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Area forecast discussion 
National Weather Service New Orleans la 
1144 PM CST Friday Nov 20 2009 


Aviation... 
mainly just sprinkles falling from middle levels clouds were 
occasionally being observed at the taf sites as a layer of dry air 
in the lower levels prevailed across the forecast area late this 
evening. As the column becomes saturated overnight...more in the 
way of light rain will be observed toward morning. This first 
batch of light rain was resulting from a lead short wave moving 
across the region. More substantial rain was occurring to the 
southwest of Lake Charles and this rain will gradually make its 
way into the forecast area late tonight and Saturday morning. 
Occasional light rain can be expected at the terminal sites during 
the day Saturday. The latest model guidance appears to be somewhat 
slower in moving the rain out of the area...therefore rain may 
linger through Saturday evening at most locations. VFR ceilings tonight 
will lower to MVFR ceilings during the middle and late morning hours 
Saturday...and then into the IFR range Saturday evening. Light 
rain and light fog will result in MVFR visibilities during much of 
the day Saturday. 11 


&& 


Previous discussion... /issued 911 PM CST Friday Nov 20 2009/ 


Update... 
there is finally some weather to talk about this evening. A 
series of well defined upper level disturbances are moving east 
across the western Gulf Coast region. The lead shortwave is 
near the western border of the forecast area and is preceded by 
patchy light rain...mostly falling as sprinkles with only trace 
rainfall amounts reported thus far due to the very dry air below 
700 mb. There is likely some measurable light rain reaching the 
ground over the coastal waters from Southwest Pass to the lower 
Atchafalaya River. Deeper convection with lightning is occurring 
farther offshore near the middle buoy /42001/. 


There is a break behind this first disturbance with no rain 
currently being reported across much of southwest Louisiana. 
Radar shows only isolated shower activity off the coast of 
southwest Louisiana. 


Farther west...the main...stronger shortwave trough and developing 
upper low is over central Texas and preceded by another 
disturbance and the main surface low near the central Texas coast. 
Thunderstorms associated with this disturbance extend farther 
north...but are mostly elevated. This system produced over 10 
inches of rain along portions of the central Texas coast last 
night and this morning. 


The complicating part of this update is how to describe the rain 
trends tonight with the aforementioned features in the previous 
paragraphs. The best way to describe the forecast is a few 
sprinkles with a slight chance of barely measurable /.01 inches/ 
early on...trending towards more steady rain in some southern 
areas...but then only a slight chance of showers for several 
hours between shortwaves...followed by more widespread rain and 
showers spreading from west to east after midnight as isentropic 
lift increases ahead of the second disturbance and approaching 
surface low. Having said that...have placed wording of sprinkles 
early on in the zone forecast then trended towards the more 
numerous to widespread showers in the western and central zones 
later on tonight. A note about the models...the 18z GFS will 
verify horribly with its heavy rain that is shows this evening 
over southern portions of southeast Louisiana. 


Looking at winds...they have been trending a bit lighter than 
forecast...so do not see the reason to raise the Small Craft 
Advisory /sca/ flags just yet. The next shift will have to 
monitor new data and decide if a Small Craft Advisory is needed on Saturday. 


On Saturday...also made some adjustments to include isolated to 
scattered thunderstorms over mainly coastal land areas and some 
inland parishes and counties to be consistent with Storm Prediction Center. Model 
soundings show very elevated instability lifting from 750-700 
early morning lowering to 850-800 mb in the afternoon before drier 
air diminishes the probability of precipitation late in the afternoon. It appears any 
true surface based instability will remain well offshore even if 
the surface low tracks slightly inland...owing to the lingering 
effects of current dry air in the low levels. Will have to watch 
this however...and am still indicating the possibility of strong 
to a few severe storms near the coast and offshore in the 
hazardous weather outlook given the strength of the middle/upper 
system and the strong diffluent flow ahead of it. 


Fortunately for US...the excessive rainfall amounts that fell in 
Texas should not even come close to southeast Louisiana and 
coastal Mississippi...and the models appear to have a good handle 
on this showing the higher amounts in the 1 to 2 inch range. 


Updated grids and text products have been sent. 


22/dew point 


Aviation... 
middle cloud deck will progressively lower to 4 to 6 kft by 08-12z 
as rain falls into drier air and the surface low approaches. Rain 
shield from this feature will gradually move in tonight an into 
tomorrow morning. Ceilings will drop to mostly MVFR around daybreak 
to middle morning...with lower cugs possible during clusters of 
stronger showers and any thunderstorms. Winds will gradually 
increase to around 10-12 knots and remain mostly northeast. 


Previous discussion... /issued 725 PM CST Friday Nov 20 2009/ 


Update... 
sounding discussion... 


No problems with the flight this evening. Moisture creeping back 
ever so slowly with a precipitable water up to 0.91 inches. Still plenty of dry 
air below 700 millibars but a very moist sounding above that 
layer. Rainfall amounts will likely be only sprinkles and very light 
showers until the lower layers become moist. Winds are light and 
variable in the lowest levels and from the southwest aloft. 


Previous discussion... /issued 327 PM CST Friday Nov 20 2009/ 


Short term... 
advancing high cloud deck a result of large convective flare-up in 
the west Gulf this morning that has since moved into the middle-Gulf 
and weakened during the past few hours. Large upper level 
divergence structure with advancing low pressure trough swinging 
across Lower Texas. Pressure falls noted along the Lower Texas 
coast vicinity of Brownsville for what appears to be cyclogenesis. 
Meanwhile...high pressure that was settled over the souteastern 
U.S. Has weakened and retreated northward...thereby softening the 
gradient more than previously expected from earlier model runs. 
Generally following continuity of past forecasts and somewhat in 
line with GFS. Biggest uncertainties lie with future track of 
surface low...which may be much farther south than any models are 
indicating with wake depression height falls behind convective 
complex in Gulf carving a more apt trajectory than model indicated. 
If such is the case...then most of the land areas will be 
subjected to isentropic lift stratiform and elevated convection 
for a duration of 12 to 18 hours onsetting around midnight in the 
west and exiting the east around Saturday late afternoon. The bulk 
of surface based convection should remain over marine areas but 
trends will have to be monitored closely overnight and 
particularly Saturday morning. Current ambient conditions are 
rather dry and continentally influenced so the saturation process 
from top-down will be a rather slow one. After low pressure 
passage Saturday...high pressure should build into the region and 
maintain a long stretch of seasonably cool to mild temperatures. 


Long term... 
seasonably cool to mild temperatures and generally rain free days 
are in store except for a brief shot at isolated showers Tuesday 
with the passage of re-enforcing cold front. Moisture does appear 
to be limited with this system. Thanksgiving travel should be 
devoid of any weather delays across the forecast area. 


Marine... trends with surface pressure gradient will have to be 
monitored next 12 hours but does appear to be hold around the 
15-20 knots range as long as the low pressure system does not deepen 
much further than 1010mb upon approach. A period of higher gusts 
in the 25 to 30 knots range may be possible Saturday late morning but 
not highly confident given the weakeing gradient. Will Post 
a'small craft exercise caution' headline valid after 00z for the 
coastal areas at this time with the possibility of small craft 
advisories if conditions exceed expectations. After low 
passes...some enhanced winds due to short duration cold air 
advection into Sunday...then flattening out for much of next week. 
Frontal passage on Tuesday does not appear to have much impact on 
coastal areas other than to re-enforce high pressure already in 
place. 


&& 


Preliminary point temps/pops... 
mcb 48 58 46 64 / 60 80 40 10 
btr 50 59 47 67 / 80 80 20 10 
msy 55 63 52 66 / 80 80 20 10 
gpt 53 63 52 64 / 40 90 40 10 


&& 


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


$$ 

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