Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 201744
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
144 PM EDT Wed May 20 2015
...Updated Aviation Discussion...
Updated 955 AM EDT Tue May 19 2015
Tweaked temps/dewpts/sky cover to match obs with this update.
Precipitation still looks to arrive west of I-65 this evening with
most locations likely receiving some scattered showers tonight.
Isolated t-storm chances still look possible over south central KY
this evening. High temps should range through the 60s to around 70
over southern IN and north central KY. Portions of central KY and
south central KY should warm into the mid to upper 70s.
.SHORT TERM (Now through Thursday)...
Issued at 255 AM EDT Wed May 20 2015
Surface high pressure over the Great Lakes continues to ridge into
the Ohio Valley, maintaining cool and dry conditions. Upper
shortwave trof working its way through the Plains has triggered
precip all along its axis. Main challenge is how this system will
impact the Ohio Valley late this afternoon and tonight.
Models converging on a solution that yields at least light QPF for
the entire forecast area, even though this disturbance will be
shearing out as it approaches. Another major source of uncertainty
is the dry low-level air mass, and how much precip will be lost to
evaporation. Will stay middle of the road with timing, ramping up
chances west of I-65 late in the afternoon. POPs tonight will be
limited to around 50% due to low-level dry air, and the very narrow
window in which the soundings actually saturate. By 06Z the deeper
saturation is lost and POPs should tail off from there. More
moisture available along the TN border where a front could wave
northward, therefore we will continue to include a slight chance for
embedded thunder roughly along and south of the Cumberland Parkway.
Could see quite a spread in temps this afternoon, with southern
Indiana just reaching the mid 60s while south-central Kentucky could
be pushing 80. Cloud cover is the most likely factor that would
dictate a sharp cutoff in between.
Once this system scoots past after midnight tonight, it will leave
behind a trapping inversion and just enough low-level moisture that
stratus will linger well into Thursday morning. This will keep temps
unseasonably cool on Thursday afternoon with highs struggling to get
out of the 60s.
.LONG TERM (Thursday night through Tuesday)...
Issued at 252 AM EDT Wed May 20 2015
The synoptic pattern at the start of the long term period will
feature northwesterly flow aloft. However, this regime will quickly
transition to southwesterly flow aloft by the middle of the upcoming
weekend as a ridge amplifies over the Ohio Valley and pushes toward
the eastern seaboard. The southwesterly flow will bring a more
active weather pattern to the region as several weak waves eject
through the flow into the Commonwealth through the end of the period.
The first part of the long term period will feature seasonably cool
and dry conditions. High pressure will build into western and
central Kentucky by Friday morning, helping winds go calm. The main
limiting factor to ideal radiational cooling conditions may be some
lingering cloud cover across the Lake Cumberland and northern
Bluegrass regions Thursday night. If these clouds hang on longer
than expected, current forecast temps will be too cool. However,
assuming these clouds do clear out, overnight lows will drop into
the 40s. Even a few upper 30s readings cannot be ruled out in the
usual cold spots!
850mb temps of around 7C into Friday afternoon will support
continued cool conditions, despite the expected sunshine. High sun
angle this time of year will help compensate somewhat, but still
think highs will be cooler than average with readings expected in
the low to mid 70s.
The aforementioned surface high pushes east of the region Saturday
into Sunday, as upper-level ridging amplifies over the Ohio Valley.
This will begin a stretch of more active weather, as deep moisture
returns to the Ohio Valley on southwesterly flow aloft, along with
warmer temperatures. Unfortunately, timing individual waves within
this regime is difficult at this timeframe, so will continue with
generally broad-brushed pops of 30-50% Sunday through at least
Tuesday of next week. There should be diurnal minima overnight with
this convection, so have lowered pops a bit in the overnight
periods. Any organized severe weather threat continues to look low,
as weak tropospheric flow should limit the organization of
updrafts. However, depending on the timing of the individual waves,
moderate instability may build in the afternoons which could support
some robust pulse cells. It appears this better chance for higher
amounts of instability will come from Monday onward next week as
low-level moisture pools into the region. Temperatures will be
highly dependent on convective timing/coverage, but a general
warming trend from the 70s on Friday into the low/middle 80s late
this weekend into early next week seems likely.
.AVIATION (18Z TAF Update)...
Updated at 140 PM EDT Wed May 20 2015
An upper level shortwave and sfc low pressure will move through the
region tonight bringing rain, low clouds, and light fog. Winds will
generally shift from NE to N to NW throughout the TAF period.
BWG...Showers and perhaps a rumble of thunder will enter the area
around 2030Z. Felt thunder chances were too minimal to enter in the
TAFs. Flight conditions should go down to MVFR around 1Z in light
rain showers. Scattered showers should be done by around 4Z, but
sprinkles or isld light rain showers may persist through tomorrow
morning. There is a pretty strong IFR signal from multiple models
from 4z-15Z. After 15Z expect improving conditions during the day
tomorrow but likely not returning to VFR until late in the day.
SDF...If rain showers hold together, they may impact SDF starting at
2130Z. SDF is located in a drier airmass than the other TAF sites
and so precipitation may be slow to reach the ground. Flight
conditions should go down around 5Z to MVFR below 2 kft. Sprinkles
or isld light rain showers may continue through tomorrow morning.
Cigs will improve throughout the day tomorrow with VFR conditions
returning by late afternoon.
LEX...Light rain showers should arrive in the LEX area around
2330Z. The NAM indicates that a better chance for rain at LEX may
occur around 5Z with passage of the sfc low pressure to our south.
Flight conditions will likely go down to low end MVFR or high end
IFR around 5Z as well. Sprinkles or isld light rain showers may
persist through tomorrow morning. After 13Z expect improving
conditions during the day tomorrow but likely not returning to VFR
until late in the day.