Area Forecast Discussion
Issued by NWS Lake Charles, LA

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FXUS64 KLCH 241201
AFDLCH

Area Forecast Discussion
National Weather Service Lake Charles LA
701 AM CDT Fri Mar 24 2017

.DISCUSSION...
24/12Z TAF Issuance.

&&

.AVIATION...
A tightening pres gradient over the region is producing southerly
winds 10-15 KT with ocnl higher gusts. The stronger winds have
kept fog at bay this morning with SCT to sometimes BKN low clouds
at TAF sites. Winds will continue to increase through the morning
with sustained speeds 15-25 KT and gusts up to 30 KT at times
through the aftn. Low clouds will increase as well but cigs should
remain mostly VFR through the day. Late this aftn into evening,
an approaching low pres system will produce widely sctd
-SHRA with a band of SHRA/TSRA expected to cross the area between
03Z and 12Z. Strong winds and hail will be possible with the
stronger storms in the band. Cigs will lower to MVFR this evening
with IFR cigs/vsbys in heavier showers.

24

&&

.PREV DISCUSSION... /issued 532 AM CDT Fri Mar 24 2017/

SYNOPSIS...
Surface and upper air analysis, along with water vapor imagery,
shows a vertically stacked cyclone in the low and mid levels of
the atmosphere located just east of the Rockies over southeast
Colorado. Southern jet rounding the system from the East Pacific
across old northern Mexico and into the immediate region. The
gradient into this system has been tightening with both KLCH and
KPOE vad wind profiles showing 40 to 50 knots around 4k to 5k feet
(roughly the 85H level)...with southerly surface winds mainly
between 15 to 20 mph with higher gusts. This allowed the first
noticeable change in the weather this morning, and that is no fog
development, as mixing too great with the winds. Despite the
increase in southerly winds, latest look at GPS-MET data, still
shows integrated precipitable water values under 0.8 inches, so
atmosphere still has some moistening up to do, and 24/12Z KLCH
sounding shortly may confirm "cap" is still in place.

Rua

DISCUSSION...
Pattern change is indeed taking place and an active conveyor belt
of southern stream storm systems will be affecting the region
through the next week.

The first will be the deep vertically stacked cyclone over
Southeast Colorado that will move eastward across the Southern
Plains today and tonight, then off to the northeast into the Mid-
Mississippi River Valley on Saturday.

Gradient ahead of this system will remain tight today with
elevated and very breezy southerly winds. Sustained wind speeds
likely over 20 mph, especially for the I-10 corridor, with gusts
to 30 mph through the afternoon hours. Therefore, a Wind Advisory
will be in effect from late morning into the afternoon. The increase
in southerly winds should also increase moisture profiles, with
the possibility of a few showers underneath the cap by afternoon.

Very good wind dynamics with this cyclone, low level jet will
continue to be 40 to 50 knots ahead of it, that will continue to
increase moisture values. Favorable right entrance region of 60
to 70 knot mid level jet streak, should help enhance ascent,
allowing sufficient moisture to break through the cap. Moisture
plume is progged ahead of the jet streak, with precipitable water
values above 1.5 inches and mean relative humidity values up to
50H over 80 percent. Therefore, still looks like showers and
thunderstorms will begin to form later today ahead of a dry line
over Texas, that will move toward the forecast area.

Still some timing differences in the guidance, with GFS about 3
hours faster than NAM12 and 6 hours faster than ECMWF. Still
looks like the best time will be during the overnight hours for
showers and thunderstorms. However, with slower timing of NAM12
and ECMWF, storms could linger into Saturday Morning.

CAPE values will be on the low side, with most unstable CAPE in
the mixing layer between 500-1000 j/kg. Wind dynamics as mentioned are
decent with low level jet between 40-50 knots and the
aforementioned mid level speed max. This still 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 still look
to be north of the forecast area, and around the Arklatex area,
where a higher probability of severe weather will occur.

However, with respectable wind dynamics, the expected squall line
will have a probability of producing strong straightline wind
gusts, with strong to borderline severe storms. Although, isolated large
hail or an isolated tornado can not be ruled out, especially if
storms along the line can tap into higher instability with better
rotation, straightline wind gusts are the favored probability if
severe weather does occur.

Therefore, looks like there is a potential for strong to isolated severe
storms along the squall line during the Friday Night into
Saturday Morning period. 10 pm to 1 am for Southeast Texas, 1 am
to 4 am for the Lake Area up to Alexandria, and 4 am to 7 am for
lower Acadiana. The Storm Prediction Center (SPC) has the
forecast area northwest of a Beaumont to Marksville line outlined
in a Slight Risk for the day 1 period (Friday Night) and a
Marginal Risk for the remainder of the forecast area. For the Day
2 period (Saturday Morning,) a Marginal Risk lingers for areas
east of a Lake Charles to Alexandria line and a Slight Risk east
of Lafayette.

High moisture content will also allow for high rainfall rates with
the storms, however, progressive nature of the line should
preclude any excessive rainfall amounts.

Actually modified Pacific cold front/dry line feature looks to be
delayed behind the convection. This feature will move into
Southeast Texas and Western Louisiana during the morning hours and
into Acadiana early Saturday afternoon before washing out. A
chance of showers or storms will continue until the drier and more
stable air behind this boundary moves through.

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.

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, looking more robust, to affect the region by mid
week. Timing still needs to be fine tuned with this system. Rather
impressive wind dynamics again, with a plume of richer and more
unstable Gulf of Mexico air, so there will be a potential for
severe weather and/or heavy rain if parameters hold with this
system. SPC already has the forecast area outlined in a Slight
Risk for severe storms on Day 6 (mainly Wednesday Night.)

Rua

MARINE...
A low pressure system is located over southeast Colorado this
early morning. This system is rather deep with an increasing
pressure gradient across the region. The result is southeast winds
have increased and are near 20 knots with higher gusts. This
system will move east across the Southern Plains tonight. Gradient
will remain tight with elevated and gusty southerly winds, along
with building seas, over the coastal waters, through tonight.
Therefore, Small Craft Exercise Caution will be in effect for the
lakes and bays, with Small Craft Advisory conditions for both the
near shore and outer waters.

Winds will decrease on Saturday, with seas gradually subsiding,
as the low weakens and moves off to the northeast off the region.

Another low pressure system will move across the Southern Plains
late Sunday into Monday. Southerly winds will again become
elevated. However, this system does not look quite as deep, and at
this time, only exercise caution conditions are forecast for the
near shore and outer waters during that time frame.

Rua

&&

.PRELIMINARY POINT TEMPS/POPS...
AEX  82  65  82  57 /  30  80  30  10
LCH  82  67  82  62 /  20  70  30  10
LFT  83  70  83  65 /  20  60  60  10
BPT  83  68  84  63 /  30  70  20  10

&&

.LCH WATCHES/WARNINGS/ADVISORIES...
LA...Wind Advisory from 10 AM this morning to 6 PM CDT this evening
     for LAZ041>045-052>055-073-074.

TX...Wind Advisory from 10 AM this morning to 6 PM CDT this evening
     for TXZ215-216.

GM...Small Craft Advisory until midnight CDT tonight for GMZ450-452-
     455-470-472-475.

     SMALL CRAFT EXERCISE CAUTION through this evening for GMZ430-432-
     435.

&&

$$

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



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