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Area Forecast Discussion (AFD)

Issued by Jackson, KY (JKL)

                            
000
FXUS63 KJKL 220248 AAB
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
1048 PM EDT Sun Jul 21 2019

.UPDATE...
Issued at 1048| PM EDT SUN JUL 21 2019

Updated the forecast grids along with the zone forecast text
product to better reflect the timing and evolution of an
approaching line of showers and storms. Based on the latest radar
imagery loops, it appears that this activity is going to arrive 
in our western counties 2 to 3 hours earlier than the previous 
forecast had been calling for. Also beefed up valley a fog a bit, 
going with areas of fog based on current trends in obs and the 
fact that a number of areas received rainfall Sunday afternoon and
evening. That, coupled with temporarily decreasing cloud cover, 
may lead to more fog tonight than originally anticipated.

UPDATE Issued at 828 PM EDT SUN JUL 21 2019

Finally issued a forecast update. Removed the outdated pre-first
period from the zone forecast text product. Also updated the
precipitation probability forecast per the latest radar trends for
this evening. May update again later in the shift if it looks the
ongoing convection is going to end and not refire until very early
Monday morning. The hourly forecast, temperatures, humidity,
dewpoint, wind, etc, were updated with the latest obs with new
trends established for the remainder of the evening. 

UPDATE Issued at 520 PM EDT SUN JUL 21 2019

Just issued a quick forecast update to better capture the
evolution and movement of a line of showers and storms that is
currently moving across the forecast area. Have a 50 to 60 percent
chance of rain forecast through early this evening for areas that
the convective line moves through. These showers and storms should
begin to dissipate later this evening as the sun begins to go
down. Also updated the precipitation amounts across the area where
the line of showers and storms moves through.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 445 PM EDT SUN JUL 21 2019

Convection is not being handled particularly well by models today,
and their guidance is being used in a more general sense. Ongoing
convection will continue to progress for a while, but likely
dwindle as we move into the evening. However, late in the night 
there is additional development expected. A cold front will 
approach from the northwest in association with a large scale 
pattern change. Geopotential height falls and cooling aloft, 
coupled with low level warm/moist advection and frontal 
convergence will all contribute to an uptick in precip. However, 
the highest POP will be during the afternoon on Monday as the
front progresses through the forecast area during peak heating.
Although, clouds and precip will limit heating, and it will not be
as hot as recent days. Flow aloft will increase, which will 
contribute to a minimal severe wind threat, and lead to a faster 
storm movement. However, precipitable water is forecast to exceed
two inches, promoting very heavy rainfall rates. Because of this,
even with more movement, there will be a risk for excessive 
rainfall where storms are able to train. Thinking is that hydro 
problems would be rather isolated. However, the situation will be
monitored for the necessity of a watch.

Most precip will taper off from northwest to southeast behind the
front on Monday night, and cooler air will already be noticed by 
dawn Tuesday.

.LONG TERM...(Tuesday through Sunday)
Issued at 445 PM EDT SUN JUL 21 2019

Much cooler and less humid weather is in store for the coming week. 
We'll also have an extended period of dry weather. Operational 
models and ensemble guidance are suggesting a very anomalous 
pattern, with the potential for some record lows being set during 
the coming week. 

The period begins with deep mid/upper level troughing in the eastern 
United States, and ridging in the west. The upper air pattern will 
deamplify during the second half of the week, with rising heights 
across our area from mid week into next weekend. This will mean a 
slow warming trend as we move into next weekend. However, the main 
story during the coming week will be the much cooler and drier 
weather. This will be a dramatic change from the recent heat and 
excessive humidity, and the frequent showers and thunderstorms we've 
been experiencing.

By 12Z Tuesday much cooler air will have invaded eastern KY in the 
wake of a cold front. In fact 12Z 850 mb temperatures by Tuesday 
should be about 10C cooler than those we've experienced this past 
weekend. 00Z NAEFS ensemble guidance is quite impressive, showing 
850 mb temperatures at 12Z Tuesday around 12C which would be in the 
bottom 2.5 percentile based on 1979-2009 climatology. 12Z Wednesday 
850 mb temperatures 10C to 12C would be in the bottom 1 percent for 
southeast KY, with a return interval of once every 5 to 10 years for 
this time of year. 

The best chance for record lows will be Wednesday morning with 
surface high pressure sprawling east from the mid MS valley into the 
OH valley, and again Thursday morning with surface high pressure 
over the central Appalachians and eastern OH valley. The 12Z GFS MOS 
shows some COOP MOS sites as low as the lower 50s Wednesday and 
Thursday mornings, and as low as the middle 50s Friday morning. 
Current planned forecast for our area will call for mostly middle to 
upper 50s for low temperatures Wednesday and Thursday mornings, with 
lows in the upper 50s to around 60 Friday morning.   

For reference here are the record lows for the coming week: 

Jackson:
Tue July 23 - 61 in 1981, 2003, 2007 
Wed July 24 - 56 in 2008
Thu July 25 - 59 in 2003, 2013, 2014
Fri July 26 - 59 in 2013

London: 
Tue July 23 - 58 in 2003
Wed July 24 - 57 in 1962, 2003, 2008
Thu July 25 - 55 in 1966 
Fri July 26 - 56 in 1966

A few showers may linger behind the cold front in far southeast 
Kentucky on Tuesday, but after that the next chance for showers and 
thunderstorms will not be until next Sunday. 

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
ISSUED AT 828 PM EDT SUN JUL 21 2019

The primary forecast issues for the TAF sites will be the overall
evolution and coverage of precipitation overnight. At this time,
it appears that ongoing showers and storms will continue to
dwindle this evening, with only a few stragglers possible
overnight. The precip chance should begin to ramp up again between
8 and 10Z on Monday, as a cold front approaches from the north.
Conditions at the TAF sites will be challenging to forecast for
tonight for a number of reasons. CIGs at any airport the received
rainfall today could see conditions varying from VFR to IFR
depending on how much fog is able to form and whether or not
anymore showers or storms directly affect any given site. For now
have MVFR VSBYs due to rain coming in toward dawn on Monday, with
MVFR to IFR CIGs forecast between roughly 9 and 11Z Monday. The
more widespread rain is expected across the Monday afternoon and
evening, as the cold front moves through. Conditions toward the
end of the TAF period could be pretty bad as well, depending on
how hard it rains and how windy it gets as thunderstorms move
through the area. Periods of IFR or worse conditions may occur
with the more intense rain showers and any thunderstorms on
Monday. Wind gusts of around 20KTs may occur as well with the
stronger storms.

&&

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

$$

UPDATE...AR
SHORT TERM...HAL
LONG TERM...SBH
AVIATION...AR