Area Forecast Discussion
Issued by NWS New Orleans/Baton Rouge, LA

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000
FXUS64 KLIX 170953
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
353 AM CST Sun Feb 17 2019

.SHORT TERM...A dense fog advisory will continue through the mid-
morning hours as areas of dense fog linger over the nearshore
waters and adjacent coastal locations. The fog should begin to
clear by late morning as an approaching cold front increases
boundary layer mixing. This weak frontal boundary will move
through the area this afternoon and evening and should clear the
coastal waters during the overnight hours. Favorable jet dynamics
will provide enough forcing to support the development of
scattered to numerous showers as the front moves through with the
highest rain chances during the afternoon and evening hours. Fortunately,
the risk of thunderstorms is very low due to a lack of instability
in the mid- levels of the atmosphere.

After the front clears the area this evening, weak cold air and
dry air advection will take hold. Temperatures tonight should drop
into the upper 40s and 50s, and highs tomorrow will be closer to
seasonal averages in the middle 60s. The weak dry air advection
into the region will drop precipitable water values down to around
an inch, and only a few light rain showers are expected through
the day tomorrow.

Tomorrow night and Tuesday will be a period of transition as a
deepening area of low pressure in the western Gulf of Mexico
begins to pull to the north. By Tuesday evening, this area of low
pressure should be generally located around the Arklatex region.
The stalled front over the northern Gulf will react to this low by
pushing northward as a warm front. The front will sweep through
the forecast area throughout the day on Tuesday. As is typical
with warm frontal processes the majority of the rain will occur to
the north of the frontal boundary, and have the highest rain
chances along and north of the I-10 corridor to reflect this.
Most of the precipitation will occur in the form of rain showers,
but increasingly unstable lapse rates will also develop above
850mb, and this will support the development of a few
thunderstorms from late Monday night through Tuesday afternoon.
Thunderstorms will tend to be most prevalent over the northern
third of the CWA, but severe storms are not expected due to a
lingering stable layer at the surface and relatively high wet bulb
zero heights. The main concern with thunderstorms will be
lightning strikes and locally heavy downpours of rain.

Tuesday night will see the area of low pressure lift from the
Arklatex region into the Tennessee Valley. As this occurs, the
warm front will continue to surge to the north and out of the
area, but a trailing weak cold front on the southwest side of the
low pressure system will begin to approach the forecast area. This
front will be very slow moving and will also serve as a focus for
repeated rounds of showers and thunderstorms. Given that storm
motions will be parallel to the frontal boundary, there will be a
potential for several rounds of heavier rainfall to train over the
same area. The most likely region that will experience the heavier
rains will generally extend from the Baton Rouge area toward
Southwest Mississippi. Lighter rain totals are expected further to
the south and east due to a lack of a low level focusing
mechanism. Of course, temperatures will be significantly warmer
with lows only dipping into the middle to upper 60s.

.LONG TERM...The slow moving to nearly stalled frontal boundary
will remain draped over the northwest third of the CWA generally
from Baton Rouge to McComb through Thursday morning. Ample omega
will remain in place aloft due to a combination of favorable jet
dynamics and a series of shortwave features passing through the
Lower Mississippi Valley. This increased forcing aloft will
interact with a warm, moist, and moderately unstable environment
in the low to mid-levels. MLCAPE values are expected to climb to
around 1000 J/KG both Wednesday and Thursday, and this will
support the development of scattered to numerous showers and
thunderstorms over the northern half of the forecast area each
day. Fortunately, wind shear values will generally remain below 50
knots in the lowest 6km, and wet bulb zero heights will be in
excess of 10k feet. A severe storm cannot be completely ruled out,
but the overall threat will be pretty low. Most of the
thunderstorm activity should remain below severe limits. The
bigger concern during this period will be the heavy rain and
localized flooding potential. Precipitable water values will
remain very high at over 1.5 inches, and storm motions will
continue to parallel the boundary. Several rounds of heavy
downpours could train over the same areas through Thursday
afternoon, and this could produce flooding concerns as localized
rainfall totals potentially exceed 3 inches.

Another area of low pressure passing to the north of the area will
draw the stalled boundary back to the north, and the heavy rain
threat should diminish for Thursday night and Friday. However, a
very warm, moist, and unstable airmass will remain in place across
all of the forecast area. Fully expect to see daytime highs
approach 80 degrees Friday afternoon, and MLCAPE values will
easily exceed 1000 J/KG. As a result, an almost summer like
pattern of showers and thunderstorms is expected to affect the
area. Fortunately, storm motions will remain high enough that
flooding concerns should be negated. However, these thunderstorms
could turn strong and produce lightning strikes and gusty winds.
Rain chances should diminish to only slight chance Friday night as
instability values decrease.

Saturday will see a region of strong difluence develop over the
Gulf South. This pattern will develop in advance of an approaching
long wave trough axis that should begin to push into the Plains
throughout the day on Saturday. With ample instability still in
place across the CWA, scattered to numerous showers and
thunderstorms will once again develop across the forecast area.
Rain chances will tend to be highest over the northwest CWA where
dynamic forcing is expected to be strongest. Fortunately, severe
parameters remain on the weaker side on Saturday, so the risk of
severe weather remains low. However, stronger storms with
lightning, gusty winds, and heavy downpours will be a concern for
any Carnival related events occurring. Temperatures should once
again climb to near 80 degrees in the afternoon.

All of the guidance indicates that a frontal boundary will slide
through the forecast area Saturday night, and have likely POP in
the forecast to reflect a band of convection that should accompany
this frontal passage. Weak cold air advection behind the front
should push temperatures down into the 50s for most areas by late
Saturday night. Clearing skies and cooler temperatures are
currently expected on Sunday in the wake of the frontal boundary.

&&

.AVIATION...Low ceilings OVC005-010 will remain through late
morning and may scatter out around noon for a few hours but will
be back by the end of the day for the overnight hours. Areas of
-RA will be around and should be high enough chances to show in
this taf set. The band of rain will move slowly southeast and
should mainly affect terminals over the northwest half of the area
during the daylight hours and southeast half after dark. Vis may
also be a concern this morning possibly lasting into mid morning
mainly for terminals near bodies of water.

&&

.MARINE...A cold front stalled just north of the coast will allow southerly
flow to remain until the next cold front moves to the gulf coast
Sunday night into Monday. The stalled front and moisture laden
air over cool waters will keep fog through much of the morning
today. A dense fog advisory will also remain for this issue
through at least 9am today, if vis continues to be an issue, the
advisory may be extended in time. The next front will also stall
but may do so over the coastal waters Monday. Winds should become
more easterly as this occurs. This will not last long as the front
will quickly retreat Tuesday bringing the southeasterly flow
back. A general weakness will remain near the coastal waters
through the remainder of the week.

&&

.DECISION SUPPORT...

DSS code: Blue.
Deployed: None.
Activation: None.
Activities: River Flood Warnings

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 rain; direct tropical threats; Events of National
         Significance.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  74  45  62  48 /  60  70  20  60
BTR  76  49  63  51 /  50  60  20  50
ASD  78  54  65  54 /  20  50  20  40
MSY  79  55  63  57 /  20  40  20  40
GPT  72  55  65  55 /  10  50  30  30
PQL  76  57  66  56 /  10  60  40  30

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Dense Fog Advisory until 9 AM CST this morning for LAZ040-050-
     058-060>064-066>070-072.

GM...Dense Fog Advisory until 9 AM CST this morning for GMZ530-532-
     534-536-538-550-552-555-557.

MS...Dense Fog Advisory until 9 AM CST this morning for MSZ080>082.

GM...Dense Fog Advisory until 9 AM CST this morning for GMZ532-534-
     536-538-550-552-555-557.

&&

$$



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