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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KJKL 210535
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
135 AM EDT Sun Apr 21 2024

.KEY MESSAGES...

- Fair weather lasts through early in the new workweek.

- Frost will be possible in some sheltered valley locations Sunday
  morning, but is expected to be most widespread Sunday
  night/Monday morning and again Monday night/Tuesday morning,
  especially through the valleys.

- Another moisture-starved cold front is forecast to pass Tuesday
  night into Wednesday morning, bringing a round of light rain
  for northern locations.

- Confidence in weather forecast late next week is lower than
  normal at this range due to significant weather model differences.

&&

.UPDATE...
Issued at 728 PM EDT SAT APR 20 2024

Minor adjustments were made to hourly grids based on recent
observations and trends. This led to no substantial changes at
this time.

&&

.SHORT TERM...(tonight through Sunday night)
Issued at 423 PM EDT SAT APR 20 2024

Sprawling surface high pressure was over the Great Plains late
today, ridging eastward to the lower Great Lakes and central
Appalachians. A nearly stalled frontal boundary was near the gulf
coast and southeast coast. The high is supplying us with a cool
and dry air surface air mass. However, moisture overriding the
frontal boundary has produced some clouds at times, with one batch
exiting to the southeast this morning, and some more high clouds
starting to show up late this afternoon.

Our flow aloft is fairly zonal, but a highly positively tilted
trough is dropping south from the north central CONUS, while at
the same time a weak shortwave trough over TX is zipping eastward.
The southern wave will progress eastward ahead of the northern
trough and is expected to send more moisture/clouds northward,
overriding our surface air mass tonight and early Sunday.
Deterministic models are generally keeping precip to our south,
and our POP has been held below 20 percent in the forecast.
However, can not completely rule out some very light precip in our
far southern counties should virga make enough headway into
moistening our low level air mass before the wave aloft departs.
The other consideration with the southern system is its impact on
our temperatures. Initially scant cloud cover should allow decent
radiating to start the night, but thickening clouds will cut into
the temperature descent during the night. The current forecast has
some patchy frost developing late tonight, mainly in our northern
valleys. This could end up less in coverage should clouds be
thicker than currently forecast, or greater should clouds be
thinner.

The northern trough will swing southeast toward us Sunday and
Sunday night, while the southern wave departs to the east with the
moisture. At the surface, high pressure ridging eastward will
continue supplying us with cool and dry air. Despite sunshine
returning on Sunday, temperatures will struggle to get out of the
50s. That will set us up for a more favorable night for frost on
Sunday night. If the current outlook persists, a Frost Advisory
will need to be considered on an upcoming shift.

.LONG TERM...(Monday through Saturday)
Issued at 400 PM EDT SAT APR 20 2024

The 20/12z model suite analysis beginning Monday morning shows a
shortwave trough over the Tennessee Valley as it rides through
cyclonic flow around a deep ~498 dam low over the northern Hudson
Bay. Upper level shortwave ridging is found upstream across the
mid-Mississippi Valley and also from the Upper Midwest into
Manitoba. An ~534 dam low is passing over Saskatchewan. A much
more substantial high amplitude longwave trough/low are found off
well off the West Coast. At the surface, high pressure ridging,
centered over the Ozarks, extends from eastern Texas through the
Ohio Valley/Great Lakes and up toward James Bay. Meanwhile low
pressure is situated over southern Saskatchewan.

Quiet weather will persist on Monday and Monday night as the
aforementioned surface high translates east across our region.
Another night of frost concern in valleys is probable on Monday
night given the dry air mass with light winds and mostly clear
skies. Heading into the Tuesday and Tuesday night, the upper low,
over Saskatchewan initially, will translate eastward to over the
St. Lawrence Valley, bringing its attending surface low with it
while also dropping a cold front toward the Ohio Valley. There is
substantial disagreement as to how much this weaker upper low will
interact with the deep upper low over eastern Canada. The
deterministic 12z GFS as well as the GEFS generally insist on the
two lows phasing and dropping down toward New England, which would
lead to a significant surge of cold air spilling southward in the
Ohio Valley behind the cold frontal passage early Wednesday. This
however, is not supported by the deterministic 12z ECWMF/GDPS nor
those model`s most recent associated ensemble systems. After high
pressure makes a brief return on Thursday, the next upper level
trough (or troughs) will approach the Ohio Valley. The significant
model differences emerging from the mid-week system continue into
the weekend, leading to lower than normal forecaster confidence
in the details. Rain chances could return as early as Friday
morning, though many ensemble members favor any notable rain
chances holding off until later in the weekend or even some point
in the following week. This could mean the difference between a
several day stretch of mainly dry and very mild weather or just
cool, damp and rainy conditions. The NBM offered a compromise
solution for the forecast during these uncertain portions of the
forecast. Significant refinements are likely once models present
a more cohesive solution.

In sensible terms, high pressure will bring fair weather and a
warming trend through Tuesday. Monday will be milder than Sunday
with widespread maximum temperatures in the lower to middle 60s
under a cool northerly breeze. Winds won`t shift to the southwest
until Monday night when the high pressure ridge axis passes. With
the high pressure still in place, another chilly night can be
expected Monday night with minimum temperatures deep in the 30s
for many deeper valley locales. Ridges will largely see minimum
temperatures in the lower to middle 40s. Milder temperatures can
be expected on Tuesday as return flow increases ahead of the next
cold front -- southwesterly breezes gusting up to around 20-25 mph
will help raise afternoon temperatures into the 68-74 range for
most locations. Rain chances rise from the northwest Tuesday
evening as that cold front approaches and peak overnight before
falling off on Wednesday. Overall QPF with this system is light
(less than 0.20 inch in northern counties ranging to little or
nothing along the Kentucky/Tennessee border). Thunder was removed
from the forecast due to lack of instability. The weather pattern
from Wednesday onward is less certain due to significant model
disagreement. For right now though,temperatures behind the front
are expected to cool off into the 60s to near 70 on Wednesday and
Thursday, under partly cloudy to mostly clear skies. Frost cannot
be ruled out again in the sheltered valley locales on late
Wednesday night and Thursday morning as low temperatures are
forecast to most likely range in the lower to mid 30s in those
areas. Temperatures up to the lower 40s on are forecast on ridges.
There are scenarios in which these values could run several
degrees warmer or cooler. Southerly return flow returns by Friday
as the next storm system approaches, but the return of substantial
rain chances in our area remains uncertain. If the drier
scenarios materialize, a warm weekend will be on tap with
temperatures climbing back to above normal. If the wetter
solutions play out, then conditions would take on a more soggy
character with near to below normal temperatures.

&&

.AVIATION...
-- Changed Discussion --
(For the 06Z TAFS through 06Z Sunday night)
ISSUED AT 135 AM EDT SUN APR 21 2024

Sfc high pressure will bring VFR conditions through the period
though some few to scattered low and mid clouds are anticipated
at times particularly during the day Sunday. Sustained winds are
generally expected to be 10 KT or less and through the period
averaging from the north to northwest.
-- End Changed Discussion --

&&

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

$$

UPDATE...JP
SHORT TERM...HAL
LONG TERM...GEERTSON
AVIATION...CMC