Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 221804
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
104 PM EST Mon Dec 22 2014
Issued at 1025 AM EST Mon Dec 22 2014
Current forecast is well on track as the flow begins to turn more
southerly through a deeper layer. Precip shield currently extends
from the Missouri bootheel NNE into west-central Indiana, and expect
it to spread eastward as the day progresses. Models have a decent
handle on this feature, but the NAM is a bit overdone with precip
over the Deep South. Therefore will not put much stock in its
depiction of precip lifting north into south-central Kentucky late
in the day.
That said, still expect the precip shield to our west to push as far
east as Interstate 65 by late afternoon, and this is already
well-reflected in our forecasts. SSE winds in the boundary layer
will help to push temps into the lower/mid 50s in spite of cloud
cover. At most will tweak the hourly-resolution products such as
AFM/PFM and point-and-click.
.SHORT TERM (Now through Tuesday)...
Issued at 320 AM EST Mon Dec 22 2014
IR satellite imagery and surface observations reveal partly to
mostly cloudy skies across the region. Low-level cloudiness was
most prevalent across southern/southwestern KY. This has kept
temperatures up in the upper 30s to the lower 40s. Further north,
where partly cloudy skies were noted, temperatures were in the upper
30s to the lower 30s. We expect cloud cover to increase across the
region as the overnight hours wear on.
For today & tonight, an increasing southerly flow will help
transport more moisture into the region. Increasing isentropic lift
and warm air advection should lead to more widespread cloud cover
with some sprinkles or drizzle possibly developing during the late
afternoon hours. Afternoon temperatures should warm into the upper
40s to the lower 50s in the north with mainly lower-mid 50s across
More widespread precipitation is expected to develop this evening
and into the overnight hours as an upper level wave punches through
the region. With a southwesterly flow remaining in place,
temperatures will not drop off that much with readings dropping into
the lower-mid 40s in the north and mid-upper 40s in the south.
For Tuesday, we`ll continue to see continued cloudiness. We`ll
remain in the southwesterly flow with several mid-level waves
passing through the region. We`re likely to see a bit of a dry
punch move in from the west (as suggested by the NAM/GFS solutions).
The atmosphere will continue to moisten with the southerly flow, but
cloudy conditions look to keep instability rather low. Can not rule
out some isolated rumbles of thunder out across the region on
Tuesday as well...where enhanced pockets of sfc based instability
could develop. It will remain a mild day with temperatures warming
back into the mid-upper 50s.
.LONG TERM (Tuesday Night through Sunday)...
Issued at 311 AM EST Mon Dec 22 2014
The forecast for the first half of the long term remains a bit
complicated due to model trends and spread along with the potential
for snow on Christmas Eve. For Tuesday night a low pressure system
will move northeast from the Gulf Coast. This low will cross the
forecast area during the day on Wednesday and into the Great Lakes
region by Thursday morning.
The models are still having some difficulty agreeing on the track of
this low, which will have big impacts on the temperatures and thus
sensible weather for Wednesday and Wednesday night. There has been a
slight trend in the models to the east with the track. The GFS/ECMWF
would bring the low directly across the forecast area. The SREF is
slightly further to the northwest while the NAM is an outlier to the
east. For now will lean heaviest on the GFS/ECMWF combined with the
Given all this, we will start out with a good chance for rain
Tuesday night and Wednesday. Temperatures will be falling Wednesday
and Wednesday night as the low pulls northeast. Highs on Wednesday
will be in the morning for most of the area and there will be a
sharp temperature gradient. As temperatures fall and the column
cools a changeover to snow is expected sometime in the evening
Christmas Eve. If solutions continue to trend easterly, this
changeover will occur earlier. At this time QPF looks to be light
after the changeover to snow. As mentioned in previous discussions,
it is expected to be gusty Wednesday night and some convective snow
showers are not out of the question along with some minor snow
accumulations. However, the ground will be wet and surface
temperatures will be marginal, so significant accumulations seem
Precipitation should move out by Thursday morning, though a few snow
showers may linger in the east. Temperatures will rise into the
upper 30s to lower 40s. The next front will move through Friday into
Saturday. This system looks to have less moisture associated with it
so precipitation should be fairly light. Depending on timing of the
precip, we could see a light wintry mix Friday night. Dry conditions
are then expected to cap off the weekend with high pressure building
across the region.
.AVIATION (18Z TAF Update)...
Issued at 100 PM EST Mon Dec 22 2014
Increasing low-level southerly flow over the Ohio Valley is having
minimal impact thus far, with only BWG having dipped into a high-end
MVFR ceiling. Lower clouds poised upstream over western Kentucky and
Tennessee, and the main near-term challenge is timing just how
quickly conditions will deteriorate this afternoon and evening.
Based on 925-950mb condensation pressure deficits, look for
fuel-alternate MVFR conditions to develop in BWG and SDF by late
afternoon, at which point we`ll also include VCSH as the precip
shield edges closer. Rain arrives in earnest by late evening, with
ceilings dropping into IFR. Low-level jetting will pick up as an
inversion sharpens around 2000 feet, which will put us into marginal
LLWS for much of the night. Can`t rule out embedded thunder
overnight either, but it`s a low enough probability that we`ll leave
it out of the TAFs.
Forecast confidence really decreases after roughly 09Z, as models
diverge on just how much of a break we`ll have in the precip. NAM
has a substantial break in the action, while the GFS appears to hang
up a SSW-NNE moisture feed across central Kentucky through the day.
Will try to depict modest improvements in ceiling/visibility,
perhaps above fuel-alternate thresholds. However, will hang on to
VCSH mention, out of respect for the possibility that precip would
not shut off.