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Area Forecast Discussion (AFD)

Issued by Louisville, KY (LMK)

                            
237 
FXUS63 KLMK 220704
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
304 AM EDT Sat Sep 22 2018

.Short Term...(Today and tonight)
Issued at 304 AM EDT Sat Sep 22 2018

A wavy cold front was draped from the northern Bluegrass southwest 
through south-central Kentucky this morning. There's a very evident 
wind shift and dewpoint gradient across the boundary. A 10-15 kt 
northerly breeze north of the front has lowered the SDF dewpoint 
from 71 to 56 degrees. Lower 70s dewpoints were still common along 
and south of the Bluegrass Pkwy as of this writing. 

The main forecast challenge today is precip coverage and timing. 
Waves of rainfall are likely today and tonight, particularly in 
central KY. A notable shortwave trough has been centered over the 
southern Plains, with mid-level energy and deep moisture spreading 
ENE along the boundary. Convection has blossomed along the elevated 
front from northern Arkansas into Middle Tennessee. Expecting 
numerous showers along with a few rumbles of thunder to continue to 
lift into southwest and south-central KY between now and 12z, so 
will maintain the highest pops across the south through mid-morning. 
The thermodynamic environment has diminished considerably over the 
past couple of hours, with only 250 J/kg MLCAPE and increased CIN in 
that area. So stronger storms are unlikely, and lightning activity 
will continue to diminish with the diurnal minimum. Southern KY 
could see a brief lull in precip from mid/late morning through early 
evening. 

Meanwhile, isentropic ascent/overrunning should result in stratiform 
rain spreading as far north as southern Indiana (as seen now in 
Missouri). Leaned on a consensus of the ARW/NMM/3 km NAM/HRRR for 
timing details, so will spread higher rain chances along the Ohio 
River from midday through the afternoon hours. This evening and 
tonight, the band of deep moisture oriented along the elevated 
frontal boundary sinks slowly south and east a bit. In addition to 
the favorable low-level convergence, some divergence aloft is 
maintained in the right front quadrant of the upper jet. Expecting 
numerous showers tonight south of the river. 

Temperatures will feature a north-south gradient, with afternoon 
highs maxing out in the mid/upper 60s in southern Indiana to the 
low/mid 70s in southern KY. Likewise, lows tonight will range from 
the mid 50s to mid 60s.

&&

.Long Term...(Sunday through Friday)
Issued at 255 AM EDT Sat Sep 22 2018

...UNSETTLED AND WET SEVERAL DAYS...

Stationary front will be draped across the Ohio and/or Tennessee 
Valleys on Sunday, supporting scattered to numerous showers 
especially across south-central Kentucky. Depending on the evolution 
of precip today and placement of the moisture axis, there could be 
some intervals of heavy enough rain in south-central Kentucky to 
cause localized flooding. Best chance of actually staying dry 
through the day will be over southern Indiana, but expect plenty of 
clouds and temps on par with mid-October normals. 

Upper flow backs SW late in the day and lifts the front northward 
across the area Sunday night. We'll carry high-end chance POPs 
heading toward morning, but at this point the heaviest QPF will 
shift north and west enough to give southern Kentucky a break. 

Broad and deep south/southwest flow on Monday will support at least 
chance POPs throughout the area. Precip chances will continue into 
Monday night, but still with low confidence in exact timing. 

Tuesday through Wednesday should bring the heaviest precip of the 
upcoming week, with heavy rain almost a given from what could be 
multiple rounds of storms to get the cold front through the area. 
Uncertainty in timing was all that kept us from going categorical in 
any time period, but we do have an extended period of 60-70% POPs. 
Main hazard will be small stream flooding, but if the timing lines 
up with peak instability, the upper dynamics could support a few 
strong or severe storms. 

Low-confidence forecast for the latter part of the week, as there 
are hints that this front will also slow down. Will carry low POPs 
on Thursday, especially in south central and east central Kentucky. 
Better confidence that temps will run solidly below normal, and by 
then the bigger deal will also be tallying up the early-week 
rainfall. Widespread 3 to 5 inch totals are possible, depending on 
how much the front meanders early in the week. Where the heaviest 
rain falls, look for rises on some of the rivers. We'll keep a 
Hydrologic Outlook in play just to provide a heads-up until the rain 
starts to fall, and we can start to pin down which rivers will be on 
the rise.

&&

.Aviation...(06Z TAF Issuance)
Issued at 158 AM EDT Sat Sep 22 2018

The surface cold front is just south of SDF as of this writing, and 
terminal has observed a healthy 10-15 kt breeze out of the north 
since about 0430z. HNB has seen similar breezes, but the boundary's 
sluggish southward progress means LEX and BWG will continue to see 
light and variable winds until fropa. 

Decided to initialize BWG with VCSH followed by VCTS in a few hours. 
Convection has lit up along the elevated front from northern 
Arkansas through Tennessee and into far southern KY. Today, enhanced 
moisture currently over Missouri and Arkansas is forecast to spread 
northeast downstream of a shortwave trough. That moisture will get 
squeezed over central KY, resulting in rounds of rain today into 
tonight. The thunderstorm potential will be limited to areas near 
and south of the surface cold front, affecting primarily BWG over 
the next 24 hours. Ceilings will gradually lower to MVFR from mid 
morning into early afternoon, and BWG could see IFR conditions by 
late afternoon. Timing widespread moderate rains is still a bit 
problematic with some question as to how far north it spreads, but 
BWG is most likely to be impacted. HNB will likely be the driest TAF 
site today with brief light showers possible.


&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&

$$

Short Term...EBW
Long Term...RAS
Aviation...EBW