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afdlix

Area forecast discussion
National Weather Service New Orleans la
826 am CDT Wed Apr 26 2017

..sounding discussion...
The sounding this morning depicts the return of moisture and the
Gulf airmass due to southerly onshore flow, though it is shallow.
A strong 17 f temperature inversion is at 900 mb and Marks a much
drier airmass still above. Winds above the inversion become
westerly. Precipitable water is below average at 0.74 inches. The strong inversion
and dry air just above the boundary layer should keep the area
nearly rain free through the day and into the early evening
hours.

Krautmann

&&



Previous discussion... /issued 347 am CDT Wed Apr 26 2017/

Short term...
a broad upper level trough is digging southeast across the southern
rockies and beginning to sharpen. It will quickly move into and
through the Central Plains throughout the day. Models are in good
agreement that the trough will then lift northeast towards the Great
Lakes this evening and tonight. A frontal boundary associated with
the upper trough will be the focus for shower and thunderstorm
activity that moves through the County Warning Area. Convection should be on the
northwestern doorstep of the forecast area Thursday at 00z and
exiting eastern areas around sunrise Thursday. The typical pop
coverage in similar weather patterns is more and stronger storms
along the la/MS border and minimal along the la coast(example: the
last system that just moved through). The current pop forecast is
based on that scenario with 80% in southwest Mississippi and 20% in
southern Plaquemines Parish. The threat for severe weather will be
in a similar with the best probabilities north of I-12 and minimal
threat on the coast. Model soundings show this as well with a mid
level warm nose that doesn't erode and much weaker winds in the
column at points further south than north. Ml cape values over
2000j/kg, Li's -6 to -8 combined with shear over 40 knots will be
sufficient for severe thunderstorms in those northern areas. Current
Storm Prediction Center day 1 shows slight risk northwest of of a btr to mcb line and
marginal for points south of there. In addition, the intensity of
storms should be on a downward trend as they progress through the
eastern half of the County Warning Area early tomorrow morning. This makes sense
with the trough lifting further away, and taking the stronger wind
field with it.

Long term...
Post frontal upper level ridge will be expanding over the
southeastern United States on Friday. Temperatures will respond by
moderating well into the 80s for the afternoon highs. Moisture will
begin to surge in as well as southeast flow strengthens. Low to mid
70 degree dewpoints will engulf the County Warning Area by the afternoon.

The second and likely more impactful event of this forecast period
will be rolling through during the second half of this weekend. An
upper trough will be closing a low and rotating negative as it digs
well into Texas and the lower Mississippi Valley Saturday into
Sunday. A wide swath of showers and embedded strong/severe
thunderstorms will March across the state on Sunday. Surface winds
look to be strong Saturday and Saturday night as a sub-1000mb
surface low develops over Texas. This will continue to bring
increased moisture well past the County Warning Area. Cape values will likely be
over 2000 j/kg as moisture and temps stay high while mid level temps
drop from the approaching deep trough. That type of moisture and
instability combined with a 50+kt mid level jet may set the stage
for quite a bit severe weather and maybe a few significantly strong
cells. All modes of severe weather possible although tornadoes and
wind seem to be the more likely threats. Timing is the biggest ??
attm as now the European model (ecmwf) is slower than the GFS. Will likely see
fluctuations until the event occurs.

Dry and cooler temps expected early next week as Post frontal
surface high pressure builds in. A progressive upper level pattern
in general will likely bring the threat for rain back into the
forecast fairly quickly.

Meffer

Aviation...
scattered-bkn015 will move into each terminal today. This will become an
ovc035 by early evening. Winds will become gusty from the south
today as south winds of 15 to 20 knots with higher gusts are
expected. Some low level ovc005 ceilings may occur overnight as the
sh/ts begin to move in as well.

Marine...
an increasing pressure gradient over the coastal waters related to a
deepening low pressure system in the Southern Plains will result in
exercise caution conditions over all waters through 4pm today. As
winds rise in advance of the front, advisory conditions will start
at 4pm and last through at least 4am Thursday. Winds on the back
side of this front are expected to collapse rapidly and all flags
should be able to lower after 9z Thursday. As a cold front stalls
over the northern Gulf tonight and Thursday, winds will remain
rather weak until Thursday night as flow will become established
once again out of the south ahead of the next much stronger cold
front expected to move through Sunday night in to Monday morning.
Winds will rise enough to become high end advisory conditions but
may actually become strong enough for gale conditions or at least
gusts to gale force by as early as Saturday night. As the front
moves through Monday morning, winds will shift to northwest and
remain at around 20-25kt early Thursday. Winds will ease through the
morning with all flags possibly dropping by noon or shortly
after.

Decision support...
dss code...green.
Deployed...none.
Activation...none.
Activities...none.

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

&&

Preliminary point temps/pops...
mcb 85 55 83 65 / 10 80 0 10
btr 87 58 84 67 / 10 70 0 10
asd 84 63 85 71 / 0 70 10 10
msy 85 63 84 72 / 0 60 10 10
gpt 78 66 83 74 / 0 70 20 10
pql 77 65 84 69 / 0 70 30 10

&&

Lix watches/warnings/advisories...
la...none.
GM...Small Craft Advisory from 4 PM this afternoon to 4 am CDT
Thursday for gmz532-536-538-550-552-555-557-570-572-575-577.

MS...none.
GM...Small Craft Advisory from 4 PM this afternoon to 4 am CDT
Thursday for gmz536-538-550-552-555-557-570-572-575-577.

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