Special Marine Warning 417, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
1241 PM CDT Wed Jul 14 2021

.AVIATION...18Z TAF Package...Overall most terminals should remain
in VFR status. The main concern is convection which luckily looks
to be a little more limited than the last...........4 months.
There will still be some storms around and the locations most
likely to be impacted would be MSY, HUM, and BTR but can not rule
out at least VCTS at any site. Convection will quickly come to an
end shortly after sunset and not anticipating any issues over
night. /CAB/

&&

.PREV DISCUSSION... /issued 614 AM CDT Wed Jul 14 2021/

AVIATION (12Z TAF DISCUSSION)...

Expect VFR conditions to begin early this morning for all area
terminals. Only exception is around KMCB with very light surface
fog in the vicinity but should diminish shortly after sunrise.
Conditions will be calm areawide through 16-17Z before scattered
SHRA/TSRA develop, becoming more widespread through the afternoon
and early evening hours. For now, timed approximate greatest rain
chances for terminals across western areas and north across the
northshore. Main risk within any one storm will be gusty, erratic
downdraft winds in excess of 25-35kts and low CIG's and VIS from
locally heavy rainfall leading to periodic lower flight
categories. Otherwise, outside of this activity expect VFR
conditions to prevail, with conditions improving areawide through
the overnight and early morning hours on Thursday. KLG

PREV DISCUSSION... /issued 344 AM CDT Wed Jul 14 2021/

SHORT TERM (Today through Friday Night )...

The dog days of summer keep rolling on across the area, just with
much more rain coverage as opposed to normal. Starting off with
early this morning, things are generally quiet. Monitoring some
nocturnal convection entering our far southeastern 20-60nm marine
zones, which will continue slowly towards the northwest remaining
generally isolated to scattered in coverage. Looking ahead to
around daybreak and beyond into early this morning, CAMs are
coming into good agreement over the idea of another round of pop-
up airmass thunderstorms very similar to yesterday. Comparing
yesterday's forecast model soundings valid for 17Z TUE to this
morning's output valid for 17Z WED shows very little differences
in the overall thermal profile aloft, other than slightly more
moisture in the H925 to H7 layer, bringing PW closer to 2" today.
Anticipating convection to start up around the 16Z-18Z time frame
as very small, isolated showers and will be confined to areas that
heat up quickest and surpass forecast convective T's in the mid
80's. Additionally as mentioned in previous forecast discussions,
light anticyclonic curvature remains anchored east to west across
the northern Gulf coast, but will drift ever so slightly north
revealing primarily E to ESE surface to low-level flow. As we
continue to heat up late this morning through the afternoon hours,
we'll get a weak lakebreeze boundary to develop around Lake
Pontchartrain/Maurepas due to differential heating but E to ESE
mean surface to low-level flow will skew the orientation expanding
more towards the W or WNW, keeping the north and southshore
boundary from spreading too far inland. This paints a better bulls
eye of convection to begin early in western/southwestern areas of
the CWA, but will fill up quickly once convection becomes more
widespread due to multiple outflow boundary interactions.
Eventually going into the early afternoon, more widespread spotty
shower/storms will get going across the northshore east to coastal
MS and far coastal SE LA. Not anticipating any severe weather
today as deep-moist ascent will reduce any extreme downward wind
potential within stronger updrafts. Calculated microburst
parameters are actually slightly lower than yesterday due to the
slight uptick in overall tropospheric moisture, however occasional
gusty downdrafts in the 30-40mph range cannot be ruled out,
requiring a few SPS's here and there. Since these storms will be
quick to develop then collapse, not anticipating storms to sit
over one area for long reducing any flash flood potential.
However, should any one storm become hung up due to boundary
collisions/interactions, some isolated flash flooding could be
possible but the threat today will be isolated.

Things quiet down tonight with only a lingering shower or storm
this evening, but likely turning dry going into early Thursday
morning. Same story going into Thursday with very similar overall
atmospheric set up leading to yet another day of airmass/pop-up
thunderstorms beginning late AM and becoming more widespread into
the afternoon hours. CAM's are coming in with another slight
uptick in coverage again mainly for the same areas across SW
portions of the CWA, north or for areas generally along/west of
I-55. With this increase in coverage, some indications of a
northward surging outflow boundary will keep an broken line of
storms surging north to BTR to SW MS. Makes sense looking at model
soundings as much deeper low to mid-level mean storm flow becomes
a bit more unidirectional from the SSE to almost due S and PW's
remain right around 2" helping to support widespread coverage.
Again, not anticipating any severe weather issues as the thermal
profile does not change much.

Rinse/repeat for Friday, but with much more widespread deeper
southerly flow as mentioned before, convection will develop around
the same times and drift north along northward surging outflow
boundaries. Main threats in any stronger storm will continue to be
wind gusts 30-40mph, locally heavy rainfall and dangerous
lightning. May need to keep an eye on a very weak tropical
inverted trough in the north- central Gulf by this time frame
rotating around the western periphery of the west Atlantic Ridge,
which takes this generally north to around central portions of the
northern Gulf coast. Doesn't necessarily develop into anything,
but may perhaps cause a slight increase in PoP coverage Friday and
into the weekend, depending on the eventual track/strength but
will continue to monitor.

LONG TERM (This Weekend into Early Next Week)...

This portion of the forecast is virtually no change from what we've
been seeing locally for the last few month, and especially over the
last couple weeks. The trough that enters the Mississippi Valley
Friday will do little to move east and erode the fringes of ridges
center west and east of the region. This will eliminate any
subsidence that would possibly keep POPs lowered. In addition, more
moisture rich air will steadily stream in from the southeast as its
being pumped in from the Bermuda ridge. GFS PW forecast shows that
those values will be around 2" at minimum and occasionally increasing
to near 2.25". This overall pattern will be conducive for high rain
chances each day with convective initiation early than what's normal
summertime afternoon storms. Expect to see coverage increasing
steadily from 10am to 2pm with some storms producing locally heavy
rainfall. On a day to day basis, most rainfall totals will be less
than an inch. However, isolated areas may see upwards of a few
inches. The good news is high rain chances means more cloud cover
and temps at to below normal.

MEFFER

MARINE...

Weak southeasterly onshore flow will persist through the rest of
the week, weekend and into early next week. Winds/seas will range
5-10kts/1-2ft for protected waters to 10-15kts/2-3ft for outer
Gulf waters. Expect nightly scattered showers and storms each day
primarily from midnight through shortly after daybreak. Not
anticipating any severe weather, however brief gusty winds 20-25kt
and isolated weak waterspouts will be the main risk, along with
locally higher waves/seas. KLG

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  73  90  73  89 /  10  70  10  70
BTR  74  89  74  90 /  10  80  10  80
ASD  75  90  75  89 /  20  70  10  70
MSY  78  91  78  92 /  20  80  10  70
GPT  75  88  75  88 /  20  50  20  60
PQL  74  90  75  89 /  10  50  20  60

&&

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