Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 222307
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
607 PM EST Sat Nov 22 2014
...Updated Aviation Discussion...
.SHORT TERM (Now through Sunday Night)...
Issued at 245 PM EST Sat Nov 22 2014
The synoptic pattern this afternoon features a split flow regime,
with a strong PV anomaly diving southeast through the southern
stream across portions of southwest Texas. Another system will move
through the northern stream and these will phase Monday into Monday
Conditions remain generally dry this afternoon, with quite a few
peaks in the sunshine being noted this afternoon. These peaks of
sunshine have allowed temperatures to overachieve quite a bit, with
temperatures now in the 60s across most areas. All precip has
pushed mainly north of the region as the flow has backed in response
to the approaching PV anomaly, helping to shunt the warm conveyor
belt off to the north.
It will be a mainly dry night, but clouds will begin to increase
from the south. SSE winds should stay up around 7-10 knots, which
should keep low temperatures in the 40s.
The main focus of the short term period will be the strong system
poised to affect the region Sunday into Monday. A low-level jet of
50-60 knots will increase over portions of the TN Valley Sunday
afternoon. This will aid in rather impressive isentropic ascent
Sunday afternoon into Sunday evening, allowing for a widespread slug
of precipitation to move from the south to the north across the
region. Will continue with a slight chance of thunder with this
initial activity given the deeply forced ascent and marginal
elevated instability, but any storm will remain elevated above a
surface stable layer thus do not expect any strong storm threat with
As a surface low deepens to the west Sunday evening, low-level
moisture will increase behind an effective warm front which will
have pushed north into portions of KY late Sunday evening into
Sunday night. Given convection expected to erupt along the
Southeast coast, it appears low-level moisture return will likely
not be robust enough to realize any substantial amount of
surface-based instability. Therefore, despite the impressive wind
fields and upper-level forcing for ascent aided by a coupled jet
structure, just don`t see much in the way of any organized severe
threat. Agree with SPC on a "marginal" risk across south-central KY
where an isolated strong convective wind gust may be possible given
the marginal instability, but the overall severe risk looks low.
The main threat with this system will likely be the gradient winds.
Once the effective warm front and main slug of precipitation pushes
north, we will lie squarely in warm sector Sunday night. Despite
the unfavorable time of day for gusty winds, the quickly deepening
low (and subsequent ageostrophic wind response) will help push
sustained winds to around 25 mph with gusts up to 35 mph through 12Z
Monday. See the long term section below for additional details on
wind potential during the day on Monday as the strong cold front
associated with this system passes through.
.LONG TERM (Monday through Saturday)...
Issued at 300 PM EST Sat Nov 22 2014
The pattern through Wednesday has pretty good model agreement, with
a surface low over Michigan Monday morning and a trailing cold front
moving across the region that day. We should still see some precip
with this system in the morning, but dry out in the afternoon. The
pressure gradient should relax a little behind this front, but we
will stay breezy with west to southwest winds. Tuesday, the upper
level pattern has a northern vortmax going across the Midwest and a
southern one over Texas. We`ll be in between these two systems, so
just looking at a chilly day.
Tuesday night, a ridge of high pressure will go across the region,
providing for a cold morning, but shouldn`t be as cold as what we
had in the past week. From this point on though the models start to
diverge. The GEM has low pressure developing with a shortwave moving
across the Northern Plains. The 00Z and 12Z Euro are stronger with
this feature, whereas the GFS is weaker with this feature and has it
travelling well north of our area Wednesday, but a trailing frontal
boundary does bring some precip either over or just north of our
northern forecast area during Thanksgiving day.
Looking upstream at the source of the low in the Euro, it looks to
be a piece of energy from a current low over the Aleutians. It
doesn`t break off on its own until 00Z Tue, when it moves over
eastern British Columbia. The GFS tracks this feature as well, but
the Euro solution is much deeper with the low. Looking at the 500 mb
fields, both models seem to keep track of a vortmax that is
associated with this low, but the GFS just doesn`t allow the surface
reflection to deepen as much as the Euro. Given both models
persistence, will maintain low-end chances for precip early
Thursday. The problem is the precip type, as the Euro solution would
mean more snow and the GFS a rain/snow mix or even just all rain.
Will not go with the all rain solution at this point, and hope for
better model agreement down the road. Would tend to like the Euro
better given its persistence and track record.
The rest of the forecast somewhat depends on which model choice is
best for Thanksgiving day, as models continue to have a large spread
of solutions. The GFS would argue for the front lingering over the
Appalachians with high pressure building in from the northwest,
whereas the 00Z Euro has high pressure over us Friday and another
system approaching from the southern Plains on Saturday. The GFS
solution may allow for some light snows in our far eastern counties,
but for now will go dry, again leaning towards the Euro solution.
.AVIATION (00Z TAF Update)...
Updated at 600 PM EST Sat Nov 22 2014
VFR conditions will continue through the night, with LLWS the main
concern as the low-level jet cranks up. Strongest jetting will be
over west-central Kentucky, with 950mb winds kicking up near 40 kt.
Will include LLWS after midnight in SDF and BWG, but keep it out of
LEX where the wind field is not as strong.
Ceilings will gradually lower on Sunday morning, dropping into
high-end MVFR with just enough of a rain chance to warrant VCSH.
Surface winds will back to the SE and gusts will resume as mixing
Precip shield moves in during the afternoon with conditions going
downhill at that point. Model guidance is quite bullish with low
ceilings, taking SDF and BWG down into IFR. Not sold on that this
early, especially as dry as we are starting out. At this time, will
take SDF/BWG ceilings down to fuel-alternate MVFR in prevailing SHRA
early in the afternoon, but will keep LEX above fuel-alternate even
as the precip spreads in. If anything, this forecast may be on the
optimistic side, given that the GFS MOS mostly agrees with NAM MOS
on the low ceilings.