Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 210709
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
309 AM EDT Mon Jul 21 2014
.SHORT TERM (Now through Tuesday)...
Issued at 305 AM EDT Mon Jul 21 2014
Near-term issue will be fog through the morning commute. Patchy fog
has developed over portions of the Bluegrass region and some other
typical fog-prone areas, especially where rain fell on Sunday
afternoon. Fog is locally dense but still not overly widespread or
persistent based on METARs and area webcams. At this point a Dense
Fog Advisory seems unlikely, but will continue to monitor trends and
could very well go with a Special Weather Statement.
Once we get through the next few hours, a strengthening upper ridge
over the southern Rockies will be the main player in our weather for
the next couple days. Max temps and precip chances will hinge on
just how far east that ridge can build into the Ohio Valley. Temps
should top out close to 90 this afternoon, and lower/mid 90s will be
on tap Tuesday afternoon as the ridge builds in even stronger. With
dewpoints in the upper 60s, heat index will creep up near 100F on
Tuesday afternoon. Main fly in the ointment, especially for south-
central Kentucky, will be the weak upper low that is progged to
close off over the Deep South. PWATs struggling toward 1.5 inches
are underwhelming for mid-July. Will keep a dry forecast for today,
and bring a 20-30 POP into the Lake Cumberland area on Tuesday
afternoon, but even that may be a bit generous.
.LONG TERM (Tuesday Night through Sunday)...
Issued at 300 AM EDT Mon Jul 21 2014
The synoptic pattern at the start of the long term period will
feature an amplified ridge across the western CONUS, with a
downstream trough attempting to build into the Great Lakes region.
This upper trough will amplify into the Ohio Valley through the long
term period, allowing for cooler conditions and unsettled weather to
return to the Ohio Valley.
Ahead of the aforementioned trough and associated surface front,
Wednesday has the potential to get quite warm as 850mb temps climb
to near 20C. However, will have to watch two separate areas of
possible convection Wednesday afternoon. One area will be pushing
into southern IN along a surface front from the north. A secondary
region of convective potential will be across the Lake Cumberland
region, where a weakening cutoff upper low across the Southeast will
sling a weak PV anomaly north into southern KY. Given only weak
capping, think this feature may be enough to spark storms across
south-central KY. In between these two convective regions, partly
cloudy skies should allow temperatures to climb into the 90s,
perhaps into the mid 90s if convection and convective debris hold
off for much of the day.
The upper trough and associated surface front will push through the
region late Wednesday afternoon into Wednesday night. As mentioned
above, convection will likely develop across central IN ahead of
this front early Wednesday afternoon, spreading south into southern
IN/KY through the evening and overnight hours. At this time, am not
overly impressed with any organized severe potential. Winds fields
are quite weak, with 0-6km shear <20 knots. 0-3km shear looks less
than 10 knots, which means any congealing cold pools will have a
tough time remaining tied to their parent convection. While a few
strong to marginally severe storms capable of gusty winds are
possible given MLCAPE of 1500-2000+ J/kg and steep low-level lapse
rates (especially if low to mid 90 temps are realized), this system
does not appear to pose an organized severe threat at this time.
After some lingering showers/storms Thursday, conditions will begin
to dry out from north to south as yet another cool airmass settles
into the region. Temperatures Thursday and Friday will be in the
upper 70s to lower 80s, with dewpoints in the lower 60s.
Uncertainty increases into the upcoming weekend as guidance is
struggling with the timing of the next trough diving into the Ohio
Valley. For now, will continue with a general model blend which
brings precipitation chances back into the region Saturday into
Sunday. This system will have better mid-level flow to work with,
so will have to watch this system for the potential of organized
convection over the coming days. Highs will be in the mid 80s for
Just outside of the long term period, it appears as if another
substantial cool down is in store for the Ohio Valley as the trough
really carves into the eastern CONUS behind the weekend system.
Highs may be similar to what we saw this past week, confined to the
70s. Stay tuned.
.AVIATION (06Z TAF Update)...
Issued at 115 AM EDT Mon Jul 21 2014
Main challenge is potential for overnight fog formation. Skies have
cleared and winds have gone nearly calm, with favorable radiational
cooling conditions expected to continue. Most concerned about LEX,
where temp is already close to the afternoon dewpoint, and over a
half inch of rain fell late Sunday afternoon. Latest GFS LAMP
guidance has backed off a bit from previous runs, but still
confident that restrictions to vis will occur. As this discussion
was about to be sent, LEX came in with IFR visibility, so will
initialize accordingly closer to the end of the issuance window, and
most likely include a TEMPO for LIFR vis. Didn`t quite hit BWG as
hard and confidence a bit lower due to some slightly lower dewpoints
that mixed down Sunday afternoon, but stuck with a similar theme of
prevailing MVFR/TEMPO IFR. SDF should stay warm enough to limit
impacts to a couple hrs of MVFR vis around daybreak.
By mid-morning the fog should dissipate, leaving VFR conditions with
only scattered diurnal cu and light ENE winds.