Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 190356
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1156 PM EDT Mon Aug 18 2014
Issued at 1150 PM EDT Mon Aug 18 2014
Area observations depict fog already building in despite any clouds
streaming through. Visibilities have been decreasing, especially in
valleys and near rivers. An SPS has been issued for areas of fog and
will continue to monitor throughout the night for a possible upgrade
to Dense Fog Advisory if conditions continue to deteriorate.
Issued at 1010 PM EDT Mon Aug 18 2014
Sufficient low-level moisture in place, along with a weak surface
gradient, set the stage for valley fog that showed up quickly on
some area webcams as the sun set. Will add patchy fog to the grids
in the typically prone areas. Otherwise we remain on track.
Issued at 855 PM EDT Mon Aug 18 2014
Series of weak disturbances in low-amplitude upper flow will
continue to cross the Ohio Valley. Most substantial impulse is the
wave moving across the Tennessee Valley, and while the more
widespread convection is suppressed well south of Nashville in parts
of middle Tennessee and northern Alabama, we have seen isolated
T-storms develop near the Kentucky/Tennessee border on the northern
periphery of the cirrus shield. Isolated showers continue to show up
at times over southern Indiana and into the Bluegrass, but believe
there is minimal thunder potential with this activity.
Expect isolated shower potential to continue overnight with the best
POPs over the Bluegrass and south-central Kentucky, and lesser
chances in between. Will see POPs diminish slightly from west to
east after midnight as impulses aloft push east of the area. These
changes are subtle and do not reflect enough of an adjustment to
previous text forecasts to merit an update. Point-and-click
forecasts and higher-resolution products have already been updated
to account for these minor tweaks.
.SHORT TERM (Now through Tuesday Night)...
Issued at 254 PM EDT Mon Aug 18 2014
Convergence along a lingering tongue of low level moisture (H85)
currently resides west to east along the I-64 corridor. Scattered
light rain showers continue to slowly move east along this boundary,
with the best coverage over the Bluegrass. This tongue will retreat
through the evening hours, and will reduce coverage over the area as
a result. Will also watch a secondary area of potential convection
along the TN border where models have tried to depict scattered
coverage this evening. Latest visible shows partial clearing in this
area with an instability axis creeping northward. Convection has
fired in western TN in response to a weak disturbance, and this will
slow move ENE over the next few hours. It will be a close call
whether these showers/storms scrape our southern counties so will
mention a chance into tonight.
Otherwise, temperatures have found their way into the upper 70s and
low 80s at this hour, and may struggle another couple degrees higher
in areas that see some partial clearing.
Have lingering small chances for a shower or storm diminishing
across south central and eastern KY overnight, with most areas
certainly staying dry. Should see diminishing cloud cover across the
NW half of our CWA toward dawn, which could allow for fog to
develop. Have included patchy mention in the forecast.
Tuesday is expected to be dry to start the day, however isolated to
widely scattered chances will be possible in the afternoon hours as
we destabilize. Overall, the lack of a trigger should keep things
pretty quiet for most people through the afternoon hours. Will watch
to the north as storms develop in response to a wave dropping
sliding across the lower Great Lakes. These storms are expected to
organize and could slide southeast across southern IN to north
central KY around or after 00z. Should this scenario be realized,
the potential for stronger storms will be there as 0-6 km bulk shear
will be on the order of 30-35 knots, along with moderate to strong
instability. Strong gusty winds, hail, and cloud to ground lightning
would be the main threats for this scenario.
Additional storms could slide into our western and southern CWA in
the best of stronger westerlies aloft, and along the periphery of
the building upper ridge. This environment will be more unstable and
may provide the potential for strong storms into the Tuesday night
Expect lows to be in the upper 60s to around 70 tonight, with highs
on Tuesday in the mid and upper 80s. Lows Tuesday night will be
slightly milder in the lower 70s.
.LONG TERM (Wednesday through Monday)...
Issued at 255 PM EDT Mon Aug 18 2014
...Unsettled pattern through Thu...
...Prodigious H590+ ridge moving in from Dixie Alley (heat is on)...
Well...another anomalous low pressure system across the Great Lakes
will allow for spokes of vort maxes to rotate through the Ohio
Valley. The OH Valley will be very unstable on Wed with MLCAPES of
2500 j/kg to 3500 j/kg, LIs of -6 to -10 C, and PWATs between 1.5
inches and 1.85 inches. Appears there will be one line of SHRA/TSRA
early Wednesday, then some temperance before more storms in the
21-06z time frame especially with additional forcing from an upper
level jet diving down from the Dakotas on Wednesday.
If cloud cover thins out then diurnal heating, decent low to
mid-level lapse rates work together to have rather organized strong
storms with heavy rainfall. Tack on 30+ knots of 0-6km shear, would
not be surprised if SPC outlooks a portion of the area on Wed.
However, once again cloud cover and the potential for ongoing
morning activity could limit storm strength/intensity.
Strong short wave will be pushing across the Buckeye State into WV
on Thu am. For CWA, the driving mechanism for rain chances will be
shortwaves riding along the periphery of the amplifying ridge over
Pelican State thru the Natural State. H8 thru H5 temps will be
rising with 595 dm high bringing torrid and sultry airmass
northward. Models are slowly starting to congeal for that the first
real heat spell could be upon us.
With vegetation so lush across the area, I am taking 2-3 degrees off
of what I expect for afternoon high temperatures. So by Monday going
97 BWG and 96 SDF and 94 LEX. Heat indices will need to be watched
as the time approaches as dewpoints also look to linger near the 70
degree mark. Lows should be a few degrees above normal, ranging from
70-76. Apparent temps will be above 100, especially for the SWRN
Have hard time justifying pops Sun-Mon with +11C at H7, and am very
against hint of the GFS to bring weak front from IL into IN. The
ridge will win and this front will dissolve. The dog days of summer
are finally going to be upon us.
.AVIATION (00Z TAF Update)...
Issued at 720 PM EDT Mon Aug 18 2014
Sloppy surface pressure pattern remains in place over the Ohio
Valley. Main challenge overnight will be fog/stratus potential, as
the air mass is quite juicy and mixing will be limited. GFS MOS
guidance suggests tonight`s issue will be visibility restrictions,
as opposed to last night`s low ceilings. Forecast soundings show
fairly shallow low-level moisture and not much wind, which backs up
the idea of a fog event rather than stratus, which the NAM hits
quite hard in its usual pessimistic fashion. Will forecast
visibilities quickly going to MVFR around midnight, with BWG and LEX
dropping into IFR toward daybreak.
Will again be slow to scour anything out in the morning as the
gradient remains quite weak. Visibilities should improve to VFR late
in the morning, with diurnal cu and west winds less than 10 kt for
the balance of the day.