Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 160718
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
317 AM EDT THU MAY 16 2013
.Short Term (Today - Friday)...
Issued at 315 AM EDT Thu May 16 2013
As of 7Z, convection was still ongoing along a sfc frontal
boundary from southern Iowa to central Indiana to eastern Ohio.
Overall convection was weakening as it continues to drift ESE. A
cluster of weak storms over central IN producing plenty of lightning
(observed from the Louisville Metro area), will continue to move ESE
through the early morning hours largely missing our region. A few
counties over southwest IN and the northern Bluegrass may see some
rain from this left over convection around or just before sunrise.
Also, a weakening upper low over the OK/KS/MO/AR region was pushing
vort waves into western TN causing rain shower activity which may
push into south central KY around sunrise as well. Overall though,
rain/t-storm chances for this morning look slim with the 3km WRF and
LMK WRF in agreement with this theory.
This afternoon may be a bit more interesting however. The sfc front
to our north looks to sag south with some possible outflow
boundaries from this morning`s storms settling further south as
well. This may put areas north of the I-64 corridor at risk for
strong t-storms to develop. However, confidence is still not great
on location since it will depend on where key trigger features
settle later this morning. Looking at soundings and high-res model
data, storms should erupt near the boundary or outflows between
18-20Z and continue into the evening hours. If they make it as far
south as our area, we could see hail up to penny size and gusty
winds up to 40 mph in the strongest storms. However, there is a
chance that the boundaries will stay to our north and keep most of
the convection out of our area as well. By mid to late afternoon,
almost all locations will have at least a shot at isld-sct
convection as plenty of instability in low and mid levels will exist
with t-storm triggers (boundaries and vort waves from an upper low
to our west) nearby. However, the strongest storms should still be
located near the boundary north of the I-64 corridor.
Tonight, expect convection to decline from 0-6Z and drift off to the
east. It looks like most of the overnight hours should be dry after
midnight. However, left a chance from some isld convection in the
forecast in coordination with surrounding forecast offices.
For Friday, the remnant trough (left over from the weakening upper
low currently to our west) will move into the Ohio Valley creating
chances for showers/t-storms starting Friday morning in the south
and spreading north throughout the day. Also, the sfc frontal
boundary will still be linger over southern IN or far northern KY
which will help convection get going Friday as well. Friday
morning`s initial convection looks to be mainly showers with
embedded t-storms with the greatest threats being brief heavy
rainfall and cloud-to-ground lightning. Friday afternoon`s storms
still don`t look all that impressive with gusty winds, brief heavy
rainfall, and cloud-to-ground lightning being the main threats.
As for temperatures, with lots of mid and upper level clouds
expected to blanket the sky during periods with no t-storms,
temperatures should be limited to the upper 70s and lower 80s for
highs both today and Friday. Low temperatures tonight should remain
rather mild in the lower 60s for the most part.
.Long Term (Friday Night - Wednesday)...
Issued at 314 AM EDT Thu May 16 2013
Friday Night - Sunday...
Active weather will continue into the weekend as a weak upper level
low embedded in the shallow northwest flow begins to meander into
the Ohio Valley. Will leave isolated shower and storm mention in
Friday night, however the bulk of the activity will be in the late
evening hours. Expect overnight lows in the low to mid 60s.
Coverage of showers and storms will pick up on Saturday as the upper
level disturbance traverses the area. The disturbance along with
enough deep moisture should warrant scattered coverage (50%) of
showers and storms in a modestly unstable airmass. Still a bit
concerned about heavy rainfall potential with tall and skinny CAPE
profiles and weak flow throughout the column. Forecast PWAT values
are not quite as impressive as depicted yesterday, however values in
the 1.5"-1.6" range are still near the 99th percentile for this time
of year. Some of the storms could even get briefly strong if
forecast instability parameters are correct, however moist column
should limit wind and larger hail threat. Feel that heavy rainers
will be the biggest concern. Look for highs around 80 on Saturday.
Coverage of showers and storms should lessen on Saturday evening with
the loss of heating and with the upper low beginning to move off to
the east. Best focus should be over the eastern CWA into the late
evening. Overnight lows will once again be in the mid 60s. Sunday
will bring less coverage (30-40%) of showers and storms, however do
expect an uptick east of I-65 in the afternoon hours. Areas west of
I-65 will have the best shot at staying dry and should see a bump in
high temperatures more solidly into the low 80s.
Sunday Night - Monday Night...
The beginning of the new week looks to offer the best shot at dry
conditions as the weak upper low moves into the mid Atlantic, and an
upper level ridge axis briefly builds over the Ohio Valley.
Meanwhile, highly amplified central CONUS trough will start to push
east. In collaboration with surrounding offices, have gone dry
Sunday night and will only mention a slight chance of showers and
storms on Monday as environment may be unstable enough to kick off
convection with heating and local effects alone. There may be some
degree of a mid level inversion in place across the area with the
upper level ridge axis, so there is still hope that Monday could be
completely dry. Choosing not to trust model forecast soundings this
far out. Will see an increase in high temperatures under this regime
as skies become mostly clear to partly cloudy. Look for lows in the
mid 60s and highs in the mid 80s.
Tuesday - Wednesday...
Models continue to generally agree on a more significant system
impacting the Ohio Valley through mid week as an impulse in the
polar jet carves out a trough in the central CONUS and pushes east.
The associated surface system looks to drag a cold front through the
region sometime late Wednesday, however significant chances for
rainfall look to be possible ahead of the front Tuesday and
Wednesday. Have gone with likely mention in the west Tuesday, and
likely mention in the east on Wednesday. Stronger westerlies should
combine with what appears to be a moderately unstable airmass to
potentially give us stronger storms during this time frame. It is
too early to nail down many details except that pattern looks
favorable for more organized convection. Stay tuned. Temperatures
should range in the 80 to 85 degree range for this period with
continued mild overnight lows.
.Aviation (6Z TAF Issuance)...
Issued at 135 AM EDT Thu May 16 2013
Prevailing VFR conditions are expected this TAF period with hard to
time convection chances over the next 24-30 hrs. A sfc frontal
boundary will remain stalled to our north and a slow
moving/weakening upper low will continue to make gradual progress
For this morning, it looks like SDF may see some left over showery
activity around 10Z or so from convection moving SE from the sfc
front over central IN as of 530Z. BWG may see some showery activity
as well from the light showers moving northeast out of western TN
due to vorticity ahead of the upper low. LEX looks too far east and
south to be at much risk of any convection this morning due to these
features. However, will maintain a close watch on convection
We should see a lull in any convection by late morning or early
afternoon before it gets going again by 19-22Z across the TAF
sites. Depending on how far south the sfc boundary sinks and where
remnant outflows from this morning`s storms settle, SDF/LEX could
see scattered t-storms this afternoon/evening. Will maintain VCTS
due to low confidence right now. Not sure if BWG will see any
action at all this afternoon/evening other than some light, late
isld convection due to peak heating instability. Therefore, will go
with only VCSH at BWG. If any t-storm hits a TAF site directly this
afternoon/evening, flight conditions could fall into the IFR/MVFR
range. Will amend and add tempos to cover this as needed.
Winds will remain out of the SSW above 7 kts for most of the day
light hours today and remaining elevating at SDF/LEX for the rest of
the early morning hours.