Wednesday, August 24, 2016

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:
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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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Saturday, August 20, 2016

Summer respite continues with a cold frontal passage

A summer-long hot streak ended earlier this week as temperatures finally abated some under abundant cloud cover and several periods of rainfall. While dewpoints remained in the lower 70s, temperatures generally stayed in the lower to mid 80s for highs this past week. However, now that the heat has relinquished it's control a bit, the rain is also getting a little tiresome (boy, we're a demanding bunch aren't we??)!



Cold front arrives

Tonight's cold front should bring a temporary end to most everyone's complaints as it pushes a good deal of humidity and our rain chances to the south for a couple days! We'll still have a good chance of showers this evening as the front moves through the Mid-South, so plan ahead if you have outdoor plans. The front pushes through the metro between midnight and dawn Sunday, bringing an end to the showers and switching wind to the northwest.

The national surface map for Sunday morning shows a cold front just to the south of the metro where it will remain for a couple of days. This will provide for drier air and north wind, leading to more pleasant conditions locally.
It will still be humid in the morning, but throughout the course of the day Sunday, north wind will escort lower dewpoints (less humid air) into the area and clouds will diminish by afternoon. Temperatures will still make it into the mid 80s but with dewpoints in the mid 60s, it should feel much more comfortable. It will also mean that the evening should be delightful as temperatures drop off a bit quicker due to drier air!

The high-resolution NAM (North American) model shows dewpoints on Sunday afternoon. It's been weeks since we have seen dewpoints in the mid 60s, but that is what is expected by this time. 70°+ dewpoints will be pushed just south of the metro. Graphic courtesy PivotalWeather.

Early week forecast

By Monday morning, temperatures are expected to be in the 60s area-wide for the first time since June 10! This will mark an end to the 72-day streak with no temperatures in the 60s at Memphis International Airport that ranks second longest of all-time behind the hot hot summer of 1980. A very nice day is ahead on Monday with highs in the upper 80s and continued low humidity (dewpoints in the lower 60s).


The pattern starts to shift Tuesday into Wednesday, as the front to our south returns to the north as a warm front and higher humidity values return. We'll be back in a more typical summertime pattern with highs in the lower 90s and lows in the mid 70s with low chances of thunderstorms each day to end the week thanks to upper level high pressure ridging over the southeast U.S.

Shown is the GFS (American) model at 500mb (18,000') level pressure and wind pattern on Sunday morning as a large trough (valley in the wind pattern) sits over the Mississippi Valley. The trough is what allows the cold front to get this far south. Graphic courtesy PivotalWeather.

By Wednesday morning, that upper level trough is gone and a ridge of high pressure has regained control over weather in the southeastern U.S. The GFS model at 500mb is shown once again. Graphic courtesy PivotalWeather.


On Long-Range Tropical Forecasts

Finally, a note on long-range tropical forecasts as it appears we head into a more active period in the Atlantic Ocean and perhaps Gulf of Mexico. Some weather entities on social media feed on the long-range models "predictions" of close encounters, or landfalls, of strong hurricanes more than a week or even two out. Our advice: "Beware the Share." Rather than sharing these "hype-casts" or "click-bait," we recommend blocking or un-following the account, especially if the post ends with "Please like/share this post!"

The only time we post longer-range forecasts (beyond 7 days) is if there is A) some model consensus between models and consecutive runs of those models, and B) it's within about a week of impact. Even then, we will also post caveats with those forecasts, as they're really only useful for trend analysis and not exact forecasting. It is near impossible to predict the exact landfall point (or even general area) of a tropical system more than about 5 days out. We simply ask that you be conscientious consumers of information and use some common sense.

This is currently the most responsible graphic for the tropical Atlantic. Two systems are being monitored for potential development in the Atlantic over the coming 5 days. The first (in orange) has a moderate probability of tropical development as it heads generally towards the Caribbean. The second, coming off the African west coast, has a high chance of development but is not currently a threat to any land areas. Tropical Storm Fiona in the central Atlantic is not a threat and is not shown.
Enjoy the respite from summer the next few days!

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

Saturday, August 13, 2016

Louisiana low produces record rain, but what about our precip chances?

You've no doubt seen the media reports of flooding that is rising to historic levels along the central Gulf Coast. It started earlier this week with a low pressure system that formed in the northeastern Gulf and brought heavy rain to western Florida and now has been sitting over Louisiana the past 24 hours. The situation in some areas of southern Louisiana, mainly west of New Orleans, is becoming dire as water rescues have been necessary and portions of towns and cities are flooding from 10-20" of rain that has fallen so far. Baton Rouge received over 11" of rain in the past 24 hours, which forced Louisiana State University to close today on what would have been move-in day for students and the governor has declared a State of Emergency for the entire state.


Unfortunately, today brings another day of heavy rain in some of the same areas. The low is centered over west-central Louisiana this morning and is combining with abundant atmospheric moisture fed by anomalously warm waters in the norther Gulf of Mexico to produce very high rainfall rates of multiple inches per hour in the heaviest storms.


With respect to how how this system affects Mid-South weather, the effect is indirect, as the low moves very little, into northwest Louisiana by Sunday morning. However, it will weaken a bit and merge with a frontal system that is loitering to our northwest. The front will move little, but act as a conveyor belt for the energy and rainfall from the low to move along the front northeast into the norther Mid-South and then the Ohio Valley.

NWS surface map valid 7pm Sunday evening shows weak low pressure over eastern TX as it gets ready to start it's journey northeast along the front that is draped out just to our north and into the Ohio Valley. That front will be the focus for the heaviest rainfall into next week.

With the metro sitting firmly in the "warm sector" south of the front, the proximity of front to the northwest, the low to the southwest, and upper level energy moving over the area will mean scattered showers and a few thunderstorms mainly during the warmer parts of the day this weekend with most people seeing a good chance of rain, especially Sunday and Monday. Abundant moisture is in place overhead, which means that any showers or storms that form will be capable of heavy downpours. Fortunately, showers and thunderstorm won't be sitting in one place very long, which should reduce the flash flooding threat this weekend in the metro.

Precipitable water values, which measure total atmospheric moisture in a column of air, will be very high this weekend, meaning any showers and thunderstorms that form will be capable of heavy rain in a short period of time. Values of 2" are considered very high and 2.5" excessive. Graphic courtesy PivotalWeather.

Closer to the front though, from northern AR into southern MO, rain and storms could "train" over the same areas repeatedly, thus setting the stage for higher risk of flash flooding.

NOAA/NWS forecast rainfall amounts through Monday evening indicate where the plume of heaviest rain to our west, although we could see more than an inch of rain the next couple days. Graphic courtesy WxBell.

Heading into next week, the weather pattern remains stagnant as the front sits just to our north. Rain chances remain at least 50/50 each day through at least Thursday. This wet pattern will mean a couple of things. First, periods of heavy rainfall will be possible each day, which will make up for our recent dry weather that for some is now going on two weeks.

Heading into next week, NOAA/NWS forecast rainfall amounts continue to build in the Mid-South and northward through the Ohio Valley into the northeast. Total amounts through Friday morning are shown, indicating 3-5" possible in the metro in the coming week. Graphic courtesy WxBell.

Also, temperatures will remain warm and humidity high, but we're not expecting to see 90° again for much of the next week. That may mean an end to the second-longest streak of daily average temperatures above 80° in the next few days. It currently stands at 64 days at Memphis International Airport. We also are in the midst of a stretch of 77 days with high temperatures at or above 85°. There's a good chance that will be broken in the next week as well with highs forecast in the mid 80s for much of next week. Finally, our streak with no temperatures in the 60s also stands at 64 days as of today. The fact that dewpoints will remain above 70 during this wet week ahead will likely keep that second-longest such streak intact.

Stay up to date with our latest forecast information and Mid-South radar via the MWN mobile apps or our website and social media streams. Links to all are provided below.

Erik Proseus
MWN Meteorologist

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Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Tuesday, August 9, 2016

As rain chances go up, temperatures come down (a bit)

A potential reprieve from summer heat is on the horizon and I doubt anyone is complaining!

A broad area of low pressure has developed along the central Gulf Coast and will very slowly move northwest towards and through the Mid-South over the next several days to perhaps a week, bringing a prolonged period of elevated rain chances. In addition, high temperatures will start to recede a bit, though that will partially be offset by increasing humidity as very moist tropical air invades the region after lower humidity to start this week.

We'll see a couple more days with temperatures in the lower to mid 90s and heat indices above 100, but scattered afternoon thunderstorms will provide some folks some relief late in the day Wednesday and Thursday. By this weekend, rain becomes likely at times, resulting in somewhat cooler, but humid, conditions.

The main threat from showers and thunderstorms the next several days, particularly this weekend and perhaps early next week, will be heavy rain and potential flash flooding as the low pressure system merges with a cold front that moves into the region. The axis of the heaviest rain is still to be determined, but the potential for 3-6" of rain through early next week exists across portions of the Mid-South. Severe weather is not currently expected the next several days.

Computer models still diverge quite a bit on when the most likely rain period comes to an end as the low moves northeast out of the region along the front, ranging from Sunday to perhaps as late as Tuesday. That will also affect rainfall totals with more rain likely if the low lingers.

NWS forecast total precipitation for the upcoming week, through Tuesday night, August 16. Heaviest rain will fall along the central Gulf Coast, though amounts of 3-6" are likely where the low intersects the approaching front early next week, currently depicted from AR northeast through the Ohio Valley. 
Besides some needed rainfall, the best news out of this forecast is probably the highs in the 80s, even if lows remain in the 70s due to abundant moisture and high dewpoints. We'll take a break from the 90s any way we can! For a complete look at the forecast, including daily rain chances and temperatures, download the MemphisWeather.net app or visit the MWN website.

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