Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 211825 AAA

National Weather Service Jackson KY
225 PM EDT Sun May 21 2017

Issued at 200 PM EDT SUN MAY 21 2017

Did a quick update to fine tune the PoPs and Wx grids through the
rest of the afternoon and also adjust the T and Td grids per the
latest obs and trends. These have been sent to the NDFD and web

UPDATE Issued at 1055 AM EDT SUN MAY 21 2017

14z sfc analysis shows a warm front just east of Kentucky with a
cold front approaching from the west. In between these boundaries
the warm sector is maintaining scattered to numerous showers
through the eastern portion of the area under overcast skies and
with light winds. Temperatures through the CWA are running in the
mid to upper 60s as the thick clouds and showers hold them back
with dewpoints not far below the air temperature. This environment
contains fairly high PW air in place making excessive rainfall a
concern for any showers and storms that redevelop later this
afternoon - particularly for places that saw heavy rains over the
past two days. The latest HRRR and NAM12 indicate that the showers
and a few storms will remain a threat over the far eastern parts
of the CWA into the afternoon. They also suggest that more
convection will bubble up during the afternoon through the rest of
the area with the approaching cold front representing a back edge
as it moves into eastern Kentucky this evening. Have adjusted the
PoPs and storm chances through the next 12 to 18 hours for these
thoughts. In the near term, tweaked the T and Td grids per the
latest obs and trends. The updated forecast grids have been sent
to the NDFD and web servers.

UPDATE Issued at 753 AM EDT SUN MAY 21 2017

Showers have become more numerous across the area in the past
hour. The short term models continue to struggle with
initialization, although the latest HRRR has come in a bit more
reasonable than previous runs. The 06z NAM was still overplayed
on the precipitation rates. Will stick with the current forecast,
having a steady increase in coverage through the rest of the
morning out ahead of the approaching cold front. Temperatures
will be slow to rise through this morning, before warming up
this afternoon, with perhaps some breaks in the action. Updates
have been sent.


.SHORT TERM...(Today through Monday)
Issued at 400 AM EDT SUN MAY 21 2017

The latest surface map features low pressure centered across
southern Minnesota, with a cold front aligned south, nearly along
the Mississippi River. Aloft, an upper level low is displaced
just slightly west of the surface low, with a stout short wave
trough lifting northeast into western Illinois. An upper level
ridge is retreating towards the Eastern Seaboard.

So far, the mesoscale has been the main convective driver across
eastern Kentucky, as yet another round of scattered to numerous
showers and thunderstorms affected portions of the area earlier,
mainly driven by outflow boundary interaction. Currently, some
scattered to numerous showers, with an isolated rumble of thunder,
is moving in from the Tennessee Valley. This uptick in activity is
mainly in response to a low level jet building in from the
southwest, but so far, the convection has been light. Most of the
model guidance suggests more intense convection to build in before
dawn; however, am a little skeptical based on the latest radar
and satellite trends.

The cold front will gradually push east today, with plenty of
moisture advecting northward out ahead it. This should lead to
numerous to widespread showers, with some thunderstorms, although
cloud cover will limit instability, as highs will be in the mid to
upper 70s. High PWATs for this time of year will continue to
bring an isolated flash flood threat; however, given the isolated
nature of the heavy rainfall coverage over the past few days,
will forego a watch at this time.

The front will shift to the east late tonight, with showers and a
few storms diminishing from west to east. Cooler air will move in
behind the front, with lows in the mid to upper 50s. A few
showers may linger in the far east into Monday morning, before
drying up by the afternoon. Highs will be in low to mid 70s.

.LONG TERM...(Monday night through Saturday)
Issued at 408 AM EDT SUN MAY 21 2017

An area of high pressure will be moving across the Ohio Valley and
exiting east to begin the period Monday night. Given the lower
confidence of cloud cover will lean away from the ridge/valley
splits. Then all eyes turn to a deepening upper level system
across the Northern Midwest. Models continue to struggle with the
overall evolution, but jet dynamics do develop a surface low to
our south on Tuesday. That said, leaned toward a compromise of the
wet NAM and dry GFS which was the ECMWF for Tuesday. After this
really leaned toward the blended POPs, as the guidance remains out
of sync with respect to the overall synoptic pattern. The GFS and
ECMWF respective 00Z solutions did make some progress toward
agreement with the overall depth of the 500mb heights. The wettest
periods right now will be Tuesday night into Thursday.
Subsequently given the amplitude of this upper level system being
fairly anomalous, the temperatures will drop to below normal
values for this time of year Wednesday and Thursday as column
temperatures lower.

This high amplitude trough is set to exit to the east by Thursday
night and guidance is in decent agreement on a flattening ridge
building east. This will bring higher heights and send
temperatures back closer normal values to round out the week. This
will also lead to the dry day by Friday. However, this ridge
becomes dirty quickly and almost more in the way of zonal. There
could be some weak waves move along the flow, but we remain fairly
displaced from most of these waves. Keep some slight POPs in the
western portions of the CWA on Friday night into early Saturday,
but thinking some height rises could bring a lull by Saturday


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)

An approaching cold front will bring periods of showers, and
perhaps a few thunderstorms to the area through the evening.
Expect a gradual lowering in the ceilings and visibilities even
outside of the storms, with most locations eventually seeing MVFR
conditions overnight. Showers will gradually diminish from west
to east tonight as the cold front passes through the area with
MVFR ceilings scattering out from west to east through the end of
the period. South to southwest winds around 5 kts will eventually
shift to west northwest after the frontal passage.




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