Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
106 PM EDT Sat Aug 29 2015
..updated aviation discussion...
Short term (now through sunday)...
issued at 235 am EDT Sat Aug 29 2015
Early morning water vapor imagery combined with 500 mb heights
showed a middle level trough extending from the Central Plains
northeastward toward eastern Wisconsin...while a broad closed low
was advancing northward from the Gulf into the deep south. The lower
Ohio Valley lied in between...as observed by infrared satellite imagery
which showed mostly clear skies and light winds. Deeper
moisture...cloudy skies...and light radar returns were off to the
The main forecast concern for today is precipitation chances as the
two aforementioned systems track toward each other. The southern
system will draw more moisture into the area...characterized by
dewpoints approaching the upper 60s to near 70 this afternoon. The
northern system will begin to wash out as it slowly advances
eastward. Overall...dynamics are rather weak with these systems but
a weakly or uncapped environment along with peak heating generating
pockets of instability should break out isolated to scattered non
severe showers and storms.
Near term hi-res models show high agreement spatially...temporally
and run to run which leads to a higher confidence forecast for
today. The hrrr has been very consistent showing isolated
showers/storms breaking out during the early afternoon hours as
convective temperatures are reached. The NSSL and Storm Prediction Center WRF solutions offer
similar solutions as well. The highest coverage looks to be along a
Bowling Green to Frankfort line and westward...along the axis of
greater instability and moisture.
Mostly sunny skies this morning will be replaced by scattered
cumulus as well as middle/high clouds...which should make for a partly
to mostly cloudy afternoon at times. Plan on highs to top out in the
middle to broken high clouds will overspread taf sites this morning...and look
for southerly winds to develop. With a weakly unstable air mass and
a couple weather systems nearby...there will be a slight chance of a
shower/storm from early afternoon through the end of the taf period.
However coverage is not expected to be very high so will continue to
leave out of taf. Mostly cloudy skies should lmiitupper 80s. The
increase in dewpoints will make it feel slightly more muggy than
Expecting a drop off in precipitation this evening with the loss of
heating...though will still carry a chance through the overnight
with the weak trough overhead. Sunday still looks to be the better
day for precipitation as we'll have a more favorable environment and
overall dynamics. Precipitation coverage still looks to be
scattered...so kept chances in the 30-40 percent range.
Long term (sunday night through friday)...
issued at 300 am EDT Sat Aug 29 2015
Monday - Wednesday...
A weak upper low near the Ohio Valley may provide enough support for
isld showers/storms mainly along and east of I-65 Sunday night and
Monday. The GFS is most optimistic with this convection and even
this notoriously wet model isn't showing much precipitation.
Tuesday looks to be dry with the next chance for showers/storms
arriving for Wednesday. Middle Range models are different in depicting the
main source for forcing/moisture. The GFS indicates the remnants of
T.S. Erika will be the cause while the European model (ecmwf) suggests a northern
stream trough will be the cause. Either way will carry 20-30% probability of precipitation
Temperatures for early to middle week will be on the increase back to seasonal
norms with highs ranging mainly the middle to upper 80s with a few
locations near 90 tues/Wed.
Thursday - Friday...
Long range models agree that a strong upper ridge axis will build
into portions of the plains/Midwest. Thicknesses will increase over
the Ohio Valley as a result bringing temperatures slightly above normal for
the latter half of the week (upper 80s/lower 90s for highs).
Although a clear trigger is not apparent, some long range signals
suggest a dirty ridge with subtle waves may cause mainly diurnal
convection peaking during the afternoon/evening hours both
thurs/Fri. Thus will have low probability of precipitation both of these days.
Aviation (18z taf update)...
issued at 105 PM EDT Sat Aug 29 2015
Isolated to widely scattered showers have developed in an unstable
airmass this afternoon. Expect coverage to increase slightly along
with a few T-storms as we go through the afternoon/early evening
hours. Any storm will be capable for gusty winds and brief heavy
rainfall. With coverage only expected to be 30% during peak heating,
will continue to only mention vcts, and amend if a T-storm or shower
looks to impact visibilities at a terminal. Expect coverage to
diminish around 6-7 PM EDT with the loss of heating and with drier
air evident on water vapor imagery moving in from the SW. Lex may
hold on an hour or two longer being further east.
Otherwise, expect VFR with scattered-broken sky cover in the 3-4 k feet range
this afternoon and a steady south-southwest wind. Should be relaively quiet
overnight with light and variable winds. Perhaps some brief MVFR visible
at bwg in the more moist airmass, but middle and upper clouds could be
a limiting factor. Expecting a repeat of conditions on Sunday
similar to today.