864
FXUS63 KJKL 091136 AAA
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
736 AM EDT Thu May 9 2024

.KEY MESSAGES...

- A few thunderstorms with brief torrential rain are the biggest
  concern for convection today in areas near the TN border, with
  this activity winding down this morning.

- Some showers and perhaps a storm or two are possible at times
  from this afternoon through early next week.

- Above normal temperatures are forecast today. However, a cold
  front passing today will usher in cooler temperatures from
  Friday through the weekend, with warming then returning.

&&

.UPDATE...
Issued at 730 AM EDT THU MAY 9 2024

Hourly grids were updated based on recent observations and trends
and with the rain rates not being all that substantial the Flood
Watch was cancelled early.

&&

.SHORT TERM...(Today through Friday)
Issued at 436 AM EDT THU MAY 9 2024

Early this morning, an upper level trough extended from the
Maritimes across portions of Ontario and Quebec to the Great
Lakes to southwest Conus. West southwest flow was in place
between this trough and upper level ridging, the axis of which
extended across Central America to portions of the Caribbean. A
shortwave trough was approaching eastern KY at this time. At the
surface, An area of low pressure was tracking across the Lower OH
Valley region with the associated frontal zone extending east to
the mid OH Valley to the mid Atlantic coast. Meanwhile an
instability gradient associated with outflow from rounds of
convection since Wednesday was in place southwest of eastern KY.
Recent mesoanalysis had very limited instability in place north
of this boundary with MUCAPE 250 to 500 J/kg in the southwest
portion of eastern KY, mid level lapse rates area wide no more
than 6 to 6.5C/km, and best shear parameters from the TN border
and locations to the south of the area. PW as high as 1.3 to 1.5
inches was in place from central KY to areas generally near and
west of Interstate 75. Convection was occurring across the OH and
TN Valley with the more robust/deeper convection from nearer to
the TN border and south in the instability gradient though
lightning was occurring into portions of central KY in a
stratiform rain regime.

The shortwave trough is expected to move east of eastern KY this
morning while the trough axis at 500 mb remains north of the the
OH Valley today. The western portion of the trough is progged to
close off/be left behind over the Four Corners region today and
then meander to the west and southwest through Friday. Meanwhile,
the 500 mb trough axis is expected to move south and southeast
across the Great Lakes and near the OH Valley and mid MS Valley
tonight, before this trough axis axis rotates across the OH Valley
and into the Appalachians to end the period. Shortwave ridging
should enter the OH Valley to end the period while another
shortwave trough moves into the evolving trough over the eastern
Conus and into the western Great Lakes and Upper MS Valley.

Chances for convection today should be greatest through the
morning though after a possible late morning to early afternoon
lull, a slight uptick in activity may coincide with peak heating
and the cold front crossing eastern KY. Recent convective
allowing models forecast as much as an inch or more over the next
few hours through about dawn in the southernmost tier or two of
counties nearest to the TN border. At this point have held onto
the Flood Watch through 8 AM across the south.

Chances for at least isolated convection will linger into the
evening and tonight as the 500 mb trough axis lingers upstream
though a lull during the evening into a part of the overnight is
possible. If and where clouds can clear out, fog may become a
concern following recent rainfall though a general increase in
cloud late tonight and toward dawn on Friday leads to uncertainty
in location and duration. Some patchy fog was included in the
grids for tonight, with somewhat lower visibilities for deeper
valleys. Chances for convection will increase on Friday during
peak heating as the trough nears with the best chances in the
southeast. Temperatures on Friday will be notably colder than
recent days with highs not expected to get out of the 60s. Friday
highs should be nearly 10 degrees below normal for this time in
May.

.LONG TERM...(Friday night through Wednesday)
Issued at 542 AM EDT THU MAY 9 2024

Our weather at the beginning of the long term period will be under
the influence of a longwave trough over the northeast CONUS and
shortwave troughs rotating around it. A cool air mass will persist
over our area in this regime. One shortwave trough will be
departing to our east on Friday night, and in its wake, transient
surface ridging will build in from the west. This results in skies
becoming mostly clear. Another shortwave trough moves southeast
over the Ohio Valley on Saturday, and with it we can look for a
potential for showers to return. The greatest probability will be
over the northeast portion of the area, closest to the shortwave.
Have included a slight chance of thunder over our northeast
counties based on NAM forecast soundings, but it looks very
marginal. This wave departs to the east by evening and precip and
most clouds dry up again.

At this point, the heart of the longwave trough also shifts east,
and ridging aloft approaches from the west on Sunday and passes
over on Monday. This brings dry weather and mostly clear skies
from Sunday night through at least Monday morning. At the surface,
high pressure passes eastward south of our area on Sunday and
departs eastward on Monday. Increased sunshine will allow for
temperatures to return to near normal on Sunday, with a further
climb on Monday as warm air advection becomes more pronounced.

Our next weather system will begin to affect the area already by
late Monday. A nearly stationary closed upper level low centered
over UT today will eventually open up and get absorbed back into
the flow. Timing of these features can be tricky, especially at
longer time ranges. At present, a model blend suggests it will
approach us from the west late Monday and arrive on Tuesday. It
is expected to support a weak surface low which will help to pull
moisture northward on the back side of the departing high. The
GFS is faster than the ECMWF with the system and shows some light
precip reaching our southwestern counties by Monday evening. Both
models have precip here on Tuesday. Based on this, have used only
a slight chance late Monday, increasing to likely by Tuesday
evening. It looks like there will be little change in air mass
behind the system, and models try to generate more convection with
heating on Wednesday. Have used a chance POP on Wednesday, with
the best upper level support departing to the east.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
ISSUED AT 735 AM EDT THU MAY 9 2024

Showers with some embedded thunder was moving across the
region at issuance time with some instances of reductions to MVFR
or IFR if not near or below airport mins. The showers should
depart in the first 2 to 3 hours of the period and improvements to
VFR should then follow toward 18Z for most areas as coverage of
convection decreases. Some increase in coverage could occur again
during peak heating as a cold front approaches. Behind this
activity, reductions down to MVFR if not IFR are anticipated in
low clouds or stratus and some reductions in vis due to fog are
also possible. Winds will average between south to southwest at
around 10KT or less before 18Z before becoming west and eventually
west to northwest late in the period after a cold front moves
through.

&&

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

$$

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