Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 152305
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
705 PM EDT SAT JUN 15 2013
...Updated Aviation Discussion...
.SHORT TERM (Today through Sunday Night)...
Issued at 300 PM EDT Sat Jun 15 2013
The synoptic pattern late this afternoon features an upper ridge
across the Ohio Valley, with more zonal flow across the Central
Plains. This ridge will deamplify through the short term period,
allowing the more active zonal flow and a slow-moving surface front
to affect the Ohio Valley through the short term period.
Latest radar imagery this afternoon depicts several clusters of
thunderstorms across portions of the Midwest and the Central
Plains. These thunderstorms are in association with better Gulf
moisture and resultant instability, which will remain west of the
region through the evening hours. Some hi-res guidance tries to
bring some of the activity currently in Illinois into southern
Indiana this evening, but with the upper ridge remaining in place
and the activity outrunning the instability, think it will continue
to struggle as it drifts east. Thus, will keep a dry forecast
through much of the night. With winds going more southwesterly
through the night and increasing mid and high level cloudiness, lows
should be a bit warmer than past nights with readings expected in
the upper 60s and lower 70s.
By Sunday morning, the ridge will begin to break down as a shortwave
trough moves through the zonal flow. Guidance continues to suggest
some decaying convection entering southern Indiana along an axis of
isentropic ascent on the leading edge of the better
moisture/instability towards 12Z Sunday morning. It remains a bit
unclear how healthy this early activity will be, and what effect
this might have on the afternoon forecast. Current thinking has
this activity pushing into southern Indiana/northern KY through the
late morning hours as it weakens. Therefore, the best prospects for
afternoon sunshine and more robust instability to build will be
across central and southern KY, where anywhere from 1000-2000 J/kg
of MLCAPE will develop. However, much of the Ohio Valley will be
displaced from the stronger westerlies, which will be located across
the Great Lakes. Additionally, the main shortwave forcing will be
pushing east of the area by 18Z Sunday, leaving the weakly
convergent (and stalling) surface boundary and any mesoscale
boundaries as the main forcing mechanisms during peak heating.
Despite little capping, subsidence in the wake of the passing trough
casts some doubt on thunderstorm coverage, thus will keep pops
confined to 40-50%. Given deep layer shear of only 15-25 knots and
lack of upper support, any severe weather threat appears quite
limited. However, given the steep low-level lapse rates, a pulse
storm or two capable of some damaging wind gusts may be possible,
especially across central and southern KY where the best instability
Thunderstorm coverage should diminish after sunset, but with an
unstable airmass in place and zonal flow continuing aloft, cannot
dry us out completely. Will taper pops to 20-30% during the
overnight hours. After highs in the mid 80s Sunday, overnight lows
will fall into the upper 60s and lower 70s.
.LONG TERM (Monday through Saturday)...
Issued at 300 PM EDT Sat Jun 15 2013
Monday through Tuesday...
Unsettled weather will prevail as a front remains nearly stationary
over the Ohio Valley, and a couple of disturbances in the zonal flow
aloft move across the area to trigger convection. Highest POPs will
be Monday into Monday night, with likely POPs area-wide on Monday
and mainly south of the WK and Bluegrass Parkways Monday night.
Would not expect either period to be a washout, but timing of any
precip depends on where and when convection fires upstream, and the
impact of any debris cloudiness on how the atmosphere recovers.
Severe weather potential is not looking as significant this time
around, as wind fields aloft are less robust than advertised by
Really low-confidence forecast on Tuesday as the upper low currently
over Saskatchewan is progged to open up and dive SE into the Great
Lakes. Much of the deep moisture will likely be pushed to the south
by the Monday night impulse, but GFS and NAM both show lighter QPF
dropping in from the north late Tuesday. Have kept slight chance for
Tuesday over southern Indiana, and a chance over Kentucky with the
highest POPs in the south. Diurnal temp ranges will be limited by
clouds and precip, so Tuesday morning lows a few degrees above
normal, and afternoon highs several degrees cooler than normal.
Wednesday through Saturday...
Wednesday will feature a surface high over the Great Lakes and a
broad upper trough over the eastern CONUS. Moisture will be
suppressed to the Deep South, and weather conditions will look a lot
like yesterday with seasonable temperatures and low humidity. The
surface high will retreat to the mid-Atlantic coast by Thursday, and
upper ridging over the Mexican desert will gradually expand into the
Ohio Valley during the latter part of the week. Low-level moisture
will gradually recover but PWATs will hover just barely over an
inch, while climo by now is creeping up toward 1.5 inches. Will keep
the forecast dry for now, but can`t rule out needing to introduce
POPs for isolated/scattered diurnal convection toward the end of the
week. By Friday and Saturday we could finally crack the 90-degree
mark again, with mins also running a few degrees above normal.
.AVIATION (00Z TAF Update)...
Issued at 705 PM EDT Sat Jun 15 2013
VFR conditions will prevail through most of the TAF period. For
tonight winds will be out of the southwest and around 5 knots or
less. A weak front and upper level disturbance will cross the area
tomorrow. This will bring scattered showers and thunderstorms.
Timing and coverage of thunderstorms is still rather in question so
will continue to mention only VCSH in the TAFs at this time. Winds
tomorrow will pick up out of the southwest to around 10-12 knots.