textproduct: New Orleans/Baton Rouge
This forecast discussion was created in the public domain by the National Weather Service. It can be found in its original form here.
SHORT TERM
(This evening through Monday night) Issued at 306 PM CST Sun Nov 3 2024
This afternoon, recent radar/obs trends indicate some very shallow/light shower activity across coastal SE LA underneath a persistent onshore, moist return flow regime. Taking a step back and looking at the state of the troposphere using the 12Z KLIX RAOB and HRRR sounding trends illustrate a moistening low-levels from the LCL around 3kft AGL to the base of a subsidence inversion at around 8.5kft AGL, which given warm afternoon temperatures helping to develop a well-mixed PBL, has provided a slither of positive bouyancy in this aforementioned layer. SPC Mesoanalysis/sfc wind obs indicate subtle surface confluence extending in an axis from well offshore south of Mobile Bay, NW to Plaquemines Parish also influencing in the support for these shallow showers. NBM PPI is not picking up on this activity (perhaps, best being the HRRR initializing it somewhat) so did add in manually to 14 PoP for drizzle mention to 15 PoP for iso showers across mainly coastal areas into tonight. Just one of those moist return flow situations where a passing rogue shower/drizzle can be squeezed out at any time, but tried to pinpoint greater chances in the forecast.
Otherwise, forecast hodographs indicate a slight increase in depth/curvature in the low-levels starting this afternoon, highlighting a steady uptick in WAA pattern building across the region. This gusty region in the vertical being at the top of the boundary layer will support downward flux transport of breezy winds this afternoon/evening, and regardless of decoupling, will keep winds "elevated" enough tonight to reduce surface fog production/maintenance. This gives scientific reasoning behind the NBM's drop in low visibility probabilities, however may still see low stratus in locations like we typically see in this type of warm pattern. Also, did catch on going back on verification to notice we've been a tad too cool with overnight lows. With this warm/moist flow intensifying and remaining parked over the area, didn't hesitate to hedge closer to the 75th percentile for overnight lows keeping them quite warm, likely breaking some record warm lows (BTR specifically has a record warm low of 69 Monday, set in 1959, 1973 and 1988 with a forecast low of 74. MSY record warm low of 71 on Monday, set in 1975 with a forecast low of 74). Just something to watch.
Starting out the new work week tomorrow fairly similar to today. Warm again with highs flirting with records in some locations yet again. Breezy at times in the afternoon, and can't rule out an isolated rogue shower or two (did manually introduce ~15% PoP in the afternoon for most areas) but generally speaking, primarily a dry day with partly to mostly cloudy skies.
Did want to briefly touch on the coastal flooding situation. Guidance this afternoon started to highlight a bit more of a concern for impacts mainly during/around high tide cycles coming up. More particularly, the Monday night into early Tuesday morning tide cycle being the peak with Waveland's current NAEFS-Based forecast touching ~4-4.2ft MLLW / 2.4-2.6ft MHHW in the 90th percentile. Went ahead an extended the advisory for coastal areas from originally ending 6AM Monday to now 6AM Tuesday, but did introduce a Coastal Flood Watch for south and east-facing shorelines from Hancock, south to SE LA coastal areas 00Z TUE - 12Z TUE to account for this impactful cycle potentially needing a warning. Will let subsequent shifts/updates take a closer look at the guidance to decide to upgrade to warning or revert to advisory, but wanted to get the messaging out for now all due to increase SE flow/fetch in the forecast. KLG
LONG TERM
(Tuesday through Saturday night) Issued at 306 PM CST Sun Nov 3 2024
Now to spend some time in the long range... we'll see how fast I get a headache by the end of it. Starting out with an overview, persistent, deep 588dm H5 ridging remains anchored over the SE Atlantic coastline, staying strong and not wanting to budge much. Meanwhile, a positive-tilt longwave trough axis over the central US, responsible for the active weather out that direction will quickly deamplify and weaken racing northeast around the periphery of the ridge. Surface reflection highlights a weakening surface low ejecting up across the Midwest into the Great Lakes region, with trailing cold front progressing slowly east across the MS valley region. The combination of the strong ridge in place and weakening upper-level support will slow the front to our north/west to a crawl, likely stopping before sliding through our area before turning back north as a warm front (or what would be left of it) into Wednesday. So locally, staying warm with the moist return flow parked in place, still a chance of an isolated shower or two but mainly dry.
Then there's the tropics. We're already starting to see slow/steady development with a tropical wave in the south-central Caribbean. NHC has now brought this system up to a Potential Tropical Cyclone #18 as this system will likely continue to develop. Will go ahead and say, for the short-term, confidence is higher with ensemble spread honing in on the movement through the next 60-84 hours continuing to drift northeast, north, then northwest towards the Isle of Youth/NW tip of Cuba in a generally favorable environment. Briefly looking at intensify forecasts shows a pretty large swath of solutions, with the possibility of either slow /steady development, or quickly growing much stronger as what some of the dynamical hurricane models show. For now, NHC's track/intensity reaching hurricane strength by the time it reaches the NW Caribbean impacting far western Cuba. But as the system continues northwest, could start to encounter a degree of southwesterly shear and even dry air meaning some weakening is possible as it gets more into the Gulf mid-week. Small additional tidbit about the wind shear/dry air: Don't discount that. Looking at water vapor right now shows the intense depth of tropospheric dry air across the southern periphery of the ridge from the mid- atlantic, west to the GOMEX and will continue to pump west as the system punches through it to the northwest with time. If the wind shear can take more of a hold of it, tilts the system more to the east and allows the dry air to work into it, could really make a difference. This time of the year, with baroclinic systems diving much farther south hosts all sorts of challenges in and variables to monitor, It's practically an atmospheric balance game. Ridge strength, position, system intensity, position, speed, degree/location of shear and dry air and how it'll interact with the system all are in question this far out. Not going to pick/choose guidance, not going to side one way or the other but wanted to emphasize the NHC's forecast seems very spot on given a collective understanding of all the variables mentioned, but could still change and bears close watching. I know a hurricane in the Gulf in early November was probably not on a lot of peoples bingo card for the 2024 Hurricane Season, but goes to show the need to stay vigilant, and ready/prepared at any point during hurricane season. As we get into around mid-week, more will become clearer so check back and we'll do our job to pass along the latest information. KLG
AVIATION
(00Z TAFS) Issued at 517 PM CST Sun Nov 3 2024
No low VIS/fog tonight as low level flow remains elevated. However, there will be some low stratus possible with brief reductions to MVFR or perhaps IFR, especially BTR, HDC, and MCB. Otherwise, VFR conditions can be anticipated. Southeast winds will increase again on Monday with some gusts up to 25 to 30kts not out of the question, especially coastal terminals. (Frye)
PRELIMINARY POINT TEMPS/POPS
MCB 69 83 68 83 / 10 20 10 40 BTR 74 88 73 87 / 10 10 20 40 ASD 70 84 69 85 / 20 20 10 20 MSY 74 85 73 85 / 10 20 10 20 GPT 71 82 71 83 / 20 20 10 20 PQL 71 86 70 87 / 20 20 10 20
PRELIMINARY POINT TEMPS/POPS
MCB 69 83 68 83 / 10 20 10 40 BTR 74 88 73 87 / 10 10 20 40 ASD 70 84 69 85 / 20 20 10 20 MSY 74 85 73 85 / 10 20 10 20 GPT 71 82 71 83 / 20 20 10 20 PQL 71 86 70 87 / 20 20 10 20
LIX WATCHES/WARNINGS/ADVISORIES
LA...Coastal Flood Advisory until 6 AM CST Tuesday for LAZ058-060-077- 080-082-084-086>088.
Coastal Flood Advisory until 6 PM CST Monday for LAZ066>070-076- 078.
Coastal Flood Watch from Monday evening through late Monday night for LAZ066>070-076-078.
GM...Small Craft Advisory from 9 PM this evening to 6 PM CST Monday for GMZ536-538-550-552-555-557-572-575-577.
Small Craft Advisory until 6 PM CST Monday for GMZ570.
MS...Coastal Flood Advisory until 6 AM CST Tuesday for MSZ087-088.
Coastal Flood Advisory until 6 PM CST Monday for MSZ086.
Coastal Flood Watch from Monday evening through late Monday night for MSZ086.
GM...Small Craft Advisory from 9 PM this evening to 6 PM CST Monday for GMZ538-550-552-555-557-572-575-577.
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