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897 FXUS64 KLIX 152111AFDLIXArea Forecast DiscussionNational Weather Service New Orleans LA411 PM CDT Sat Jun 15 2024...New SHORT TERM, LONG TERM, MARINE...KEY MESSAGES: 1. A surge of tropical moisture will enhance rain chances startingSunday and will continue into the work week. Locally heavyrainfall could lead to flash flooding especially in urban and poordrainage areas on Monday and Tuesday. A Flood Watch may be warranted in subsequent forecast packages.2. Increasing onshore winds into the work week will bring a largeenough fetch to cause coastal flooding concerns. Latest forecastsindicate coastal flooding of up to 2 to 3 feet above normally dryground will be possible for east and south-facing shores by Tuesday and into the latter half of the week.3. Hazardous marine conditions, especially for small crafts, are expected as winds and seas increase starting Sunday. Sustained winds are expected to be approaching 25 to 30 knots with gusts as high as gale-force (~34 knots) by midweek. Expect headlines to be hoisted for all of the coastal waters by Monday..SHORT TERM...(This evening through Sunday night)Issued at 404 PM CDT Sat Jun 15 2024Dry air above 750mb is keeping our area hot and dry today, with highs across the area expected to reach the upper 90s. This is largely due to the ridge over the southeast CONUS. The ridge and suppression will cause the highs today to flirt with daily records at some sites. Once we get into tonight and tomorrow, a subtle inverted shortwave from the Gulf will bring in deep tropical moisture across the area, especially south of I-10/12. The moistening up of the air column will feature PW from 1.4 in to over 2 inches by Sunday afternoon, which will be flirting with the daily max for that day. This deep tropical moisture from the gulf will lead to very efficient showers with the threat of heavy rainfall and flash flooding. South of I-10/12 will have the highest deep tropical moisture, so the threat seems to be maximized there Sunday afternoon through the overnight. The rainfall seems to be more scattered in nature Sunday afternoon and evening, so rainfall totals are about an inch during that timeframe. Higher PW of 2.2 inches look to advect into the area during the late evening Sunday and through Monday morning. This is when the high-res guidance has the bulk of the rainfall in the short-term. During the 7pm Sunday to 7am Monday timeframe, places south of I-10/12 could see rainfall up to 2 or 3 inches with a reasonable maximum of 5-6 inches in localized areas. Of course, the urban areas such as the New Orleans and Houma-Thibodaux metro area would be most susceptible to flash flooding if the high totals occur during that time period. With the main threat being overnight and continuing into the long term, its best to be weather aware.&&.SHORT TERM...(Today through Sunday night)Issued at 250 AM CDT Sat Jun 15 2024A highly subsident airmass associated with deep layer ridgingcentered directly over the forecast area will remain in placetoday. The end result will be very warm and dry conditions continuing with little in the way of cloud development anticipatedeven during peak heating hours. Temperatures will once again climb into the mid to upper 90s and heat index values will rise tobetween 100 and 105 degrees this afternoon.Tonight and Sunday will be a period of transition from the drypattern in place the past few days to a much wetter period ofweather expected for much of the upcoming week. This will be dueto a flux of deep layer tropical moisture currently parked overthe southern and eastern Gulf sliding to the northwest on thewestern periphery of the deep layer ridge axis that will remaincentered over the Southeast CONUS. Tonight will remain dry through the evening hours, but gradually increasing moisture in the low levels will result in scattered showers and few weak thunderstorms pushing onshore by daybreak tomorrow. Further inland, the airmass will remain sufficiently dry and stable to keep rain shower activity at a minimum. This is well reflected byan expected PWAT gradient ranging from around 1.4 inches insouthwest Mississippi to near 2 inches along the immediate coastof Louisiana by daybreak on Sunday. As the deep layer moisture continues to feed into the region on Sunday, PWATS will further increase to around 2 inches over the northern third of the CWA to around 2.3 inches along the Louisianacoast. These values are between the 90th and max PWAT readings typically seen in mid-June. Rainfall rates of 2 to 3 inches per hour can be expected by the afternoon hours, but storm motions of around 20 mph will help to limit rainfall totals a bit. Still, there will be a localized street flooding concern for areas with poor drainage capacity. Despite the very moist airmass in place, some lingering dry air in the mid- levels could become entrained into the deepest convection Sunday afternoon. Thus, there could besome stronger gusts of wind that occur with the most potent convective cells that form. This is supported by both fairly steeplow level lapse rates in excess of 6.5 C/km and downdraft CAPE values of around 1000 J/KG. Conditions will remain unsettled Sunday night as a weak upperlevel vorticity maxima sliding up from the central Gulf taps intothe very moist airmass in place. Despite some loss of instability as temperatures cool into the low to mid 70s, the slight increasein upper level lift will allow for continued development ofthunderstorms offshore where the diurnal maximum will be reached.Mean storm motion will be onshore from the south-southeast ataround 20 mph, and this will push these offshore storms inland asfar north as the I-10 corridor through the night. Rainfallprocesses will be highly efficient at this point as any mid-leveldry air is flushed out and temperatures start to warm aloft and would not be shocked to see rainfall rates of 3 to 4 inches per hour develop during the overnight hours. This will keep the risk of street flooding in poorly drained areas in place through the overnight hours. A flood watch for flash flooding will likely be needed for portions of the forecast area starting Sunday night. &&.LONG TERM...(Monday through Friday night)Issued at 404 PM CDT Sat Jun 15 2024Deep tropical moisture (>2.2" PWAT, near daily records) will continue to stream into the area from the southeast through the early part of the work week bringing with it heavy rainfall and subsequent flash flooding concerns especially for urban and poor drainage areas. The primary focus for our heaviest rainfall potential will be Monday and Tuesday where high PWATs coupled with favorable diffluent flow aloft from upper-level troughing will yield periods of heavy rain with high rain rates of 2 to 3 inches per hour possible. NBM guidance favors rainfall totals of 6 to 8 inches along coastal areas and 4 to 6 inches along the I-10/12 corridor, lower further inland, with reasonble worst case being about 2 inches higher than the forecasted totals currently. However, it's worth noting that the rainfall gradient will probably much tighter from south to north depending on the track of the broad surface low and the deep tropical moisture position across the northern flank of the surface trough axis. With that said, the flash flood risk is evident so a Flood Watch may be warranted for Monday into Tuesday with the subsequent forecast package. As this broad area of low pressure moves toward the western Gulf coast on Wednesday, the surge of deep tropical moisture will follow it and allow for driercontinental air from the northeast to sneak back into the area and reduce PWAT values. Onshore southeasterly flow will keep rain chances elevated, but more typical of afternoon convective regime we see in the summer. This will also allow temperatures to reboundsome with more sunshine during the first half of the day on Thursday and Friday.Additional wind-driven hazards will also materialize as the broad area of low pressure attempts to consolidate over the southwestern Gulf of Mexico in association with the Central American Gyre (CAG) and the Bermuda High noses in across the southeast CONUS. This will quickly tighten the pressure gradient across the Gulf of Mexico and generate a corridor of stout 850mb winds (35-45 knots) primarily to the southwest of our area Monday through Wednesday. We can expect upwards of 20-25 mph winds and gusts up to 30 mph across coastal areas during this period. Though the highest winds will not reach our area, the stout southeasterly flow will generate gusty winds across the coastal areas and marine zones that will drive a large fetch of water inland. Coastal flood concerns have increased since yesterday with latest PETSS forecasts indicating inundation of water above normally dry ground beginning on Monday and increasing through midweek. Water heights 2 to 3 feet above normally dry ground are now in the forecast by Tuesday into Wednesday. While southeasterly winds do relax slightly into the latter half of the week, onshore flow persists through the period and this could prolong coastal flood concerns through the remainder of the week.Aside from the broad low in the southwestern Gulf of Mexico, no other entities are outlooked at this time and we do not expect significant impacts from any tropical systems for the next 7 days. That said, we will continue to monitor trends in the latest global guidance regarding a weak surface trough that attempts to consolidate in latest 12Z guidance and moves west under the influence of the large Bermuda High toward the Florida Peninsula. It is far too uncertain and inconsistent at this time to be discussing anything with regards to potential impacts to the local area as it pertains to any of these systems suggested by long range global guidance.&&.AVIATION...(18Z TAFS)Issued at 1245 PM CDT Sat Jun 15 2024Dry mid-level air will keep VFR conditions and no rain across thearea for the rest of today and into tomorrow. Much more moist airis expected to advect into the area from the Gulf tomorrowafternoon. When this happens, expect convective coverage toincrease, mainly at the southern terminals (MSY, NEW, and HUM)where the moisture is more prevalent. Expect the scatteredconvection to continue into the evening as well beyond this TAFcycle.&&.MARINE...Issued at 404 PM CDT Sat Jun 15 2024A tightening pressure gradient between a high over the Southeast CONUS and a strengthening area of low pressure in the Bay of Campeche will result in an extended period of small craft advisoryconditions over the coastal waters during the upcoming week. These small craft conditions with winds of 20 to 25 knots and frequent gusts in excess of 30 knots are expected to develop as early as Sunday night, but more likely on Monday and then continuethrough Thursday. These winds extend across a long fetch from theeastern Gulf, and this will allow for a decent swell of 2 to 3 feet to move into the area. This swell will combine with wind waves of 5 to 7 feet to push offshore seas to around 10 feet during the week. Wave run-up along east and south facing shores will also be an issue through the week, and coastal flooding is anticipated to occur during high tide cycles starting on Tuesday and continuing through Thursday.&&.PRELIMINARY POINT TEMPS/POPS...MCB 70 90 72 84 / 0 70 40 90 BTR 75 93 76 87 / 0 80 40 90 ASD 75 90 76 87 / 10 80 70 90 MSY 79 90 79 87 / 10 90 80 90 GPT 78 89 77 84 / 10 80 70 90 PQL 75 91 76 89 / 20 80 70 90 &&.LIX WATCHES/WARNINGS/ADVISORIES...LA...None.GM...None.MS...None.GM...None.&&$$SHORT TERM...TJSLONG TERM....TJSAVIATION...JZMARINE...TJS
