752 
fxus64 klix 231256 
afdlix 


Area forecast discussion 
National Weather Service New Orleans la 
656 am CST Monday Nov 23 2009 


Update... 
sounding discussion... 


The atmosphere is saturated up to 875 mb and very dry 
aloft...precipitable water value is 0.55 inches. A stratus deck is 
present again this morning with visibilities mainly less than 5 sm 
due to mist or fog across the County Warning Area. An inversion is present near 
740 mb with west-southwest winds dominating the atmosphere. 


&& 


Previous discussion... /issued 319 am CST Monday Nov 23 2009/ 


Synopsis... 
a surface low is centered over the northeastern Gulf of Mexico 
with a front extending southward into the Bay of Campeche. Surface 
high pressure is centered over New England with the ridge axis 
extending southwestward through the Appalachians and into the 
local area. A second surface low is taking shape near The Four 
Corners region. 


Locally...ample low level moisture has led to the development of 
low clouds and areas of fog. Temperatures are currently hovering 
in the middle to upper 50s...and should only drop another degree or 
two before sunrise. 


Short term... 
high pressure will continue to shift eastward today as an upper 
level trough digs into the plains states. The associated surface 
low and cold front are expected to strengthen as they move 
eastward into the middle Mississippi Valley. Not sure why...but 
the NAM is carrying 60 to 70 percent probability of precipitation Tuesday night as the 
front moves through the area even though it forecasts no actual 
quantitative precipitation forecast. The GFS and European both forecast a dry frontal passage with 
10 to 20 percent probability of precipitation. With little to no return flow ahead of the 
front...moisture should be rather limited and only isolated 
showers are expected to develop. Will continue to carry 20 percent 
probability of precipitation across the area for Tuesday night...but this is probably 
still overdone. Expect near normal temperatures behind the front 
Wednesday and Wednesday night. 


Long term... 
a reinforcing high will bring colder and drier air into the region 
beginning Wednesday night. Strong cold air advection will result 
in low temperatures Wednesday night that are 5 to 10 degrees lower 
than Tuesday night. This trend will continue into Thursday night 
with low temperatures dropping about another 5 degrees. Expect 
lows Thursday night to be in the middle to upper 30s north and west 
of the tidal lakes and in the low to middle 40s south of the tidal 
lakes. High should generally top out in the lower 60s across the 
area. 


By Friday night...the surface ridge should be centered over the 
area. Low dewpoints and clear skies will allow for efficient 
radiative cooling and we should see the coldest night of the 
season thus far. The models typically tend to under forecast the 
amount of cooling in this kind of situation...so will undercut 
model guidance by a few degrees. The result is a forecast for near 
freezing temperatures across the far northern zones...and 
temperatures in the upper 30s and low 40s across the southern 
portion of the area. 


The surface high will shift eastward Sunday and Sunday night 
leading to a warming trend as winds become more southeasterly. 
Another low and cold front are forecast to affect the area early 
in the next work week. 


Aviation... 
mostly IFR to LIFR ceilings expected through the early to 
middle morning hours. Most of the terminals will continue to deal with a 
stratus deck between 300 and 1k feet before slowly lifting around 15z. 
After 15z all taf sites will continue to slowly improve with VFR 
conditions finally expected around 18z. Visbies will also be an 
issue through the morning as light fog and br continue to develop. 
Also as ceilings fall they could temporarily drop all the way to the 
ground causing visbies of 1/2-1sm to occur. Visbies should improve 
to above 6sm by 15z. 


Marine... 
weak high pressure will remain over the region providing 
light winds and lows seas through midday Tuesday. Late Tuesday a weak and 
mostly dry cold front will move through the area. At the same a wave 
will develop on the southern end of the front across the Gulf. Both of 
these features will lead to winds and seas picking up some with 
exercise caution headlines possibly needed Wednesday. Stronger high 
pressure will begin to build in behind this front and should 
dominate the area through the week and at least the first half of 
the weekend. The center of the surface high will remain northwest of the area 
through Friday leading to offshore flow during that time. A secondary 
surge of colder air will occur Thanksgiving day and overnight. The 
cold air advection associated with this will likely lead to gusty winds once again 
and small craft advection may be necessary Thursday and Thursday night over the 
coastal waters. Onshore flow looks to redevelop sun as the surface high 
moves through the region more twrds the Atlantic coast. 


&& 


Preliminary point temps/pops... 
mcb 66 48 71 48 / 0 0 10 20 
btr 70 50 73 50 / 0 0 20 20 
msy 69 54 72 54 / 0 0 10 20 
gpt 66 50 71 52 / 0 0 10 20 


&& 


Lix watches/warnings/advisories... 
la...none. 
GM...none. 
MS...none. 
GM...none. 
&& 


$$