Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 292248
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
648 PM EDT Wed Oct 29 2014

...Updated Aviation Discussion...

.SHORT TERM (Now through Thursday Night)...
Issued at 250 PM EDT Wed Oct 29 2014

The synoptic pattern this afternoon features a general trough across
much of the central and eastern CONUS, with an upstream ridge
located across the western seaboard.  This regime will substantially
amplify through the short term period, as a deep trough begins to
build into the Ohio Valley by tomorrow night.  This pattern will
make for declining temperatures along with increasing chances of
precipitation by the end of the short term period into the long term
section below.

Despite slightly cooler temperatures than we`ve seen the past few
days, this afternoon has turned rather pleasant as temperatures have
climbed into the upper 50s and 60s under mostly sunny skies.  Have
continued to watch clouds to the north attempt to push into the
region, but the moisture with these clouds is shallow.  Therefore,
our relatively deep mixing today will likely continue to mix out
this deck as it tries to push in from the north.  Have upped clouds
a bit for this evening, but still think we will see only partly
cloudy skies at worst.

Otherwise, the main story for tonight will be cool temperatures and
the potential for frost formation.  Surface ridging will slide into
the Ohio Valley towards Thursday morning, resulting in winds going
calm. Given the expected clear skies, it will be rather favorable
conditions for radiational cooling.  Therefore, have gone on the
lower end of guidance, which puts lows in the mid and upper 30s.
These temperatures will likely result in some patchy frost
formation, especially in the more sheltered locations.  Was on the
fence on whether or not to issue a Frost Advisory, but in
coordination with neighboring offices, it was determined that the
frost would not be widespread enough to warrant an advisory at this
time.  The evening shift will continue to monitor the latest data
coming in and adjust the forecast as necessary.

Thursday will be another pleasant late October day, as quite a bit
of sunshine rules for at least the first part of the day.  However,
by the late afternoon into the evening hours, clouds will begin to
increase ahead of a quick-hitting shortwave diving through the mean
trough aloft.  Highs on Thursday will be in the upper 50s and lower
60s.

The precipitation associated with the incoming shortwave trough and
synoptic forcing will mostly hold off until after 03Z Friday.
Forecast soundings show pretty deep moisture up to around 600mb with
this system, and cross-sections show good ascent within the
surface-600mb layer.  Therefore, have upped pops substantially (now
in 40-60% range) for late tomorrow night into Friday morning as this
forcing coupled with more-than-adequate moisture should lead to good
coverage of some light showers.  With the increasing cloud cover and
precip, overnight lows will be warmer than tonight, with readings
expected in the low to mid 40s.

.LONG TERM (Friday through Wednesday)...
Issued at 311 PM EDT Wed Oct 29 2014

...Blast of Cold Air Coming This Weekend. Wintry Precipitation and
Sub-Freezing Temperatures on the Menu...

Friday - Saturday Morning...

Several issues to talk about for the long term period of this
forecast. Will begin Friday with coverage of showers increasing from
NW to SE across the CWA as the left exit region of a 120 knot upper
impulse dives into the Ohio Valley. The strength of this impulse
coupled with the depth of moisture above 700 mb has increased
confidence in precipitation enough to warrant mention of scattered
to numerous coverage, generally in the 40-60% range east of I-65.
Wouldn`t be surprised if coverage needs to be increased more as this
will be a low QPF/higher coverage setup. Will also have to watch for
the potential for some thunder in our NE as low and mid level lapse
rates quickly steepen Friday afternoon and yield a somewhat unstable
thermal profile. One thermal profile suggested by the NAM would even
support small hail/graupel due to the weak instability and very low
freezing levels (this could be mistaken for sleet given the imminent
cold surge behind it).

Temperatures are expected to find their way to right around the 50
degree mark by midday Friday, before strong cold front associated
with the upper impulse swings through and temperatures begin to fall
late Friday afternoon/evening. Time heights show vertical theta
surfaces, suggesting that NW winds will become quite gusty behind the
frontal passage, so can`t rule out some gusts up between 25-35 mph.

Cold air continues crashing after sundown with steep low and mid
level lapse rates. Thermal profiles begin to support the potential
for snow to mix in with rain, despite surface temperatures still
above freezing before midnight. Saturation up to 700 mb gets you
very near the -7 to -10C threshold, so ice crystals will be marginal
for pure snow once surface temps drop enough. As cold air continues
to crash in toward dawn, it can`t be ruled out that some of our
eastern counties might change over to all snow or snow grains. We
will start losing deeper moisture by this time so again it will be a
race for the cold air to catch up to the moisture. Overall, do think
we will see some wintry precipitation Friday evening and overnight,
however don`t really expect much in the way of impacts as surfaces
and surface temperatures will be above freezing for much of it. Will
have to continue to monitor however as the somewhat convective
nature of these showers could be quite intense at times. Best
coverage of any wintery precipitation should be along and east of
I-65, especially toward the Bluegrass.

Will hold onto a few lingering wintery showers in our far east until
around dawn on Saturday, before surface high starts to take over.
There has been a shift in model data toward colder temps by Saturday
morning, with many suggesting sub-freezing by then. Will hold off on
going just that cold yet as NW cold advective component will still
be quite steady and turbulent mixing may keep things a bit milder
(relatively speaking!). Will go with around freezing.

Saturday - Sunday Morning...

We try and dry out on Saturday with surface high pressure slowly
building in. Slower trend may end up keeping showery activity and
low cloud lingering in our east a bit longer than anticipated. Either
way, temps aren`t getting out of the 40s for highs, some in our NE
may barely get into the low 40s.

The surface high centers itself over our area by early Sunday
morning setting the stage for our coldest air of the season and lows
likely in the upper 20s. Watch for Freeze headlines coming in future
forecasts should current data hold.

Sunday - Wednesday...

A classic warming pattern takes hold the rest of Sunday into Monday
night as upper ridging takes over and we get on the back side of
surface high pressure. The resultant rise in heights/thicknesses
combined with southerly flow and near full sun will allow temps to
rise into the upper 40s and low 50s Sunday, finding their way to the
56-61 range by Monday. Lows will also recover from the 30s on Sunday
night to the 40s on Monday night.

The next precipitation chances come Tuesday into Wednesday as a
frontal boundary becomes nearly stationary, parallel to the
southwesterly upper flow. This SW flow through a deep layer will
advect plenty of moisture along the boundary and could set the stage
for more widespread light rain. Will continue mention of solid
chance for rain from Tuesday through Wednesday.

&&

.AVIATION (00Z TAF Update)...
Updated at 645 PM EDT Wed Oct 29 2014

Prevailing VFR conditions will continue for the valid TAF period.
This afternoon`s wind gusts are already starting to lay down with
the loss of heating, so may even be able to initialize with wind
speeds 6 kt or less. Light and variable winds overnight may open the
door for fog formation around daybreak at BWG and LEX. Even the most
pessimistic model guidance suggests dewpoint depressions of at least
3-4 degrees, so not intending to hit it very hard. Will continue to
mention 6SM visibility as a heads-up at both BWG and LEX, although
the former has the better chance of actually going MVFR. Light winds
and increasing cirrus on Thursday.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...NONE.
IN...NONE.
&&

$$

Short Term........KJD
Long Term.........BJS
Aviation..........RAS





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