Southwest Pass of the Mississippi River to Port Fourchon, LA 20 - 60 NM Marine Forecast
|Rest Of Today...South Winds 15 To 20 Knots. Seas 3 To 6 Feet. Dominant Period 6 Seconds.|
|Tonight...South Winds 15 To 20 Knots. Seas 3 To 5 Feet. Dominant Period 6 Seconds.|
|Monday...South Winds 10 To 15 Knots. Seas 3 To 5 Feet. Dominant Period 5 Seconds.|
|Monday Night...South Winds 10 To 15 Knots. Seas 3 To 5 Feet. Dominant Period 6 Seconds.|
|Tuesday...Southeast Winds 10 To 15 Knots. Seas 3 To 5 Feet. Dominant Period 6 Seconds. Slight Chance Of Showers And Thunderstorms.|
|Tuesday Night...Southeast Winds 10 To 15 Knots. Seas 2 To 4 Feet. Dominant Period 6 Seconds. Slight Chance Of Showers And Thunderstorms Through The Night.|
|Wednesday...East Winds 10 To 15 Knots. Seas 2 To 4 Feet. Dominant Period 6 Seconds. Chance Of Showers And Thunderstorms.|
|Wednesday Night...East Winds 10 To 15 Knots. Seas 2 To 4 Feet. Dominant Period 6 Seconds. Chance Of Showers And Slight Chance Of Thunderstorms.|
|Thursday...Northeast Winds 10 To 15 Knots. Seas 2 To 4 Feet. Dominant Period 6 Seconds. Chance Of Showers And Slight Chance Of Thunderstorms.|
|Thursday Night...Northeast Winds 15 To 20 Knots. Seas 3 To 5 Feet. Dominant Period 5 Seconds. Slight Chance Of Showers.|
| Synopsis for Pascagoula to SW Pass Mississippi - GMZ501|
Synopsis for SW Pass Mississippi River to Atchafalaya River- GMZ500
1016 AM CDT Sun May 9 2021
A frontal system will approach the Louisiana coast tonight into tomorrow. Several low pressure systems will move along that front through Wednesday. The front will push through the area Wednesday night with high pressure building in for the latter part of the week.
| Area Forecast Discussion|
National Weather Service New Orleans LA
1030am CDT Sunday May 9 2021
.MORNINGA review of the morning sounding along with water vapor imagery analysis indicates that the strong capping inversion currently in place around 925mb will continue to hold strong through the afternoon hours. A deep pool of dry air in the mid-levels will persist through the day day, and have removed any mention of Probability of Precipitation from the forecast for the afternoon hours across the entire forecast area due to the strong capping inversion induced by this dry airmass. No big change in thinking for tonight as a convective complex forms to the northwest of the CWA and begins to impact the northern third of the area late tonight into tomorrow morning.
A bit more cloud development is occurring beneath the inversion as lapse rates below 925 mb are fairly steep. The forecast has been updated to show a scattered to broken cumulus field persisting through the day. This steep lapse rate in the low levels has alllowed for strong boundary layer mixing and winds have increased faster than forecast over land areas. Adjusted wind speeds up slightly and these winds should remain in the 10 to 15 mph range with higher gusts through the afternoon hours. Temperatures and dewpoint forecasts are tracking nicely with observed values, so no changes were made to those grids this morning. PG
AVIATION (12z TAF package)... Expect primarily VFR conditions for all area terminals today through the afternoon hours. Only noted some low clouds near KMCB bringing tempo MVFR (Marginal Visual Flight Rules) but is expected to improve thereafter. Otherwise, thin to patchy alto-cumulus and cirrus can be expected today, with some Cumulus clouds development but will refrain from becoming a significant impact with breezy SE winds bearing 190 to 210. Spotty showers may develop after 00Z this evening across coastal SE LA, drifting north and increasing in coverage through the late evening/early night time hours tonight. Additionally, a line of showers and storms may approach from the northwest around 04Z, drifting south and east through Monday morning. Have approximated timing for each terminal, with impacts being the potential for gusty, erratic downdraft winds, lightning and heavy rainfall leading to reduced flight categories. This activity may persist beyond 12Z but should drift primarily more east across coastal MS and far SE LA, but more scattered shower/thunderstorm development will be possible thereafter through the day on Monday. KLG
Short Term - Today Through Tuesday
Quite a nice way to start off Sunday across the area, with a light southerly breeze helping to draw up a steady increase in surface moisture. Only notable feature on GOES-16 Night Fog Channel Difference is some patchy low clouds developing across the Northshore east across parts of coastal MS. KHDC places these at around 3000ft AGL, but may sag down to around 2500 to 2000ft AGL around daybreak. Starting off daybreak today, model soundings illustrate a rather dry subsidence inversion aloft right at 3500ft AGL, which appears to be the same lower tropospheric dry air intrusion noted yesterday building in from the western side of the eastward progressing low-level ridge. Deeper low-level moisture will continue to build in through lateam and into the afternoon hours, suppressing the strength of this inversion with time, but residual warming in the 850mb to 600mb layer will remain in place (~4.5C/km lapse rates) leading to a dry day today. There may be enough moisture at the top of the PBL/near the LCL for some patchy cumulus, but an overall pleasant day with highs in the mid to upper 80's (still leaning on the warmer bias above deterministic NBM guidance).
Going into tonight is where we will apply the most focus as we will have quite a complicated forecast on the horizon. Beginning with 00Z MON (Sunday evening)... widespread severe convection will be ongoing along a cold front across the Arklatex region, extending north and east into the Tennessee Valley. The eventual storm mode transition of this convection will be a significant player in what we can expect here locally Sunday night into Monday morning. Peeling apart each HREF member reveals some interesting and complicated aspects to closely focus on. As mentioned for a few nigheights now, one concern was eventual storm evolution becoming linear or into a QLCS, developing a local cold pool/meso high behind the convection which would propagate this activity more east (and potentially southeast). Seeing several CAM's illustrate this possibility which leads to a higher confidence that a potential, messy east or southeast moving MCS or QLCS will try to advance into our NW CWA beginning before midnight, pushing across the entire area through the early morning hours on Monday.
Now, few things to note here that lacks high enough confidence on this solution... It has been observed over the past month or more an obvious struggle in the NAM/NAM3KM solutions, which bring lower confidence in this guidance. For days now, the NAM has been stubborn at keeping the front up north, perhaps likely struggling with downstream convective response and never really develops deep enough convection to develop a cold pool. While the SUN 18Z and MON 00Z HRRR (High-Resolution Rapid Refresh) are in strong agreement on a line of strong to severe storms racing across then area, the 06Z threw in a wrench by igniting cells earlier ahead of the line, killing the inflow ingest of the MCS leading to a large mess of convection earlier than anticipated. Closely comparing both run's model soundings shows deep-moist ascent underway after 02Z, rising and shrinking the subsidence inversion with time which will support shallow showers initially south of I-10/12 between 00 and 02Z, growing vertically with time into eventual cells. The vertical wind profile will be lacking in shear magnitude, but supportive in direction for shallow rotating updrafts (0-1km SRH in the 150-170m2/m2, nearing 200m2/n2 in the 0-3km layer). Mean storm- layer motion is around 237/21kts (towards the NE) but will monitor for right moving cells (Bunkers right motion averaging due E). This brings a low, yet non-zero risk of a few isolated weak tornadoes/waterspouts in any deeper/stronger cells.
The overall amount of pre-MCS cells will be dependent on the MCS's forward speed itself. Slower? more time for deep-moist ascent to break the cap and develop widespread cells, cutting off the main inflow to the complex. Faster? (which, keep in mind typically is the case)... this complex will race east with only a few pre-MCS cells merging into the line. Should this complex solidify enough, 0-3km Bulk Shear will be lacking enough for a distinct linear spin-up potential, instead would be a mainly cold- pool dominated system with damaging winds the main risk. Worthy of noting however, we can see some supportive lapse rates in the mid-levels which is typical above residual inversions, which does pull up 700-500mb LR's in the 7.5C/km range, utilizing maximized energy in the -10 to -30C hail growth zone. This, combined with H5 temps of ~ -11C supports some hail in any stronger storm which will need to be monitored as well.
Now that we have made it into daybreak Monday, it will get even trickier as this point forward will be dependent on just how the lead MCS materializes and potentially leads behind a lingering east to west oriented boundary. This is where there could be a potential for localized flash flooding depending on where this sets up, as storm mean layer winds continue parallel the boundary. Upwind corfidi vectors also line up well with 850mb moisture transport in this type of scenario for back building storms possible (not to mention anomalously high PW's at or slightly above 2.0" which would absolutely break the MON, May 10th 12Z KASD record of 1.82" according to Storm Prediction Center Sounding Climo) Since this is all dependent on boundary location, will only mention flash flooding as a potential but will let subsequent shifts monitor and determine the need for any highlighted areas to focus on, meaning if a watch is required, it would likely come much later due to lower confidence and will let the D1/D2 MRGL ERO from WPC ride for now. But please be aware that there is a distinct flash flooding potential somewhere in our CWA come early Monday, and be aware there could be changes or upgrades as we go through the rest of the day/night tonight.
Going into the day on Monday, we actually reach a temporary drying period in between waves/disturbances. Have nudged NBM projected highs at or slightly below deterministic suggestions due to a mostly cloudy, drizzly day expected. As mentioned before, any additional development later Monday and into Monday night will be focused on small-scale remnant boundaries, which could quickly develop training convection yet again in either same areas or different areas impacted prior Monday morning. PW's remain high in the 1.7 to 1.8" range with the stalled boundary draped across the region.
By Tuesday, our next mid-level shortwave impulse rides the quasi- zonal flow aloft on top of the stalled boundary. Tuesday will be yet another day to focus on the potential for excessive rainfall and potential strong thunderstorms. Will wait going in-depth here, but be aware Tuesday could be another active day across the area with the primary threat being some areas of flash flooding, which may persist into the evening hours Tuesday. KLG
LONG TERM (Tuesday night and beyond)... By Tuesday evening, a frontal boundary will be stretched east-west somewhere in our area, probably close to the coast. Southern stream shortwaves are expected to track across the area Tuesday evening and again Wednesday night. A northern stream shortwave will eventually push the moisture out of the area by Thursday evening. With precipitable water values near 2 inches in many areas, each shortwave is expected to produce localized heavy rainfall. Cumulative rain totals by Thursday morning are likely to be in the 2- 4 inch range. As long as this doesn't occur in a couple hours, rivers are likely to be able to handle this rainfall, much like we did this past week. Will be carrying likely to categorical Probability of Precipitation for much of the area Tuesday night through at least Wednesday night. Drier weather on tap for the weekend after the passage of the northern stream shortwave. Will go with the blended temperatures as they seem to agree pretty well with the operational medium range guidance. 35
High pressure building east offshore eastern Florida will continue to promote breezy onshore flow. Beginning with today, gusty winds reaching Advisory criteria will persist for far western marine zones through about noon today, with headlines in effect. Locally enhanced waves/seas in the 3 to 5ft range can be expected, but winds and seas will steadily subside going into later today and tonight with Exercise Caution headlines to follow. By tonight through Monday, a slow moving front will eventually stall along the coastline, with a potential for a line of strong to severe storms to race across the tidal lakes/sounds tonight and into Monday night. In these regions, strong winds will be the primary risk, along with some hail or waterspouts in any storms developing out ahead of the line. Expect active weather to persist all through the middle and later part of the week, as several disturbances ride the stalled frontal boundary. High pressure will attempt at building in Thursday and into the day on Friday, with improving conditions expected next weekend. KLG
NOAA New Orleans LA Office: Watches - Warnings - Advisories
LA...None. GM...None. MS...None. GM...None.