Heat Advisory 15, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
1209 PM CDT Fri Aug 20 2021

.UPDATE...
Updated for the 18Z aviation discussion.

&&

.AVIATION...
18Z TAF Package...
VFR conditions prevail at all area airports and will persist
through the forecast period. MCB may touch MVFR conditions due to
fog around daybreak tomorrow. NEW will have wind shifts greater
than 30 degrees throughout the forecast period. MSW

&&

.PREV DISCUSSION... /issued 413 AM CDT Fri Aug 20 2021/

SHORT TERM...Another hot day in store which will begin rather
uncomfortable for portions of the area. Surprisingly locations
across southwest MS and adjacent LA parishes have cooled a little
faster than the previous night but temps should stall out for the
most part now with those areas likely remaining the mid 70s. Closer
to the coast and near the marshes it is miserable with dewpoints
near 80 and temps as well. Even at 8z we still had locations with
heat index values in the lower 90s. In addition there is actually a
few showers out there with the greatest concentration over coastal
MS.

Main impact in the short term will be the heat potential. Nothing
has changed in the thinking from the previous few days. High
pressure will continue to dominate and temperatures will nudge up a
degree or two each day for the next 3 days and with no real dry air
advection or strong mixing the humidity will be quite high and thus
lead to oppressive conditions.

Today the mid level ridge will finally begin to build in directly
overhead. Mid lvl hghts will slowly rise and this will have an
impact on rain potential. There is a subtle s/w trying to ride over
the building ridge and through the Ozarks and Lower MS Valley but
this feature will likely remain north of the area. With that rain
chances will be quite minimal for much of the area. The one location
that may have the best chance of seeing any rain could be coastal MS
late this afternoon as that weak disturbance tries to round the
ridge and drift southeast. Elsewhere few to scattered clouds will
probably be the best we can see. However, it is not completely out
of the realm of possibility that an isolated storm or two could
develop given the amount of moisture in place and the ridge still
not overpowering.

Rain chances will be even lower on Saturday as the ridge continues
to build across the region. Any subtle disturbance will likely
remain north and northeast of the region. However, on Sunday there is
a slight increase in rain chances across coastal MS. The ridge will
continue to build with the ridge axis stretching from the ARKLATEX
into the Great Lakes. Coastal MS could be right on the periphery of
the ridge. At the same time the mid lvl low that was over the Mid-
Atlantic states may try to steer Henri towards the New England
states Sunday with a weak disturbance working down the backside.
This could work just far enough into the region to provide some
late convection over coastal MS. Mid lvl hghts of 591-593dm would
already cause convection to develop slowly over the area but combine
that with northerly flow and if we do get some storms it will likely
be late afternoon if not early evening. Mid lvl winds near 20 kts
could be just enough of a kick to get a few strong storms and with
northerly flow in the Summer we can have storms overachieve so this
may be something to keep an eye on late Sunday.

As for the heat, again like the last few days we have had numerous
locations see HI's around 110 and this should not be a problem again
today if not possibly a touch higher in spots.

Multiple factors to point out:
1. This will be the 3rd day now with lows in the mid 70s to near
   80 for pretty much the entire area. With that it is not allowing
   vulnerable populations and people with no AC to cool at night.
2. Highs are forecast to be a degree or two warmer than yesterday
3. Even with the area mixing out to over h85 there isn't a great
   deal of dry air to mix down to the sfc so dewpoints should still
   remain at or abv 73 with many locations abv 75.
4. With highs forecast to get into the mid 90s, maybe even a few
   upper 90s, locations near water, and that is a lot of our area,
   conditions could be just nasty, especially if we get those upper
   90s.
5. Heat is a cumulative issue and after 2 days already and today
   likely just the middle of the current heat wave, the
   potential for heat illness will only become easier.
6. Again school is also back in session with numerous activities
   occurring during the afternoon around and just after peak
   heating.
7. We will be moving into the weekend and there is a good chance
   that many people will be outside which is when we expect the
   hottest temperatures in this forecast period.
8. Even with some expectation of the general public being
   acclimated to this, not everyone will be and these people could
   be caught off guard quickly.

Current forecast has heat index values ranging from 106-112 today
and 106-115 Saturday and Sunday. The highest values will likely be
seen around the tidal lakes, coastal MS, and the marshes of SELA
around Livingston, St James, Upper Terrebonne, and Upper Lafourche.
Confidence is high in the forecast and that is why the heat advisory
was expanded to include southwest MS and adjacent LA parishes. This
is also the case for expanding the Excessive Heat Watch across
coastal MS and into Sunday. That said it doesn't take much to keep
these extreme values from occurring. First and foremost would be if
we actually have slightly more storms than anticipated. Also if
temperatures are a degree or two cooler than forecast or if
dewpoints can mix out just a touch more then those heat index values
will not occur. That said it will still be hot and humid and
conditions will be quite oppressive for many outside. /CAB/

LONG TERM...(Monday through Wednesday)... As the dome of high
pressure that has ridged over the area continues to maintain its
position. Heat risk will become a factor once again on Monday with
high pressure continuing to dominate the area. High temperatures
will surge into the mid 90s and heat indicies will be knocking at
advisory criteria with indicies peaking near 110. A weak cold front
will emerge from the northeast sometime on Monday moving down
through the southern region right over our area. This will allow
some increase in convection for the Monday- Tuesday timeframe and
also give us a break in the excessive heat values with temperatures
cooling back near normal. Model soundings show PW's just above the
75th percentile in SPC Climatology ranging from near 2.04 to 2.30
inches so locally heavy rainfall will be possible where storms
develop, with Monday being the best day for efficient rainfall. By
Wednesday the boundary would have weakened but continued to linger
over the area , and  a tropical wave moving through the Gulf will
bring sufficient moisture to the area, thus increasing rainfall
potential even more for the week. With continued easterly flow
through the area, we will be able to return to our summer climate
with diurnally driven afternoon showers and thunderstorms through
mid week. /TDB/

MARINE...Actually quite the interesting impact in the coastal
waters with 13-15 second swells being generated by Grace. Otherwise
it is relatively quiet in the northern Gulf. Light winds will
continue with higher pressure in control. Winds near the coasts will
be diurnally driven and as mentioned in last nights discussion look
for that nocturnal jet to develop each night south of the MS coast
and east of the MS river. /CAB/

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  94  73  96  74 /  20  10  10   0
BTR  95  74  96  74 /  20  10  20   0
ASD  96  76  96  77 /  20  10  20  10
MSY  98  83  97  82 /  10   0  20   0
GPT  93  77  95  77 /  20  10  20  10
PQL  94  76  96  77 /  30  10  20  10

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Heat Advisory until 7 PM CDT this evening for LAZ034>037-039-
     046>050-056>061-063>065-071-072-075>078.

     Excessive Heat Watch from Saturday morning through Saturday
     evening for LAZ049-050-057-058-060-061-063-064-072-075>078.

     Excessive Heat Watch from Sunday morning through Sunday evening
     for LAZ049-050-057-058-060-061-063-064-072-075>078.

GM...None.
MS...Heat Advisory until 7 PM CDT this evening for MSZ068>071-077-
     080>082.

     Excessive Heat Watch from Saturday morning through Saturday
     evening for MSZ080>082.

     Excessive Heat Watch from Sunday morning through Sunday evening
     for MSZ080>082.

GM...None.
&&