Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service New Orleans la
649 am CST sun Dec 4 2016

..sounding discussion...

A very moist airmass is over the region with the precipitable
water at 1.67 inches, which is a record value for this date/time.
This moisture is a result of deep southwesterly flow. Some drier
air is in the mid-levels, which contributes to a dcape value near
900 j/kg. Low level (surface to 850mb) relative humidity is at 95% with
saturation to about 2400 feet may prevent wind gusts from being
entrained to the surface. K index value of 34 indicates heavy
rain is certainly a possibility. Recent rain amounts support this
with almost 2.5 inches at Baton Rouge in the last 6 hours and a
storm total over 4 inches at Grand Isle and likely 3 to 4 inches
somewhere over southern Plaquemines Parish given current legacy
and dual pol rainfall estimates. Thunder may be limited this
morning as lapse rates are on the weak side near 6c/km and
instability is rather limited for surface based and mixed layer

12z balloon info: a routine flight ascending for 68 minutes to a
height of 13.8 miles above the ground bursting near Wiggins 54
miles downrange from the office.



Previous discussion... /issued 316 am CST sun Dec 4 2016/

latest surface analysis showed over a broad 1025 millibar high
from the Great Lakes region to the mid Atlantic states. In
addition, an inverted trough/warm front was along the Texas and
Louisiana coast with dewpoint readings 65 to 70f degrees.
Precipitable water values on a planer view revealed moisture axis
of 1.7 to 1.9 inches from Lower Texas coast to southwest Louisiana
to southwest Mississippi. Latest upper air analysis showed a
closed low south of New Mexico with a moist southwest flow over
Texas and mid and lower Mississippi Valley. Regional radars showed
widespread rain from northwest Gulf to west central Alabama,
thunderstorms were along the leading edge of rain Florida parishes
to northwest Gulf of Mexico.

Short term...
moisture axis is expected to drift east to northwest Gulf of
Mexico to Atchafalaya Bay to Mississippi coast with values 1.8 to
2 inches. Moderate lift ahead of the main trough will yield
mainly moderate precipitation through today. Rainfall amounts between
0.75 and 1.5 inch will be common with heavy amount up to 4 inches
this morning through this evening. Near and south of the warm
front, south winds will increase low level shear to create a few
rotating storms. The warm front will remain nearly stationary this
morning through Monday morning and any storm north of the coast that
do show signs of rotating will likely remain elevated.

Upper level low over Mexico will finally open and lift northeast
tonight. The approaching upper level system will assist the
development of a surface low over Texas coast early Monday. As the
system lifts northeast over East Texas Monday afternoon, the
surface low will push northeast, placing the forecast area in the
warm sector Monday afternoon and evening. Southerly winds at the
surface and lower levels will create shear and create a
environment conducive of a few two rotating storms Monday
afternoon and evening. While the threat of isolated strong to
severe storms increases, rain chances remain elevated with heavy
rainfall due better lift. Pooling moisture along an axis is not
expected in the warm sector on Monday. Damaging winds from a few
severe storms will be another primary threat on Monday too.

Storm Prediction Center continue to include a 15% severe weather risk area for Monday
and Monday night. This risk area currently encompasses all of
southeast Louisiana and southern Mississippi.

Long term...
the associated cold front will sweep through the area Monday
night and we should see a brief lull in the rain chances Tuesday
afternoon through Wednesday evening before a reinforcing front
brings another shot of rain through the area Wednesday night and
early Thursday. With little instability in place, however, we
should only rain showers ahead and along the front and have
maintained low end chance pops. Behind this second front, we could
be in for a blast of colder air with temperatures well below
normal for the end of next week. 18

latest radar imagery shows a large swath of moderate heavy rainfall
approximately 100 miles wide slowly moving east across south
Louisiana and southwest Mississippi. This band will progress
eastward through the morning and likely leave a lull of isolated
showers late morning thru early afternoon. After that, another large
band of showers and isolated thunderstorm are expected to move in
from the Gulf of Mexico. Thus, MVFR to LIFR conditions will continue
this morning with VFR returning as the rain subsides. Rain coming
back later in the day will lead to low ceilings and visibility restrictions
this afternoon and Onward.


a surface ridge centered over the Great Lakes extends to the central
Gulf of Mexico while an inverted surface trough extends along the
northwestern Gulf. The pressure gradient between these 2 features is
still strong enough to produce small craft wind speeds but not for
too much longer. Winds will being weakening throughout the day as
the gradient relaxes. The Small Craft Advisory will expire accordingly. Winds will
shift to the west and increase to near small craft Monday night as a
cold front associated with that low pressure moves through. Expect
this wind shift to be brief as the pressure field breaks down before
a very strong cold front marches through Thursday morning.


Decision support...

Dss code: blue.
Deployed: none.
Activation: none.
Activities: Small Craft Advisory. Monitoring convective and heavy
rainfall trends through Monday night.

Decision support services (dss) code legend
green = no weather impacts that require action.
Blue = long-fused watch, warning, or advisory in effect or
high visibility event.
Yellow = heightened impacts with short-fused watch, warning or
advisory issuances; radar support.
Orange = high impacts; slight to moderate risk severe; nearby
tropical events; hazmat or other large episodes.
Red = full engagement for moderate risk of severe and/or
direct tropical threats; events of National significance.



Preliminary point temps/pops...
mcb 61 53 65 54 / 90 70 80 70
btr 66 55 69 54 / 90 70 80 50
asd 72 58 69 57 / 70 60 80 70
msy 73 61 71 58 / 70 60 80 60
gpt 71 59 69 59 / 70 60 80 70
pql 73 60 70 59 / 80 60 80 70


Lix watches/warnings/advisories...
GM...Small Craft Advisory until 3 PM CST this afternoon for gmz555-

MS...coastal Flood Advisory until 9 am CST this morning for msz080.

GM...Small Craft Advisory until 3 PM CST this afternoon for gmz557-



National Weather Service Glossary of Abbreviations