Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
628 am EDT sun Aug 2 2015
..updated aviation discussion...
Short term (now through monday)...
issued at 245 am EDT sun Aug 2 2015
An early morning surface analysis showed high pressure centered over
south central Kentucky, extending east/west from the Appalachians
toward the middle Mississippi Valley. Aloft, water vapor imagery and
500 mb heights continued to show ridging through the western Continental U.S.
With downstream troughing over the Great Lakes region. Locally,
light/variable winds prevailed with passing cirrus and readings in
the middle 60s to lower 70s.
Main forecast concern in the short term is shower/storm chances
Monday afternoon associated with a surface cold front that will pass
but stall near the lower Ohio Valley this week.
For today though, plan on another seasonable day with highs around
normal /upper 80s to near 90/ for early August. Light/variable winds
this morning will become southerly to southwesterly this afternoon
and tonight as the surface high pressure moves east. Outside of
passing cirrus at times, mostly sunny skies are expected. Humidity
levels will remain comfortable, as dewpoints mix into the upper 50s
to lower 60s this afternoon. Plan on mild readings tonight, mainly
in the upper 60s to middle 70s.
On Monday, a shortwave will drop south into lower Michigan within
the large scale trough over the Great Lakes. A surface low is
expected to track from northern Wisconsin toward Ohio over the
course of the day, which will drag a surface front through the middle
MS and Ohio valleys by afternoon. Moisture return ahead of the front
is limited, though expect dewpoints to creep back up into the
middle/upper 60s. Precipitable waters will also trend upward, to around 1.5 inches
across southern Indiana and northern Kentucky. As this moisture
pools along/ahead of the front, peak heating will contribute to 2000
to 2500 j/kg of SBCAPE by middle/late afternoon. Deep layer shear
across the area is on the order of 20 to 30 kts. Given the limited
moisture and the lack of strong forcing though, will continue to
keep probability of precipitation in the 20 to 30 percent range as activity may end up more
isolated in nature. However, if storms do manage to develop they
could pose a damaging wind and heavy rain threat. At this time, the
greatest concern is across southern Indiana into northern Kentucky
and locations northeast.
Long term (monday night through saturday)...
issued at 255 am EDT sun Aug 2 2015
Low amplitude ridge west/trough east pattern still in the cards for
the long term portion of the forecast. A lead shortwave trough coming
in from the west late Tuesday night or early Wednesday morning will
be followed by several more Wednesday through Saturday.
Meanwhile at the surface a cold front approaching from the northwest
during the short term will, according to the more consistent ECMWF,
become quasi-stationary as it lays out west-east from the Central
Plains to the mid-Atlantic. The operational GFS pushes the front on
through on Tuesday, but then replaces it with a new front Wednesday
through Saturday so the resulting sensible weather is about the same:
periods of scattered showers and thunderstorms. The gefs plumes do
show pretty good agreement among the individual members of showers
and thunderstorms...especially Wednesday and Thursday.
Precipitable water amounts will be slightly above normal in the 1.5
to 2 inch range, along with k index values generally in the middle 30s
and tall thin cape on European model (ecmwf) sounding forecasts. We could see some
training along the surface boundary as well. Storm Prediction Center said it well in
their day 4-8 outlook that while strong to severe storms can't be
ruled out, there are too many meteorological details and vagaries in
the models to reliably pinpoint when they might occur. Right now
locally heavy downpours looks like the more pervasive threat.
Tuesday will be the driest and thus warmest day with highs in the
middle 80s to lower 90s. The rest of the week clouds and periodic
convection will keep afternoon temperatures in the 80s.
Aviation (12z taf update)...
issued at 620 am EDT sun Aug 2 2015
Plan on VFR conditions at sdf/Lex/bwg through the taf period as high
pressure remains in control combined with a dry air mass in place.
Light/variable winds this morning will become southerly this
afternoon and tonight ahead of a weather system approaching from the
northwest. Look for cirrus clouds to pass overhead at
times...otherwise skies will be clear.