Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 181912
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
311 PM EDT SAT MAY 18 2013
.Short Term (Now - Sunday Night)...
Issued at 300 PM EDT Sat May 18 2013
An area of low pressure continues to spin across the forecast area
this afternoon. Early morning light showers/drizzle gave way to a
few hours of dry conditions. However, now, with diurnal heating,
scattered showers have developed. Not seeing any lightning, but an
isolated thunderstorm cannot be ruled out given the steep lapse
rates aloft. Think the morning cloud cover helped keep instability
down a bit. The heavier rain producing showers should reside across
southeast Indiana and into the Kentucky Blue Grass, with isolated to
scattered showers elsewhere. Temperatures have reached the low to
mid 70s, with some warming still possible for several locations.
The coverage of showers and any storm that may develop will slowly
diminish this evening and tonight, as the upper low moves east. With
the departing low, an upper-level ridge axis will begin to build
across western Kentucky tonight. High resolution models continue to
indicate we will go briefly dry by around midnight through dawn.
Skies should become at least partly cloudy overnight, which will
allow temperatures to drop into the low and mid 60s and perhaps some
patchy fog or low stratus to form. Depending on the amount clearing,
some locally dense fog could develop, especially for locations that
receive rainfall today.
The upper low will move to the central Appalachians Sunday, with the
upper-level ridge axis continuing to build eastward. Expect diurnal
scattered convection across our eastern CWA, associated with the
upper low. Should also see some scattered convection across our
south and southwest with a weak warm front edging into the forecast
area. Heavy rain and of course lightning will be the main threats
with any convection that develops. High temperatures Sunday should
reach the upper 70s across the eastern CWA and low to mid 80s across
the west. Temperatures will drop back into the low and mid 60s under
partly cloudy skies. Once again, could see some patchy fog develop
around dawn Monday.
.Long Term (Monday - Saturday)...
Issued at 310 PM EDT Sat May 18 2013
The latest multi-model solutions continue to be in fairly good
agreement with a large upper level trough remaining nearly
stationary out over the Plains through the first part of the
forecast period. With the upper trough to our west, the Ohio Valley
will see a relatively deep southwesterly flow aloft which will also
good moisture advection and continued warm temperatures through the
first half of the week.
Increasing humidity levels will be evident by Monday afternoon as
the southwesterly flow kicks into high gear. Despite having good
moisture pooling across the region, latest model proximity soundings
do show expanding mid-level height rises across the region. The
height rises along with a fairly evident capping inversion will
likely keep convection at a relative minimum Monday afternoon.
Highs Monday should top out in the 85-90 degree range. With
dewpoints in the upper 60s to around 70, it will feel like the lower
90s across the region. Lows Monday night look to cool into the
upper 60s to around 70.
By Tuesday, the upper ridge axis looks to shift a bit more to the
east. Correspondingly, mid-level heights lower through the day and
the capping inversion is not as strong as what is indicated for
Monday. As temperatures warm again into he 85-90 degree range,
we`re likely to see isolated to scattered convection develop during
the afternoon. Latest data does not have a well defined lifting
mechanism that stands out...other than differential heating and
convective overturning. Therefore, still am inclined to run low
chance PoPs equally across the forecast area. Given the elevated
instability on Tuesday, a few of the storms that pop up could be on
the strong side. The combination of high instability and weak shear
are suggestive of pulse type convection with damaging winds and hail
being a threat.
By midweek, the model solutions suggest that the upper trough axis
will slowly transition eastward from the Plains into the Great Lakes
region. Corresponding occluding surface cyclone will move from the
Midwest into the Ohio Valley dragging a frontal boundary across the
region. As this boundary pushes eastward, it will encounter ample
instability and moisture to produce a large area of thunderstorms
from the Great Lakes southward into the lower Mississippi Valley.
Strong to severe storms will be possible as this front moves
through. Complex and unresolvable mesoscale interactions at this
time scale preclude defining a particular hazardous weather threat
at this time. We feel those feature will come into clearer view
over the next few days.
The upper trough will slowly head eastward through the southern
Great Lakes on Thursday with a secondary frontal cruising through on
Thursday bringing yet another round of convection to the region.
Upper trough then looks to push on off to the east on Friday with
mid-level height rises taking place and surface high pressure
building back into the region. This should result in dry conditions
from Friday into Saturday.
Highs Wed/Thu will be cooler due to clouds and precipitation and
readings in the upper 70s to the lower 80s are expected with
overnight lows in the 60s. Cooler temperatures look increasingly
likely on Friday/Saturday with highs in the lower 70s and overnight
lows cooling into the lower 50s.
.Aviation (18Z TAF Issuance)...
Issued at 145 PM EDT Sat May 18 2013
Upper low moving across the area continues to bring extensive cloud
cover and scattered showers to our region. Ceilings at the TAF sites
are fluctuating between MVFR and VFR. Anticipate mainly VFR
conditions. Showers should increase in coverage through the
afternoon, with the LEX and SDF areas having the best coverage, at
least for the next couple of hours. Coverage should be less around
BWG. Not seeing any lighting early this afternoon, but cannot rule
it out for later. However, given the cloud cover this morning,
thunderstorms might be more isolated. Showers and clouds should
decrease with loss of heating. Could see a mix of reduced visibility
and/or low stratus form in the pre-dawn hours. Winds will remain
southeasterly through the evening before become light and variable
overnight. Winds will be more south-southwesterly Sunday.