Area Forecast Discussion
Issued by NWS Lake Charles, LA

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FXUS64 KLCH 231145
AFDLCH

Area Forecast Discussion
National Weather Service Lake Charles LA
645 AM CDT Thu Mar 23 2017

.DISCUSSION...
23/12Z TAF Issuance.

&&

.AVIATION...
LIFR/VLIFR vsbys prevail at southern terminals this morning with
IFR cigs at KAEX. These conditions will persist for another hour
or so gradually improving by 15-16Z. VFR is expected to prevail
through the remainder of the day with southerly winds 8-12 KT.
Winds are expected to remain elevated into this evening and
overnight as low pres deepens west of the region. Because of this,
expect more of a low stratus layer developing late this evening
rather than dense fog, with MVFR cigs after 03-04Z tonight,
possibly lowering toward daybreak Friday.

24

&&

.PREV DISCUSSION... /issued 504 AM CDT Thu Mar 23 2017/

SYNOPSIS...
The weather map this early morning shows a quasi-stationary front
from the Arklatex to the Gulf South coast, and is barely into the
northeast corner of the forecast area. Meanwhile, a weak surface
ridge is noted over the north central Gulf of Mexico. Between the
front and the surface high, a light southerly flow has brought in
a shallow moist layer near the surface. With stable conditions
aloft, low stratus and fog has again formed for much of the
forecast area. Fog has been dense in patchy locations. However,
see enough visibility restrictions of around 1/4 of a mile to keep
the Dense Fog going for all but Rapides and Avoyelles Parishes
until 9 am local.

Looking at the bigger picture. Water vapor imagery and upper air
analysis shows Pacific energy moving out of Southern California
into the Desert Southwest.

Rua

DISCUSSION...
Areas of low clouds and fog will again mix out by mid morning once
daytime heating takes affect and surface winds increase. That will
leave again mainly sunny, warm and humid conditions as upper level
ridging building ahead of the west coast system moves across.

Mainly persistent pattern will begin to transition tonight. Pacific
upper level energy will move across the Rockies, this will form a
surface and upper level low on the lee-side of the Rockies. The
result will be height falls beginning over the forecast area with
ridging moving off to the east. The increase in pressure gradient
will increase southerly winds in the boundary layer that will
dissipate old frontal boundary as it lifts to the north.

The first affect of this change will be the lowering of fog
potential for tonight into Friday Morning. Boundary layer wind
speeds expect to be 10 to 15 knots in the evening hours,
increasing to around 20 knots during the overnight. These
conditions should allow decent mixing to take place that any
shallow ground fog that may be trying to form during the evening
hours will dissipate overnight.

The southerly breezes will continue during Friday as the low moves
eastward into the Southern Plains. Sustained wind speeds could
reach over 20 mph by Friday Afternoon for portions of the forecast
area, and a Wind Advisory will be needed if this occurs. The
increase in southerly winds will also increase moisture depth and
lift for a few showers to begin to form by afternoon, especially
over Southeast Texas.

As cyclone moves eastward across the Southern Plains on Friday
Night, southerly flow will continue to increase, and thus, so will
moisture profiles and lift. Therefore, still looks like a very
good chance for the forecast area to see showers and thunderstorms
during the Friday Night period into early Saturday.

Moisture plume is progged ahead of the surface dryline/trough and
dry slot aloft, with precipitable water values above 1.5 inches
and mean relative humidity values up to 50H over 80 percent. Right
entrance region of 50-70 knot mid level speed max rotating around
the low, will help in lifting moisture and merging into a likely
squall line of showers and thunderstorms ahead of the dryline.
Still some timing differences in the guidance, with GFS about 3
hours faster than NAM12 and 6 hours faster than ECMWF. Therefore,
have opted to just broad brush the pops on Friday Night for the
12 hour period instead of trying to break it up.

Still looks like the best time will be during the overnight hours.
With timing, CAPE values will be on the low side, with most
unstable CAPE in the mixing layer between 500-1000 j/kg. Wind
dynamics are decent with low level jet between 35-40 knots and the
aforementioned mid level speed max. This produces 0-6 km shear
numbers between 45-55 knots. 3-6 km lapse rates are also decent
with values between 6.5-7 c/km. So as is typical, looks like a
highly sheared/low cape environment. Best parameters look to be
north of the forecast area. However, with respectable wind
dynamics, squall line will have a probability of producing strong
straightline wind gusts. Also, if storms can tap into some higher
instability, then some rotating storms with isolated tornadoes or
large hail could also happen along the line, or if any cells can
become discrete ahead of the line. High moisture content will
also allow for high rainfall rates, however, progressive nature of
the line should preclude any excessive rainfall amounts.

Therefore, looks like there is a potential for strong to severe
storms along the line during the Friday Night into Early Saturday
Morning period. The Storm Prediction Center has most of the
forecast area outlined in a Slight Risk for the day 2 period
(mainly Friday Night) and the far eastern zones in a Slight Risk
for the day 3 period (mainly early Saturday Morning.) The main
hazard will be damaging straightline winds. Isolated tornadoes,
isolated large hail, and torrential downpours will also have a
probability.

Dry slot looks to move in on the first part of Saturday, and that
should lower chances for showers and thunderstorms as that occurs.
Therefore, the best chance for rain on Saturday will be in the
early part of the day.

Remainder of the weekend should be warm with limited chance for
rain as more stable conditions move in behind the departing low.
Just somewhat lower dew points behind the surface boundary, so not
a whole lot of change in temperatures.

A progressive pattern looks to be in store next week. The next
southern stream system to move across the Southern Plains on
Monday. Some shower activity will be possible by late Sunday
Night, with better chance during the daytime Monday. Dynamics with
this system at this time do not looks as good as the Friday
Night/Saturday morning time period. Another system, possibly
pretty robust, to affect the region by mid week.

Rua

MARINE...
A low pressure system will develop later today east of the
Rockies. This system will move east across the Southern Plains on
Friday Night. Ahead of this system, gradient will increase over
the coastal waters, and thus, southerly winds will increase, along
with building seas. Looks like Small Craft Exercise Caution to
Small Craft Advisory conditions will be possible during the
overnight tonight through Friday Night.

Winds will decrease on Saturday, as the low moves off to the
northeast.

Rua

&&

.PRELIMINARY POINT TEMPS/POPS...
AEX  86  65  79  64 /   0  10  20  80
LCH  82  65  80  67 /   0  10  20  70
LFT  85  68  80  68 /   0  10  20  70
BPT  84  67  80  67 /   0  10  30  70

&&

.LCH WATCHES/WARNINGS/ADVISORIES...
LA...Dense Fog Advisory until 9 AM CDT this morning for LAZ027-
     030>033-041>045-052>055-073-074.

TX...Dense Fog Advisory until 9 AM CDT this morning for TXZ180-201-
     215-216-259>262.

GM...None.
&&

$$

PUBLIC/MARINE...07
AVIATION...24



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