As September approaches, summer continues and an update on the busy tropics

It's been 79 days since Memphis International Airport, and the urban core of Memphis, have seen a low temperature below 70°. (If you're in the suburbs or outlying areas, you saw a morning or two of mid to upper 60s early this past week.) That is just 2 days shy of the all-time record for consecutive days with minimum temperatures at or above 70°, set back in - you guessed it - the scorching summer of 1980. (As a spoiler, we'll break that record on Wednesday morning.) For the period of June-August (commonly defined as "meteorological summer"), the average temperature this year has been 84.1°, which is good for 6th hottest "summer" on record. And we're just 0.2° from 4th place with daily averages the next two days around 86°. I'm done with summer.

Unfortunately, as average temperatures come off their peak for the year and head towards typically more comfortable values in September, our weather pattern is acting like a two-year-old that isn't getting her way. It isn't budging.

Early week weather

Summer-like high pressure at the mid levels of the atmosphere (about 20,000') will continue to dominate our weather pattern for the first half of the week, which results in subsiding air and above normal temperatures. Couple that with high pressure at the surface over the Mid-Atlantic and we're getting an easterly surface wind component. So while the Gulf of Mexico is not sending us moisture directly, we do still have a fair amount of humidity in the air.

The mid-level weather map shows broad, hot high pressure dominating the region early this week. with weather systems that could bring cooler weather and organized thunderstorm activity well north into Canada. (PivotalWeather.com)

The good news is that in the afternoons as the air "mixes down" from aloft due to heating, drier air above us also comes down to the surface, so our afternoon dewpoints are lower than in the mornings, resulting in a little relief from the heat. It also makes for easier heating of the air due to lower moisture content, thus the 96-98° highs this weekend! This pattern continues early this week with afternoon showers being few and far between and triggered only by atmospheric heating, while temperatures continue to reach the mid 90s. Heat indices will be in the 100-105° range from lunchtime through late afternoon. Act like it's summer, because it is, and don't overdo it.

The surface map for Monday evening shows high pressure dominating the eastern U.S. with wind into the Mid-South from the east due to high pressure well to our north (clockwise wind around the high). (NWS - weather.gov)

Late week/weekend weather

As we get into the latter half of the week, the upper air patter breaks down as a trough forms in the northeast U.S. and an upper level ridge rebuilds to our west, over the Plains. This puts the Mid-South in "northwest flow" aloft, which will allow a weak front to move through the area by Thursday.

As compared to the map above, the late week mid-level weather pattern shows high pressure well to our west with almost northerly flow into the Mid-South between the high and a large trough over the eastern U.S. This pattern allows a weak front to move to our south and bring slightly cooler and drier air into the region. (PivotalWeather.com)

While temperatures may only be a bit cooler (right now I still expect 90° each day late this week), dewpoints will fall thanks to drier air from the north moving into our area. The drier air will mean more comfortable, if still warm, conditions and most likely some morning lows in the 60s, especially outside the urban core. Rain chances drop to nil for the end of the week and next weekend - good news for Tigers fans looking forward to the first tailgate opportunity of the year on Tiger Lane and kickoff to the 2016 college football season!

Unfortunately, the long-range forecast continues to lean heavily towards above normal temperatures well into September. Fortunately, "above normal" is relative and 90° will be above normal in another week as average highs fall into the 80s. Hopefully a good taste of fall is coming soon!

The long-range outlook shows favorable conditions for above normal temperatures continuing into mid-September. Memphis has about a 60% chance of above normal temperatures, a 33% chance of near normal temperatures, and almost no chance of below normal temperatures. (NOAA/Climate Prediction Center)

A few notes on the tropics

In the past week, as we near the "peak" of tropical weather season in a couple weeks, the Atlantic and Pacific basins have gotten very busy. While nothing appears imminent to affect the Mid-South, here are a few takeaways:

Tropical Depressions 8 & 9 formed today, while Hurricane Gaston churns in the central Atlantic. The fate of each is discussed below. (NOAA/National Hurricane Center)

• "99L" has been watched closely for well over a week as it passed through the Caribbean with some "reliable" models portending doom and gloom for portions of the U.S. Today, between south Florida and Cuba, it finally achieved "closed circulation," which means it is now a tropical depression (#9). It is on it's way to becoming a tropical storm, or potentially, a hurricane in the Gulf of Mexico. The future track is still uncertain, but should get clearer in the next couple days. Most likely scenario is a trek into the south-central Gulf and then a turn to the north or northeast with landfall somewhere between the MS/AL coasts east into Florida north of Tampa. Florida hasn't had a hurricane landfall in 11 years. Could this be it? 

The NHC forecast track of Tropical Depression 9. This track is still fairly uncertain and likely to change somewhat this week. All interests in the eastern Gulf of Mexico need to keep a close eye on it.

•  A new system, "91L," also became a tropical depression (#8) today, much more quickly than it's predecessor 99L. T.D. 8 is located between Bermuda and South Carolina, but moving west towards the Carolina coast. It should make a close pass to the Outer Banks on Tuesday before also making a hard right and retreating back into the Atlantic. The first of these two depressions to strengthen to a Tropical Storm will be called "Hermine" while the latter will be "Ian."

The NHC forecast track of Tropical Depression 8, which will have a close encounter with the NC coast.

• Hurricane Gaston is well out to sea and poses no threat to the U.S. as it moves northeast.

Hurricane Gaston as seen by infrared satellite earlier this evening. (NOAA)

• Finally in the Atlantic, a new wave that emerges off the African coast in a day or two is already forecast by long-range models to make the long trek across the Atlantic and make a run at the Caribbean and/or U.S. in about 10 days to 2 weeks. That's a LONG way out, but it's one that shows potential for strong development and will be worth watching.

An expected area of tropical development will need to be closely monitored as it could pose a threat to the Caribbean and perhaps the U.S. in 10-14 days or so. (NOAA/National Hurricane Center)

• In the Pacific, a rare "double threat" is being monitored in Hawaii as Tropical Storm Madeline and Hurricane Lester both eye the big island over the next week or so. 

The NHC forecast track of Tropical Storm Madeline as it threatens the Big Island mid-week, remaining a Tropical Storm.

Following Madeline, Hurricane Lester also has Hawaii in it's sights, according to the NHS forecast track.

Hang in there, cool weather fans!  The pumpkin spice is appearing, which means that fall can't be that far away!

Erik Proseus
MWN Meteorologist

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Commentary: On forecasting tropical systems that haven't yet formed

You've likely heard the hype. A tropical wave that has not yet formed into a storm is "bearing down on the Caribbean" and sure to pose the gravest threat to the United States since Twitter was born. Millions are in the potential path! Maybe you've even heard of a "triple threat" to the U.S. because there are two other storms in the Atlantic, even though they have virtually no chance of reaching American shores.

Tonight I thought I would take a few minutes to explain why responsible meteorologists (as I consider myself to be) can't, and won't, predict what the yet-unnamed tropical cyclone approaching the Caribbean is going to do. It's part of the education process I engage in with MemphisWeather.net, particularly because I display the Weather-Ready Nation and NWA Digital Seal logos that mean I am a responsible purveyor of weather information.

But first, why even bring this up? We aren't near the coast, so why does it matter? Because I get your questions and answer almost all of them! "What does the European model say today? It's always right!" Also because many of us have interests in near-coastal locations - relatives, friends who have moved, or maybe even our favorite beach spots. The Mid-South is also near enough to the central Gulf coast that when a big storm heads that way, we need to pay attention. If it doesn't directly affect our weather, it likely will indirectly.

Forecasting in the early stages of development

So here's the problem. A tropical wave that is still a few clusters of thunderstorms, with no "closed circulation" (meaning it is not yet an actual low pressure system), is not well-observed by the computer models. And if it's not well observed, it isn't going to be well-forecast. There are simply too many variables, including whether or not it will actually form and then how strong will it be and where will it go. Models, and the human "interpreters" that forecast these systems, are getting better, but it's still very difficult especially in the early stages of a system.

Some examples

Look at these examples from today. Here's this evening's model "spaghetti plot." It looks like a pot of noodles! The colored lines are forecast tracks of this particular system by various computer models. The tighter they are together, the higher the confidence in the track as they all have roughly the same solution. The farther apart they diverge, the lower the confidence, because the solutions are very spread out. In this case, a couple models say the system will miss the east coast altogether while a couple point it towards the mouth of the Mississippi River. In other words, the "spread" of the models doesn't tell us who should be preparing for the storm! Sure it looks like southern Florida has the best chance, but when? And how strong will it be? And then where does it go??

The next graphic shows the forecast strength of the system, going out in time. What does it tell me? That we could have a system that remains a weak tropical depression with little impact other than rain for the next week, or we could have a category 4 hurricane in 5 days! The models don't have any idea, even if you throw out that aggressive GFNI model.

What does history tell us about how well the forecasts from the National Hurricane Center (you know, the "experts" at this stuff) pan out? See below. Forecast error is on the left (vertical) axis and time is along the bottom. Each colored bar represents a certain type of storm based on intensity. As you can see, the weaker the system being forecast, the worse the track error is, especially as you get further out in time. The average error for a weak system at 5 days is over 350 miles! And these are the experts that interpret all the data, not the models!

The point here is that we don't know where "99L" or "Hermine" will go or how strong it will be yet. Or even if it will become Hermine for that matter. What we do know is that it's something we, as meteorologists, need to keep a keen eye on. In addition, your responsibility is to keep in touch with your trusted sources and not fall for the bait of a bold headline, slick graphic, or doomsday prediction. Here's how I handle long-range forecasts of impactful weather events, including tropical tracks, beyond more than a few days:

The only time we post longer-range forecasts is if there is A) some model consensus & B) it's within about a week. Even then, with caveats.

— MemphisWeather.net (@memphisweather1) August 20, 2016

Thanks for making MemphisWeather.net one of your trusted sources! I'll continue to work diligently to ensure that trust is not broken, including saying "I'm not sure" if I'm not.

Stay tuned to our social media feeds listed below for the latest "truth without the hype" outlooks and forecasts.

Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

MWN is a NOAA Weather Ready Nation Ambassador

Meteorologist Erik Proseus is an NWA Digital Seal Holder