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fxus63 kjkl 121142 
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Area forecast discussion 
National Weather Service Jackson Kentucky 
642 am EST Friday Mar 12 2010 


Short term.../today through Saturday night/ 


Upper level low will pivot southeastward from the upper Midwest into 
the southern Appalachians during the period. Recent infrared and water 
vapor satellite imagery indicate a rather potent shortwave is 
currently rounding the base of the trough over Arkansas. Forecasts 
indicate this shortwave will continue northeastward into the 
Tennessee Valley today and tonight. This system appears poised to 
bring a rather healthy rainfall to the area late today and 
tonight... along with a few strong thunderstorms. 


First off... the clearing which took place overnight has allowed 
areas of dense fog to develop in our eastern valleys. The fog is 
reducing the visibility to near zero along the rivers and larger 
creeks. While the fog is not widespread and most surface observation are not 
reporting any significant visibility reductions... many area roads 
parallel the rivers and so the dense fog will be a concern this 
morning. Will highlight this threat in a Special Weather Statement. 
A light southeast breeze should keep the fog from being a problem 
outside of the deeper eastern Kentucky valleys. 


Once the fog Burns off... a period of partly cloudy skies will allow 
the atmosphere to destabilize during the morning and early afternoon 
hours. Forecasts indicate marginal instability by afternoon as the 
shortwave and attendant surface low/frontal boundary to our 
southwest approaches. The forcing from this system along with very 
good upper divergence should allow for the development of widespread 
showers with scattered embedded thunderstorms by the late 
afternoon/early evening hours from southwest to northeast. Forecast 
soundings indicate the potential for large hail with wet-bulb zero 
heights between 7k and 8k feet... plus a bit of dry middle level air. 
Forecasts also indicate a decent low level jet pushing northward from 
Tennessee in advance of the shortwave... so gusty winds are also a 
possibility with any thunderstorm. Our severe threat will depend 
heavily on how much sunshine we see but it does appear to be a 
decent threat right now and one that will continue to be highlighted 
in the hazardous weather outlook. 


The thunder threat will end quickly after sunset... but a band of 
showers will continue to lift northeast across the area overnight. 
This rain will likely fall heavily at times and most locations can 
expect upwards of an inch of rainfall by Saturday morning... with 
locally up to 1.5 inches. No significant flooding problems are 
anticipated however thanks to the recent dry spell. More scattered 
shower activity is then anticipated Saturday and Saturday night as 
the upper low passes overhead. 


Temperatures will push 70 in our eastern counties today but a cooler 
air mass will filter back in tonight and remain in place through the 
rest of the period. Most forecasts indicate the core of the coldest 
temperatures aloft will actually reside over our area Saturday and Saturday 
night so we nudged temperatures down a bit each period to account for this 
fact and the abundant cloud cover which is expected to be in place. 


Long term.../Sunday through Thursday/ 


The models start out in good agreement with upper low pressure 
systems located over the Tidewater and near The Four Corners. The 
East Coast low moves off to the northeast. As it does...it continues 
to spin short wave troughs across the area keeping the weather 
somewhat unsettled through Monday. The low over The Four Corners is 
a bit more problematic to the forecast after Monday. The low moves 
off to the southeast and eventually some weak troughing develops 
over the plains. The models vary greatly with the positioning of the 
lows and troughs and as such...confidence GOES way down after 
Monday. None of the models are showing much more than some weak 
troughing...and the worst case would be some showers moving across 
the area. Have chosen to go dry until the models become a bit better 
defined. For temperatures...stayed slightly below MOS through the period. 


&& 


Aviation.../12z to 12z/...updated 


Patchy dense valley fog still an issue this morning...particularly in 
the deepest valleys an near lakes...rivers...and streams. IFR 
conditions could occur where near lakes and rivers where the fog 
would have the most potential to become dense. The latest nam12 and 
sref models dont bring any precipitation into the area until between 
18 and 21z. Based on the latest forecast data an radar imagery...a 
nasty squall line could potentially form in Tennessee late this 
evening and then move across east Kentucky. A few storms could 
produce damaging winds and large hail. The latest forecast soundings 
also feature a period of good instability after 18z today...which 
would also aid in the formation of thunderstorms given the other 
favorable factors. The primary focus the 12z taf was to further zero 
in on when showers and thunderstorms would directly effect the tafs 
sites today and how strong and numerous will the thunderstorms be 
today. The current run of GFS time heights reveals a very deep layer 
of moisture that will likely persist through Sunday morning. This 
fuel will be what leads to occasional periods of moderate to heavy 
this evening and tonight as low pressure inches toward the area. 


&& 


Jkl watches/warnings/advisories...none. 


&& 


$$ 


Short term...Abe 
long term....jj 
aviation...Arkansas