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Area forecast discussion...updated 
National Weather Service Paducah Kentucky
635 am CDT Sat Apr 25 2015

issued at 625 am CDT Sat Apr 25 2015

Updated the aviation discussion for the 12z taf issuance.


Short term...(today through Monday night)
issued at 307 am CDT Sat Apr 25 2015

Forecast focus with this package will be within the first 12 to 18
hours (today through this evening)...tied to severe potential with
the approaching low and warm sector instability and shear.

Forecast confidence for timing of severe weather is moderate to
high for expansion this afternoon and early this evening.
Confidence is moderate for placement...mainly due to antecedent
rainfall...or the lack thereof...between now and 15z (10 am cdt)
this morning. That could throw a slight wrench in the initiation
point for severe convective development today.

From a forecast initialization standpoint...the 4km NAM-WRF (arw
version) is doing fairly well with the current convective
elements. Overall...the NAM-WRF guidance (12km) seemed to have a
grasp on the generation and propagation of the convection through
the weather forecast office pah forecast area. Must assume that the northeastward
shift of the enhanced risk area outlined in the Storm Prediction Center day 1 outlook
may show some influence of the 00z Saturday NAM-WRF model output.

In reviewing the convective parameters from the NAM-WRF guidance
family and other high resolution model guidance...there may be a
small window...mainly between 15z-17z Saturday...where storms may
flare up over southwest Illinois and over northwest
cape...helicity...and a gradient of 0-6km bulk shear of 50-70
knots briefly phase. This may produce an isolate severe storm
before the cap build in from the south.

However...recent and current model guidance suggests the cap will
be short lived...especially between 18z-21z Saturday. There should
be an explosion of convection across the northeast half of the weather forecast office
pah forecast area. During this convective inhibition
drops to near zero. Decent cape of 1500-2000 j/kg2 will be average
over most of the area...with the highest...up to 3000 j/kg2 will
be centered over west Kentucky during the afternoon. Although
respectable...0-3km and 0-6 km shear range spans mainly between
35-55 knots during the event...with the highest...45-65 knots
tiedl closely to an east-southeast axis...running from the I-64
corridor in southern Illinois...eastward into southwest Indiana
and northwest Kentucky. The vorticity generation parameter (vgp)
becomes quite impressive between 18z Sat through 00z the
aforementioned corridor...suggesting decent updraft strength for
hail. The 05-06z Saturday runs have been hinting that maximum
updraft helicity and vertical velocities increase markedly north
of a line from Ava to Carmi Illinois between 19z-20z this
afternoon. This...combined with a sharp gradient of -20c heights
suggest sharpening lapse rates for hail generation in this area.

With all that said...could be brief...but isolated chance for
severe over a couple of spots in southern Illinois and west
Kentucky...mainly along and north of a Carbondale to Calhoun
Kentucky line late this morning...with the main severe weather
impact starting up in southwest Illinois on or after 19z...then
working east-southeast during the remainder of the afternoon until
about 01z in the pennyrile region of west Kentucky. Rather than
get to picky on coverage of severe potential...defined severe
potential in grids for the period from 18z Saturday through 01z
Sunday for all grids that contained thunderstorms.

Kept a mention of temperatures near 80 degrees in the south
today...but some guidance suggests that 76-78 degrees may be more
reasonable given antecedent cloud cover.

Incorportated a blend of NAM-WRF/hrrr guidance for sensible
weather elements in the very short term (today and tonight)...then
added in European model (ecmwf)/GFS into the longer term.

The key to any changes in the enhancement and inital location of
strong to severe thunderstorms will really depend on where the
strongest warm air advection gradient (warm front at surface and
aloft) is focused this morning.

Long term...(tuesday through friday)
issued at 307 am CDT Sat Apr 25 2015

Medium forecast confidence in the long term period as models
continue to show some disparity.

The only weather of any consequence in the long term continues to
hinge upon the development and evolution of a storm system forecast
to move out of the Southern Plains Tuesday into Tuesday night.

Latest model runs continue to hold fast to their solutions where the
aforementioned storm system is the GFS track brings
the far northern edge of the precipitation shield across
approximately the southern half of our County Warning Area Tuesday and Tuesday night
while the European model (ecmwf) and Canadian keep all precipitation skirting by to
the south of our area. For probability of precipitation in those two periods just continued
with a blend which only impacts our far southern counties.

Not sure what to do about probability of precipitation on Wednesday. The GFS shows a broad
500 mb trough over the area with a smattering of precipitation over our
entire County Warning Area. The European model (ecmwf) shows an 500 mb trough dropping due south out of
the Great Lakes region pulling moisture and light precipitation
southward across our far eastern counties. The Canadian shows an 500 mb
trough just to our south with no precipitatin forecast. Kinda went
with a GFS/European model (ecmwf) hybrid for mainly schc probability of precipitation.

Beyond Wednesday...high pressure at the surface and ridging aloft
should keep the region dry with slowly moderating temperatures.
Readings starting off the period below normal will gradually warm
back to near normal by the end of the period.


issued at 625 am CDT Sat Apr 25 2015

With the approach and passage of a storm system in this taf
period...cigs/vsbys will range from IFR to VFR along with showers
and thunderstorms. A few storms may become severe...especially
near kevv/kowb between 00-05z. Winds out of the southeast at or below 10
knots early will gradually veer around to the northwest with
gusts up to 24 knots starting this afternoon.


Pah watches/warnings/advisories...


Short term...Smith/fire weather
long term/

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