Scientific Forecaster Discussion
fxus63 klmk 222253
Area forecast discussion
National Weather Service Louisville Kentucky
653 PM EDT Mon may 22 2017
..updated aviation discussion...
Short term (now through Tuesday night)...
issued at 230 PM EDT Mon may 22 2017
..rain chances return late Tuesday...
Just some passing high clouds disturbing an otherwise fine May Day.
Southwest flow aloft will keep bringing in those clouds tonight into
tomorrow morning. High pressure over US now will shift eastward
allowing for a light easterly flow tonight. A very broad area of
lower pressure will replace it. Given the southwest flow in place,
and precipitable waters around 1.2-1.3 inches in out south Kentucky
counties, cannot rule out an isolated storm developing in the late
afternoon/early evening hours. Some of the better performing models
for rain chances in the short-term, the met/GFS/ecs as well as our
consall and consraw models, indeed bring in some chances in that
area, so have trended the forecast to slight chances in the late
afternoon along and south of a Morgantown to Lexington, Kentucky line.
Weak isentropic lift kicks in Tuesday night ahead of that broad area
of low pressure which looks to be deepening over far western
Kentucky overnight...as an upper level low drops south out of Iowa.
Precipitable waters increase to about 1.3-1.4 inches. The surface
low will lift northeast as the upper low continues southward by
daybreak Wednesday. With the deepening low, cannot rule out a
stronger cell developing near the warm frontal boundary associated
with the low, and we may see some stronger wind gusts as the low
deepens. The NAM/GFS drop the central pressure 7-10 mb from 00 to
12z, whereas the hi-res wrfs, CMC, 0.1 degree European model (ecmwf) only go 2-4 mb.
Another potential concern by daybreak Wednesday could be minor
flooding, especially in areas rain soaked a few days ago. Model quantitative precipitation forecast
is not more than in the 0.5-1 inch range for any given 6-hour period.
Long term (wednesday through monday)...
issued at 230 PM EDT Mon may 22 2017
..chilly weather for mid week...
The surface low will continue northward before stalling over
southern Indiana as the upper low continues to pivot with it
somewhere over our longitude Wed. And Thu. The surface low should
fill back in some. This pattern however will continue to bring good
rain chances, cloud cover, and much cooler temperatures, some 10-15
degrees below normal.
The upper low should clear east of the region some time during the
day Thursday, allowing a brief warmup as shortwave ridging crosses
the region Friday. A stationary frontal boundary looks to set up
somewhere along the vicinity, oriented from west-northwest to east-southeast. Upper level
disturbances passing along this boundary could lead to some mesoscale convective system
development for the Holiday weekend, but should the boundary stay
far enough north of US, any associated convection would be over the
north half of the region. Better performing models over the last 6
months in this time period, consall, MOS guidance, wmodel, and the
new nbm regional blend, would argue for high-end chances to likely
range during various periods from Sat-Mon. Given some uncertainty,
would prefer to keep US out of the likely range though. The
superblend model agrees with this thinking as well, so did not make
any changes here.
Temperature forecast beyond the cold Wed/Thu values will depend
greatly on that frontal location as well as associated convection.
So, this part of the forecast has a lower confidence.
Aviation (00z taf update)...
issued at 653 PM EDT Mon may 22 2017
High pressure over the upper Ohio Valley will keep winds light and
skies VFR tonight, with cirrus as the only cloud type. Tomorrow low
pressure will form over the Tennessee Valley and drift northward
into Kentucky. Initially, though, there won't be a lot of moisture
for this system to work with, so significant chances of rain will
hold off until after this taf period.