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FXUS63 KLMK 251911

Area Forecast Discussion
National Weather Service Louisville KY
311 PM EDT Thu May 25 2017

.Short Term (Now through Friday Night)...
Issued at 245 PM EDT Thu May 25 2017

Rain is finally tapering off as the stacked low pressure over 
southern Ohio continues to pull away from the area, giving way to a 
more mixy westerly flow in the low levels. Still seeing patchy light 
rain across the Bluegrass region in Kentucky, but expect that to end 
by sundown as the low continues to exit the region. 

Expect benign weather tonight and Friday as shortwave ridging aloft 
builds across the Ohio Valley. Strong mid-level cap will develop 
with 700mb temps rising to near 9C, so no worries about convection 
on Friday afternoon even with sfc dewpoints creeping back up toward 
60 and steepening lapse rates above and below the cap. 

Warm front well to our north will fire convection later in the day, 
but will limit POPs in the Ohio Valley to 20-30% Fri night as the 
cap guides most of the storms across central Indiana into Ohio.

.Long Term (Saturday through Thursday)...
Issued at 255 PM EDT Thu May 25 2017

...Organized Severe Weather Potential Late Saturday and Sunday of 
the Holiday Weekend...

An active and potentially volatile weather pattern may be setting up 
for late Saturday into Sunday for the lower OH Valley. The pattern 
suggests one or more bowing mesoscale convective systems (MCSs) are 
possible with wind damage.

Surface dewpoints on Saturday will rise to near 70F as the strong 
mid-level cap limits the depth of the boundary layer. By late 
afternoon, expect convective initiation somewhere over the mid-
Mississippi Valley. With strong instability and unidirectional 
shear, expect this to organize into a cold-pool-dominated MCS, which 
will push quickly to the east or ESE across some part of the Ohio 
Valley through the night. Main threat will be damaging winds, but 
very strong mid-level lapse rates will also support a large hail 
threat. Most likely track will be dependent on just where convection 
initiates, with small but significant model differences in both 
track and timing. However, strong to severe thunderstorms are likely 
to roll through part or all of the area at some point on Saturday 

We looked at CIPS analogs for this pattern, and a mean composite for 
the top 15 analogs suggest an axis of heavy rain and severe weather 
(mainly wind damage and large hail) roughly from St. Louis to 
Louisville (but with definite spread among individual past events). 

Confidence in the Sunday forecast is limited as convective 
redevelopment will be dependent on the boundaries laid out by 
Saturday night storms. Air mass still looks to destabilize ahead of 
the synoptic cold front, so we will carry likely POPs for Sunday 
afternoon. Less inhibition will likely mean less organization, but 
there is still plenty of instability to support at least an 
isolated damaging wind threat. 

Memorial Day figures to feature Chamber-of-Commerce weather as a 
bubble of high pressure builds in behind the front. Expect max temps 
very near climo, with dewpoints only in the 50s.  Much lower 
confidence Tuesday through the middle of the week with low-amplitude 
trofiness over the Great Lakes. Will keep Tue-Wed dry as most of the 
forcing stays to our north, but carry a slight chance Thursday as we 
tap into a better moisture feed.


.Aviation (18Z TAF Update)...
Issued at 152 PM EDT Thu May 25 2017

As the low pressure system moves east of northern Kentucky, 
conditions at the forecast sites will improve. BWG ceilings have 
raised to VFR levels. The last of the rain is moving away from SDF, 
but low ceilings will keep SDF in MVFR conditions this evening. LEX 
will continue to see passing showers, reduced visibilities, and IFR 
ceilings for a few more hours. Tonight SDF, BWG, and LEX will all 
continue to improve during the overnight hours. SDF and LEX will 
join BWG with VFR conditions. 

Winds will remain in the 10 to 15 knot range with gusts in the 20 to 
25 knot range.




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