Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
849 PM EDT Tue Apr 21 2015
Issued at 848 PM EDT Tue Apr 21 2015
Have made some major changes to the forecast this evening to account
for the latest radar trends and trends in the hi-res guidance. A
line of showers with a few embedded rumbles of thunder has entered
portions of southwestern Kentucky this evening. Kentucky mesonet observations
show pretty widespread gusts of 35 to 40 mph with this activity
across pah's cwa, even in the weaker parts of the line. This is due
to the very well-mixed and dry boundary layer, aiding in evaporative
cooling and associated downward momentum Transfer in the
shower/storms. Have been issuing spss to cover this 35 to 40mph
wind possibility. Recent trends suggest this activity is beginning
to wane with the loss of daytime heating, so gusts around 40 mph
should become less likely over the next hour or two.
Otherwise, this weakening batch of showers will continue to push
east across southern Kentucky early tonight. Thereafter, there will
likely be a lull in the activity. Have reduced pops overnight to
account for this. A cold front, now across portions of northern MO
and central IL, will approach towards dawn. Shower activity should
begin to increase once again ahead of this feature. Am hesitant to
nudge pops up too high given the very dry low-levels and guidance
overestimating precip this evening, so will leave pops in the 50-60%
range. Updated products already out.
Short term (now through Wednesday night)...
issued at 254 PM EDT Tue Apr 21 2015
Winds will lessen some as the sun sets this evening, but will remain
brisk overnight around 10 mph (with higher speeds and gusts in the
early evening hours).
Scattered showers will remain in the forecast tonight as a cold
front over the Midwest this afternoon approaches the region tonight.
Lows tonight should mostly be in the 40s...maybe around 50 just
ahead of the incoming front.
That front will cross the region early Wednesday morning, with
showers tapering off during the morning hours and then the sun
coming out in the afternoon. It won't be as windy as today was, but
afternoon gusts to 25 mph will be possible. High temperatures should
be a few degrees either side of 60.
Skies will clear and winds will go nearly calm Wednesday night as
high pressure centers itself right over Kentucky by sunup Thursday.
Temperatures will tumble into the middle and upper 30s, with the
usual cold spots dropping further, leading to the possibility of
some patchy frost.
Long term (thursday through tuesday)...
issued at 315 PM EDT Tue Apr 21 2015
..much cooler temperatures and frost possible towards the end of
The synoptic pattern at the start of the long term period will
feature a large trough across much of the northeastern Continental U.S.. this
regime will persist through much of the period, bringing seasonably
cool conditions and some chances for precipitation.
Surface ridging will persist over the Ohio Valley Thursday into
Friday, keeping conditions dry. However, given the troughing aloft
and light northwesterly surface flow, temperatures will struggle in
low/mid 60s despite the expected sunshine. On Friday morning, the
center of the high looks to settle somewhere across eastern KY,
helping winds to go light and variable. The light winds coupled
with mainly clear skies will lead to a cool night, with lows in the
mid/upper 30s and lower 40s. Frost formation is looking
increasingly likely Friday morning, especially over the northern
Bluegrass given their proximity to the center of the surface high.
Will introduce some patchy areas of frost into the grids for the
coolest areas on Friday morning.
Saturday is shaping up to be a pretty raw day across the Ohio
Valley. A strong pv anomaly will dive southeast through the trough
aloft, helping to induce a surface low across portions of the
Central Plains, which will dive southeast into the Tennessee Valley by
Saturday night. This track places much of the Ohio Valley to the
north of the low, which means cool northeasterly winds and
rain/clouds will prevail through much of the day. May even be some
embedded rumbles of thunder given some elevated instability above
the stable boundary layer. Given the agreement in the 12z
deterministic guidance suite along with the 12z gefs regarding the
low track, have trended highs lower for Saturday. Areas along and
north of the Ohio River will likely struggle with temperatures in
the 50s throughout much of the day. Certainly not looking to be an
ideal day for outdoor activities.
Guidance diverges significantly for the beginning of next week. The
latest European model (ecmwf) has come in much more amplified with a trough for early
next week, which is quite a bit different solution than its
previous run. The GFS has also shown its inconsistencies with this
feature. Therefore, will continue with a general model blend which
gives some consistency with the previous forecast, leaving pops in
the 20-30% range for Monday into Tuesday of next week.
Aviation (00z taf update)...
updated at 717 PM EDT Tue Apr 21 2015
VFR conditions are expected through the valid taf period. The
strong wind gusts from this afternoon will continue to subside
through the evening hours. However, the region will remain in a
tight surface pressure gradient so sustained surface winds will
remain up around 10-15 knots tonight. A line of showers and a few
embedded thunderstorms has formed across western Kentucky. This will
likely weaken as it heads toward kbwg, but think it will hold on
enough to affect kbwg with some -shra.
Otherwise, while a stray shower remains possible tonight, ksdf and
klex should mostly remain dry until around dawn Wednesday. That is
when a cold front will sweep through all terminals, bringing a wind
shift to more west-northwest and a few additional showers. Winds could gust to
around 25 knots once again tomorrow afternoon mainly at ksdf and