Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 151718
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
118 PM EDT WED MAY 15 2013
...Updated Aviation Discussion...
Issued at 117 PM EDT Wed May 15 2013
Current forecast continues to be on track. Temperatures across the
region have now warmed into the 80s and will continue to rise
through the afternoon hours. Only tweaked the hourly T/Td grids to
bring them closer to the observed data. Otherwise, highs of 83-87
still look good across the east with 85-90 in the western sections.
Update issued at 959 AM EDT Wed May 15 2013
Clear skies were noted across much of the Ohio Valley this morning.
Temperatures have already warmed into the lower 70s. Expect
temperatures to continue to rise throughout the morning and into the
afternoon hours. Highs out in the east will be in the 83 to 87
degree range with readings out in the central and west ranging from
85 to 90. It will also be quite breezy today with a strong pressure
in place across the forecast area. Current forecast has this well
handled at the moment, so a simple grid refresh was performed. This
will result in an updated AFM/PFM and grid/web products.
.Short Term (Today - Thursday)...
Issued at 300 AM EDT Wed May 15 2013
Expect mostly sunny skies today with gusty southwest winds as we
remain in a tight sfc pressure gradient between systems. The main
forecast challenge for today will be temperatures and exactly how
warm we`ll get. According to the latest model guidance, h85 temps
should be around 16-17 degrees C which would translate to temps in
the mid to upper 80s at the sfc using dry adiabats. Based on
observed temps for Tues, think that this sounds reasonable. As a
reference though in case temps over achieve, here are record highs
for May 15th:
SDF 92, 1900
BWG 92, 1962 & previous years
LEX 89, 1962
FFT 91, 1962 & previous years
Forecast highs for today are about 4-5 degrees below these records.
Late this afternoon we`ll see convection light up along a warm front
located over central IL/IN. As we get into the evening hours, some
models indicate that this convection will work southward into our
southern IN and northern KY counties between 0-6Z. This would most
likely be due to either the boundary sagging a bit farther south
and/or convection re-firing on outflow boundaries of initial storms
moving south into our area. Either way, as convection becomes
farther displaced southward from the boundary and a nocturnal stable
layer starts to develop, think that showers/t-storms should become
elevated as they enter our area Wed night. Perhaps a few could
contain gusty winds or small hail, but nothing near severe limits is
Late tonight, a weakening upper low pushing eastward out of the
southern Plains will spread vorticity into our region producing more
elevated showers/t-storms for the area late tonight into Thurs
morning. This convection will move from WSW to ENE throughout the
morning hours Thurs. By the time they exit southwest IN and north
central KY early in the afternoon, a few may once again be
able to become sfc based with some gusty winds or small hail
possible. However, the overall wind column is not supportive of
strong storms. With morning showers/storms spreading cloud cover
over most of the area, think that Thurs afternoon chances are less
than previously anticipated so have lowered POPs to 20-35% over most
areas Thurs afternoon into the evening hours. However, it should be
noted that this is a complicated situation to follow so POPs may
need to be adjust Thurs afternoon based on the evolution of Thurs
With clouds/convection over the area tonight and Thurs, expect a
lower diurnal range of temps. Lows tonight should remain in the low
to mid 60s for the most part. Highs on Thurs will be highly
dependent on the exact timing of convection and whether or not we
get some breaks in the clouds Thurs afternoon. For now, will base
temps on a mostly cloudy afternoon and go on the conservative side
with forecast highs in the upper 70s to around 80.
.Long Term (Thursday Night - Tuesday)...
Issued at 314 AM EDT Wed May 15 2013
Thursday Night - Friday Night...
A frontal boundary will remain draped across central Indiana and
southern Ohio as we end the work week. Meanwhile, shallow and weak
northwest flow will hold aloft. There are a few subtle shortwaves
forecast to traverse the area, most notably Thursday night and again
on Friday afternoon. Will continue the trend of putting the most
focus for coverage (scattered) of showers and storms across the
northeastern CWA close to the frontal boundary through this period.
South of I-64, coverage should be more isolated to widely scattered,
however enough moisture and modest instability will warrant pops
mainly from a diurnal convection standpoint. The overnight periods
should be mostly dry, but will still carry low pops. Expect lows
each night mainly in the lower 60s, and highs on Friday around 80.
Saturday - Sunday...
Upper level flow will begin to amplify across the CONUS over the
weekend as upper level ridging begins to develop over the east and
upstream troughing develops to the west. The Ohio Valley will find
itself to the northeast of the ridge axis on Saturday, with a moist
and more unstable airmass to work with. There is also some data
supporting a weak upper level disturbance traversing the region.
This will provide focus for better coverage of thunderstorms.
Forecast soundings offered by the GFS would suggest a heavy rain
threat as a tall and skinny CAPE profile combines with PWATs up
around 1.8 inches. Adding to the threat of heavy rain will be
generally weak flow through the column. Convection may linger into
Saturday night. Sunday becomes a little less certain as focus shifts
upstream to a developing system over the central Plains. Will leave
scattered showers and storms in the forecast to account for the
unstable and uncapped environment, although it is tough to really
place a triggering mechanism at this point due to difference in
model solutions. Overall, highs on Saturday and Sunday should be in
the low 80s, with mild overnight lows in the mid 60s.
Sunday Night - Tuesday...
As we head into early next week, upper level blocking ridge axis
will be east of the CWA, with an upstream trough across the central
Plains. A baroclinic zone will set up between these two features,
with the bulk of precipitation to our northwest on Sunday night and
Monday. As we head into Tuesday and Wednesday, it appears the
front will slowly slide eastward over the Ohio Valley, which
could provide a prolonged period of scattered to numerous showers
and storms as upstream blocking keeps the boundary from progressing
east too quickly. Better deep layer flow, along with instability may
lend to a stronger storm threat. Highs are expected to be in the low
80s, with lows in the mid 60s.
.Aviation (18Z TAF Issuance)...
Issued at 115 PM EDT Wed May 15 2013
VFR conditions are expected to prevail at the terminals through the
upcoming TAF period. High pressure across the southern US combined
with a wave of low pressure moving through the Midwest will keep the
pressure gradient up across the region this afternoon. Thus, expect
a breezy southwesterly wind to continue. Sustained speeds of
10-13kts with gusts up to 21-22kts are expected through at least
16/00Z. After 16/00Z, expect winds to remain out of the southwest
but with sustained speeds of 6-9 kts.
Latest models have backed off precipitation chances for this evening
and into the overnight hours. The 15/00Z 3km WRF runs were
suggesting that convection would develop to the north of the region
late tonight and then sag southward toward morning (16/12Z). The
large scale models (NAM/GFS) still show this to some extent, but not
until after 16/12Z. For now, have trended the forecast dry at KBWG
and KLEX but will keep VCSH in at SDF after 16/12Z.