Severe Thunderstorm Warning 57, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
644 PM CDT Tue Apr 28 2020

.AVIATION...
Main concern for this forecast cycle is the broken line of
thunderstorms currently moving through the area. The line
currently is from roughly Tylertown, MS to Hammond to
Donaldsonville moving SE. Main impacts to terminals ahead of this
line ASD, MSY, NEW and HUM are gusty winds up to around 40 kt. Ceilings
will drop down to MVFR to IFR. Additionally visibilities may be
obscured due to heavy rainfall. Put best timing as a tempo group
for these terminals. Behind this line of thunderstorms will be
mostly light rain with maybe embedded tsra. Still monitoring a
secondary line of thunderstorms possible in the early morning
hours as well. -BL

&&

.PREV DISCUSSION... /issued 422 PM CDT Tue Apr 28 2020/

SYNOPSIS...
Mosaic radar showed convection from southwest Mississippi to
southeast Texas. In addition, IR satellite imagery showed cloud
top cooling over southeast Texas and southwest Louisiana. Latest
surface analysis showed a 1024mb off the Georgia coast and 1004mb
low over west Texas. Southerly flow was present across Texas and
Lower Mississippi Valley. Surface dewpoints were in the 60s and
70s over east Texas, 60s over Louisiana, and 50s over Mississippi.

Upper level analysis showed a short wave from northwest
Mississippi to southeast Texas and clipper system over the
northern and central plains diving southeast. Isotach analysis
showed the northern stream jet of 120 knots from Montana to
northern Colorado on the backside of the clipper.

DISCUSSION...
HRRR and RAP continue to show convection associated with the
short wave pressing east southeast this afternoon and this
evening. NAM12 and GFS showed lapse rates of 7.0 to 7.5 C/km from
700 to 500mb from southwest Mississippi to southwest Louisiana
this evening. LCH sounding from 12z this morning showed a lapse
rate of 8.6 C/km and surface base cape value 420j/kg. Southerly
winds have modified instability values across east Texas and west
Louisiana. Forecasted surface base cape values increase 600 to 800
j/kg this afternoon but this convection is driven by the short
wave. As the short wave push east, expecting this convection to
push east weakening before midnight.

Wave over northern and central plains will continue to dive
southeast and deepen the main trough over the mid Mississippi
Valley overnight into Wednesday. An associated trough axis and
frontal zone will push into the area early Wednesday. With the
ongoing convection, cautiously looking at this second wave of
convection along the front.

Surface base cape values are expected 300 to 600 j/kg directly
north and east of Lake Pontchartrain but pooling along this
boundary with cape values 1000 to 2000 j/kg west of Lake
Pontchartrain after midnight. Moisture will pool along the
frontal with precipitable water values 1.8 to 1.9 inches. As a
result, will maintain high rain chances with more showers and
scattered embedded storms along the front early Wednesday. A few
strong to severe storms in a squall line are possible with hail
and strong to damaging wind gusts being the main factor but can
not rule out an isolated tornado. Showers and storms will linger
through Wednesday day period as rainfall amounts of 1 to 2 inches
can be expected. Dry conditions are expected through Thursday
through the weekend with warm conditions.

AVIATION...
Short range models are suggesting convection developing over
southwest Louisiana tracking southeast affecting KBTR, KHUM, KMSY
and KNEW between 23z today and 03z Wednesday. Will use tempo TS to
cover this threat. Cigs will move into the area just before
midnight at OVC030 and fall to OVC010 before daylight while sh/ts
will also be noted as prevailing at NW terminals before daylight
and all remaining terminals after daylight. Some of these TS could
produce strong wind speeds as well.

MARINE...
Return flow will continue through Wednesday morning ahead of an
approaching cold front. Winds will increase on Wednesday afternoon
and Wednesday night especially behind the front as a moderate
offshore flow once again returns to the coastal waters. Offshore
flow will continue on Thursday and gradually lull by Thursday
night remaining light but shifting around to a southerly direction
over the weekend.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  63  77  55  76 /  50  70   0   0
BTR  63  78  54  76 /  50  60   0   0
ASD  63  79  55  79 /  40  70   0   0
MSY  66  79  61  78 /  40  60   0   0
GPT  66  77  58  76 /  20  60  10   0
PQL  64  80  55  79 /  20  70  20   0

&&

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

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE NEW ORLEANS LA
829 PM CDT TUE APR 28 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0745 PM     TSTM WND DMG     SCHRIEVER               29.74N 90.83W
04/28/2020                   TERREBONNE         LA   EMERGENCY MNGR

            EMERGENCY MANAGER REPORTS SEVERAL TREES DOWN ON ROADS IN
            SCHRIEVER AND GRAY. TIME ESTIMATED BY RADAR.

0802 PM     TSTM WND DMG     1 N HOUMA               29.59N 90.71W
04/28/2020                   TERREBONNE         LA   EMERGENCY MNGR

            EMERGENCY MANAGER REPORTS TREES DOWN IN THE HOUMA AREA.
            TIME ESTIMATED BY RADAR.

&&

BL