Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 121446
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1046 AM EDT Sat Jul 12 2014
...Updated Aviation Discussion...
Issued 1030 AM EDT Sat Jul 12 2014
A convective complex is maintaining it intensity across Illinois and
extreme western Indiana. These storms developed during the pre-dawn
hours along a north south axis coincident with an elevated warm
front and a low level jet transporting humid air from the southwest.
These storms will work its way into less humid air across southern
Indiana and especially central Kentucky. The trend for these storms
will be to weaken. However, some showers will, I think, make it into
southwestern Indiana during the late morning hours before
dissipating. Later this afternoon, isolated to scattered
thunderstorms may develop across southern Indiana, especially if
they can initiate along any residual boundaries left over from
earlier in the day. Made changes to the ongoing forecast, to add a
couple of hours of likely showers late this morning across Dubois
County and scattered wording to the rest of southwestern Indiana.
.SHORT TERM (Now through Sunday)...
Issued at 310 AM EDT Sat July 12 2014
Weak high pressure - both at the surface and aloft - continue over
the area early this morning. Because this high pressure is so weak,
though, a weak disturbance aloft was able to generate a line of
light showers that has persisted through the night while moving
southeast over Illinois. As of 3 am EDT, this line was knocking on
the door of our CWA. While likely little more than sprinkles, none
of the models handled this area very well. The only matching
feature lining up with it was a vort lobe in the 12km NAM. With
nothing else even close, will use the NAM guidance for at least the
first part of the forecast. With that in mind, have inserted low
POPs for thunder this afternoon as vertical cross sections of the
NAM show a small area of instability continuing across the CWA
immediately behind this weak short wave. Again, it is not anything
big, but will likely be enough dynamics with it to generate a few
garden variety thundershowers.
Any convection that does develop today will quickly die with the
setting sun, leaving the overnight hours dry. On Sunday, though, a
better chance for thunderstorms returns to the northern half of the
CWA in advance of a cold front poised to make the first day of the
extended forecast period more interesting.
Weak southerly winds will work to return higher dew points north of
the Tennessee and into the Ohio Valley over the next 36 hours. This
will work against the increase in cloud cover to boost temperatures
by a couple of degrees today and Sunday. Look for highs around 90
today and in the lower to mid 90s Sunday. Overnight lows, thanks to
the influx of gulf moisture, will remain in the 70s, save for a few
upper 60s along the I-75 corridor.
.LONG TERM (Sunday night through Friday)...
Issued at 310 AM EDT Sat Jul 12 2014
The long term will start out with upper level ridging building
across the western CONUS as a deepening trough slides southeastward
towards the Great Lakes region. A cold front will lie to the north
of the region across central Indiana. There is still some
disagreement in the models as to how fast this cold front will push
through the forecast area with the GFS being the fastest and having
it clear out by Monday evening. Think this may a bit too fast, so
will lean more towards having the front move through by Tuesday
Given the model differences, there is naturally still some
uncertainty in the precipitation evolution with this system.
Sounding show quite a bit of instability will be present Sunday
night and Monday. In addition, precipitable water values will rise
to around two inches. Scattered to numerous thunderstorms look to
develop overnight Sunday night into Monday as the front nears and a
weak disturbance crosses the area. There is the possibility of MCS
development overnight. The most widespread storms overnight look to
be across southern IN and north central KY. With the high PWAT
values, these storms have the potential to produce torrential
The evolution of storms overnight and how much convective
debris remains on Monday will impact the development of storms
Monday afternoon. However, if we are able to clear out or storms
overnight are less than expected numerous storms may develop Monday
afternoon with strong to severe storms possible. Damaging winds would
be the main threat with the strongest storms.
Scattered storms will continue until the passage of the cold front
late Monday night to Tuesday. Behind this front much cooler and
drier air will push into the region. Dewpoints will fall back into
the mid 50s by Tuesday night. Highs Tuesday and Wednesday will be
much cooler than normal for this time of year, topping out in the
mid to upper 70s. Lows will drop down into the the mid to upper 50s.
We will see a gradual warming for the end of the work week. A few
showers or storms may move into south central KY Friday afternoon,
as a system passes by to the south, but given model differences and
uncertainty will keep the forecast dry for now.
.AVIATION (12Z TAF Update)...
Issued at 650 AM EDT Sat Jul 12 2014
Showers mentioned in previous discussion have all but dissipated
with just some altocumulus remnants remaining, so will remove VCSH
from SDF and BWG TAFs. Will continue with VCTS for LEX and SDF this
afternoon, but confidence in this is waning given current radar and
satellite water vapor trends.
In the absence of any convection this afternoon, the most
significant wx during valid time may be brief MVFR vsbys overnight
at BWG as south winds bring 70+ dewpoints back in for the second
half of the weekend.