LIX issues Area Forecast Discussion (AFD) at Sep 4, 1:34 PM CDT ...New UPDATE, AVIATION...100
FXUS64 KLIX 041834
AFDLIX
Area Forecast Discussion
National Weather Service New Orleans LA
134 PM CDT Wed Sep 4 2024
...New UPDATE, AVIATION...
.UPDATE...
Issued at 1049 AM CDT Wed Sep 4 2024
Update sent to increase PoPs across the southshore earlier and
decrease slightly across the far north, mostly southwest MS.
Convection will begin to take a lull through the early afternoon
hours but could see an uptick late this afternoon. Main concern
for thunderstorm development though is overnight and tomorrow. In
addition to the PoP adjustment we issued a coastal flood advisory
for the east facing shores along the typical problem areas i.e.
Hancock, far southeastern St Tammany, Orleans, St Bernard, and
Plaquemines parishes and county. Even though we are in Neap tide
this is all being driven by the winds which have been increasing
out of the east and starting to pile the water up along the east
facing shores. That said there is not much response in the lake as
the east winds only just started to increase and to really get the
water through the Rigolets it does take some astronomical tidal
influence to force the water through that narrow channel. /CAB/
&&
.SHORT TERM...
(Today through Friday)
Issued at 336 AM CDT Wed Sep 4 2024
Key Message:
1. Confidence is increasing in excessive rainfall occurring
Wednesday through Friday especially along and south of the I-10/12
corridor. This could lead to flash flooding especially in urban
and poor drainage areas. The threat for excessive rainfall is
highest on Thursday.
Scattered showers and storms are gradually developing
over the coastal waters and moving inland over the southern tier
of parishes this hour. This is indicative of the deep tropical
moisture beginning to work itself back inland with increasing
southeasterly flow. Expect this surge of moisture to continue
throughout the day as the inverted surface trough continues to
sharpen and enhance the pressure gradient with the high pressure
system situated over the Ohio River Valley. 00 UTC LIX upper air
sounding measured a PWAT of 1.86" which is right around the daily
mean for this time of year, but latest global ensemble suites
indicate that PWATs will be exceeding daily maximums of 2.5" by
the end of the day. The combination of this deep moisture, plenty
of convergence near the surface in association with the surface
trough, and favorable diffluent flow aloft from the exit region of
an upper trough over Texas will increase the risk for excessive
across southern LA and MS today through Friday.
While coverage today will be more scattered in nature as deep moisture
continues to mix in and residual capping from mid-level ridging
abates, by tonight we anticipate more widespread convection to
develop especially across the eastern marine waters. The onshore
flow and orientation of this convection could lead to training of
showers and storms that could result in flash flooding, especially
in urban and poor drainage areas. Right now we have two different
camps in how the distribution of heaviest precip will shake out.
CAM guidance is likely being biased by the over-development of a
better defined surface low within the surface trough in the GOMEX
which creates a more focused area of particularly heavy rainfall
along the coastal areas of LA and MS. Meanwhile, the global
guidance has favored a more elongated (SW-NE) and broad surface
trough that could bring heavier rains further inland and spread
out rainfall with less of a gradient in heaviest totals. The key
focus over the next 36 hours will be monitoring the progress of
the inverted surface trough along the Gulf coast and its relative
orientation to our CWA. For now, this forecast is keeping closer
to WPC and NBM guidance where heaviest totals of 3 to 5" remain
along the immediate coast (as one would expect when these areas
are likely to see more frequent showers and storms at all hours).
Areas north of the I-10/12 corridor are generally expected to see
2-4", but there exists much greater uncertainty in the lower and
higher end scenarios still dependent on exact how this axis of
heavier rainfall sets up on Thursday. NBM 90th percentile still
suggests locally higher amounts could exceed 6-7" along and south
of the I-10/12 corridor and thus the Flood Watch has been expanded
to cover these areas.
The surface trough will linger over the area into Friday,
exacerbating excessive rainfall concerns for much of the day.
Simultaneously, the longwave trough over Manitoba will begin to
dive and deepen into the eastern CONUS pushing a more substantial
frontal boundary across the central CONUS. This will be the
primary mechanism that begins to kick the surface trough out of
the area by the weekend.
&&
.LONG TERM...
(Friday night through Tuesday)
Issued at 336 AM CDT Wed Sep 4 2024
Overnight Friday and into Saturday we will gradually begin to see
the surface trough get shunted southward into the coastal waters.
The first fall front of the year gets to our doorstep, but will be
losing steam as the large longwave trough over the eastern CONUS
starts to lift out and its support of cold air spilling down from
Canada is cut off. We'll have to keep an eye on just how far south
this boundary sinks into the coastal waters before stalling as it
is possible we don't get rid of it fully from the coastal areas
and thus slight chance of showers is remaining in the forecast. The
post-frontal high pressure and steady northerly winds will still
be sufficient in freeing us from the deep tropical air mass that
we have been stuck with and bring dew point temperatures down into
the 50s and 60s! This will allow overnight temperatures to get to
nearly 10 degrees below average, some areas might even see their
first temperatures below 60F in southwest MS on Sunday and Monday
morning.
&&
.AVIATION...
(18Z TAFS)
Issued at 1049 AM CDT Wed Sep 4 2024
TSRA and SHRA are impacting most terminals currently or will do so
soon. Deep tropical moisture will continue push onshore with
sightly stronger southeasterly winds gusting to 20-25 knots along
the coastal terminals. MVFR to IFR impacts will be likely and
continue at terminals as SHRA and TSRA. Convection may have a
slight lull late this afternoon and early evening but expect
redevelopment overnight especially along and south of the 10/12
corridor with greatest risk along the coast. /CAB/
&&
.MARINE...
Issued at 336 AM CDT Wed Sep 4 2024
An area of low pressure is beginning to slowly develop over the
northwestern Gulf and will slowly deepen over the next few days.
This will tighten the pressure gradient across the region. This
will result in an increase in easterly to east-southeasterly
winds through the day today and possibly into Thursday as the low
continues to deepen and slowly work east-northeast. Small craft
headlines remain in effect through this evening. The highest
winds will be focused over the eastern waters (Lake Borgne,
Sounds, and open waters east of Plaquemines Parish) where
sustained winds will be 15-20 knots with gusts up to 25 knots and
seas of 3 to 5 feet. Waters will remain unsettled with numerous
showers and storms across the marine areas Wednesday through
Friday which could result in locally higher winds and seas. A
frontal passage will bring more offshore winds by the weekend and
potential headline criteria again.
&&
.PRELIMINARY POINT TEMPS/POPS...
MCB 87 72 79 69 / 40 60 80 60
BTR 91 76 84 74 / 70 70 90 70
ASD 87 74 83 72 / 80 80 90 80
MSY 87 76 83 74 / 100 80 90 80
GPT 86 74 81 72 / 80 80 90 80
PQL 88 75 85 73 / 70 80 80 80
&&
.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Flood Watch through Friday evening for LAZ046>048-056>060-
064>070-076>090.
Coastal Flood Advisory until 7 AM CDT Friday for LAZ069-070-076-
078.
GM...Small Craft Advisory until 7 PM CDT this evening for GMZ532-534-
536-538-557-577.
MS...Flood Watch through Friday evening for MSZ083>088.
Coastal Flood Advisory until 7 AM CDT Friday for MSZ086.
GM...Small Craft Advisory until 7 PM CDT this evening for GMZ534-536-
538-557-577.
&&
$$
SHORT TERM...TJS
LONG TERM....TJS
AVIATION...CAB
MARINE...TJS
Source:
LIX issues Area Forecast Discussion (AFD) at Sep 4, 1:34 PM CDT ...New UPDATE, AVIATION...---------------
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