Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 220736
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
336 AM EDT Wed Apr 22 2015
.SHORT TERM (Now through Thursday)...
Issued at 300 AM EDT Wed Apr 22 2015
...Some Potential for Frost Tonight...
Cold front moving into our northern counties at this time and will
continue to drop steadily through the region the rest of this
morning. Seeing some showers form along this boundary, but coverage
appears to be limited by how little moisture the atmosphere has.
Still, with a stronger upper trough now over central IL, there
should be enough forcing to squeeze out more showers as the front
passes us. Have gone with chance-range pops (30-50%) for now, given
that lack of moisture. Totals should be fairly light.
High pressure will nose into western KY this afternoon, allowing for
gusty winds under a tighter pressure gradient this morning then
lighter winds by late afternoon. Lows look to fall into the mid 30s
for many locations. The question mark for frost potential will be
lingering cloud cover down south and how much of a gradient wind
remains through the night over the north. Thinking at this point is
that we will have a stripe of best frost potential across the
central forecast area, around a line from Tell City, IN to Etown and
Campbellsville, KY. Areas farther away from this line could see some
patchy frost. Will let the day shift decide if a frost advisory is
warranted based on how the cloud/wind forecast changes.
High pressure will continue west of the region Thursday, making for
a pleasant though cooler day.
.LONG TERM (Thursday night through Tuesday)...
Issued at 320 AM EDT Wed Apr 22 2015
Thursday Night - Friday...
Dry and cool conditions are expected to end the week as dry NW flow
aloft and surface high pressure dominate the pattern. Frost (patchy
at least) is going to be a concern toward dawn on Friday as the
combination of clear skies and calm winds allow for excellent
radiational cooling. Will call for lows mostly in the mid and upper
30s across our CWA, however a few of our NE cool spots may be able
to briefly get into the low 30s. As is typical, the Louisvile metro
area may stay around 40.
After the cool start, temperatures should still be able to recover
to the low and mid 60s on Friday. However, this will still be a bit
below normal for this time of year. Do expect to see increasing mid
and upper level clouds as we move through the afternoon hours. A few
sprinkles are possible across our extreme SW late in the afternoon.
Friday Night - Saturday Night...
The wettest portion of the long term will occur late Friday night
into Saturday evening as deeper moisture overruns a warm front
extending eastward from a Plains surface low. Have gone with
numerous showers and perhaps a few rumbles of thunder by dawn on
Saturday. These showers will continue at times through the day on
Saturday as the surface low reaches the Ohio/Mississippi River
confluence by early evening. Expect to see a fairly tight
temperature gradient from south to north across the CWA Saturday
afternoon as a warm sector gets established. Right now, will call
for upper 60s to around 70 across our southern third, with low and
mid 60s south of a Tell City, IN to Richmond, KY line. North of that
line, mid and upper 50s are expected.
The surface low will weaken and drop southeast across the Tennessee
Valley by Saturday evening as a mid level trough axis drops through
the Ohio Valley. There should be enough lingering deep moisture to
keep isolated to scattered rain showers in the forecast Saturday
night as the mid level disturbance moves through. Expect cooler lows
as surface winds shift to northerly behind a cold front. Low 40s
north to upper 40s south.
Sunday - Monday...
Confidence begins to lower as we end the weekend as models diverge
on upper features. Save for a few lingering light showers across our
southern half Sunday morning, dry conditions are expected Sunday
through Monday as surface high pressure dominates. Expect highs in
the low and mid 60s Sunday, with mid and upper 60s on Monday. Lows
Sunday night should be in the low and mid 40s in most spots.
Monday Night - Tuesday...
We go from low confidence to no confidence by Monday night as models
struggle with placement of a southern Plains closed low and its
interaction with another impulse diving sharply out of Canada. ECMWF
gives the Ohio Valley a dry forecast as the main feature stays well
south of the area.
The GFS depicts a scenario where a much stronger/north system
experiences the Fujiwhara Effect right over the Ohio Valley as
another closed low dives sharply over Canada. This scenario would
bring a prolonged period of cold and rainy/dreary conditions well
past the end of this forecast package.
Will lean on the GFS ensemble mean compromise and go above climo,
calling for a chance of rain until models work themselves out. Highs
should remain below normal into Tuesday. How far below normal will
remain to be seen.
.AVIATION (06Z TAF Update)...
Updated at 115 AM EDT Wed Apr 22 2015
A cold front remains north of the region this hour, and this front
is generating some light to moderate showers. Expect showers in the
vicinity from roughly 09-15Z at SDF, and a little later at LEX and
then BWG. Ceilings look to stay VFR in general, but cannot rule out
a brief drop to MVFR. Winds will shift to northwesterly as the front
passes the region late in the morning. Gusty winds will remain until
late afternoon when high pressure will ridge into the region.