801 
fxus63 kjkl 211510 
afdjkl 


Area forecast discussion 
National Weather Service Jackson Kentucky 
1110 am EDT sun Mar 21 2010 


Short term.../today and tonight/...updated 


Have not made any significant changes...just some tweaks based on 
overnight model runs. Have lowered maximum temperatures a bit today...mainly in 
the SW...due to clouds and advancing precipitation. Have also lowered probability of precipitation 
slightly in the extreme eastern part of the area for today...where 
low level downslope flow will be more likely to keep precipitation at Bay 
longer. Also made some adjustments to the timing of the slight 
chance of thunder...and included it in all areas. 


The previous short term forecast discussion follows... 


/today through Tuesday/ 


06z surface analysis shows a mature...well defined...and nearly stacked 
low centered over central Arkansas. A warm front from this stretches 
northeast through southern Illinois and across the Northern Ohio 
valley while an occluded front is sweeping through the deep south. 
Well east of the low...east Kentucky is just now seeing the effects 
of it in the form of high clouds. However...the clouds have not been 
sufficient to prevent another night of decent radiation... 
particularly in the sheltered valleys of the east where readings have 
fallen into the upper 30s and lower 40s...middle to upper 40s are found 
in the more open valleys elsewhere...while balmy middle and upper 50s 
reside over the thermal belt ridgetops. A small increase in winds 
later this morning will mix much of these differences out well before 
any showers threaten the area. 


With the upper low now well initialized within the models their 
solutions are quite similar through the bulk of the short term with 
any differences hard to discern from the noise. As such...they all 
take the bowling Ball of an upper low from East Texas eastward 
through the deep south with a turn more northeastward Monday morning 
and then up through the southern and central Appalachians by Monday 
evening. The European model (ecmwf) has a slightly slower solution at this point than 
the others and while the Gem is the quickest. Have stayed more with a 
consensus scenario offered by a mega blend for timing and placement 
of the upper low and its surface features. This low then slowly moves 
east out of the mountains on Tuesday...loosening its chilly and damp 
grip on east Kentucky/S weather. 


A bigger model differences to note revolve around the low level 
thermal structure represented by each. Most of the discrepancies 
appear to be due more to vertical and horizontal resolution 
differences in the models than any alternative vision. Accordingly... 
have relied on the highest resolution nam12 for guidance in this 
important aspect of the forecast. This results in a cooler low level 
thermal profile that would suggest the strong potential for snow 
showers mixing with the rains late tonight in the southwest and 
across much of area Monday night...far more so than the European model (ecmwf) in 
particular and the others in general. Regardless...the threat of 
accumulating snow will be remote beyond the higher peaks given the 
relatively warm ground of these past days...the boundary layer 
staying above freezing...and the warm/moist nature of any snow. Will 
hold off on any accumulations in the grids for most of the area at 
this point. This should end up...for east Kentucky...mainly a Novelty 
to go from highs near 70 over the weekend to pockets of snow late 
tonight and again Monday night. 


Otherwise...sensible weather will have a cloudy and warm day turned 
wet from southwest to northeast with time during the afternoon and 
early evening as the front pivots into the County Warning Area. With warm temperatures in 
place and marginal instability anticipated aloft will keep a slight 
chance for thunder in the grids and forecast coincident with the 
highest probability of precipitation and progress of the instability east...with respect to 
the front. Most locations will see a band of breezy showers this 
evening...followed by a dry slot move across the County Warning Area later in the 
night. In conjunction with the system/S Post frontal cold air advection...the 
showers associated with the wrapped up low at the surface and aloft will 
then proceed to further dampen east Kentucky. A thermal profile 
favorable for a mix of snow then moves into our southwest zones 
toward morning and spreads northeast during the day. However...during 
the day any snow will have to compete with some insolation and a 
minor diurnal rise. Later that night...though...snow showers should 
fare better...especially over the higher elevations where some light 
accumulations will be possible by Tuesday morning. The showers will 
become more scattered during the day Tuesday and ease off to the 
northeast by evening. 


Through the short term period a gradient of temperatures will affect 
east Kentucky thanks primarily to the cold pool aloft passing right 
overhead from late tonight through Tuesday morning. The adjmet 
numbers looked the best to represent this and were followed most 
closely. As for probability of precipitation...also ended up closest to the met values 
through the forecast. 


Long term.../Tuesday night through Saturday/ 


Closed upper level low will be exiting...with a few lingering 
showers across extreme southeast Kentucky ending early in 
the evening. A weak cold front will then be dropping down on 
Wednesday and Wednesday night. Have maintained a slight chance pop 
Wednesday night. 


Progressive southern stream system will then take aim at the area 
from late Thursday through Friday morning...providing another shot 
of rain. Have made some adjustments to the timing of this based on 
the latest model data...mainly to speed up the onset a bit as well 
as the exit. Have the overall best chance of rain for Thursday 
night. This faster trend seems reasonable given the progressive 
pattern in place. 


The only adjustments to the previous temperature forecast were made 
between Thursday and Friday...mainly to favor a more diurnally 
limited temperature range with clouds and precipitation around. 


The previous long term forecast discussion follows... 


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 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.../12z to 12z/ 


Clouds will thicken and lower during much of the day in advance of a 
slow moving occluded front sweeping in from the southwest. Eventually 
this will reach the taf sites later this afternoon bringing showers 
and a potential thunderstorm to all sites. In the tafs...initiated 
the precipitation threat with a vcsh starting late in the afternoon and then 
went with a tempo for MVFR conditions in the form of heavier showers 
during the bulk of the evening as the visible falls to around 3 miles and 
ceilings come down to between 2k and 2.5k feet. The rest of the night 
should see ceilings down to between 1.5k and 2k feet along with MVFR visible 
in occasional showers. Light southeast winds will pick up ahead of 
the front later today...becoming gusty toward evening. Allowed for a 
bit higher gusts to near 20 kts in the showers/stray thunderstorms 
during the evening hours. Thereafter...expect winds from the 
southwest at around 10 kts. 


&& 


Jkl watches/warnings/advisories...none. 


&& 


$$ 


Short term...greif/hal 
long term....geogerian 
aviation...greif 

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