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

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FXUS64 KLIX 220228 AAA
AFDLIX

Area Forecast Discussion...UPDATED
National Weather Service New Orleans LA
928 PM CDT Sat Apr 21 2018


.SOUNDING DISCUSSION...
No problems with today`s flight which was accompanied by a
Brownie tour this evening. Flight terminated 20.0 miles up, 9
miles south of Pass Christian, MS in the Mississippi Sound, about
36 miles downrange ESE. Temperature profile is strongly suggestive
of marine layer influences on deep layered onshore flow to about
6kft. An inversion is noted from 832 mb / 5700 ft to 781 mb /
7500 ft, then a bit of a warm nose to 710 mb / 9900 ft before
becoming moist adiabatic to tropopause at 179 mb / 42.5kft.
Marine moisture to the base of the inversion, then a dry mid-layer
to about 13.0kft, then moist to 22.6kft, capped with another dry
layer to 26.1kft, then moist through the tropopause. This type of
profile is favorable for wet-microbursts if upper level dry layer
becomes more pronounced in time.

Winds are showing early signs of a favorable hodograph with SE
winds 10-20kt from surface to about 3700 ft, then veering SW
15-25kt to about 20kft, then W-NW 45-80kt above. SFC to 3K
helicity is only 93 at this time due to lighter velocities in the
lower levels, but likely to increase steadily as nocturnal low
level jet develops in tightening gradient flow. Peak winds was
289/80kt at 39.9kft. Precipitable water is 1.14 inches and should
increase overnight to around 1.35 inches or more by daybreak.

CHAP output from both the NAM and GFS modeled soundings do not
show much convective integrity due in large part to marine layer
influences. GFS does ping on about 1.75 inches of rainfall on
average but only gust potential 20-25 kt, whereas the NAM is
showing 1.16 inches and similar gust potential with RI values in
the 60-72 range at peak impact times. 24/RR

&&

.PREV DISCUSSION... /issued 411 PM CDT Sat Apr 21 2018/

SHORT TERM...

An upper low and associated shortwave trough will move east-
southeast across the southern Plains tonight and eastward over
western Arkansas by Sunday morning. Surface low was over central
Texas. An associated warm front extended southeast from the low to
the Texas coast to north central Gulf. This low will advect in
low level moisture. Surface dewpoint of 65F will push north to
1-20 and 70F south of tidal lakes from 12z to 18z Sunday. A warm
frontal boundary will push north and should cross the western to
central Gulf coast region by later tonight. 0-3km helicity values
are expected to increase 250 to 350m/s just ahead of the warm
front 10 to 15z Sunday across the Florida parishes and Southern
Mississippi. These higher values will shift northeast as the warm
front shifts north of the forecast by noon Sunday. Therefore, the tiny
window of atmosphere conducive for rotating storms will shift
north as instability arrives a little behind the warm front.
Therefore, the main threat will be gusty winds from strong storms
Sunday morning.

The orientation of the upper level low maintain the cold core
north of the forecast and hail threat is low. The cold front is
expected to move through the forecast area Sunday afternoon and
evening, so rain chances should diminish as the front moves
through with only a few lingering showers expected (mainly in
northern and eastern areas) Sunday evening. Entrainment of dry air
on the south side of the low will quickly displace the moisture
east Sunday evening, decreasing rain chances to nil after 06z
Sunday.

LONG TERM...

Series of shortwave troughs in the larger longwave trough over
the eastern U.S. are expected to move southeast from the northern
Plains and upper Mississippi valley across the mid to lower
Mississippi valley and southeast states through the middle to late
part of the week. Some model differences still exist from run to
run consistency with timing, strength and orientation of these
systems as they move across the lower Mississippi Valley. Because
of this, there is low confidence in exactly which periods will
experience mentionable chances of rainfall. Therefore will
maintain a slight chance to chance of showers is on Tuesday
across southeast Mississippi, Wednesday and Wednesday night across
all but the southern coastal areas, and on Friday when a more
potent storm system could move through the central Gulf coast
region. Temperatures are expected to be generally slightly below
the seasonal averages. 18

AVIATION...

Lowering lifted condensation levels associated with deeper
moisture advecting in from the Gulf of Mexico should push ceilings
into MVFR range of 2000 to 3000 feet. Further lowering of
ceilings to around 1500 to 2000 feet is expected to occur after
00z. Additionally, have included a mention of vicinity showers in
the forecast for all of the terminals by 05z. After 06z,
prevailing showers and MVFR and IFR visibilities are forecast for
KBTR, KMCB, KHDC, KHUM, and KMSY. Further to the east, VFR
visibilities should continue through 12z as the band of rain
remains west of KASD, KGPT, and possibly KNEW. At KMSY, VCTS
wording will be added after 12z.

MARINE...

Tight pressure gradient over the coastal waters allowed southeast winds
to increase to 15 to 20 knots this afternoon. Winds will decrease
slightly this evening only to increase back to 15 to 20 knots over
the coastal waters Sunday. As a result, exercise caution flags
will be raised again. Only the tidal lakes are expected to remain
below exercise caution levels.

A passing cold front Sunday evening will shift winds to the
northwest. A decrease in the overall pressure gradient over the
northern Gulf should allow winds to relax back to 10 to 15 knots
Sunday night. Next week should see a persistent offshore flow of
around 10 to 15 knots from Monday through the end of the week as a
series of weak reinforcing fronts slides through the waters. 18

DECISION SUPPORT...
DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Assessing thunderstorm risk for Sunday
            Sandhill Crane NWR Outreach Support
            New Orleans Navy Week Support

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; direct tropical threats; Events of
         National Significance.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  63  74  54  70 /  60  90  10  10
BTR  64  78  54  73 /  70  90  10   0
ASD  63  75  58  74 /  40  90  20   0
MSY  66  77  58  74 /  40  90  10   0
GPT  66  73  60  73 /  30  90  30   0
PQL  64  74  59  73 /  30  90  30   0

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...None.
MS...None.
GM...None.
&&

$$



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