520 
fxus64 klix 091111 
afdlix 


Area forecast discussion 
National Weather Service New Orleans la 
511 am CST Friday Jan 9 2009 


Short term... 
dense fog over portions of the area is the main concern through 
middle morning today. This is mostly caused by radiational cooling 
with clear skies and near calm winds...but there is also surface 
convergence near a stalled frontal boundary and advection of Gulf 
moisture above the surface inversion. 11-3.9 micron satellite 
imagery shows the fog is more widespread south of metropolitan Baton 
Rouge and over the West Bank of the Mississippi River and areas 
south from the river parishes to west and south of New Orleans. 
Satellite imagery shows a northward movement towards Baton Rouge 
and a large portion of metropolitan New Orleans on the South Shore. 
Visibilities from the affected metars are around one quarter mile 
but the Sheriffs Office in Lafourche Parish earlier reported 
visibilities as low as 50 feet over sections of US Highway 90 near 
Raceland. Based on surface reports and satellite...have issued a 
dense fog advisory through 9 am for areas along and south of 
Interstate 10 in south central and southeast Louisiana. After the 
fog Burns off...expect a mild and slightly more humid day. 


Will continue to watch a positively tilted shortwave trough 
advancing east through the western states. There have been some 
timing issues with this system...and have gone with a slightly 
slower solution again this morning. Am still expecting a cold 
front to move through most land areas of the forecast area on 
Saturday as the middle/upper shortwave moves east and gradually 
shears out across upper portions of the lower Mississippi Valley. 
There is still a slight chance of some showers starting over 
mainly the northwest half of the area late tonight...then showers 
and likely on Saturday. The GFS has slowed down enough to show 
similar cape values as the NAM...and both models show most 
unstable convective available potential energy on the order of 500-1250 j/kg developing by midday. 
Most of the better shear occurs ahead of the better instability... 
and forcing is not all that great. With that in mind...have only 
kept a slight chance of thunderstorms forecast with the showers on 
Saturday...and the 5% or less /see text/ severe threat from Storm Prediction Center 
day two convective outlook looks good. 


The main change was to keep a higher pop Saturday evening as 
overrunning moisture and light rain behind the front will be a bit 
slower to clear out. 


Long term... 
a broad and fairly deep upper trough will become positioned over 
most of eastern Canada/Continental U.S. Through most of next week. The devil 
will be in the details as numerous shortwaves and associated 
surges of cold air will make their journey south through the base 
of the trough with varying amounts of alternating return flow 
moisture. Given the uncertainty...have not changed much from the 
previous forecast package. The Canadian and European model (ecmwf) show a better 
chance of return flow and rain chances with a more potent 
shortwave trough Monday night and Tuesday. Have kept a dry 
forecast at this time...but that will have to be watched. Have 
maintained the next chance of showers starting on Thursday...and 
am still expecting some near freezing temperatures over portions 
of the north Monday through Wednesday mornings. 


22 
&& 


Aviation... 
low clouds and reduced visible beginning to creep northward towards msy. 
Added tempo for deck and visible likely dropping to around 1/4sm . 
Not expecting any other sites to be affected...but will need to 
closely monitor btr. Winds have stayed light and variable overnight 
as high pressure continues its influence. The ridge will shift 
eastward today and lead to southerly winds around 10 kts. Winds will 
drop slightly after sunset but should remain in the 5 to 10 knots range 
overnight. Middle cloud deck will move in around 00z and lowering ceilings 
late in the period. 


Meffer 
&& 


Marine... 
light easterly winds will increase and become more southerly through 
the day. High pressure will shift eastward through the day. As an 
surface trough/cold front will move towards the area today...tightening 
the surface pg and thus increasing winds. May need small craft exercise 
caution for tonight in the coastals...at least in the outer waters. 
Cold front is expected to push through the coastals Saturday night. 
Winds and seas will likely both be on the fringe of scy criteria. 
Later shifts will need to re-evaluate as the event would be quite 
short at maybe 12 hours. High pressure will move back in behind the 
front. Another re-enforcing front is expected Monday night. This 
front looks be weaker than the first. Late next week could see a 
potent system move through the area. 


Meffer 
&& 


Preliminary point temps/pops... 
mcb 72 59 68 38 / 0 30 70 20 
btr 74 60 69 40 / 0 20 70 20 
msy 74 62 74 46 / 0 20 70 30 
gpt 67 60 72 43 / 0 10 70 40 


&& 


Lix watches/warnings/advisories... 
la...dense fog advisory until 9 am CST this morning for laz034- 
046>050-056>070. 


GM...none. 
MS...none. 
GM...none. 
&& 


$$