Saturday, August 28, 2021

Hurricane Ida to pummel the LA coast, then head into the Mid-South: what to expect locally

All eyes are on the Gulf of Mexico, as this morning Hurricane Ida is poised to go through what meteorologists call "rapid intensification," and what those in the path must call terrifying. 

Saturday morning "sandwich" RGB satellite imagery of Hurricane Ida in the central Gulf (COD)

16 years to the day after Hurricane Katrina barreled into southeast Louisiana, Ida is poised to do the same with forecasts of 130 mph maximum sustained wind and 10-15 FEET of storm surge are expected along coastal southeast LA on Sunday evening. Local officials are urging all in the path in Louisiana to finalize preparations, and leave if necessary, today before wind and rain picks up Sunday morning. New Orleans, in the right front quadrant of the storm, is bracing for high impacts, including flooding rain, hurricane force wind, and a few feet of surge even in Lake Pontchartrain. Hopefully this time the levees hold.

The latest forecast track from the National Hurricane Center (NHC) has been remarkably consistent as forecast models have a strong lock on the path of the storm, at least until after landfall. The path thereafter, as the storm takes a turn to the north and northeast into Mississippi, has also been fairly steady the past 24-36 hours with only minor adjustments east and west on Monday and Tuesday. Once again, models are fairly consistent in their guidance.

What does that mean for Memphis? 

Ida is a relatively slow-mover into early next week. This means a couple of things. First, that considerable weakening of the storm over land will take place before it gets into north MS. And second, when it does pass by, heavy rain will be the primary threat due to the proximity to Memphis and duration of its passage across north MS. Let's dig into the details...

Heavy rain

The flooding threat will likely be moderate for the Memphis area and points south and east. Models bring anywhere from 2-6" of rain to the general area over a roughly 24-hour window, starting as light showers Monday (mainly afternoon) and picking up overnight in Tuesday morning as the center of T.D. Ida passes by roughly 40-50 miles to our southeast on the current track. Rain should let up as it pulls away Tuesday afternoon and evening. Most of the urban area can handle that amount of rain in 24 hours, but if there are squalls of heavier rain, maybe an inch or an hour or two, the impervious surfaces in the city could see low lying flooding. We'll be keeping a close eye on this threat, particularly if forecast totals rise above 4".

The National Blend of Models (NBM) shows roughly 2-4" of rain for the larger area around Memphis, roughly 3" in the city, in approximately 24 hours from late Monday through late Tuesday.

Strong wind

The probability of tropical storm force wind (sustained at 39+ mph) graphic tells the wind story for us, which is that damaging wind is unlikely. The chance of occurrence is less than 20%, according to NHC (below).

Probability of tropical storm force wind (sustained at 39 mph or higher) from Ida. Memphis' chance is near 10%, although gusts could be strong at times. (NHC)

However, that doesn't mean it won't be breezy or even windy for a time Monday afternoon through Tuesday morning in particular. Wind will be from the east Monday night, shifting to the north on Tuesday as the circulation around the center of Ida moves by to our southeast. Early estimates are that sustained wind could reach 20-30 mph with wind gusts possibly into the 40s ,though 30s is a more realistic guess.

The GFS model prediction of maximum wind gusts from Ida. For Memphis, that number is 40 mph and actually occurs Monday evening. (Not shown: the latest European model puts our max wind gust closer to 60 mph. I just don't believe that at this point.) (WeatherBell)

The good news is that the center is passing closest to us early Tuesday in a weakening state and the wind field around tropical remnants tends to "contract" towards the center of the storm with fewer gusts during the coolest part of the day with gusts picking back up as heating from the sun occurs during the day. So, plan to secure loose outdoor objects and other potential airborne projectiles (and small pets!) by Monday afternoon and keep them secure through Tuesday. No sense giving your neighbor a free patio umbrella or trampoline!

Tornado threat

Regarding wind of the spinning variety, remnant low pressure systems with tropical origins tend to sometimes be spin-up tornado producers. This phenomena is most likely to the RIGHT of the track of the center and during the daytime hours. Thus, we judge the metro tornado threat to be very low, as we'll be on the left side of the track and mainly during the cooler hours of the day. Northeast MS might see a brief twister or two on Monday afternoon or evening, so be aware if that includes you.

Day 3 (Monday) severe weather threat is currently forecast as "Marginal" for a possible brief spin-up tornado in north MS south of the metro. (SPC)

The calm after the storm

The good news is that once all of this leaves, which I expect by Wednesday morning, a drier and a bit cooler airmass moves over the area and likely stays with us into early Labor Day weekend! Dewpoints should drop into the 60s with abundant sunshine, highs in the mid 80s, and pleasant mornings. On the "12 Seasons Scale," I label this #FalseFall. Real fall night be still a little ways out as we can get some hot days in September, and I wouldn't really call mid 80s high temperatures true fall weather, but we're getting closer!

Erik Proseus
MWN Meteorologist

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