A too-early look at the cloud forecast for the solar eclipse
A too-early look at the cloud forecast for the solar eclipse

First things first: you’re not going to get anything here resembling a confident cloud forecast for a few minute window 12 days out. But if you have plans to watch the total solar eclipse on April 8, especially travel plans, we think you’re desperate enough to keep reading.

In all seriousness, there are a few things we can say about this day’s early outlook, and a few things worth learning before you start obsessively checking the forecast every day, multiple times a day if you haven’t already.

Why are clouds so hard to predict?

Clouds are one of the most difficult weather parameters to predict, even just a few days in advance, let alone more than 10 days.

It’s not just large storm systems that create extensive cloud cover. These clouds are relatively easy to predict, especially within a few days. Smaller, weaker disturbances in the atmosphere can also generate clouds, even when they do not have enough moisture to form precipitation. Meanwhile, skies can go from completely cloudy to completely clear within about 50 miles of a weather front, while model predictions of the front’s location can be off by twice as much a day or two beforehand.

The challenge is that cloud cover often depends on processes occurring at very small scales in the atmosphere — small enough that models don’t have enough detailed data to resolve them accurately.

Why do I need to know what an “ensemble” is?

We usually try to keep technical jargon to a minimum. However, for those of you invested in this forecast, it’s worth learning what an “ensemble” is and why it matters.

There are two main variants of forecasting models: deterministic and ensemble.

The deterministic model is the forecast you are used to seeing. This is a single-decision forecast: the temperature will be X, the chance of precipitation will be Y, and the cloud cover will be Z. Such forecasts are usually most accurate up to two to three days in advance.

For forecasts from a few days to about two weeks in advance, most meteorologists prefer to look at ensembles. In essence, these are multiple implementations of a deterministic forecast. In each run or simulation, the original meteorological observations fed into the models are slightly modified to represent imperfections in both the observations and the models themselves. By analyzing the similarities and differences between multiple simulations, forecasters can gain a better understanding of the range of possible weather outcomes and their probabilities, as well as overall forecast confidence.

What does the deterministic forecast for April 8 show?

The image above is a deterministic model forecast for noon on April 8. It shows lots of clouds across much of the path of totality, the roughly 115-mile-wide swath that stretches across the United States from Texas to Maine. It could be completely right and it could be completely wrong. There are simply too many days left to base anything on a deterministic forecast.

What it shows is similar to the climatology — or what the average cloud conditions are for this time of year — with clearer skies over Texas and increasing clouds in the northeast. However, there are pockets of clear skies and over the northeast associated with a forecast area of ​​high pressure nearby off the east coast.

What do ensembles show?

The image above shows an ensemble forecast of atmospheric pressure using the average of the pressure predicted by 30 simulations from the US Modeling System. Generally speaking, low pressure areas — shown in shades of blue — tend to be cloudier, while high pressure areas — shown in yellow and orange — tend to be sunnier.

Several of the individual simulations suggest lower pressure and therefore cloudier skies for a significant portion of the path of the ensemble. There are some that hint at higher pressure and sunnier skies for the eastern or northeastern parts of the path of totality, but still show lower pressure moving into the western parts of the path of totality, especially in areas north of Texas.

The cloud forecast shown at the top of this article suggests that the area of ​​low pressure is far enough north of Texas that cloud cover may be limited if its simulation of the location of the low pressure is correct.

Of course, this far out not only is confidence in the presence and location of weather systems low, but models can be up to a day or two slow or fast on how the systems will develop across the country.

Excarta, an AI weather forecasting startup, has launched a solar eclipse tracker providing cloud forecasts along the path of the eclipse.

The company is one of several that have developed AI weather models that make predictions by learning to recognize patterns in historical weather data, while traditional models crunch complex mathematical equations that represent the physics of the atmosphere.

“This allows us to produce highly accurate, hourly, global forecasts in minutes instead of hours,” Vivek Ramavajala, Excarta’s CEO and founder, said in an email. “We can also use the improved speed and cost to produce ensembles of weather forecasts that are critical for quantifying forecast uncertainty several days out.”

A news release from the company said its predictions can be up to 20 percent more accurate than traditional models.

Here is the AI ​​model’s current forecast for several cities in the path of totality during the peak eclipse:

  • Dallas: 53 percent cloudy (+/-20 percent uncertainty).
  • Little Rock: 58 percent cloudy (+/- 15 percent uncertainty).
  • Indianapolis: 54 percent cloudy (+/- 19 percent uncertainty).
  • Cleveland: 55 percent cloudy (+/- 19 percent uncertainty).
  • Buffalo: 53 percent clouds (+/- 19 percent uncertainty).
  • Burlington, Virginia: 47 percent cloudy (+/- 22 percent uncertainty).

(The uncertainty figure means it predicts, for example, that Dallas could have anywhere between 33 and 73 percent cloud cover.)

Is the forecast off to a great start for those hoping for clear skies? Not exactly. But we’re still at least a few days away from being able to take any cloud forecast too seriously, and the reliability of the forecast may not be terribly high until a day or two before April 8th.

The Washington Post will launch its own eclipse cloud forecast tracker on Friday, so stay tuned.

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