817 
fxus64 kjan 082156 aaa 
afdjan 


Area forecast discussion 
National Weather Service Jackson MS 
356 PM CST Thursday Jan 8 2009 


Discussion...a quiet dry start to the forecast will give way to 
rains followed by cold...colder and coldest as we head through the 
7-day cycle ahead. Unfortunately a pattern shift away from our more 
tolerable Pacific air mass regimes comes into play next week. This 
amplifying pattern will result in at least 2 Arctic intrusions for 
the deep south...a brief incursion Monday night/Tuesday and then the 
motherload arriving Thursday. 


In the short term(tonight through saturday)...weak high pressure(1017mb) 
along the MS River Valley this afternoon will move east tonight with 
near calm winds and clear skies over the County warning forecast area. Radiational cooling will 
yeild a decent rural frost for locations east of I-55 but lows should only 
approach the freezing mark along the Alabama border. Southerly breezes returning 
along the MS River Valley late tonight should allow readings to hold 
closer to around 40. 


On Friday...we will see plenty of sunshine early with a rapid warmup 
for the most gorgeous day we'll see for the next week to ten days. 
High temperatures will be in the upper 60s to lower 70s as warm air advection pattern kicks 
in. Isentropic upglide develops in the 295-300k layers in the late 
afternoon hours for some increase in cloud cover by dark...especially 
over western half of County warning forecast area. Expect these cloud layers in the 5-8k foot 
range to gradually thicken through Friday evening. The warm air advection increases 
Friday night to keep lows early on Saturday some 12-20f warmer than 
tonight. A few rain showers could break out over the Delta before midnight 
but rain showers/isolated thunderstorms and rain should really begin after midnight working 
their way from northwest to southeast across the County warning forecast area. A 35-45kt 850mb transport of 
324k Theta-E air should combine with 5.5-6c 7-500mb lapse rates and k 
indexes of around 30 for the rainfall. This pre frontal axis of rains 
moves across the region with quantitative precipitation forecast amounts of maybe one half to one 
inch from precipitable waters  that peak close to 1.3-1.4 inches. 


Even though favorable low level wind shear profiles will exist from 
late Friday night through early Saturday...MLCAPE/sbcapes prognosticated to 
be less than 150j/kg ahead of the frontal passage. On Saturday...that front 
arrives in the Delta just after daybreak and moves through the Natchez 
trace area by midday and then southeastern MS before sunset. There could be a 
few briefly stronger storms in southeastern MS Saturday afternoon if the front 
is slower per the nam12. For now we are staying with the sref/GFS 
timing of the boundary with much cooler air settling across the region 
heading into Saturday night. Highs Saturday should generally occur by 
early afternoon with falling readings into the evening hours as we 
head toward the first of several freezes starting Sunday morning. 
Sunday should be sunny and dry but temperatures will top out several 
degrees below normal and mainly 48-53f. 


Long term remains generaly on track with slight adjustments to the 
details on the Arctic air mass intrusions. The sharpening mean 
longwave trough axis in the eastern Continental U.S. Has several weak short waves 
moving through Sunday night and Monday. These help weaken the Pacific 
surface high over the Southern Plains as it shifts eastward. Low level 
southerly flow on Monday should do little to increase moisture over 
the region and no rain is expected with the shortwaves. 


Meanwhile...the deepening upper trough to our east will send the first 
chunk of Arctic air breaking off with a cold front leading the charge 
into our County warning forecast area Monday evening with increasing cold air advection all day Tuesday. 
The GFS appears to be having difficulty with the old shallow cold 
regime and once again tries to develop a Central Plains surface low 
in an erroneous attempt to hold back the dense Arctic air. Have 
leaned more heavily on the GFS ensemble means/European model (ecmwf) from midday 
Monday Onward with near 1030mb high over southern MS by Tuesday night. 


While we catch our breath on Wednesday with below normal temperatures...the 
next motherload of Arctic air takes over first thing Thursday on 
gusty northwest winds with temperatures dropping through the day. A 
massive 1045-1055mb high descends into the northern plains Wednesday night 
with the leading edge marching into our region before Thursday 
morning. This system should deliver the coldest air we have seen thus 
far this winter as it plunges down the MS valley through Friday under 
a strong polar jet on the east side of a very impressive Gulf of Alaska 
upper ridge axis. Considering the current strength of the air mass 
heading our way...we expect the coldest readings to be about 15-20f 
below normal next Thursday or Friday morning(i.E. Lows 11-17f). 
Earlier model runs also hinted at some winter style precipitation 
during the end of next week and we will need to be watching closely 
for that potential. /22/40/ 


&& 


Aviation...VFR conditions will persist at all taf sites over the 
next 24 hours. /19/ 




&& 


Preliminary point temps/pops... 
Jackson 37 70 52 55 / 0 3 59 65 
Meridian 33 68 53 59 / 0 1 47 71 
Vicksburg 40 71 50 53 / 0 4 67 53 
Hattiesburg 40 73 55 63 / 0 1 27 75 
Natchez 45 73 55 55 / 0 2 57 60 
Greenville 37 67 45 49 / 0 15 78 36 
Greenwood 35 69 47 51 / 0 9 76 46 


&& 


Jan watches/warnings/advisories... 
MS...none. 
La...none. 
Arkansas...none. 
&& 


$$