406 
fxus63 kjkl 201902 
afdjkl 


Area forecast discussion 
National Weather Service Jackson Kentucky 
302 PM EDT Sat Mar 20 2010 


Short term.../tonight through Monday night/...updated 


Deepening cyclone over the Southern Plains will cut off and slowly 
trudge eastward into the Tennessee Valley by Monday. Ridging aloft 
will hold on strong through tomorrow morning and this should keep 
cloud cover thin tonight. Light winds will allow for decent 
radiational cooling and our eastern valleys will likely once again 
dip into the 30s... while ridges should remain in the low 50s. 


Latest forecasts continue to slow down our approaching system... 
delaying the increase in deeper moisture and precipitation. Low level 
southeast flow will also be freshening on Sunday and this will help 
scour out any low clouds over most of our area during the day and 
allow temperatures to warm toward 70 again for our eastern counties... with 
middle to upper 60s elsewhere. It is beginning to look doubtful we will 
see any rainfall east of hazard until after nightfall on Sunday... 
so will keep probability of precipitation in the chance range for far eastern Kentucky with 
likely probability of precipitation for our western half. Have also held onto the slight 
chance for thunder west of I-75 late in the day into the early 
evening... but dropped it for the remainder of the area with 
negligible surface based or elevated instability prognosticated east of 
I-75. 


Band of showers associated with occluded front will lift northeast 
across the area Sunday night with categorical probability of precipitation everywhere. A 
lull is then possible Monday morning before additional spokes of 
energy rotate northward from the parent upper low located just to 
our south later in the day. These shortwaves will spawn additional 
rounds of showers into Monday night and inched probability of precipitation upwards for most 
areas. Temperatures will struggle through the 40s on Monday... with coldest 
temperatures likely near the Tennessee border closer to the core of the 
colder air aloft. There remains a remote possibility for a few wet 
snow flakes to mix in with the rain from time to time late Monday 
into Monday evening but we will continue to leave this mention out 
as the boundary layer just looks too warm. The 20/12z GFS does look 
intriguing Monday night though with a period of strong forcing which 
could lead to some dynamic cooling and lowering of the freezing 
level to near the surface. If this trend continues... we may have to 
add some snow back in at a later time. 


Long term.../Tuesday through Saturday/...updated 


There is good agreement among the models to start the period. A 
large storm system with a strong closed upper low will be centered 
over the Middle-Atlantic States. Any remaining rain in our area on the 
back side of the system will be on the decline during the day and 
evening on Tuesday as the system pulls out. The greatest pop...in 
the chance range...is in east and southeast. 


A cold front dropping south will be approaching or passing in the 
Wednesday and Wednesday night time frame...while a southern stream 
weather system moves east into the Southern Plains. Models disagree 
on timing and strength of an upper level impulse crossing the Great 
Lakes in the northern stream during this time...leading to 
differences in the progress of the cold front in the various models. 
The Gem is weak enough with the upper level wave so that the surface 
front stalls to our north as the southern stream system begins to 
influence it. Meanwhile...the GFS is considerably stronger with the 
northern stream wave and it advances the cold front much further 
south...reaching the deep south by Thursday. The GFS is also the 
wettest...but even so...its precipitation and probability of precipitation look meager. In light of 
this...will undercut the GFS precipitation outlook and go with only a 20 
percent pop for Wednesday night. 


Models generally agree on timing for the upper level portion of the 
southern stream system through Thursday night. However...there is 
not good agreement on the depth or placement of surface low 
development. By Friday night this leads to considerable 
discrepancies in the amplitude and placement of the upper level wave 
as well as the surface low. A mentionable pop is certainly in order 
from Thursday night Onward...but due to low confidence...only in the 
chance and slight chance range. 


The realm of possibilities concerning the midweek front and the late 
week storm system also create an uncertain temperature forecast. The 
GFS ensemble mean appears to be a reasonable compromise...and will 
use it. However...the reality could easily be warmer or colder than 
what it suggests. 


&& 


Aviation.../18z to 18z/ 


High pressure will give way to a low pressure system and associated 
frontal boundary which will approach from the west over the next 
24 hours. As a result... scattered to occasionally broken middle level 
cloudiness will frequent the area. Deeper moisture will remain to 
our west through the end of the period however so ceilings will remain 
VFR and no precipitation is expected this taf period. Winds will remain 
light... shifting from the southwest this afternoon to the east- 
southeast tonight into Sunday morning. 


&& 


Jkl watches/warnings/advisories...none. 


&& 


$$ 


Short term...Abe 
long term...hal 
aviation...Abe