area forecast discussion 
National Weather Service Louisville Kentucky 
549 PM EDT Tuesday Jun 18 2013 


Mesoscale update... 
issued at 549 PM EDT Tue Jun 17 2013 


Now that the thunderstorms in Indiana have gelled into a nearly 
solid line of convection, have increased pops in our southern 
Indiana and far northern Kentucky counties in the very near term 
(next few hours) using propagation timing from radar data. 
Mesoanalysis still suggests that the storms will begin to weaken as 
they approach and cross the Ohio River, encountering a slightly less 
unstable atmosphere. 


&& 


Short term (now through Wednesday night)... 
issued at 310 PM EDT Tue Jun 18 2013 


..Severe Thunderstorm Watch 324 in effect until 02z tonight... 


Thin line of cumulus is developing along the front, across central 
Illinois/in/OH, this hour. Still expect storms to develop here, as they 
get enhanced by a shortwave trough, now over northwest Illinois. We may 
also see some development over the Bluegrass, where cumulus are 
hanging around and along a differential heating boundary. Still 
think the best chance for strong to severe storms will be over the 
north associated with that line as it shifts southward with the 
shortwave. 


The best chance for storms over southern Indiana will range from 5-8 
PM and 6-9 PM for the sdf/Lex Metro. The line should accelerate 
southward, getting a little ahead of the shortwave forcing. Thus 
expect it to weaken as it crosses the river. Thus a Severe 
Thunderstorm Watch has been posted for a couple of tiers of counties 
across my northern forecast area. 


The front to our north will move across the region overnight, 
shifting rain chances into the southern area. Then by daybreak all 
rain chances should be down for a few days. We probably will at 
least have patchy fog where the heaviest rains fall tonight and in 
the usual river valleys. Have painted the best chance for fog in the 
grids over the southern half of the region, where it will take 
longer for drier dewpoints to get in by daybreak. 


Wednesday will be a nicer day with skies clear and highs ranging 
from 79-85. Lows Wednesday night will be a few degrees below normal. 


Long term (thursday through tuesday)... 
issued at 300 PM EDT Tue Jun 18 2013 


The extended forecast period will be dominated by an expanding ridge 
of high pressure aloft over the southern and eastern United States, 
in line with a summerlike pattern. The main belt of westerly flow 
aloft will retreat a bit northward to the northern third of the U.S. 
Where most of the active weather will occur during this forecast 
period. In between, 500 mb heights will build across the Ohio Valley 
as general high pressure at the surface anchors over the eastern 
Continental U.S.. gefs ensembles from 06z shows good member continuity and 
agreement with this scenario. 


This pattern will result in generally dry weather throughout a good 
portion of the forecast period. However, as the air mass 
destabilizes again later this week into early next week, can't 
preclude a few isolated diurnal thunderstorms. But, determining when 
and if precipitation will occur is a challenge as no organized 
forcing is apparent in the benign pattern. In addition, medium range 
models will not be able to resolve nor time subtle shortwaves moving 
into or through the ridge that might be enough to touch off isolated 
or scattered thunderstorms in an unstable air mass. 


On a daily basis, it does appear that Thursday will be a dry day 
with relatively low humidity levels for mid-late June. Prefer the 
lower GFS dewpoints Thursday (except at 00z Friday since GFS 
dewpoints have a high bias at 00z) more so than the higher NAM 
dewpoints. On Friday, GFS continues to indicate that some moisture 
over the southern Appalachians could push west-northwestward into 
southern Kentucky on the underside of the mid-level ridge axis. This 
could increase moisture enough for isolated diurnal storms Friday 
afternoon, but this is tenuous and perhaps overdone by GFS. Canadian 
Gem tends to keep any activity over Tennessee which is possible. 


Over the weekend, our forecast area looks to be basically sandwiched 
between westerly flow aloft and better convective potential well to 
our north, and diurnal scattered convection in a tropical air mass 
across the southeastern U.S. As a result, it appears our area will 
remain dry and very warm Saturday and Sunday, and possibly Monday as 
well. Nevertheless, again can't rule out a few isolated storms in 
the afternoon to early evening. Then on Tuesday, the westerlies over 
the Great Lakes may dip a bit farther south allowing a chance for 
scattered thunderstorms to develop or propagate southeast into our 
area at that time. 


Temperature-wise, afternoon highs will slowly moderate into the 
upper 80s and lower 90s over the weekend and early next week, 
although the core of the heat will remain to our west over the 
plains states. Morning lows will moderate from the lower and mid 60s 
Thursday morning to the upper 60s and lower 70s later in the 
forecast period. 


&& 


Aviation (18z taf update)... 
issued at 105 PM EDT Tue Jun 18 2013 


A shortwave trough crossing the Iowa/Illinois border now will continue 
dropping southeast today. This system, along with a frontal boundary 
to our north, will force scattered storms to develop late this 
afternoon. Think the best chance for timing purposes will run in the 
evening hours at the ksdf and klex terminals. The threat is not zero 
at bwg later, but think it is low enough to keep out of the tafs at 
this time. As storms develop to our north and moving 
south/southeast, we likely will amend for tempo groups. Once the 
front drops south of the region tonight, the rain chance will end 
and next up will be chances for fog overnight. For now have all 
three sites going to MVFR, as dewpoint drop behind the front should 
not occur till later in the morning. 


&& 


Lmk watches/warnings/advisories... 
in...none. 
Kentucky...none. 
$$ 




Mesoscale........13 
short term.......rjs 
long term........twf 
aviation.........Rjs