455 
fxus63 kjkl 212319 
afdjkl 


Area forecast discussion 
National Weather Service Jackson Kentucky 
619 PM EST Sat Nov 21 2009 


Short term.../tonight through Monday night/ 


An shortwave was moving through the Great Lakes and upper Ohio Valley 
early this afternoon...with moist of the moisture associated with it 
well northeast of the area. Drier air in the middle and upper levels 
worked into the area today...allowing for a thinning out of the 
cirrus. Cirrus continues to be more prevalent to the south of the 
area...with much of it having moved more east than north. However... 
cirrus over middle and western Tennessee appears to be on course to move 
into the region this evening. At the surface high pressure remain 
anchored over the Ohio Valley region. 


Models continue in decent agreement in the near term period...though 
the models have trended slower than yesterday with the shortwave to 
approach the area from the southwest later tonight and then gradually 
move into the commonwealth on Sunday night...before exiting to the 
northeast late Sun night. Models hint at another weak shortwave 
moving across the area on Monday. Given the slower timing...increased 
moisture and lift also appears to be delayed from yesterday by as 
much as 12 hours or so later...similar to the 12z/20th European model (ecmwf). The 12z 
GFS slowed the timing down from the 6z run and yesterday...but is 
still slightly faster than the 12z European model (ecmwf) and 12z NAM or 15z sref. 
Even though deeper moisture will depart after the main shortwave 
exits early on Monday...low level moisture is expected to remain in place. 
At the surface...the reflection is expected to be weak with only an 
inverted trough north of the surface low moving across the Gulf Coast 
states. 


With a ridge of high pressure gradually shifting to the northeast...a 
Ridge/Valley split is anticipated again tonight as the cirrus is not 
expected to impede radiational cooling in the valleys...especially 
eastern valleys tonight. Lower 30s in some valleys are not out of the 
question again given current dewpoints in the low to middle 30s in most 
areas. 


Clouds will thicken and lower on sun...especially from early 
afternoon on. This should hold back temperatures from todays readings a 
bit...though more northern and northeastern locations will experience 
the least cloud cover and should end up warmest. This was already 
depicted in the previous forecast...but with timing having slowed 
some more...opted to raise maximum T a couple of degrees...even across 
the south...though with increasing clouds the 12z met numbers appear 
to be too warm...so compromised between the 12z mav and the previous 
forecast...leaning bit more toward the 12z mav consistent with the 
slower timing. As far as probability of precipitation...the atmosphere will only slowly 
moisten up from the top down...and with that in mind...only have 
slight chance in the extreme south late. Continued with probability of precipitation on Sun 
night similar to the previous forecast...and then continued with 
chance into Monday and slight chance far southeast early Monday night with the 
secondary shortwave also moving into the Ohio Valley. The rest of the 
period should be mostly cloudy to cloudy with temperatures holding back Monday 
highs and holding up Monday night lows...as the region will be in 
between systems late in the period. 


Long term.../Tuesday through Saturday/ 


With quite a bit of uncertainty amongst the models for days 
3-7...decided to live and die by the GFS solution again today. 
Adjusted the pop...sky...and weather grids toward the timing of the 
12z GFS run. The GFS is 6-12 hours faster and further north with the 
track of a large of area of low pressure as it moves out of the 
Central Plains on Wednesday. Temperatures will remain warm enough 
for the precipitation to remain all rain. The next system on 
Thursday and Thursday could bring a large area of cold Canadian air 
into the Ohio Valley...particularly overnight Thursday. With the GFS 
forecast soundings showing about an 8 hour of period of at or below 
freezing temperatures for Thursday night into early Friday morning 
for a second day in a row...decided to lower hourly and min 
temperatures for that time frame and to introduce a mixture of light 
rain and snow flurries into the forecast. The two very limiting 
factors for snow for the end of the extended forecast will be the 
lack of moisture in the critical -10 to -20c ice Crystal growth 
zone...and a less than favorable westerly wind flow. Therefore...no 
snow accumulation is expected across the area at this time. 


Also made a few adjustments to temperatures across the board...going 
a couple of degrees warmer for highs and lows Tuesday and Tuesday 
night. The rest of the forecast temperatures and dewpoints required only 
the most minor of adjustments. Winds were also adjusted slightly per 
the changes made to the timing of the two areas of low pressure 
expected to traverse the area next week. 


&& 


Aviation.../00z to 24z/...updated 


Expect VFR conditions to prevail through the forecast period with 
some patchy MVFR fog present in the valleys. Have included a tempo 
group at ksme around dawn to account for a possible drop around the 
VFR/MVFR threshold. Otherwise...surface winds will remain light and 
generally out of the east through the forecast period. Upper level 
clouds will stream into the southern County Warning Area overnight with gradually 
lowering ceilings to around 15 k feet after middle morning. Any rain should 
hold off until after the forecast period is over. 


&& 


Jkl watches/warnings/advisories...none. 


&& 


$$ 


Short term...jp 
long term....AR 
aviation...schoettmer