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New Orleans Local Weather thread - dkmac - 11-07-2008 06:01 AM

000
fxus64 klix 070936
afdlix

area forecast discussion
national weather service new orleans la
336 am cst fri nov 7 2008

.short term...

A cold front will continue to slowly move eastward across the
forecast area today. The front will be forced eastward by a vort
lobe swinging through the southern plains. Strong forcing along
the front...and ample lift aloft associated with the right
entrance region of 110 kt jet streak will allow for the continued
development of convection along and ahead of the frontal boundary.
In the low levels...ample instability is in place with lifted
indices of -1 to -2 and some cape running up to around 500 j/kg.
However...overall low level shear profiles are limited...with some
weak speed shear at best. With these factors in mind...do not
expect any severe thunderstorm activity today. However...a few
strong thunderstorms cannot be ruled out.

The front will quickly pull eastward...as the vort lobe slides
through...and strong negative vorticity advection take hold.
Strong cold air and dry air advection...as well as ample
subsidence will allow for rapidly clearing skies and much cooler
temperatures. In addition...strong boundary layer winds aloft will be
easily transported down across the coastal waters due to the
subsidence aloft...which will allow for small craft conditions
later tonight. A cool and dry ridge of high pressure will settle
in for the weekend...while the flow aloft becomes zonal. Clear and
seasonably cool conditions are expected.

.long term...

Going into next week...overall forecast confidence is fairly low
as the models have changed significantly between runs. In
addition...there is very little agreement between the ecmwf and
gfs on the speed of the next system as it exits the desert
southwest early in the week. The gfs is much more progressive with
this system...while the ecmwf continues to dig a cut off low into
the southwestern states through the end of the week before lifting
the low out next weekend. Given this wide spread in the
models...have spread out the pops through the week...as timing
uncertainties are very high. The biggest change overall...has been
to slow up the initial short wave energy expected to pull out of
the four corners early this week. Both the gfs and ecmwf show a
slower solution...and have removed pops for monday and monday
night. As a broad area of ascent develops over the area on
tuesday and a surface warm front pushes through..expect some shower
activity to form...especially in northern and western zones. The
vort max should slide through tuesday night into wednesday...and
have chance pops in for this time period. Then keep chance pops in
through thursday...as it appears that a broad area of upper level
lift associated with strong positive vorticity advection in
advance of the next trough over the four corners will remain over
the region. In the low levels...a warm and unstable airmass will
also remain in place...as little change in airmass is expected
behind the initial vort max. This secondary trough axis should
slip through the lower mississippi valley friday into
saturday...but overall timing of this system is highly uncertain.


&&

.aviation...
Cold front extending from northern mississippi to southwest
louisiana continues to make eastward progress. Shower activity has
steadily increased this morning...but very little lightning. Ifr
ceilings have moved into kmcb and kbtr and will continue to develop
and spread eastward this morning. Showers and possibly a few
thunderstorms will affect kmcb and kbtr possible after 10z and kmsy
and kgpt after 12 or 14z. The front should move through all of the
taf sites by late friday morning. Vfr conditions outside any
precipitation should return friday afternoon. 18

&&

.marine...
Southeast flow should remain below sustained 15 knots before frontal
passage this afternoon. Considerable amount of very dry air
behind front will allow for momentum transport and a few hours of
gustiness through much of today and tonight night. 1000 to 700mb
thickness layers drop from 3000m to 2955m through sunday afternoon.
A small craft advisory has been issued for the coastal waters this
evening through saturday morning with winds of 20 knots and seas
building up to 6 feet. With cold air advection continuing through
sunday...winds may kick up a little saturday night too...but below
sca. 18

&&

.fire weather...
Wettable rains still expected today in advance of the front.
Forecast will indicate a 50-60 percent chance. Dewpoints will be
falling off significantly post frontal with humidities lowering into
the 25-30 percentile saturday and again sunday. 18


New Orleans Local Weather thread - ROLLTIDE - 11-07-2008 11:19 AM

[Image: kmob_br248.png]


New Orleans Local Weather thread - dkmac - 11-08-2008 06:38 AM

000
fxus64 klix 080935
afdlix

area forecast discussion
national weather service new orleans la
335 am cst sat nov 8 2008

.short term...

Strong subsidence and dry air advection in the wake of a trough
axis that passed through this evening will continue to bring clear
skies to the region through tomorrow night. Some weak cold air
advection is also noted tonight...with temperatures a good 20
degrees cooler than last night. 1000-500 mb thicknesses will
remain around 5600 meters through the weekend...which will support
near to slightly below normal readings.

Going into monday...the pattern will change as deep the trough
over the eastern third of the nation begins to lift out. Across
the gulf south...deep northwest flow will break down...allowing a
a weak upper level ridge to build in. In the low levels...surface
ridging to will slide eastward...allowing southerly return flow
off the gulf to take hold. The atmosphere will modify slowly at
first...but a few showers will be possible late monday afternoon
over northern and western zones. These showers will be supported
by a fast moving vort max swinging through the southern
plains...and a broad area of pva pushing through the region.

.long term...

The forecast becomes less certain beyond monday...as a great deal
of difference exists between the global dynamical models. The gfs
is much more progressive with a second...much stronger...trough
expected for later in the week. However...the ecmwf cuts the low
off over northern mexico before shearing it out over the gulf of
mexico next weekend. Given these timing and phase
uncertainties...there is very little confidence in the extended
forecast package. At this time...have decided to make little
overall change in the forecast...with more of an emphasis on the
gfs solution overall.

There is greater certainty on the initial vort max...which was
mentioned in the short term section. This trough will continue to
impact the area monday night into tuesday...and have chance pops
in the forecast to reflect this. These higher pops are supported
by a combination of decent surface instability with lifted indices
running around -2...deep mid-upper level lift due to the trough
axis sliding through and a strong jet streak settling over the
region...and a weak frontal boundary stalling over the
area...which will serve as a low level focus for convective
development. It appears that the boundary could remain stalled
over the area through wednesday...which has led me to keep chance
pops in tuesday night and wednesday as well. Overall instability
will be lower...but strong dynamic forcing aloft associated with
the jet streak and strong pva in advance of rapidly deepening
trough over the southern plains will support the continued
development of rainfall along the stalled low level boundary.
There is some concern that some heavier rains could develop along
the boundary...but have left this out of the forecast for now due
to the overall uncertainty going out beyond tuesday.

The second...much stronger trough should sweep through the gulf
south on thursday. Once again keep chance pops in for thursday and
thursday night as this trough axis slides through. This looks to
be very strong system...if the gfs solution is correct...and given
the dynamic structure of the system...there could be a round of
severe thunderstorms thursday afternoon into thursday ahead of a
surface low expected to form over the forecast area.
However...given the timing differences between the medium range
models...have not mentioned any severe thunderstorm activity in
the forecast package. This scenario will have to be monitored over
the next few days. As the trough axis slides to the east on
friday...deep northwest flow will usher in a cooler and drier
airmass...which will allow for clearing skies...lower
humidities...and a return to below normal temperatures.

&&

.aviation...
/12z taf issuance/
isolated restrictions in light fog/br may impact kbtr with mvfr
vsbys right around 12z this morning...otherwise...clear skies/vfr
weather will prevail the next 24 hours with high pressure in
control. Winds will mostly be less than 10 knots. 22

&&

.marine...
Subsidence behind mid level trough passage and cold air advection
have brought winds up near small craft advisory criteria in the
offshore waters east of southwest pass where surface pressure
gradient is also supportive of 20 knot winds. Seas have only reached
4 feet at buoy 42040 as of 3 am...but they may reach 5 to 6 feet
briefly before settling down with winds after sunrise. Have
cancelled small craft advisory and replaced with /small craft
exercise caution/ for all western waters and eastern waters 0-20 nmi
offshore. Will have /small craft advisory for winds/ in the eastern
leg 20-60 nmi offshore until 6 am this morning. Winds and seas will
diminish considerably by late morning and afternoon. Cool high
pressure over land may cause offshore winds to increase to 15 to 20
knots late tonight across the eastern waters for a few hours...then
winds will subside again during the late morning and afternoon on
sunday. A couple weather systems may bring surface low pressure
areas across the central gulf coast region during the middle to
latter part of next week. Hazardous winds and seas look most likely
beyond wednesday. 22

&&

.fire weather...
The light rainfall from yesterday will quickly finish evaporating
and soaking into the parched ground this morning as relative
humidities plunge into the 25 to 30 percent range by late morning/
early afternoon. Fortunately...winds will be less than 10 mph in
most areas today...so fire spread will not be a problem. Sunday will
be cooler with a light breeze...but still dry with lowest humidities
dropping to around 30 percent. 22


New Orleans Local Weather thread - dkmac - 11-09-2008 06:49 AM

000
fxus64 klix 091034
afdlix

area forecast discussion
national weather service new orleans la
434 am cst sun nov 9 2008

.short term...
The dry and mild weather pattern will hold for a couple more days
as surface high pressure remains in control. A cold autumn storm
system over the great lakes and adjacent canadian provinces will
linger today before moving east towards the atlantic coast monday
and monday night. A mid/upper low/trough moving across california
early this morning will move across the desert southwest today...
Southern rockies late tonight...then northeast across the
southern/central plains monday and monday night. Increasing
southwest flow aloft and warm air advection will bring a low
chance of showers...possibly a thunderstorm or two monday night
across the far north and west.

.long term...
A shortwave trough will move southeast across the southwest conus
and northwest mexico tuesday and then east with a positive tilt
across texas and north central mexico on wednesday. The latest
06z gfs and 00z ecmwf are in better agreement with this
pattern...and both models suggest a heavy rain scenario for
portions of the western and central gulf coast region tuesday into
wednesday with the highest shower and thunderstorm chances and
qpf north and west of lake pontchartrain.

The models diverge with the southern energy thursday through
saturday with the ecmwf closing off an upper low over northeast
mexico and moving it slowly east into the western gulf of mexico
saturday morning. The gfs only briefly closes off this system and
then moves it faster east into the western gulf friday morning.
The surface pattern will be complicated with a weak low and
initial cold front moving towards the area and weakening
wednesday. Later in the week...a deep upper trough will set up
over the eastern half of the conus with cold air pouring south
into the southern states friday through sunday with a cold frontal
passage. There will be a continued chance of showers and isolated
thunderstorms thursday...then dry air will push any threat of rain
south into the coastal areas/waters late friday into saturday.
Much cooler and drier air is expected in the wake of the cold
front next weekend. 22

&&

.aviation...
/12z taf issuance/
strong surface ridging across the region...combined with strong
subsidence aloft will continue to bring clear skies and vfr
conditions to all of the terminals through 12z monday. 32

&&

.marine...
A cool and dry surface ridge...combined with strong subsidence aloft
will dominate the region through tomorrow. Expect a fairly
persistent northerly to northeasterly flow around 10 knots over the
coastal waters...which should put a light chop out there.
However...no small craft conditions are expected. A fairly potent
low pressure system will then begin to take shape for midweek going
into tuesday and wednesday. The flow should turn more
southeasterly...and the pressure gradient should tighten up a
bit...allowing for possible exercise caution conditions wednesday
into thursday. High pressure should then briefly build in on
friday...with a return to calmer winds and seas. 32


New Orleans Local Weather thread - ROLLTIDE - 11-10-2008 01:05 AM

...Fall Severe Weather Season...
...Review Severe Weather Safety Rules...


As the first cool fronts of the fall season arrive, the threat of a significant tropical weather event is decreasing for southeast Louisiana and coastal Mississippi. However the likelihood of severe weather and associated hazards of tornadoes and damaging winds is increasing. Often the severe weather develops as warm and unstable air moving northward from the Gulf of Mexico interacts with cold fronts or upper level disturbances moving eastward through the area. Severe thunderstorms and tornadoes can develop in this unstable environment.
CLIMATOLOGY

Historical records show that the fall season, particularly November, has produced a large share of severe weather in Gulf Coast region. The fall season is only secondary to the primary severe weather season that occurs during the spring months. In addition, the severe weather often occurs at night. The severe weather may take the form of damaging straight-line winds, tornadoes, and large hail.
Examples of several recent damaging severe weather events in southeast Louisiana and south Mississippi include:
  • November 21, 1997 - a strong tornado moved across St Tammany Parish, Louisiana, and continued into Pearl River County, Mississippi. Covington, Louisiana, and surrounding areas were particularly hard hit with nearly 70 homes damaged and 4 downtown buildings damaged. 47 people suffered mainly minor injuries.
  • October 29, 2002 - a weak tornado moved across the southeast portion of the Baton Rouge area during the early morning hours, knocking down trees and damaging house roofs. Straight-line wind damage occurred in several other areas from Baton Rouge area eastward to Pearl Rive County, Mississippi.
  • November 24, 2004 - damaging straight-line winds and several tornadoes were reported during the early morning hours in an area from near Baton Rouge eastward into coastal Mississippi. A strong tornado touched near the Slidell airport damaging 152 homes in a nearby subdivision. The same storm system produced another strong tornado that touched down around sunrise just north of Gulfport. The tornado damaged homes and businesses. Fortunately only a few relatively minor injuries were reported. Straight-line wind damage occurred in other areas of southeast Louisiana and south Mississippi.
  • November 15, 2006 - Severe thunderstorms produced tornadoes as they tracked across areas north of Lake Pontchartrain into southwest Mississippi during the early morning hours. Several strong tornadoes touched down from east of Greensburg, Louisiana, across northern Washington Parish to east of Tylertown, Mississippi. Unfortunately, one fatality occurred near Montpelier in St Helena Parish when a strong tornado struck a house and adjacent trailer.
SEVERE WEATHER DEFINITIONS, WATCHES, AND WARNINGS

Severe thunderstorms, by definition, can produce wind gusts of 58 mph or higher, and hail greater than three quarters of an inch. Wind gusts associated with severe thunderstorms can down trees, power lines, blow out plate glass windows, and heavily damage house roofs.
Tornadoes can also develop from severe thunderstorms. Tornadoes can inflict damage ranging from minor damage to roofs and trees to total destruction of well constructed buildings.
The National Weather Service issues Watches and Warnings to help inform you of the approaching hazards of severe weather.
A Severe Thunderstorm or Tornado Watch is issued when atmospheric conditions are conducive for severe thunderstorms and tornado. Typically watches cover large portions of a state or states and are in effect for around 6 hours. If a watch is in effect, be alert and monitor weather conditions and listen to television or radio for the latest watches and warnings.
A Tornado Warning or Severe Thunderstorm is issued when reliable ground truth reports have been received or when doppler radar indicates there is a high threat of tornadoes, damaging winds, or large hail.
SEVERE WEATHER SAFETY RULES

If a warning is issued for your area...take action immediately. Seek shelter in an interior room on the lowest floor of a well constructed building away from doors or windows. An interior small room such as a bathroom or closet, or even a hallway away from doors and windows is the best location
If you are living in a travel trailer or mobile homes make sure it is anchored securely as these structures are more vulnerable to severe thunderstorm winds and tornadoes. If possible, move to stronger and well constructed building if severe weather develops.
RELIABLE SOURCE OF WEATHER INFORMATION

One of the most important features of your severe weather safety plans is to have a reliable means to receive severe weather warnings if severe weather is forecast, monitor commercial television, radio or cable television for the latest weather information,including watches and warnings.
A battery operated NOAA Weather Radio is an excellent means to receive severe thunderstorm and tornado warnings. Weather radios can be programmed to automatically alert if warnings are issued for your parish or county. Weather Radios are especially helpful at night when people may not be monitoring the weather or television. The Weather Radios can be programmed to automatically alarm if a warning is issued.
For additional information on severe weather visit the National Weather Service web site at http://www.srh.noaa.gov/lix or contact the National Weather Service forecast office in Slidell at telephone number 985-649-0357 or 504-522-7330.


New Orleans Local Weather thread - dkmac - 11-10-2008 06:37 AM

000
fxus64 klix 101058
afdlix

area forecast discussion
national weather service new orleans la
458 am cst mon nov 10 2008

.short term...
A more changeable weather pattern is taking shape the next couple
days. Dry high pressure continues to hold over the lower
mississippi valley and southeast states this morning with
generally clear skies reported. However...satellite shows a series
of shortwaves over the southwest states and northwest mexico
rotating around an upper low over arizona. A couple branches of
the upper jetstream are also noted...one over the southwest
conus/northwest mexican border area...and another sub tropical
branch all the way from the east pacific to the northern gulf of
mexico. Pva...upper divergence...and warm air/moisture advection
are causing cooling cloud tops and recent isolated thunderstorm
development from southern oklahoma to the lower texas coast. These
systems will bring an increase in cloud cover to the forecast area
from west to east today and tonight. Have backed off on rain
chances somewhat...starting only in the extreme northwest late
tonight...and only north and west of lake pontchartrain on
tuesday. Have held off on mention of thunderstorms until tuesday
night as a weak surface low pressure area and cold front move
slowly east into northern and western louisiana.

Temperatures should be fairly close to the mav guidance and are
generally warmer tonight through tuesday night.

.long term...
A positively tilted upper trough will move across the southern
plains and northern mexico tuesday night through wednesday and the
lower mississippi valley to the extreme western gulf of mexico
wednesday night into thursday. Numerous disturbances will move in
the belt of stronger mid/upper level southwesterlies ahead of the
main upper trough. It appears the initial low and cold front will
stall and weaken over our area wednesday which will provide the
best chance of showers and thunderstorms. Severe weather is not
expected...but there could be locally heavy rain over about the
northwest half of the area. The models still differ on the timing
of the stronger secondary cold front...the deep upper trough
developing to our north...and rainfall during the thursday to
friday time frame. Have trended towards the drier gfs...but keep
some mention of rain. For the timing of the cold front...have gone
fairly close to the latest 06z gfs which is slower than the 00z
gfs and faster than the 00z ecmwf. The deep upper trough over the
eastern states will deliver some very cool and dry air to the
forecast area this weekend as strong surface high pressure moves
in. Have blended mex temperatures with some of the cooler
gfs/ecmwf surface temperature fields. 22

&&

.aviation...
/06z-12z taf issuances/
vfr conditions through the period. As high pressure drifts toward
the east coast...some mid and high level clouds could move into the
area overnight monday night. Any ceilings below vfr range will hold
off until at least the daytime hours on tuesday. Winds should
generally remain 10 kts or less through 12z tuesday. 35

&&

.marine...
High pressure will gradually drift eastward over the next 24-36
hours. While overall...winds should remain in the 10 to 15 knot
range through wednesday...although winds over the western coastal
waters may get near exercise caution criteria for a time today. A
series of fast moving impulses will move through the area beginning
tuesday night. At this time winds and seas are expected to remain
below headline criteria through the daytime hours on friday. A
strong cold front is expected to move through the coastal waters
friday evening and small craft advisories are expected to be issued
following the frontal passage. 35


New Orleans Local Weather thread - ROLLTIDE - 11-11-2008 01:03 AM

000
FXUS64 KLIX 102137
AFDLIX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
337 PM CST MON NOV 10 2008

.DISCUSSION...
LATEST UPPER AIR ANALYSIS AND IR SATELLITE IMAGERY SHOWED A
PROGRESSIVE PATTERN WITH A TROUGH OVER THE ROCKIES...ANOTHER
TROUGH EAST OF THE HUDSON BAY AND FAST WEST FLOW FROM BAJA
CALIFORNIA TO FLORIDA. SURFACE ANALYSIS SHOWED A 1024MB HIGH OVER
SOUTHERN INDIANA WITH THE RIDGE AXIS SOUTHEAST TO NORTHEAST GULF.
IN ADDITION...A 1002MB ELONGATED LOW OVER EAST COLORADO TO WEST
TEXAS WAS NOTED AND SOUTHEAST FLOW OVER EAST TEXAS AND WEST LOUISIANA.
EAST FLOW WAS NOTED OVER SOUTHEAST LOUISIANA AND SOUTH MISSISSIPPI
THIS AFTERNOON. SHORT RANGE MODELS CONTINUE TO SHOW AXIS
ASSOCIATED WITH THE ROCKIES TROUGH BECOMING NEGATIVELY TILTED OVER
THE CENTRAL PLAINS TONIGHT...THEN EVOLVING INTO A CLOSED LOW OVER
CENTRAL PLAINS. A GENERAL MOIST SOUTHWEST FLOW WILL DEVELOP OVER
REGION AND SLOWLY PUSH THE DRY AIR EAST. THE UPPER LEVEL LOW WILL
OPEN AND LIFT NORTHEAST. HOWEVER...THE SOUTHERN STREAM WILL BUCKLE
INTO WEDNESDAY ALLOWING MOISTURE TO POOL AHEAD THE TROUGH AXIS AND
OVER THE FORECAST AREA WEDNESDAY INTO WEDNESDAY NIGHT. WILL
MAINTAIN LIKELY POPS FOR THIS PERIODS. SOME SCOURING OF MOISTURE
WILL LIKELY...GFS IS AGGRESSIVE WITH DECREASING MOISTURE LAYERS
AND BRINGING THE NEXT SYSTEM IN FRIDAY NIGHT AND ECMWF IS A LITTLE
SLOWER WITH LESS DRYING. WILL GO WITH A BLEND...KEEPING A CHANCE
OF RAIN INTO SATURDAY AND SLIGHT WARMER TEMPS THROUGH SATURDAY
AFTERNOON.

&&


New Orleans Local Weather thread - dkmac - 11-11-2008 06:36 AM

000
fxus64 klix 111108
afdlix

area forecast discussion
national weather service new orleans la
508 am cst tue nov 11 2008

.short term...
Satellite loops show several features that will be impacting our
weather over the next few days. Initial upper low/shortwave is
moving northeast across the central plains while a couple
disturbances can be seen rotating along and north of the well
defined branches of the upper jet stream over the desert southwest
and southern plains. The first impulse over the southern plains
will eject northeast today and tonight while a broad belt of the
upper jet stream sets up from northern mexico/texas to the lower
ohio valley and tennessee valley. The main 500 mb trough axis will
shift east into west texas and north central mexico this evening.
Showers and thunderstorms will continue to develop and move across
texas into northern...central...and western louisiana...and
portions of western mississippi today and tonight. Have kept a
slight chance of showers today for the northwest including mccomb
and baton rouge...and lowered to slight chance tonight for much of
the same area as dry air should hold to the east and south.

On wednesday...the main mid/upper trough axis will shift only
slowly east across the southern plains as a series of shortwaves
on the back side sharpens and holds back the southern portion of
the trough over northern mexico. The retreating low level/surface
high and an approaching cold front with weak surface lows should
allow a warm front with mid 60s dewpoints to lift north into
southeast louisiana and coastal mississippi. The combination of
these features along with upper divergence and deep moisture will
bring the likelihood of showers and thunderstorms with locally
heavy rain to areas north and west of lake pontchartrain...
Especially from around metro baton rouge to mccomb. Spc day two
outlook has a 5 percent chance of severe thunderstorms...but there
could be a better chance in some southern/western areas if the
quality of low level moisture return is as great as the gfs
indicates. The propagation vectors look quite favorable for
regenerative or back-building convection...similar to what has
been occurring over portions of north texas over the last 18 hours
or so. Fortunately...the ground is very dry from the extended dry
spell...but will have to watch for locally excessive rainfall on
wednesday and possibly wednesday night.

.long term...
The last two runs of the gfs have been showing a cut-off low
developing over the lower rio grande valley which moves east into
the northwest gulf of mexico friday. The nam is weaker and a bit
faster with this feature while the ecmwf is also weaker and 12-18
hours slower. Given the uncertainty...have not changed too much
from the previous forecast package and am keeping continued
chances of rain thursday across the area...and mainly coastal from
late thursday to friday night...possibly into saturday morning
over the coastal waters. The much cooler and drier air still
looks on track for the weekend...especially sunday into monday as
a deep upper trough slides east across the eastern conus. 22

&&

.aviation...
/preliminary for 12z taf issuance/
vfr conditions will prevail through the daytime hours today. After a
period of clearing this morning...anticipate mid level clouds to
move into the area to stay this afternoon. Could see isolated to
scattered showers at kbtr this afternoon...spreading eastward to
kmcb this evening. As airmass becomes saturated...mvfr ceilings
could move into kbtr and kmcb during the overnight
hours...especially after 06z wednesday. 35

&&

.marine...
Will add eastern inner waters to exercise caution and carry until
issuance of afternoon package. Buoys 42007 and 42040 continue around
16 or 17 knots sustained...and anticipate this to continue at least
through the morning hours. Winds over tidal lakes remaining just
below 15 knots and will not include headline there.

Winds should generally stay in the 10 to 18 knot range through
wednesday before falling below 10 knots for about 36 hours. With
approach of stronger front on friday...gradient will ramp up again.
With frontal passage friday night...expect winds to increase to
greater than 20 knots over the coastal waters...and advisories will
be needed at that point. 35


New Orleans Local Weather thread - ROLLTIDE - 11-12-2008 01:30 AM

FXUS64 KLIX 112133
AFDLIX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
333 PM CST TUE NOV 11 2008

.DISCUSSION...
MOSAIC COMP REFL OFF REGIONAL RADARS SHOWED A LARGE OF RAIN SHIELD FROM
CENTRAL MISSISSIPPI TO SOUTHEAST TEXAS...WITH EMBEDDED
THUNDERSTORMS OVER EXTREME WEST CENTRAL LOUISIANA AND SOUTHEAST
TEXAS. NAM MODEL SHOWED MARGINAL INSTABILITY WITH 400 TO 800 J/KG
CAPE VALUES ACROSS WEST CENTRAL AND SOUTHWEST LOUISIANA AND
SOUTHEAST TEXAS TODAY. UPPER LEVEL ANALYSIS AND IR SATELLITE SHOWED THE
CLOSE CYCLONIC CIRCULATION OVER EAST NEBRASKA WITH THE AXIS
EXTENDING SOUTHWEST TO THE FOUR CORNERS REGION. IN ADDITION...FAST
SOUTHWEST FLOW WAS NOTED FROM WEST TEXAS TO THE MISSISSIPPI
VALLEY. ISOTACH ANALYSIS AT 250MB REVEALED A 115 KNOT JET ACROSS
THE CENTRAL TEXAS AND JET DIVERGENCE OVER THE ARKLATEX REGION
TODAY. AS THE MAIN TROUGH MOVES EAST...THE TROUGH DEEPENS AND FLOW
BECOMES MORE SOUTHWESTERLY TONIGHT THROUGH WEDNESDAY EVENING.
MOISTURE POOLING AHEAD OF THIS TROUGH AXIS EXPECTED AND NAM AND
GFS MODELS SHOW AT MINIMUM PRECIPITABLE WATER VALUE OF 1.6 INCHES
ACROSS WEST LOUISIANA LATER TONIGHT...THEN SHIFTING SLOWLY EAST
OVER THE FORECAST AREA WEDNESDAY. AT THE SURFACE...NAM AND GFS DEPICTED A
FRONTAL BOUNDARY/INVERTED TROUGH FROM SOUTHWEST LOUISIANA TO NORTH
MISSISSIPPI WEDNESDAY AFTERNOON INTO WEDNESDAY EVENING. IN
ADDITION...LOW LEVEL FLOW AT 850MB WILL BE SOUTHWESTERLY DURING
THIS TIME. RAINFALL IS EXPECTED IN THE SOUTHWEST MOIST FLOW FOR
WEDNESDAY AND TRAINING OF STORMS IS POSSIBLE. WE HAVE BEEN DRY FOR
SOME TIME...BUT COOL GROUND MAY INCREASE RUNOFF...POSSIBLY
EXCESSIVE RUNOFF. RAINFALL AMOUNTS OF 1 TO ISOLATED 3 INCHES
COULD POSSIBLY OCCUR FROM WEDNESDAY AFTERNOON THROUGH THURSDAY
NIGHT AND GENERALLY SOAKING IS MORE LIKELY. OUR OFFICE WILL
MONITOR IF RAIN EVENT SITUATION BECOMES PRESSING. THE SECONDARY
STRONGER BOUNDARY WILL SWEEP THROUGH FRIDAY NIGHT AND WIND SPEEDS
OF 20 KNOTS SOUTH OF THE LAKE WILL NULLIFY ANY WARMING FROM THE
LAKE SATURDAY AFTERNOON. WIND ADVISORY IS POSSIBLE DUE TO THE WIND
0N SATURDAY. DRY CONDITIONS SATURDAY THROUGH TUESDAY. 18


New Orleans Local Weather thread - dkmac - 11-12-2008 06:34 AM

000
fxus64 klix 121109
afdlix

area forecast discussion
national weather service new orleans la
509 am cst wed nov 12 2008

.short term...
This will be the first substantial rain in nearly 2 months for
much of the area...so this is much needed rain. The advertised
weather changes are coming to fruition this morning as satellite
and radar shows a large area of rain and thunderstorms heading in
our direction.

Water vapor loops and upper air data show a positively tilted
mid/upper trough advancing east across the plains with well
defined vort lobes over west texas. Looking at 250 mb...jet level
winds are very strong over the north pacific into southern british
columbia...then arcing southeast and south across the rockies...
Four corners region...to northwest mexico. On the front side of
the trough...the belt of stronger jet stream winds extends from
west central/northeast mexico to the ark-la-tex region...then
arcing anticyclonically across the ohio valley region to the mid
atlantic coast. The most active thunderstorms and heavy rainfall
over southeast texas into southwest louisiana are occurring where
pva...strong upper divergence...and a warm front/low level
convergence boundary are coincident.

The features mentioned above will all be shifting east today. The
only complication is the wedge of slightly drier air at the
surface associated with the stubborn surface ridge that remains
strong from the carolinas and georgia. This drier air is expected
to get flushed out as winds continue to shift to veer to more
southerly today. It appears the thunderstorms will remain slightly
elevated...however...cannot rule out a few strong to severe
thunderstorms with strong straight-line winds possible near bowing
line segments. Like we have been advertising...locally heavy
rainfall is anticipated...starting first in the northwest from
the baton rouge area into southwest mississippi...then spreading
east across southeast mississippi to areas near lake pontchartrain
and the i-59 corridor northward into mississippi this afternoon.
Fairly widespread one half to one inch rainfall amounts can be
expected today with localized 2 to 3 inch amounts possible where
echo training of stronger thunderstorms occur.

Heading into tonight...thursday...and thursday night...the models
continue to show significant spread with timing...movement...and
strength of the mid/upper low that should develop over the lower
rio grande valley thursday and track east or east-northeast and
weaken to a wave across the northwest gulf of mexico and central
gulf coast region. Refer to the latest model diagnostic discussion
from hpc for the details on the models. There is quite a spread in
the mos pop guidance in the latest one to two runs and also some
inconsistency. Feel more forecast confidence with a much wetter
than 00z gfs solution tonight...carrying forward activity from
today with a gradual decreasing trend overnight. Have not included
fog in the forecast at this time tonight...but that may be a
problem if rain and deeper layer clouds clear out late.

Thursday and thursday night should see less rainfall and lower
pops. Have kept higher than gfs pops for now to keep better
forecast consistency and allow for additional weaknesses in
southwest flow.

.long term...
Have gone along with gfs timing with the gulf shortwave and then
the strong cold front that surges through the area friday night.
Pva...plenty of moisture...and low level convergence ahead of the
front should provide a decent chance of additional showers and
thunderstorms on friday...possibly lingering into friday night
ahead of the cold front. After cold frontal passage...the models
are showing rather cold air at 850 mb...near to below zero
celsius...on saturday with slow warming sunday and monday. Have
gone a bit below the mex guidance for highs over the weekend. This
means some locations may only reach the upper 50s for highs.
Clouds will clear out and humidity will drop substantially. The
dry weather and slightly cooler than normal temperatures will hold
early next week as a mean upper trough will cover eastern north
america. 22

&&

.aviation...
/preliminary for 12z taf issuance/
mvfr conditions gradually approaching from the west...with
occasional mvfr cigs at kmcb already...and likely at kbtr in the
next few hours. Moisture return should trigger general ceilings
around 2.5-3k feet for the afternoon hours. As jet max lifts out of
texas...convection across sw la and se tx should approach the area
while in a weakening trend. Thunderstorms possible at kbtr as early
as 13-15z...but probability not high enough for a tempo until around
lunchtime. Potential for convection will spread eastward during the
afternoon and early evening hours. Vfr conditions should return
by late evening for several hours...but with light winds and
abundant moisture...expect mvfr to ifr ceilings and vsbys around
sunrise thursday morning. 35

&&

.marine...
Will continue to assess the exercise caution on the coastal waters.
Buoys 42007 and 42040 continue to sit around 16 or 17 knots.
Guidance indicates easing of winds somewhat during the morning
hours...so will only continue headline until next forecast issuance.

Surface boundary to the west is expected to become stationary near
or over our coastal waters in the next 24 hours. This will allow
gradient to be somewhat slack for about 36 hours or so...until main
front approaches late friday. Winds will quickly turn more southerly
again on friday as that front approaches the area. This front will
arrive some time friday night or saturday morning. As a strong high
builds into the region behind this front...small craft advisories
will likely be necessary on the coastal waters and tidal lakes. 35