Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 182021
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
321 PM EST Thu Dec 18 2014
.SHORT TERM (Now through Friday Night)...
Issued at 300 PM EST Thu Dec 18 2014
The synoptic pattern this afternoon features zonal to WSW
upper-level flow across the Ohio Valley, through which a dampening
shortwave was traveling. This shortwave will push through the
region this evening, but another weak southern stream disturbance
will push in towards the end of the period and into the long term
As expected, the very dry airmass between 900 and 750mb has largely
won out this afternoon over the region, as isentropic ascent above
this layer has not been enough to overcome it. A few locations have
seen a few flurries/ice pellets/sprinkles, but most have remained
dry and should remain dry through the rest of the evening.
The cloud and resultant temperature forecast will be very tricky
tonight, as it does appear a few breaks in the clouds may occur
towards Friday morning. The latest RAP/HRRR depict similar
solutions in breaking up the cloud cover tonight across portions of
central KY and southern IN. Forecast soundings from the synoptic
models also support this potential as drier air works in in the
800-600mb layer. Am always hesitant to forecast skies clearing this
time of year, but given the model agreement, will reduce sky cover
amounts overnight. Still think we will see at least some cloud
cover almost all night, but large breaks coupled with generally calm
winds may allow temperatures to drop into the low to mid 20s,
perhaps even lower if the clouds break sooner. If they don`t break
at all, then obviously these temperatures will be way too cool.
Certainly a tricky forecast and one the evening shift will monitor.
Friday will be dry as high pressure remains in control.
Temperatures will once again be dependent on the amount of cloud
cover we see (likely more clouds in the afternoon/evening than in
the morning). Will go with highs in the upper 30s and lower
Attention will then turn to the southwest as the next system
approaches the OH and TN Valleys. Guidance continues to suppress
this system further south, with the GFS/ECMWF on the northern end of
the envelope, while the hi-res parallel GFS/NAM/WRFs all point to a
more southern solution. Looking at the forcing progs, really don`t
see anything to support widespread precip in our area. The coupled
jet structure seen a day or two ago is largely gone as guidance has
now backed off the idea of a stronger northern stream jet streak.
All the low-level ascent associated with frontogenesis and low-level
moisture transport are focused well to the south of the region.
Therefore, all we really have to work with will be weak upper-level
ascent brought on by the left-exit of the southern stream jet streak
and very modest mid-level height falls as the dampening wave pushes
through. With all this in mind, have continued to trim pops with
this package. Kept 20-30 pops in the far southeastern CWA mainly
just to keep some forecast continuity, but suspect the drier hi-res
models will end up being right with little to no measurable precip
in our area.
.LONG TERM (Saturday through Thursday)...
Issued at 300 PM EST Thu Dec 18 2014
High pressure will be over the Upper Ohio Valley at the start of the
period. This high will ridge across our area through the weekend.
Will keep in some lower chance pops for light precip during the day
Saturday as a system passes by well south of the region. The next
precip chance is Sunday night as a weak vortmax crosses the Midwest.
This system is the first in a series of disturbances crossing our
area the next work week as a large upper low deepens across the
Great Lakes and south central Canada. These systems will bring
multiple precip chances to Christmas Day. One such system looks
to drag a surface low across the region Tuesday and then another by
the end of the work week. The exact placement of these systems will
determine the type of precipitation we get. For now, have a band of
rain/snow chances along the Ohio River Sunday night, with little if
any accumulations. The GFS has the Tuesday low going up the Ohio
River, which brings a typical line of showers through with winter
precip fighting drier air filtering in on the back side.
Much colder thicknesses come in for midweek, with an impressive
fetch of moisture from the Great Lakes potentially bringing some
rain or snow showers Christmas Eve. Then we warm up slightly on
Thursday as the next low develops across the southern Plains and
lifts a warm front back across the region.
.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.