Scientific Forecaster Discussion
Area forecast discussion
National Weather Service New Orleans la
415 PM CST Thursday Nov 20 2014
return flow will continue to bring warm and moist air into the
area from the Gulf of Mexico as high pressure continues to shift
eastward. Gradual warming and moistening trend will continue
Dewpoints currently running in the low to middle 40s...and skies
remain mostly clear so we should see some fairly efficient
radiative cooling again tonight. Lows are forecast to bottom out
near normal tonight...generally in the middle 40s north and in the
upper 40s and lower 50s south. Another relatively pleasant day on
tap for tomorrow with forecast highs in the lower 70s...though
dewpoints will continue to gradually increase throughout the day.
With the increased moisture and some cloud cover forecast to move
into the area as well...lows Friday night will be above normal...
generally in the low 50s north and middle to upper 50s south.
Our weather becomes significantly more active on Saturday as the
next frontal system begins to impact the area. Looks like we/ll be
dealing with multiple hazards...so I/ll handle them in sequential
order. First...as the pressure gradient tightens ahead of the next
low/cold front...winds will strengthen out of the east/southeast.
Right now forecast wind speeds do not look like they/ll reach Wind
Advisory criteria...but we/ll be keeping an eye on trends over the
next few forecast cycles. Stronger onshore winds over the Gulf
waters will peak right around the time of high tide resulting in
elevated tides on east facing shorelines...and tide modeling
indicates potential for some minor/nuisance coastal flooding
outside of levee protected areas. With that said...have gone ahead
and hoisted a coastal Flood Advisory for east facing shorelines
from the mouth of the river eastward to Bay Saint Louis from noon
Saturday until 6 am Sunday.
On to the thunderstorm and severe weather threat...there will be
plenty of moisture in place by Saturday after a few solid days of
return flow and model forecast precipitable water values in the 1.75 to 2 inch
range by Sunday evening. Model forecast soundings indicate
generally 500 to 1000 j/kg of ml cape across the area.
However...there are a couple things that could curb thunderstorm
development. The first is the potential for a shallow marine layer
moving inland across coastal areas. Models also suggest a large
area of showers and thunderstorms ongoing over the Gulf waters and
parts of coastal Louisiana through much of Saturday. These showers
and thunderstorms could limit the northward advance of the more
unstable air. Either way though...we should see some
destabilization during the day...and am carrying high end chance
to likely probability of precipitation for Saturday.
Going into Saturday evening and night...the upper trough will
move across the local area and take on a negative tilt. This will
help support development of severe thunderstorms and we are
currently expecting a squall line to develop ahead of the cold
front. This squall line will pose the greatest threat of severe
weather as this is when the dynamical support will be greatest
over the local area. Large right turning hodographs suggest
potential for development of rotating updrafts...especially in any
individual cells that develop ahead of the line. These rotating
storms lead to the potential for isolated tornadoes. Along the
line...the main threat will be strong/damaging wind gusts...but
there could also be a few embedded rotating storms which would
allow for tornado and/or large hail formation. Right now...Storm Prediction Center is
indicating a slight risk of severe weather across the entire local
area. Unfortunately...timing does appear to be during the late
evening and overnight hours...with the trough axis moving through
the local area between 06z and 12z Sunday.
Another consideration here will be the rainfall totals. Area wide
averages of 1 to 2 inches are currently forecast with some areas
possibly seeing 3 or more inches. With antecedent conditions being
so dry for most areas...this shouldn/T lead to any significant
negative impacts. However...if an area sees repeated storms or if
rainfall rates exceed drainage capacity...we could see some street
flooding or ponding in low lying/poor drainage areas.
We will continue to narrow down timing and specific impacts of
this system over the next few forecast cycles.
conditions should rapidly improve from west to east Sunday
morning after sunrise as the cold front moves through the area.
The remainder of the week should be dry and cooler as a deep
trough sends another strong cold front through the area Monday and
then again Tuesday night with a reinforcing boundary.
VFR conditions are expected to continue through tonight and Friday
though marine layer clouds will return to the area generally based
at or above sct-bkn035.
the main concerns will be a tightening of the easterly
gradient wind Friday into Saturday with warm frontogenesis taking
place. As strong dynamic upper system lifts northeast from south
Texas Saturday night...onshore flow is expected to become quite
strong as low pressure area moving into middle-Mississippi Valley draws
the flow northward. The period of strongest winds will likely
coincide with astronomical high tide to produce enhanced tide
situation of 1-2 feet above normal. A coastal Flood Advisory has
been issued to highlight this impact potential. Small Craft Advisory
conditions are highly anticipated for Saturday through Sunday. A
marine weather statement will be issued to bring attention to the
inclement conditions expected for mariners this weekend.
assessing severe weather threat for Saturday/Sunday
monitoring coastal flood threat for Saturday night.
Decision support services (dss) code legend
green = no weather impacts that require action
blue = long fused watch/warning/advisory in effect or high
yellow = heightened impacts with short fused
watch/warning/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 to high risk severe
and/or direct tropical threats; events of National
Preliminary point temps/pops...
mcb 65 44 69 48 / 10 10 10 10
btr 68 48 72 49 / 0 10 10 10
asd 67 44 71 49 / 0 0 10 10
msy 68 53 71 57 / 0 0 10 10
gpt 65 47 69 51 / 0 0 10 10
pql 65 41 69 46 / 0 0 0 10
la...coastal Flood Advisory from noon Saturday to 6 am Sunday for the
following zones: lower Plaquemines...lower St.
Bernard...Orleans...and upper St. Bernard.
MS...coastal Flood Advisory from noon Saturday to 6 am Sunday for
the following zones: and Hancock.
rest of discussion...95/dm