area forecast discussion
National Weather Service Louisville Kentucky
914 am EDT Tuesday may 21 2013
Forecast update...
issued at 905 am EDT Tue may 21 2013
Precip from the overnight mesoscale convective system is dissipating over central Kentucky
as we speak. Expect to see a break in the action for the next few
hrs, but we remain in a juicy air mass and a boundary-rich
environment. No changes to the afternoon forecast for scattered
shower/T-storm activity, nor the potential for these storms to pulse
up to marginal severe levels. Expect to update text and point-and-click
within the next hour, after we give this dissipating trend a little
more time.
&&
Short term (today - wednesday)...
issued at 251 am EDT Tue may 21 2013
A difficult and complicated forecast as a storm system approaches
from the plains...
Early this morning a long line of showers and thunderstorms
stretched from Michigan to Oklahoma. This line will weaken
considerably as it pushes to the southeast away from the jet energy
that was supporting it earlier. Will have likely pops in southwest
Indiana before dawn, and just chance pops across the area after dawn
as the line continues to fall apart.
We should see a break in the action for several hours before
redevelopment takes place this afternoon. Widespread clouds and
scattered morning showers will prevent US from destabilizing fully,
but some strong storms could still fire in the warm and juicy
atmosphere. It will be interesting to see where the outflow boundary
from this morning's convection ends up, as that could serve as a
focus for renewed convection. As for any severe weather, it looks
like marginally severe hail is the primary threat, with marginally
severe wind gusts a secondary threat. Tornadoes look unlikely,
though local boundary intersections will need to be watched for the
possibility of a brief spin-up.
We may see another break this evening as afternoon convection
wanes. Renewed shower and thunderstorm activity is then expected
during the late night hours as a low level jet revs up from Memphis
to Fort Wayne. A strong low level inversion is forecast to set up,
so small hail would be the main threat with any stronger storms.
On Wednesday the cold front that has been producing the rough
weather across the nation's midsection recently will draw nearer
still to the lmk cwa, reaching the lower Wabash and middle
Mississippi valleys by evening. Additional shower and thunderstorm
development can be expected on Wednesday, particularly in the
afternoon. Once again, widespread morning clouds and scattered
showers will prevent US from destabilizing much. A few strong
storms will still be possible, though, with marginally severe hail
as the primary threat...marginally severe wind gusts secondary...and
tornadoes unlikely other than where local boundaries meet.
Temperatures are also very challenging given the clouds and rain.
For today will shoot for highs mostly in the middle 80s, though that
may be generous if we don't get some sun this afternoon. Muggy lows
tonight will be in the middle and upper 60s once again. Model data
vary widely on how warm we will get on Wednesday. For now will go
with highs around 80.
Long term (wednesday night through monday)...
issued at 245 am EDT Tue may 21 2013
Upper air pattern at 500mb late Wednesday is forecast to become
somewhat amplified with thin ridging across the northern plains
sandwiched between a deep closed low off the Pacific northwest coast
and a slow moving trough centered over the western Great Lakes. This
Great Lakes trough will deepen, with its base swinging across the
commonwealth by early Friday morning. Showers and thunderstorms
Wednesday will diminish during the evening hours and overnight as
veering mid-level winds will bring lower pwats east from Missouri.
Pwats of 1.6 inches earlier Wednesday will diminish to under one
inch by dawn Thursday. Still, will maintain at least a chance of
showers and an isolated thunderstorms overnight through Thursday
morning.
By late Thursday, a surface trough will move southeast of the Ohio
River, switching our winds to the northwest and eventually bringing
in substantially cooler air. Despite less available moisture, the
presence of the 500mb trough axis and a diffuse frontal boundary
will not preclude scattered low topped showers and isolated thunder
Thursday afternoon.
High pressure of Canadian origin will build southward along the
Mississippi Valley Friday, sliding east towards Ohio by Sunday
morning. Distinctly cooler air with low dewpoints will arrive by
Friday morning and continue through the entire weekend. Despite
clearing skies, Friday will stay quite cool and refreshing, with
highs only rising into the lower 70s at best. We can expect several
cool nights from Friday to Monday mornings with lows initially
around 50, rising into the upper 50s by Monday morning. Under mostly
clear skies, highs Saturday and Sunday will remain in the 70s.
By late Sunday, a slowly progressive 500mb wave pattern will have
moved the Great Lakes trough eastwards over New England. Ridging
will begin to build across the Southern Plains, with northwest flow
developing across the lower Ohio Valley. Moisture will begin to pool
late Sunday along and north of a warm front expected to develop
along a line from Iowa southeast through northern Kentucky. Will
introduce a chance for isolated to scattered convection late Sunday
and Monday, especially across our northern counties. Humidities will
also rise by Monday as southwesterly surface winds develop.
&&
Aviation (12z taf issuance)...
issued at 644 am EDT Tue may 21 2013
A complex and, as a result, low confidence forecast. A storm system
approaching from the plains will throw unsettled weather at US
throughout the taf period.
A line of storms is advancing into central Kentucky at 1045z. Have
used radar animations to bring this line into the taf sites over the
next few hours. The line should be weaker by the time it gets as
far east as the airports, and should be mostly in the form of
showers with isolated thunderstorms and rain. Sdf stands the best chance at receiving
the most adverse weather from this line.
Once that clears out, we should have several quiet hours from mid
morning through mid afternoon. The amount of destabilization we'll
be able to realize this afternoon is in question, given the morning
clouds and showers. Nevertheless, will forecast scattered
thunderstorm redevelopment during the heat of the day. Winds may
get a little gusty this afternoon as well...especially if we can get
some decent sunshine.
Another quiet period is expected this evening as the afternoon
convection wanes. Then, another round of showers and storms should
break out late tonight as the plains storm system continues its
approach and a low level jet Cranks up from Memphis to Toledo.
Needless to say, with all this going on, changes will certainly be
made to the tafs over the course of the day.
&&
Lmk watches/warnings/advisories...
in...none.
Kentucky...none.
$$
Update...........Ras
short term.......13
long term........jsd
aviation.........13
