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Area Forecast Discussion (AFD)

Issued by Jackson, KY (JKL)

FXUS63 KJKL 221758

National Weather Service Jackson KY
158 PM EDT Thu Aug 22 2019

Issued at 1248 PM EDT THU AUG 22 2019

Only a few isolated showers/storms remain across the CWA, mainly
along the TN/VA border. Made some small adjustments to the pops to
reduce to isolated in the northern CWA, and keep low end chances
across the southern CWA. Since we are headed into the afternoon
hours, things may begin popping up fairly quickly, so did not want
to expand isolated to far southward. Otherwise, forecast is still
in good shape. Did another refresh to the temps and winds to make
sure they were on track with the current conditions. All updates
have been published and sent to NDFD/web. 

UPDATE Issued at 930 AM EDT THU AUG 22 2019

Still tracking mainly isolated to scattered storms across our
southern CWA, moving northeastward. Reduced pops from likely down
to scattered, and changed placement of highest pops to match up 
better with the current conditions. Also loaded the latest
observations in to make sure the near term forecast for temps and
dew points was on track with the current conditions. All updates
have been published and sent to NDFD/web. A new ZFP/SAF was also
sent out to remove likely pop wording. 

UPDATE Issued at 704 AM EDT THU AUG 22 2019

We have isolated to scattered storms rolling across parts of the
area, with some gusty winds of 15 to 20 mph not out of the
question along the outflow. Adjusted PoPs to better manage the
current trends and also adjusted winds closer to HRRR which seems
to handle the outflow best. There should be a lull before more
convection becomes possible this afternoon, with some convection
moving out of the Tennessee Valley this morning so far. All this
has been sent to NDFD.


.SHORT TERM...(Today through Friday)
Issued at 403 AM EDT THU AUG 22 2019

The morning surface analysis shows a frontal boundary now entering
the Ohio Valley this morning. Meanwhile, we are monitoring a
thunderstorm complex moving into north central Kentucky near 
Louisville and tracking east. These storms are slated to move into
the Bluegrass through the morning, but the weaker CAPE will 
probably lead to a lessening of strength as it does so. The 
outflow from this will also be problematic, as this could help 
generate new convection through the morning and early afternoon. 
Given the trends will keep the better PoPs across the Bluegrass 
early before showing chance to likely PoPs across much of the area
with outflow convection potential. This afternoon there could be 
a period where there is a lull in convection before more 
development is possible in the afternoon as front approaches from 
the northwest. The potential for severe weather is in question 
given the cloud cover and timing of early on convection/outflow. 
Thinking this marginal risk remains the best call given the 
uncertainty and more isolated nature expected at this point.

Tonight we see quasi-zonal flow aloft, with multiple waves riding
through the flow. This could help induce a weak low in the
Tennessee Valley that will progress northeast into Kentucky late 
evening into tonight. There will be a strengthening low level jet
and departing influence from the right entrance region of the 
300mb jet for additional lift. This in conjunction with higher 
PWAT values in the 2 inch range could be an issue for some 
isolated flash flooding at least despite the dryer weather we 
have seen. Thus think the WPC slight risk for excessive rainfall 
looks justified. For now will make mention in the HWO and let 
later shifts evaluate further. The boundary will push farther into
the region on Friday, with best chances of convection pushing 
along and near the Tennessee border through the day. 

.LONG TERM...(Friday night through Wednesday)
Issued at 403 AM EDT THU AUG 22 2019

The period is expected to begin with a shortwave trough extending 
from the Maritimes into the OH Valley. This trough will gradually be 
moving east with a narrow shortwave ridge building into the Great 
Lakes and into the OH Valley at least briefly to begin the weekend. 
At the same time, a shortwave trough will move into the Plains to 
start the weekend. The axis of the ridge is expected to move east of 
East Ky to end the weekend with a shortwave trough moving into the 
OH Valley by late Sunday into Sunday night. A stronger trough is 
then expected to dig into the Plains to start the week as a ridge 
moves into the wester US, gradually shifting east through midweek. 
Downstream of this ridge, a central to eastern US trough is expected 
to take shape and linger through the end of the period. 

At the surface, a cold front should settle south of East KY to begin 
the period while high pressure moves across the Great Lakes and into 
the northeast over the weekend. Southern portions of the area, 
generally along or  will be close enough for the frontal zone for 
south of KY 80/Hal Rogers/Cumberland Pkwy corridor for isolated to 
scattered convection on Saturday. However, the bulk of the area will 
be under the influence of the drier airmass arriving with the upper 
ridge which should cap cumulus development. In advance of the 
shortwave trough to end the weekend, on Sunday into Sunday night, an 
inverted trough may develop and extend from the TN Valley into the 
Central to northeastern parts of the Commonwealth. The shortwave 
trough and moisture return near and west of this surface feature may 
lead to some convection by Sunday evening, particularly near the TN 
border and west of I 75. With the shortwave moving through the area, 
isolated to scattered convection should occur from Sunday night into 
Monday accompanied by a considerable amount of cloud cover. 
Convection will remain possible through the end of the period as the 
trough begins to take shape west of the area , though recent model 
runs have a bit of spread in bringing the next shortwave trough axis 
east of the MS River in the Tue to Wed period. This would also lead 
to some possible timing differences for a cold front as well. 

Temperatures during the extended time frame should be more typical 
of late August from Saturday through the middle of next week if not 
slightly below normal initially for highs on Saturday. Highs should 
average in the upper 70s to mid 80s with lows mostly in the upper 
50s to mid 60s. 


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

Scattered showers and thunderstorms are expected throughout much
of the afternoon, increasing in coverage as we head into the
overnight and a cold front sinks south into the state. Best
chances for convection early this afternoon will likely be along 
the TN and VA border, with less likely chances across the north 
(include KSYM). That being said, can't rule out a stray shower or 
storm anywhere in the CWA. Went with VCTS and P6SM for this time 
period given the uncertainty. As POPS begin to ramp up this
evening and overnight, increased to likely coverage of showers 
and VCTS (generally between 4 and 8Z). VIS and CIGs will likely 
be impacted, with CIGs expected to fall throughout the second 
half of the night and remain MVFR to IFR through the end of the 
TAF period. VIS will be more variable, but expect generally IFR 
conditions once the rain begins, with some brief lowering in the 
heavier showers/storms. Winds will be light and variable 
throughout the period, though any storms may experience higher