Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 202325
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
724 PM EDT MON MAY 20 2013
...Updated Aviation Discussion...
.Short Term (Now - Tuesday Night)...
Issued at 300 PM EDT Mon May 20 2013
Low-level debris clouds from convection across the Middle
Mississippi Valley overspread the Lower Ohio Valley early this
morning. This caused a slow warming trend through this afternoon.
However, clouds are breaking up with breezy south-southwest winds
aiding in mixing. Still some time for warming, with mid and upper
80s being common. Louisville could still make a run at 90, but
believe the record of 91 (1934) will remain. Latest guidance and
aircraft soundings continue to illustrate the ridge aloft has us
under a cap. A very thin line of light rain showers and isolated
thunderstorms has developed across eastern Illinois. This will have
a tough time reaching us this afternoon, but could skirt our
northern most counties this evening if it holds together. How much
the cap weakens between now and then will be the determining factor.
The best chance will be farther north in Indiana, where the cap is
Heading into tonight and Tuesday, the upper-level ridge over our
area will break down as the central CONUS trough axis moves into the
eastern Plains and upper Midwest, where convection is expected to
develop this afternoon and evening. This upstream convection should
progress eastward into the Ohio Valley late tonight. Latest guidance
indicates this activity should approach the forecast area in a
weakening state during the pre-dawn hours. However, westerlies are
increasing and the ridge aloft will be weakening, so some storms may
hold on. This convection should wane early Tuesday, but will leave
plenty of mesoscale boundaries around the region.
Additional development is expected in the afternoon and evening hours
Tuesday, to our west along a frontal boundary and over our area with
the remnant mesoscale boundaries. SPC has placed areas along and
west of a Bowling Green to Frankfort line in a slight risk. With
increasing 0-6km bulk shear of around 25-35 knots and strongly
unstable airmass, multicell clusters would be the convective mode.
Heavy rainfall and cloud to ground lightning, along with isolated
large hail and damaging winds will be the main threats. So, severe
storms are possible Tuesday afternoon and evening. The convection
will push east and weaken in strength, but some models are
indicating another round of showers and thunderstorms late Tuesday
night. Will not get too fancy with the PoP forecast given great
uncertainty among models with timing of these events.
As for temperatures, we will remain rather mild tonight, with temps
ranging from the middle 60s to lower 70s. Highs Tuesday will be
cooler with the potential outflow/convection in the morning, debris
clouds, and the ridge axis having shifted east. Middle to upper 80s
and humid conditions are on tap. Temperatures Tuesday night should
remain mild, as temperatures drop into the middle and upper 60s.
.Long Term (Wednesday through Monday)...
Issued at 255 PM EDT Mon May 20 2013
Unsettled weather is expected Wednesday and Thursday, in association
with a slow and complex frontal passage. The best chances for precip
and the strongest storms will be on Wednesday, with a deep
southwesterly moisture feed across the Ohio Valley. Not too excited
about SVR potential as instability will be the limiting factor and
our best guess is that precip will already be ongoing at daybreak.
Still, it merits likely POPs through the day on Wednesday.
Much lower POPs on Wednesday night as the deep moisture will be
shunted to our south and east. Will see high-end chance POPs again
on Thursday as the positively-tilted upper trough swings through the
Great Lakes and the Ohio Valley, and decent low-level moisture
remains in place. Best rain chances will be over the Bluegrass,
closer to the more favorable dynamics. This activity will be
shallower and less intense than any storms we see on Wednesday.
Temps through Thursday will be near normal by day, and a few degrees
above normal by night due to a moist air mass and abundant cloud
Upper trough axis will slip to our east by Thursday evening, opening
the door for a very mild area of Canadian high pressure.
Picture-postcard weather is on tap for Friday and Saturday with
plenty of sunshine, temps a few degrees below normal, and low
Temps will recover to near normal during the latter half of the
holiday weekend. However there is some question as to where a
backdoor front will set up, and how efficiently that will focus
precip. Therefore will include a slight chance of showers and storms
from Saturday night onward.
.Aviation (00Z TAF Issuance)...
Issued at 720 PM EDT Mon May 20 2013
The main TAF concerns for this forecast period will be marginal LLWS
overnight tonight and then timing of convection for tomorrow. A
potent low pressure system over the Plains states will continue to
make slow progress eastward toward the Midwest through tomorrow.
Ahead of this system, a 30-35 kt LLJ will get going tonight.
However, according to model soundings, the atmosphere should stay
mixy with sfc winds staying in the 7-11 kt range overnight and no
real good inversion becoming established. Therefore, decided to
leave LLWS out of the TAFs at this point.
For tomorrow, waves of convection are expected to push through the
TAF sites. According to the latest high-res model data, the first
wave should occur during the mid morning to early afternoon hours
with a second wave occurring during the evening hours. Have tried
to indicate these 2 different waves especially in the longer SDF
TAF, but timing will likely need to be adjusted. The strength of
these waves is in question as well. We`ll likely see a broken line
of showers and t-storms during the first round tomorrow
morning/early afternoon, but just how intense this first wave will
be is still unknown. Will need to monitor trends overnight as
convection heads this way. Have remained on the conservative side
in the TAFs right now with VFR conditions, VCTS, and wind gusts
around 20 kts. Look for further refinements to t-storm timing and
strength in coming TAF packages.
Sfc winds will generally remain out of the SSE this evening and then
shift to SSW during the early morning hours. Wind gusts of 18-22
kts will be possible outside of any t-storm tomorrow afternoon.