Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 281537 AAA
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
1137 AM EDT Mon Sep 28 2020

.UPDATE...
Issued at 1137 AM EDT MON SEP 28 2020

The surface cold front has made it across western Kentucky.
The latest IR satellite imagery reveals generally warming cloud
tops so far across the Commonwealth, with better convection
located further south. Partial clearing across eastern Kentucky
has allowed temperatures to jump up quickly this morning, with
currently readings in the mid to upper 70s for many locations.
The thicker cloud shield will be moving into our western tier of
counties in the next hour, and cumulus is already developing
further east. As such, will stick with the forecast highs, besides
adjusting up a degree or so at some places, given the quick head
start. The latest SPC Mesoanalysis page shows weak instability
currently in place across our area, with ML CAPE values of only
around 100 J/kg. Looking out ahead in time, forecast ML CAPE
values from the RAP will get to around 250 J/kg by early this
afternoon, with perhaps a few places west of I-75 making it into
the 250-500 J/kg range for a short period of time. 925-850 mb
winds will increase to between 30 and 40 kts early this afternoon,
especially west of I-75 and near the I-64 corridor. As such, an
isolated stronger wind gust is still possible this afternoon. All
this is well handled in the current forecast, so have mainly
adjusted the PoP timing, according to the latest radar trends as
well as more recent higher resolution model data. Updates have
been sent.

UPDATE Issued at 743 AM EDT MON SEP 28 2020

We are seeing a few showers develop amid an increasing low level
jet, but more isolated with greatest coverage in the far
southeast. Overall we will see an increase in rain through the
afternoon, with timing still on track at this point. Outside some
adjustments to deal with the latest obs and trends the overall
forecast remain on track.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 420 AM EDT MON SEP 28 2020

The surface analysis this morning shows low pressure developing
across the Great Lakes and frontal boundary extending southward
into the Midwest and further south into Texas. In the upper level,
a sharp trough axis is push southeast toward the Midwest evident
on the water vapor imagery from GOES-16. This trough axis will
continue to push into the Ohio Valley through the period. This
will help slowly push the previously mentioned cold front across
eastern Kentucky. Ahead of this feature, we have a weak low level
jet that will bring a few showers into the southeast this morning
and a few are already showing up on scans from the WSR-88D radar
here at JKL. The bulk of the showers and isolated thunderstorms
will arrive after 18Z and slowly progress west to east through the
evening. There is the potential for some instability to develop
ahead of this feature amid strong flow aloft. However, the mid-
level lapse rates are underwhelming to say the least. Given this
the outside chance of a marginally strong storm remains possible
due to the strong flow aloft, but unlikely given the lack of
better instability. The main hazard would be strong gusty winds if
a storm can become more organized. The greatest threat area looks
to be around the I-75 corridor where the HREF mean MUCAPE values
might get around 500 to 1000 J/kg.

Tonight, this front will push across the area through the evening,
but a spoke off of the upper low in the north will push southward
closing off as it does so. This will slow the 850 mb front and
greatest precipitation chances will continue in the far east and
southeast through the night and slowly decrease through the day
Tuesday. The biggest change will be a sharp drop in temperatures
through the night given the northwest flow and cold air advection.
This will drop temperatures into the lower 50s by Tuesday
morning. The showers and cloud cover on Tuesday will make for a
cool fall like diurnally limited day, with highs in the upper 50s
to lower 60s and high elevation spots like Black Mountain likely
not making it out of the 40s. The total precipitation from today
into Tuesday will be around a half an inch to about three quarters
of an inch for most, with higher amounts of around an inch in the
far east and southeast where precipitation lingers longer.

.LONG TERM...(Tuesday night through Sunday)
Issued at 603 AM EDT MON SEP 28 2020

Model agreement is generally good through Saturday, with
significant differences emerging for Sunday onward. Model
consensus shows broad, deep troughing prevailing over the eastern
half to two thirds of the nation through the long-term period with
multiple shortwaves and associated vorticity lobes rotating
through the mean trough. The period begins 00Z Wednesday with a
multi-centered sub-540 dam low along the southern shores of the
Hudson Bay while its trough axis extends southward over the
western Great Lakes and Lower Ohio Valley. A developing closed low
is situated over the TN valley along the southern periphery of
the the upper level trough. At the surface, strong low pressure
will be situated over northern Quebec with a cold front extending
southward into the the Champlain Valley and along the eastern
slopes of the Appalachians to a surface wave developing over the
Carolinas. High pressure is located to our southwest over Texas
with ridging extending northeastward toward the lower Ohio Valley.
Another more subtle surface low is positioned near Moosonee, ON
with a secondary cold front trailing to the SW-W across Lakes
Superior and Central Plains.

The battle lines will be drawn Tuesday night as surface high
pressure tries to nose in from the southwest while the low
pressure wave slowly lifts across the Carolinas. This will likely
cause thicker cloud cover to hang on most of the night over
eastern areas while clouds break up over the Pennyroyal Plateau
and Bluegrass. Given the antecedent rainfall, building surface
ridge and slowing winds, any clearing will likely lead to valley
fog development in sheltered valleys. Overnight lows are expected
to range from the lower 40s over the Bluegrass and Lake Cumberland
areas, which will see the greatest clearing, to near 50 over far
eastern Kentucky where thick cloud cover and spotty showers
persist. The closed upper low and associated surface low will
finally lift northeast along the Atlantic Seaboard on Wednesday
while surface high pressure builds across the area. Skies should
turn mostly sunny area-wide by afternoon while warming westerly
flow aloft sends 850 temps rebounding to 8-10C by late in the day
supporting high temperatures in the mid 60s to around 70, warmest
in valleys west of the Pottsville Escarpment.

The secondary cold front drops in from the north Wednesday night
and Thursday supported by a robust shortwave rotating through the
mean trough. However, moisture will be severely lacking so nothing
more than increased cloud cover, a few sprinkles/light showers
and a SW-WNW wind shift are expected. The frontal boundary should
keep the PBL rather mixy through the night supporting low temps
within a few degrees of 50. The less than ideal post-frontal wind
direction and modeled theta-e gradient suggest a weak CAA regime
for Thursday, allowing high temperatures to reach the mid to upper
60s in most areas. Steepening low-level lapse rates will be
supportive of deep mixing to 5k+ft AGL, leading to a breezy
afternoon. The low-level winds should turn more northwesterly
behind the shortwave Thursday night-Friday night increasing
perhaps bringing some low-level moisture from the western Great
Lakes. This moisture will be the wild card for nighttime
temperatures both nights. If the moisture is sufficient for a
stratus/strato-cu deck then night lows with a few degrees of 40
are probable; but, if skies clear, valleys could easily decouple,
allowing temperatures to drop well into the 30s. 850 temps in the
1-4C range on Friday will only support highs reaching the mid 50s
to lower 60s.

Another chilly day is in store for Saturday with 850 temps
remaining in the lower to mid single digits, but decreasing low-
level moisture should allow for slightly warmer highs rebounding
into the lower to mid 60s for most. Clouds begin to increase from
the west Saturday night-Sunday as a clipper-type system passes
from the Central Plains to the Ohio Valley. Saturday nights
lows will depend on how quickly this cloud cover arrives. Models
continue to struggle with the timing and track of the system but a
few showers are probable at some point Sunday into Monday. A
reinforcing shot of colder air follows the systems cold front
early next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
ISSUED AT 721 AM EDT MON SEP 28 2020

We are seeing mostly VFR CIGs and Vis to begin the period, as
mostly mid-level clouds push across the region. We have
maintained some valley fog in mainly the Kentucky and Big Sandy
river valleys, which has only managed to cause LOZ to see some
period of lowering Vis this morning. This will burn off through
the morning as mixing takes place and more clouds push into the
region. After this all eyes turn toward an approaching cold front
that will bring showers and even an isolated thunderstorm from
west to east by this afternoon and into tonight. This could begin
causing periods of restrictions to the TAF sites and have begun
mentioning MVFR CIGs and Vis by this afternoon. The winds will
remain light through the morning, but mixing and approaching front
will lead to sustained winds of 5 to 10 knots and gusts of 15 to
20 knots through the afternoon before relenting this evening.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...GEOGERIAN
SHORT TERM...DJ
LONG TERM...GEERTSON
AVIATION...DJ


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