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Area Forecast Discussion (AFD)

Issued by Louisville, KY (LMK)

                            
000
FXUS63 KLMK 202303
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
603 PM EST Wed Feb 20 2019

...Updated Aviation Discussion...

.Forecast Update...
Issued 500 PM EST Wed Feb 20 2019

The flood watch has been cancelled a little ahead of schedule as the 
heaviest rains have pushed well east of the area, and calls to 
several counties have revealed some improving conditions in regards 
to flooded roadways. Water levels will still remain high along 
creeks, streams, and rivers through the end of the week.

&&

.Short Term...(This evening through Thursday)
Issued at 227 PM EST Wed Feb 20 2019

The surface boundary that entered western Kentucky this afternoon 
will continue to move eastward across central Kentucky and southern 
Indiana this evening, pushing scattered showers and isolated 
thunderstorms ahead of it. The region should be free of rain by 
midnight. Clouds will likely linger through the night, though, as 
moisture gets trapped under a sharp inversion. A few patches of 
drizzle aren't out of the question, especially in the Blue Grass. 
Low temperatures will mostly be in the middle 30s, but a few spots 
in southern Indiana may dip into the lower 30s, especially if the 
clouds break up sooner than expected.

Though rainfall amounts will be relatively light this evening, there 
are many spots around the area that are still waiting for earlier 
minor flooding to recede, so will hold on to the Flood Watch while 
the rain is still falling. The watch will be cleared early from west 
to east as the front passes through.

Tomorrow high pressure nosing in from the Midwest will give us a 
much welcome break from the wet and unsettled weather. We should 
actually see some partial sunshine with a light breeze and 
seasonable highs in the upper 40s and lower 50s.

.Long Term...(Thursday night through Wednesday)
Issued at 325 PM EST Wed Feb 20 2019

...Additional Heavy Rainfall and Thunderstorms This Weekend...

Thursday's dry weather will be short-lived as rain moves back into 
southern Kentucky Thursday evening, then drifts north toward the 
Ohio River by Friday morning. This is in response to ripples moving 
through the southwest flow aloft, and amounts will be light due to 
low level northeast flow around a surface high drifting eastward 
across the Great Lakes region.

However, hot on the heels of this weak system, a vigorous trof over 
the southwest U.S. will make its presence known for the weekend. As 
a surface low develops over the southern Plains ahead of the digging 
upper trof, the Gulf opens back up, priming the lower atmosphere 
with moisture to combine with mid and upper moisture and energy 
moving in as the upper system moves into the Mississippi Valley 
Saturday.  This tightly wound system will bring strong southerly 
winds to the area that will not only send temperatures into the 60s 
across the area, but dew points into the 50s and lower 60s as well.  

With the airmass saturated from the surface through 40K feet, 
rainfall rates will be high at times, but the thick clouds will also 
work to limit instability somewhat, preventing any insolation to 
heat the surface.  Current guidance shows a weak thermal cap at 
850mb, which could suppress thunder development somewhat, but with 
the strong low level jet in place - and a 140kt jet aloft, synoptic 
forcing could overcome this cap.  If that happens, not only will 
there be the threat for torrential rainfall, but for severe 
thunderstorms Saturday as 60kt winds look to be just a few thousand 
feet above the surface, and any mixing down would wreak havoc on 
trees already struggling to stand in the quicksand-like topsoil 
across the area. Fortunately, with no leaves yet on the trees, they 
won't have that added weight and wind resistance to worsen the 
situation. 

The center of both the surface and upper low will move into the 
Great Lakes by Sunday morning, and rain will end across the area 
with a trailing cold front moving through with it. A rare set of dry 
days follow with nearly zonal upper level flow before the next 
chance of rain returns Wednesday. This far out, however, it remains 
to be seen exactly how significant this will be, with the major mid-
long range models all doing their own things by then.  

Temperature-wise through the period, highs will be in the 50s Friday 
before warming into the 60s Saturday, then drop back into the 50s 
Sunday and the 40s Monday before rebounding.  Low temperatures will 
be in the 30s to lower 40s Thursday night, then stay above 40 
through Sunday night, when they will drop back into the 20s before 
returning to the 30s for the remainder of the period.

&&

.Aviation...(00Z TAF Issuance)
Updated at 600 PM EST Wed Feb 20 2019

Showers ahead of a front are beginning to push east of the I-65 
corridor, and should push past the I-75 corridor over the the next 
several hours. Post-frontal stratus will linger through the 
overnight hours, with gradual clearing occurring from west to east. 
Areas near the I-75 corridor may not see clearing until after 
sunrise tomorrow.

One low-confidence concern for tonight is the fog potential west of 
I-65. Most guidance does not develop fog, but some (like the NAM) 
hit dense fog really hard overnight in areas that see clearing. 
Think winds will stay up enough overnight behind the front to keep 
the lower boundary layer mixed enough to keep fog from being too 
much of an issue. This will be monitored in subsequent forecast 
updates.

&&

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

$$

Update...DM
Short Term...13
Long Term...JBS
Aviation...DM