Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 151921
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
321 PM EDT WED MAY 15 2013
.Short Term (Now - Thursday Night)...
Issued at 320 PM EDT Wed May 15 2013
Afternoon satellite imagery shows mostly clear skies across southern
Indiana and central Kentucky. With nearly full solar insolation
occurring and a decent southwesterly breeze, temperatures have
warmed into the middle to upper 80s. Expect temperatures to hit
their maximum readings in the next hour or so. Highs of 83-87 in
the east with 85-90 in the west are still expected. Temperatures
are expected to slowly drop off through the late afternoon and
evening hours with readings cooling back into the lower-middle 70s
by late evening.
Overnight, the models are keying in a mid-level wave moving through
the upper Midwest and into the southern Great Lakes region. This
feature should ignite a batch of convection across Illinois and into
Indiana late tonight. The majority of the activity looks to stay up
toward the I-70 corridor. However, as the night wears on, surface
boundary to the north will drop south and associated outflows from
the nighttime convection may result in additional convection
developing across southern Indiana. Based on the current datasets,
we feel that the best chances of precipitation tonight would be
generally north of the river. Thus, we`ll continue the scattered
thunderstorm wording for late tonight in that region. South of the
Ohio River, generally dry conditions are expected overnight. An
isolated shower or storm is not out of the question very late
tonight, mainly north of the WK/BG parkways. Organized severe
weather threat looks limited due to the lack of instability
overnight. Nonetheless, heavy rains, gusty winds and plenty of
lightning will be the hazardous weather threats overnight. Lows will
be mild with readings cooling into the upper 50s to around 60 in the
south with lower 60s across southern Indiana and the northern half
For Thursday, the short term multi-model consensus continues to be
in agreement with the surface frontal boundary pushing into northern
Kentucky during the mid-morning to early afternoon hours. Next
mid-level wave is forecast to pass through the region during the day
on Thursday and this combined with the frontal boundary, pooling
moisture, modest surface convergence, and some instability will
result in more widespread convection developing. High resolution
3km WRF and local LMK WRF models do develop some sort of NW-SE
oriented line of convection tomorrow afternoon from southern Indiana
through the Bluegrass region. This convection is likely to push
eastward into SE OH and then into WV during the evening hours.
Further southwest, more isolated-scattered convection will be
possible during the afternoon hours, but the most coverage still
looks to be across the NE third of our forecast area. Severe
weather threat for Thursday continues to look rather marginal, given
the anticipated cloud cover in the morning which would result in
less destabilization during the afternoon hours. However, deep
westerly flow aloft looks to approach 40kts or so which would bring
our bulk shear up enough for multi-cellular convection to develop.
It appears that the main threat, if any, would be damaging winds and
some possible hailers. High temperatures will be influenced by
clouds and precipitation. Overall, temperatures look to warm into
the upper 70s to around 80 in the south with mainly upper 70s across
southern Indiana and the Bluegrass region of central Kentucky.
With the upper wave departing to the east Thursday night, combined
with the loss of heating, convection is expected to diminish
markedly after sunset...though some convection may persist into the
overnight hours. Lows Thursday night will cool into the upper 50s
to the lower 60s.
.Long Term (Friday - Wednesday)...
Issued at 314 PM EDT Wed May 15 2013
Weather associated with early summer has finally arrived for an
extended time over the Lower Ohio Valley. Our days where we have
widespread soaking all_day rains associated with frontal boundaries
and closed lows have ended. As is typical this time of year, any
precipitation that we receive through the early portion of next week
will arrive in the form of scattered afternoon and evening
Currently, the polar branch of the jet stream is dominate, and lies
along the US/Canadian border. A weak closed 500mb low currently over
southern Oklahoma will weaken further and tend to shear out as it
approaches the Commonwealth early Friday. A weak nearly stationary
boundary is forecast to lie across central Indiana southeast through
southern Ohio during the day on Friday. Despite weak surface
convergence and no upper air support, PWATs approaching 1.4 inches
coupled with moderate afternoon surfaced-based instability will
likely spark some scattered slow-moving thunderstorms through the
evening hours. Expect highs Friday in the lower 80s.
Over the weekend, a longwave trough will move across the northern
Rockies, approaching the upper midwest by late Sunday. Ridging will
build northward towards the western Great Lakes ahead of this
trough, while weak northwest flow will continue across the
Commonwealth. We will remain within a warm semi-humid airmass with
afternoon and evening scattered convection quite possible both days.
Warm early June-like temperatures will continue through the entire
weekend into Monday, with afternoon highs in the lower 80s and lows
near or slightly above 60.
Both the ECMWF and the GFS develop a trough over the upper midwest
by Tuesday, although the orientation of this feature differs between
these two models. The GFS Shows the 500mb trough, centered over the
Dakotas, assuming a negative aspect as strong westerlies punch south
of the upper feature across the Lower Missouri Valley. The ECMWF on
the other hand shows a closed 500mb low over Nebraska as a part of a
developing block over the western Great Lakes. In general, southerly
winds will tend to increase Monday and Tuesday, bringing warmer and
more humid weather. Isolated convection at best on Monday may become
more numerous by late Tuesday and Wednesday as the eastern edge of
this trough approaches.
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.