NWS New Orleans, LA wrote its last area forecast discussion before the first event at Dec 16, 1:20 PM CST, 1 hour 10 minutes earlier.
Area forecast discussion, Dec 16, 1:20 PM CST, 1 hour 10 minutes before
Area Forecast Discussion
National Weather Service New Orleans LA
120 PM CST Mon Dec 16 2019
.UPDATE...
Updates have been sent to refine timing of the thunderstorms and
severe potential. Area radars, satellite imagery and hi-res
lightning data has indicated quick severe thunderstorm development
in the highlighted SPC Enhanced to Moderate risk area in the Day
One Convective Outlook, and there has already been long-track
supercells that have produced tornadoes over central Louisiana and
central Mississippi. Severe thunderstorms were set to enter the
far northwest portions of the forecast area at this time, so
previous forecast timing of activity developing or moving into
northwest portions of the areas after 2 pm still looks on track,
and strong to severe thunderstorms could develop as far southeast
as a Franklinton to Hammond to Plaquemine line by 6 pm, that area
was coordinated with SPC to be the southeastern extent of Tornado
Watch 700. It is possible to likely that another Tornado Watch
will be needed after 6 pm for much of the forecast area possibly
into more southeastern portions of Louisiana and Mississippi.
Please refer to the latest SPC Watches, Mesoscale Discussions, and
Outlooks along with previous Area Forecast Discussions for more
details on the meteorological set- up.
Also updated for 18z aviation discussion.
&&
.AVIATION...
Lingering IFR category CIG at KHDC was the lone location that low
with all other local airports ranging from MVFR to VFR. Additional
mixing/scattering out of the lower CIGS should occur the
remainder of the afternoon with more concern centered around the
increasing threat of strong to severe TSRA from the west and
northwest. Highest confidence for a period of two of significant
wind gusts and lowered VSBY is at KMCB, KBTR and KHDC, so have
indicated TEMPO conditions to time out the most likely occurrence.
Airports to the east and south were lower confidence on adding
TEMPO lower conditions, so utilized prevailing SHRA with VCTS
this evening to overnight. Future amendments/routine TAF issuances
may need to consider adding more significant impacts in these
more southeastern areas. Otherwise, expect strong northwest to
north winds behind the cold front overnight and Tuesday morning
along with lower level CIGS ranging from MVFR to IFR.
&&
.MARINE...
Marine fog mostly dissipated before the end of the Noon
expiration, so the main emphasis is on the potential impacts of
what should be a weakening squall line with the strong cold front.
It is likely at least a few Special Marine Warnings will be
required later this evening or overnight for potential
thunderstorm wind gusts of 35 to 50 knots. This will be followed
by high end Small Craft Conditions with sustained winds up to 25
to 30 knots and gusts up to 35 to 40 knots, especially Tuesday
night and early Wednesday morning. Maintained Gale Watch at this
time for all waters except the tidal lakes, and will consider an
upgrade to a Gale Warning with the next routine forecast issuance.
Latest Coastal Waters Forecast update issued a Small Craft
Advisory for Lakes Pontchartrain, Maurepas and Borgne valid from 3
am Tuesday morning to 9 am Wednesday morning.
&&
22/TD
.PREV DISCUSSION... /issued 519 AM CST Mon Dec 16 2019/
UPDATE...
Based on latest observations, placed a Dense Fog Advisory over the
MS coastal counties and easternmost LA parishes through 9 am. This
should be relatively short-lived as warm air advection should lift
the fog deck shortly after sunrise. 24/RR
PREV DISCUSSION... /issued 349 AM CST Mon Dec 16 2019/
SHORT TERM...
The main concerns for this forecast package fall squarely on the
day one portion, namely with the threat of severe thunderstorms
along and ahead of a cold front that arrives on the western flank
this afternoon around max daytime heating. To start, went with NBM
temperatures, but bumped up 2-3 degrees across the board to
overcome typical cold bias in the guidance. This means BTR and MCB
are expected to reach the lower, possibly mid 80s, prior to the
front. This will aid in destabilizing the air mass with the added
compressional heating but this is not a clear cut scenario. As far
as messaging, going to maintain vigilance for the SLIGHT to
ENHANCED outlook given the amount of speed shear over the area,
better than lately surface based CAPES and anomalously high
precipitable water values that exceed the daily max value of 1.55
inches by about 0.15-0.25 inches. The high resolution convective
allowing models indicated discrete supercellular structures in
advance of the frontal QLCS with several mergers. It would be this
behavior to cause some potential tornado development on right
of mean moving cells. Some inhibitors to possibly nix severe
integrity include frontal elongation away from the main dynamics
well north of the area; stable marine layer influences, and
somewhat lacking deep layered CAPE. The higher resolution model
soundings do indicate a veer-back-veer nature to the winds in the
lower part of the column in deference to an elevated marine
layer, which could detach any MARC structures aloft from extending
and penetrating fully to the surface. The CHAP output(which is
sensitive to layer stability) from yesterday's model runs were
pinging on severe potential around 17/00Z, but this morning's runs
have settled back to to sub-severe levels with probability of
severe generally dialed back to around 10%, when it was in the
30-50% range in previous runs of the GFS. The NAM soundings shows
no severe, generally gust potential 20-25 mph on the frontal
QLCS. BUT, despite these negatives, it does remain prudent to
maintain alertness and messaging to severe threat and to continue
monitoring latest observations, high res model trends and updated
sounding data at 12Z and special release tentatively scheduled for
21Z. The timing of the main activity still appears to favor
between 2 pm and 5 pm for SW Mississippi and the adjacent
Louisiana Parishes. If the marine layer considerations come into
play, then sunset and loss of daytime heating will cut into the
intensity of the line considerably farther east and south, but a
decent squall line with gusts probably 30-40 mph still likely
downstream in the evening hours. Front should be east of the
forecast area by daybreak Tuesday with cold high pressure building
into the region. This will produce a non-diurnal nature to the
temperatures, typically warmest in the night time hours and about
20-25 degrees colder than previous day during the daylight hours.
LONG TERM...
Wednesday and Thursday should be seasonably cold with morning lows
in light freeze range for the interior and drainage prone areas,
and mid to upper 30s for marine influenced locations. Daytime
highs only muster the 50s both days but under mostly sunny skies
and diminishing winds. Meanwhile, upper level troughing becomes
more amplified on Friday to induce QG frontogenesis in the base
for cold air re-enforcement Friday and Saturday as the sharper
trough axis moves through. This is followed by steady height rises
aloft Sunday into early next week for a steady warming trend
closer to normal levels.
AVIATION...
Marine layer cloud deck in places at 003-005 for LIFR conditions
and very patchy light fog where cloud cover is less pronounced
this morning. Then all locations improving to VFR by mid-morning
through the afternoon. Cold front and pre-frontal convection is
expected to impact the western terminals this afternoon, generally
KBTR 20-22Z, KMCB 20-23Z, KHDC 21-24Z, KASD/KMSY/KNEW/KHUM 22-01Z,
KGPT 00-03Z. Some convective gusts near severe 45-50 kts and hail
surface and aloft seem possible at KBTR and KMCB. Sub-severe
35-40kt gusts at KASD/KMSY/KNEW, 25kt at stable marine layer influenced
locations KHUM and KGPT. Wind shift to follow shortly after
convection then increasing to moderate NW winds, maybe 1-2 hours
of frontal LLWS until the cold column deepens by early morning Tuesday.
MARINE...
Rather light winds and low seas today to undergo some increase on
tightening gradient in the pre-frontal zone this afternoon and
evening. Pre-frontal squall line anticipated to bring gusty
conditions 35-40 knots with wind shift, settle for a period, then
increase to strong NW with frequent gale gusts as cold air
advection deepens over the coastal waters Tuesday morning and
continuing through Wednesday morning before settling down. A Gale
Watch has been posted for Tue and Wed for the coastal waters, with
an upgrade to warning coming later today, along with issuance of
Small Craft Advisories for the tidal lakes and sounds not covered
by Gale Watch. Another shot of re-enforcing cold air becomes
established as broad low pressure attempts to develop in the lower
gulf this weekend.
DECISION SUPPORT...
DSS code: Orange
Deployed: None.
Activation: None.
Activities: Support for the City of New Orleans
Slight to Moderate Risk of Severe Thunderstorms for this
afternoon/evening.
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; Marginal risk severe or excessive rain.
Yellow = Heightened impacts with short-fused watch, warning or
advisory issuances; radar support for slight risk severe
or excessive rain.
Orange = High Impacts; Enhanced risk severe; nearby tropical events;
HazMat or other large episodes.
Red = Full engagement for Moderate to high risk of severe and/or
excessive rainfall, or direct tropical threats; Events of
National Significance.
&&
.PRELIMINARY POINT TEMPS/POPS...
MCB 77 45 54 32 / 70 90 10 0
BTR 81 44 53 33 / 70 80 10 0
ASD 80 46 58 32 / 30 90 30 0
MSY 80 49 57 38 / 30 90 30 0
GPT 75 49 59 34 / 20 80 40 0
PQL 79 50 64 32 / 20 80 40 0
&&
.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...Gale Watch from Tuesday morning through Wednesday morning for
GMZ532-536-538-550-552-555-557-570-572-575-577.
Small Craft Advisory from 3 AM Tuesday to 9 AM CST Wednesday for
GMZ530-534.
MS...None.
GM...Gale Watch from Tuesday morning through Wednesday morning for
GMZ536-538-550-552-555-557-570-572-575-577.
Small Craft Advisory from 3 AM Tuesday to 9 AM CST Wednesday for
GMZ534.
&&