Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
124 am EDT Thu Apr 2 2015
..updated aviation discussion...
issued at 930 PM EDT Wed Apr 1 2015
Current forecast remains in good shape this evening. Regional
radars show a weakening area of convection moving up through
west-central Tennessee. This activity was focused near a warm front,
easily seen by the dew point gradient to the south of it. The
activity over west-central Tennessee will continue to move northeastward
this evening as the warm front make a trek to the north. Based on
the latest trajectories, this activity would start to impact our
western/southwestern zones after midnight local time.
Otherwise, clear to partly cloudy skies will be seen across the
region this evening. This will help temperatures drop into the
Lower-Middle 40s across the NE sections while readings across the
south/southwest only drop into the upper 50s. Temperatures will
gradually warm up overnight as the warm front pushes in from the
south and cloudiness increases. In addition, we'll see surface
winds remain out of the southeast this evening. Winds will shift to
the south and then to the southwest towards morning while picking up
in strength. We expect winds of 10-15 miles per hour with gusts to 20 miles per hour
picking up after sunrise and continuing throughout the day on
Late afternoon model data continues to suggest a threat of very
heavy rainfall with the expected convection over the next few days.
While our hydrological service area is in good shape, soils remain
rather damp and the expected heavy rainfall may result in some
hydrologic issues over the next 24-48 hours. For this reason, we
issued a hydrologic outlook (esflmk product) earlier this evening.
If later forecasts continue to support heavy rainfall, a Flood Watch
may be required in later forecast issuances.
Short term (now through Thursday night)...
issued at 320 PM EDT Wed Apr 1 2015
..strong to severe storms and flash flooding possible
Sfc high pressure and upper level ridging will remain in place for
much of tonight providing dry conditions. With mostly clear skies
for the first half of tonight, temps should drop into the mid 40s to
lower 50s. A weak warm front will lift north through the area just
after midnight turning the temp trend around to a slow warming
during the pre-dawn hours.
For Thursday, expect on and off rounds of showers and storms
throughout the day. The first round will arrive around sunrise
bringing mainly elevated clusters of multi-cells through the
morning. As we get closer to the late morning and afternoon hours,
convection may become more sfc based with strong winds and some hail
possible. Another shortwave is progged to push through thurs mid
afternoon/evening sparking more storms. Although we'll be in a
moisture rich environment with improving wind profiles throughout
the day on Thursday, morning convection and resultant cloud cover
may hinder storm strength thurs afternoon/evening. Right now the
best area for strong to potentially severe storms would be areas
along and west of I-65 near the better wind fields and Theta-E axis.
Thursday night a frontal boundary will settle over the Ohio Valley
and slow as flow aloft becomes parallel to the boundary. This will
result in multiple rounds of showers/storms thurs night with the
potential of training storms along the boundary. With precipitable
water (pwats) amounts rising to 1.4-1.5 (near the upper end of
climatologically precipitable waters for early april), storms will become very
efficient rain producers. The combination of these factors could
lead to some flash flooding where repeated thunderstorms occur.
Considered hoisting a Flash Flood Watch this afternoon, but much
model spread still exists on if/where an axis of very heavy rain
will set up. Stay tuned!
Long term (friday through wednesday)...
issued at 145 PM EDT Wed Apr 1 2015
..strong to severe storms possible Friday afternoon...
Focus in the long term period is on Friday and the potential for
strong to severe storms across parts of the area. The upper level
pattern Friday morning is expected to feature southwesterly flow
with an approaching shortwave trough during the course of the day.
At the surface, a messy pattern is likely, characterized by ongoing
showers and storms associated with a slow moving boundary draped
across parts of the area. A developing surface low to the west will
eventually drag a cold front through during the afternoon and
There remains some timing and placement differences between the
01.12z guidance as to where the surface boundary lies up Friday
morning and how fast the surface low works through the area. The
slower solutions would allow more destabilization/recovery from
morning showers, putting more of the area under a severe threat
later in the day. This idea is supported by the European model (ecmwf)/Gem while the
GFS/NAM are more progressive, which would bring a narrower window of
opportunity for stronger storms for a smaller portion of the area.
What happens Thursday night / Friday morning will dictate the
overall threat for Friday.
During the day, increasing moisture characterized by dewpoints in
the 60s will be advected northward into Tennessee and Kentucky. By
afternoon, MLCAPE values of 500 to 1000 j/kg is forecast to develop
while 0-6km bulk shear values increase to 30 to 50 kts. Soundings
show some low level curvature in the wind profile but most is speed
shear. If the air mass destablizes and as the cold front approaches,
discrete storms would be the primary storm Mode. Main hazard would
be damaging winds and large hail and at this time, south central
Kentucky is the higher threat area. Main timing is early/mid
afternoon to mid evening. As the cold front moves through, the storm
threat would end northwest to southeast during the evening hours.
Saturday into Sunday will feature drying conditions as surface high
pressure builds into the Ohio Valley. Look for seasonably cool but
mostly sunny conditions Saturday with highs mainly in the upper 50s
to near 60. Sunday will be about the same but tack on several
degrees. Readings look to top out in the mid/upper 60s. Should be a
pleasant Spring weekend.
Heading into next week, the upper levels turn more southwesterly
ahead of a digging trough across The Rockies. Southerly return flow
will pull warmer and eventually more moist air into the region by
Monday. A series of disturbances look to rotate through so rain
chances will increase early next week. Too far out to pin down
details but another unsettled pattern looks possible Monday into
Issued at 700 PM EDT Wed Apr 1 2015
As indicated in the short term and long term discussions...several
rounds of convection with heavy rainfall will be possible across the
region over the next 24 to 72 hours. Deterministic model guidance
has various placements for the axis of heavy rainfall. With The
Spreads in the deterministic runs, an ensemble approach suggests
that the axis of heaviest rainfall with this event would generally
be from southeastern Missouri northeastward through southern
Illinois and into southern Indiana. Some of the Kentucky areas
along the Ohio River could also be affected.
Total rainfall amounts of 1.5 to 3 inches look likely between
Thursday and early Saturday morning. While area rivers are all well
below flood stage, the soils across the area remain relatively
moist. This, combined with the potential for repeated and heavy
rainfall may result in excessive runoff. Initially, small streams
and creeks will be the first to see the effects of the rainfall,
then the larger river basins.
Given the current spread in the model data, we have gone ahead and
issued a hydrological outlook (product esflmk) to cover the
potential hydrological problems. If future forecasts converge on a
more stable solution, a Flood Watch may be required with later
Persons in low lying and typical flood prone areas should be on the
lookout for possible flooding problems over the next few days.
Aviation (06z taf update)...
issued at 115 am EDT Thu Apr 2 2015
VFR overnight with light southeast winds ahead of the warm front. While
klvx and upstream 88-ds are showing a 30+ knot low-level jet, there
doesn't seem to be enough of an inversion to focus the shear into an
low level wind shear-worthy layer. Mid-level ceiling will hold, but lower strato-cu
will come into play by daybreak. Not ramping up precip too quickly
as there is a lot of dry air to overcome.
Looks like there will be two waves of precip in the valid taf
period. Warm front will likely lose most or all of its precip to
evaporation, but a mid-level wave close on its heels should bring a
round of showers and storms by mid-morning. Will keep prevailing
VFR, but can't rule out a brief drop to MVFR both in ceiling and vis
during the precip. South winds will crank up to around 15 kt with
gusts close to 25 kt through the middle portion of the day.
Could be a lull in the precip for several hours during the mid to
late afternoon. Will hang on to vcsh in this time range. If we clear
out at all, winds could be a bigger deal than anything else, with a
solid 20 kt and gusts pushing 30 kt. Fortunately with the south-southwest
direction there shouldn't be much crosswind at sdf or Lex.
Next round of precip in the evening should be enough to take US back
to prevailing rain showers. With the planning period extending through the
night at sdf will take ceilings down into MVFR but not
fuel-alternate. Could go lower at least briefly, but not expecting
it on a prevailing basis.
Forecast confidence is moderate in sdf and Lex, but lower in bwg
given the potential for most of the action to remain farther north.