Scientific Forecaster Discussion
fxus64 klix 211434
Area forecast discussion
National Weather Service New Orleans la
834 am CST sun Jan 21 2018
the local atmospheric column has begun to dry considerably as a
result of northwesterly flow in the mid levels. A 30 to 50 degree
dewpoint depression resides in that region of the atmosphere.
This should keep most of the area from seeing any shower
development today. There is good saturation but only from the
surface to about 50m. This depth is too shallow for fog of any
significance to develop.
Previous discussion... /issued 340 am CST sun Jan 21 2018/
some patchy fog this morning will lift while the remainder of the
day stays mostly cloudy to cloudy. The main timing for best fog
production will be overnight tonight into Monday morning as the
incoming front helps strengthen the inversion at 950mb capping
the moisture beneath while pressure continues to slowly lower. At
the moment, we will continue with patchy fog for tonight as well,
but this could also end up being fog's twin...a stratus deck at
500 ft since the base of the inversion is high enough.
The main thing with this front is the marginal risk of severe
thunderstorms. Numbers are not impressive, hence the marginal
risk. But they are in The Ball park as profile soundings Show Low
cape high shear. Instability is lacking a bit as Li numbers are
sitting at -1 to -2 and cape values around 450 at best. 0-3km
helicity values are 300-400m2/s2. These numbers show the slight
potential for a spin up. Dry air is also locked in at 700mb as
well, so if a thunderstorm is capable of penetrating this region,
strong straight line winds would be the result. So the main two
things that could result from this marginal risk would be an
isolated tornado &or damaging wind. Storm Prediction Center has the risk area outlined
very well as the best potential energy that can be found is over
southern Mississippi and a portion of adjacent Louisiana parishes.
There is a weak disturbance that moves over the South Shore, but
does not get going until it gets to the eastern coast line. This
disturbance causes thunderstorms to develop from the coast
southward well into the Gulf and begins to moisture starve the
southern end of the cold front. The front moves through Monday
morning clearing things out and cooling US back down, but not as
cool as previous fronts. Matter of fact, we will be hard pressed
to get into the 30s. But some locations over the northern tier of
counties may fall that far.
by the end of this week, another system will be in the area. Winds
will veer around to more easterly while remaining in the 15-20kt
range by Thursday. This continues into the weekend with a long
fetch of moderate to strong east to southeast winds Thursday
night through Saturday. Winds will ramp up quite a bit(25-30kt)
over the Gulf as a sfc low develops over the southwest Gulf and
move north into our area by Saturday. This will cause tide levels
to rise by at least a foot if not two. Rainfall and the
possibility of severe weather will depend greatly on where the sfc
feature moves inland. At the moment, models barely agree with
keeping this low along the coast or just inland as it moves north
then northeast ahead of the cold front that should move through
early Sunday. This current scenario would rob moisture away from
the cold front causing thunderstorm activity associated with the
front to decrease. But, if the sfc low is nudged westward, then
the heavy rainfall and severe weather would show much better
chances of occurring.
VFR conditions expected for much of the first half of the forecast
period with the exception of khum, which is reporting MVFR to IFR
visibilities in fog/mist. Can't rule out LIFR there around sunrise,
but should see improvement by mid-morning. As onshore flow increases
today, expect the column to saturate from the mid-levels downward.
Will likely see ceilings around fl050 by late morning today. By late
evening, prior to 06z, expect ceilings to fall through MVFR to IFR
at most locations. Moisture levels would indicate the potential for
advection fog near/after midnight, especially at kasd/kgpt/kpql, but
guidance lending to the idea of more of a low stratus event.
Precipitation likely to affect some or most terminals prior to 12z
Monday, but will only carry potential of thunder at kbtr in the 06z-
12z range. The late morning package will probably need to add it at
the rest of the terminals beyond 12z Monday. 35
First 24 to 30 hours should be fairly benign for the tidal lakes,
sounds and inner open waters as southeast winds 10-15 knots
expected. Will probably need exercise caution headlines for the
outer waters tonight. After monday's cold frontal passage, there is
likely to be an extended period of conditions worthy of exercise
caution headlines on the open waters through much of the work week.
Since there will not be strong cold advection behind this front
(pacific in origin), the tidal lakes and sounds likely to stay below
criteria for much of the week. As the next storm system approaches
next weekend, a prolonged period of easterly to southeasterly
onshore flow will produce some swell, which could pose coastal
flooding issues in the usual places. Small craft advisories will
also likely be necessary as we get to Friday or Saturday. 35
Dss code: green.
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 73 58 68 39 / 10 40 80 0
btr 73 61 68 39 / 20 60 80 0
asd 71 59 69 42 / 10 20 80 0
msy 72 60 70 45 / 20 20 80 0
gpt 68 58 66 44 / 0 20 60 10
pql 69 57 68 44 / 0 10 60 10