to a -NAO developing (Greenland Block) in the later periods.
These features then lead to a very strong negative height anomaly
developing over the Midwest and shifting into the Great Lakes by
Christmas day. Beyond Christmas, a significant pattern change will
be in progress. The continued -AO/-EPO/-NAO should result in a
rather large mid-level gyre over central Canada...paving the way to
a well advertised return to cold over much of the eastern United
Over the past few days, we`ve seen a decent amount of forecast
convergence within the guidance. The early weekend system was
originally progged to shift through the northern Gulf coast states,
but the models abandoned that idea in the last day and half once
they sensed the fast flow coming in from the Pacific. The early
week Canadian GEM picked up on this more sheared out, less poleward
track, while the Euro adjusted fairly quickly toward the GEM as the
week wore on.
This morning we find ourselves with a better forecast agreement for
the most part. The GFS/GEM/ECWMF all have the early weekend system
brushing the southern Ohio Valley late Friday night into Saturday.
The system does not have much in the way of cold air to work with,
other than mainly diurnally driven cooling. Thus, mixed P-type
issues remain...mainly across southern KY. Have leaned close to the
Euro/GEM/GFS solution with highest weights in that order. This
keeps good continuity with the previous forecast. Overall
confidence with regards to precipitation remains medium to high
across southern KY...with mainly medium confidence as one gets near
the Ohio River. Temperature confidence here remains generally
medium as well.
Into the middle part of next week, there seems to be decent
agreement with the large height anomaly developing over the northern
Plains and Midwest...then shifting into the Great Lakes. The
pattern for us will remain mild into the early week and then turn
colder by the Holidays. NAEFS ensemble guidance points to a very
strong height anomaly by the end of the forecast period...with
generalized return periods of every 10 years (decent anomaly). The
MSLP anomalies are off the chart compared to climo suggestive of a 1
in 30 year event (quite impressive). The anomalies from the
ensembles are quite high indicative of good ensemble agreement (can
not get high anomalies with high spreads in your ensembles). Thus
forecast confidence even out at day 7 is medium. For now have gone
close to the Euro Ensembles and NAEFS ensembles for the forecast.
This again maintains good continuity with the previous forecast.
Temperature confidence is slightly below medium given timing spreads
in the cold air arriving.
Sensible Weather Impacts
As for sensible impacts, we are entering a high travel period this
weekend with the holiday traffic picking up both on the ground and
aloft. The early weekend system is likely to bring mixed
precipitation to the state late Friday night and into Saturday.
Initially, thermal profiles are suggestive of a rain/snow mix,
quickly changing over to snow by late Friday night. Highest
precipitation coverage looks to be across south-central KY here. A
slight warming of the boundary layer and lack of cold air at the
surface may result in mixed p-type again during the day. Some minor
snow accumulations still look possible over the southern end of the
state. Overall, the model trends, have been less and less on each
run. Nonetheless, there could be some negative travel impacts
Friday night and Saturday morning. This system will quickly pass
out of the region by Saturday night with mainly dry conditions for
Sunday and into Monday.
A secondary wave may swing south of the region on Monday bringing
another round of showers. However, main weather story will be the
developing strong low pressure system over the upper Plains and
Midwest. This will drop into the Midwest on Tuesday and then into
the Great Lakes by Wednesday. We`ll see numerous rain showers out
ahead of the system followed by an influx of colder air into the
region by Christmas Eve and into Christmas Day. This should result
in a quick change over of rain showers to snow showers. Coverage of
snow shower activity is still questionable at this juncture for the
late period given that the low really wraps up into the Great
Lakes. Perhaps more of a sensible weather impact will be the gusty
winds associated with the surface pressure gradient. Sustained
winds of 15-20 MPH with gusts of up to 35 MPH or higher are not out
of the question by the late forecast period.
As for temperatures, generally have stuck close to our SuperBlend
guidance which is really close the Euro 2m raw temps. This result
in highs in the 40s for Mon/Tue with some low 50s possible by
Tuesday. A return to more seasonal cold is expected by Wednesday
with highs back in the 30s to low 40s.
.AVIATION (18Z TAF Update)...
Issued at 1150 AM EST Thu Dec 18 2014
The low-level stratus around FL018-FL020 has finally cleared out of
all sites late this morning. Despite better moisture trying to
approach from the west, a pocket of very dry low to mid-level air
will keep skies VFR through the valid TAF period. An isolated
flurry or sprinkle will be possible through this afternoon at any
site, otherwise expect dry conditions through the period. Winds
will be light and variable today through Friday afternoon.