Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 220509
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
109 AM EDT Tue Apr 22 2014
...Updated Aviation Discussion...
Issued at 958 PM EDT Mon Apr 21 2014
Did a quick update to adjust PoPs slightly. Initial band of showers
now pushing into the region...mainly along and west of I-65. These
will continue to move eastward and into the eastern
sections/Bluegrass region later this evening. Not overly confident
on thunderstorms overnight...thus have removed the mention in the
forecast. Though...a rumble or two can not be ruled out in a
stronger shower that moves through. Surface front still located out
to our west. Frontal passage still looks on target after sunrise
Tuesday morning. Also adjusted temperatures upward slightly. Much
of the guidance was a little aggressive on the diurnal fall in
temps. Adjusted temps close to the short term multi-model consensus
for the overnight period.
.SHORT TERM (Now through Sunday)...
Issued at 315 PM EDT Mon Apr 21 2014
A cold front will approach the area tonight. Ahead of it, showers
and isolated t-storms will be possible. We`ve had light radar
returns over the area throughout the afternoon, but most of the
precip has been virga with maybe a lucky sprinkle reaching the
ground under the best radar returns. Most precip has remained aloft
due to very dry low levels. However, as we get into the late
afternoon/evening hours, low level moisture will increase allowing
showers to reach the ground. Rain showers will continue on and off
throughout the night with most places seeing at least a sprinkle or
shower. Thunderstorm chances are in question as we`re not seeing a
whole lot of lightning activity downstream. However, most model
soundings do indicate some elevated instability overnight so will
continue an isld t-storm mention. Due to such a dry airmass in
place already, overnight temps will likely fall into the low-mid 60s
as the atmosphere saturates with overall night time lows bottoming
out in the upper 50s and lower 60s.
For Tuesday, the cold front will pass through the region early in
the morning with showers and isld t-storms gradually pushing east of
the area through the morning hours. Skies will turn sunny from NW
to SE during the afternoon hours. Expect cooler afternoon high
temps in the mid 60s to around 70. Also, NW winds will be a bit
gusty (up to 25 mph gusts) in a tight pressure gradient behind the
frontal passage Tues afternoon.
Tuesday night expect clear skies with high pressure sinking south
into the Ohio Valley. The cold airmass and decent rad cooling
conditions will likely result in temps dropping into the mid 30s to
lower 40s across the area. Although low level moisture is in
question, some locations over the Bluegrass that drop into the mid
30s may experience patchy frost. Sheltered locations would stand
the best chance at any frost formation.
.LONG TERM (Sunday night through Friday)...
Issued at 315 PM EDT Mon Apr 21 2014
Wednesday - Thursday...
Ridging from a surface high centered over the Great Lakes will be in
place over the Ohio Valley Wednesday, sliding east of the area
through Thursday. Meanwhile, a progressive upper ridge will build
over the area, then slide east by Thursday evening. This pattern
will result in dry conditions, with temperatures making a recovery
from Tuesday`s cold front. Highs on Wednesday look to range from 63
to 68, with the warmer temps being across the west closer to the
building upper ridge/increasing thicknesses. Wednesday night should
be slightly milder than Tuesday night, with lows mostly in the low
and mid 40s. A few of the cooler eastern valleys might drop into the
upper 30s closer to the surface high. A few of the models try and
depict some very light rain associated with isentropic lift over a
warm front across southeastern IN, but will not bite on these very
small chances attm. Thursday will see a nice increase in
temperatures as thicknesses rise and a steady southerly flow takes
hold. Mainly upper 70s above guidance looks like a good call under
this pattern, as has been the case recently.
A progressive shortwave and its associated surface low will cut
across the upper Midwest and into the Great Lakes Thursday night.
Meanwhile, the trailing cold front will approach our region. Look
for pretty solid coverage of showers and scattered thunderstorms
Thursday night ahead of this feature. Have continued mention of
likely Pops. Looks like the best dynamics will scoot to our north so
not concerned about severe potential at this time. A few of the
stronger storms may be able to produce some gusty winds, brief heavy
rain, and cloud to ground lightning.
Friday - Saturday...
The cold front passes through quickly and have all of the
precipitation chances east of our area by dawn on Friday morning. We
look to go into a fairly benign zonal flow, gradually transitioning
to shallow northwest flow by Saturday morning. Will go dry through
this period, although very small chances of a shower or storm can`t
be ruled out as a moisture starved front drops south toward the Ohio
Valley. Look for highs in the low and mid 70s on the warm side of
this boundary as we head into the weekend.
Saturday Night - Monday...
Complicated forecast to end the weekend and start the new week as
forecast models diverge in solutions. The problem appears to lie in
how amplified an omega block gets over the central CONUS and its
impact on the resulting downstream flow over our region. The latest
ECMWF seems to have some support from the GFS ensemble mean in less
amplified flow, which would allow the frontal boundary to stall out
near our region as it becomes nearly parrallel to the upper flow.
The GFS is much more amplified with the omega block and the
downstream flow, allowing the front to progress through our area and
well south. The ECMWF solution would be much warmer/wetter, while
Pretty much have to pick a path at this point. Will side with the
warmer/wetter ECMWF and its supporting cast. This will lead to
gradually increasing shower and storm chances late Saturday night
along and north of the Ohio River, with several waves riding along
this boundary through at least Monday. As the front hangs out or
very slowly slides south across our area, expect temperatures to
become slighly cooler each day. Highs Sunday should generally be in
the mid/upper 60s, with Monday in the low to mid 60s.
Please note that if it becomes more clear that the more amplified
solution will work out. Look for big changes to the forecast in the
direction of cooler/drier during this time.
.AVIATION (00Z TAF Update)...
Issued at 100 AM EDT Tue Apr 22 2014
Approaching cold front currently extends from roughly LAN to LAF to
just south of STL, with scattered showers out ahead of it. Believe
that any precip through the overnight period will be intermittent
enough that VCSH will cover it. The timing of fropa so close to
sunrise is problematic, as that is favorable to trap enough
low-level moisture to bring down ceilings. Took all three central
Kentucky TAF sites down to high-end MVFR after 09-10Z, with fropa
slated for 12-14Z.
Behind the front, NW winds will pick up, with MVFR ceilings lowering
into fuel-alternate. Confidence is limited because these lower
ceilings have yet to develop upstream, but this forecast is fairly
well in line with GFS and not as pessimistic as the IFR ceilings
advertised by the overly bullish NAM.
As mixing deepens in the afternoon, expect ceilings to lift and
scatter out by around 18Z. NW winds will be a bit more frisky, with
gusts near 20 kt through the latter half of the afternoon.