Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 212002
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
302 PM EST Fri Nov 21 2014
.SHORT TERM (Now through Saturday Night)...
Issued at 240 PM EST Fri Nov 21 2014
The synoptic pattern this afternoon features split flow across the
CONUS, with a strong PV anomaly diving through the southern stream
into the southwestern United States as of this writing. This PV
anomaly will help to induce a surface low in the lee of the Rockies
on Saturday, which will affect our weather beginning tonight and
lasting through the weekend.
For tonight, isentropic ascent will ramp up across portions of
MO/IL/IN on the nose of a 40-50 knot low-level jet. The bulk of
this moisture transport and deeper moisture content will remain well
north across portions of central IN and northern IL. However,
forecast cross-sections do show saturation between about 900 and
600mb. Beneath this saturation, there will be a pretty substantial
near-surface dry layer, but it will not be overly thick (only around
2000-3000 feet). Therefore, do think we could see some light
precipitation punching through this dry layer overnight, mainly
across southern Indiana and far northern KY where the better
isentropic upglide will reside.
The main challenge will be determining the potential for any light
freezing rain, mainly in our southern IN counties. Given the
surface dry layer, surface wet bulb zeroes will lag air temperatures
significantly. Therefore, if we are able to get any measurable
precipitation to the surface, it should cool surface temperatures a
couple of degrees. With this thought in mind, think we will have a
brief window where we could see some light freezing rain tonight.
However, this window will only be 2-3 hours or so at any given
location before surface wet bulbs climb above freezing, and amounts
will be quite light. Given these factors coupled with the time of
night (5-10Z) limiting the amount of people on the road, do not
expect too many impacts from this event. However, cannot rule out a
few isolated slick spots for a couple hours tonight.
Otherwise, temperatures will warm through the overnight hours and we
will start in the upper 30s to lower 40s Saturday morning. Have
reduced pops a bit for Saturday, as the warm conveyor belt with the
developing system lifts more to the northwest of the region. Given
less expected precipitation and even a few peaks of sunshine, have
bumped high temperatures up a bit from the previous forecast. This
puts highs in the 50s and lower 60s, making Saturday one of the
nicer days we`ve had in a few weeks!
A secondary surface low will begin to deepen across portions of
Oklahoma on Saturday night. This will draw additional moisture
northward from the GoMex Saturday night into early Sunday morning,
which means precipitation will begin to creep into far southern KY
near dawn on Sunday. Refer to the long term section below for
additional details on this storm system.
.LONG TERM (Sunday through Friday)...
Issued at 300 PM EST Fri Nov 21 2014
...Quite wet but mild on Sunday...
...Looking cold for Thanksgiving...
Models continue in good agreement with storm system that will
develop across the southern Plains Sunday morning then lift and
deepen rapidly northeastward during the day and Sunday night. The
surface low will pass to our west, with a sizable low-level jet to
its east that will bring substantial warm air/moisture advection and
isentropic lift across our region Sunday. Widespread rain is
expected, especially in the afternoon, with 0.5 to 1 inch of rain a
decent bet across parts of the area. Precipitable water values are
progged to reach from 1.1 to 1.4 inches, about 2 standard deviations
above normal or in the 80-90 percentile for mid-late November. Most
of the precip will be just rain with a rumble of thunder or two
possible in the afternoon over south-central KY.
The steady rain will push quickly northeast by evening as the
surface warm front lifts north as well. Behind this front, milder
surface air will push north in the afternoon across south-central
KY, then reach north-central KY and southern IN Sunday evening from
south-to-north. Across southern IN and north-central KY, maximum
temperatures for the day will be reached in the evening after dark
following warm frontal passage.
While widespread rain will end, models suggest a band of additional
showers may race northeast across our area sometime Sunday evening/
night. The air mass should destabilize marginally, with low-level
lapse rates increasing somewhat in the warm section. Thus, there
could be a few embedded low-top thunderstorms with the shower
activity within a moderate-to-strongly sheared air mass. However,
the lack of any real instability should prevent robust storm
development. Nevertheless, surface winds Sunday evening should
become quite gusty over 30 mph at times ahead of the approaching
cold front, so will need to monitor this.
On Monday, models differ in their timing of cold frontal passage.
GFS is fastest while GEM/ECMWF are a bit slower keeping scattered
showers possible Monday morning along and just ahead of the front.
Frontal passage will end any precip, although temperatures behind
the front won`t be bad as highs top out in the 50s (probably in the
early to mid afternoon) in many locations.
Benign weather is forecast early next week. The next weather system
of concern will be for Wednesday or Thursday (Thanksgiving)
depending on the model of choice. The overall pattern does favor a
system moving southeast across the Plains and into the mean/average
trough position over the eastern half of the U.S. All models show a
system but have varying solutions and timing as expected.
Word of note - this system is NOT expected to be significant at this
time, but still could have some impact given heightened travel
during that time. The GEM is fastest suggesting rain showers,
possibly mixed with light snow Wednesday. Meanwhile, the ECMWF is
slowest with light snow/show showers Thanksgiving day. Given the
spectrum of potential solutions, will go middle ground here with
some light snow (possibly mixed with rain at the start) Wednesday
night, lingering over the east and north Thursday morning. Either
way, expect a cold Thanksgiving day with highs no higher than in the
lower-to-mid 30s, with colder wind chills.
.AVIATION (18Z TAF Update)...
Updated at 1207 PM EST Fri Nov 21 2014
VFR conditions are expected to prevail through the valid TAF
period. Mainly clear skies will prevail through this afternoon,
although some VFR cigs (around FL040) will approach from the
southwest by late this afternoon into this evening as a storm system
organizes to the southwest.
This storm system will help to ramp up a SSW low-level jet of 35-40
knots over KSDF tonight. Given generally light ESE surface winds,
this may be just enough to include LLWS in the KSDF TAF overnight.
This is definitely a marginal setup but given the amount of turning
from the surface to 2000 ft AGL, went ahead and decided to include
it. Otherwise, a passing light shower may affect KSDF overnight but
think all sites will likely stay dry and within VFR conditions.
VFR conditions will persist into Saturday. Winds will increase out
of the south at 10-15 knots, with gusts approaching 20 knots at
times especially at KBWG.