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How to read an aurora forecast (correctly)

Catching a display of the aurora often seems like catching smoke in New England; it’s falsely branded as random, unpredictable, and a let-down. While it certainly is tricky to chase the lights here, myself and dozens of other photographers have the receipts to prove that it is possible, and I’m here to tell you that the aurora isn’t random; it’s actually somewhat predictable and with the help of a forecast, you can catch them for yourself!



Space Weather: It’s a real thing! And it can be forecasted!


Let’s fly through some (grossly oversimplified) basics first: the aurora is caused by solar wind and other things that are ejected from the sun towards the earth. We call this space weather, and when it disturbs our planet’s magnetosphere, it often causes auroras. The word “weather” being used to describe invisible wind in the vacuum of space may be confusing here, but the key point is that space weather can be forecasted to some extent: similar to, but far less accurate than earth weather.

 

The people in charge of predicting space weather are called the Space Weather Prediction Center, or SWPC (Swip-See). SWPC publishes a forecast twice a day in a bizarre robot language of numbers, letters, and an obscure scale called the “Kp Index”. It’s not exactly intuitive, but I’ll do my best to decode a forecast like the one below into useful information.



An example of a SWPC forecast

The example above was taken from SWPC’s 3-day forecast, available from their website at swpc.noaa.gov (Products and Data > Forecasts > 3-Day Forecast). Before we start decoding, there's some basic definitions to get out of the way first:


Some Basic Space Weather Definitions
Kp

Kp is a measure of how disturbed the earth’s magnetosphere is. The scale ranges from 0-9, and from what I can tell, Kp is directly related to the strength of the Northern Lights: The higher the Kp, the stronger the lights.

UT

UT refers to the UTC Timezone. All times in SWPC forecasts are in UTC (Universal Time Coordinated, also known as GMT or Zulu time). UTC time is 5 hours ahead of New England (4 hours ahead during Daylight Saving time).

G Scale

The G scale is the Geomagnetic Storm scale. When strong Space Weather impacts us and the Kp level reaches 5, it is considered a Geomagnetic Storm. The G scale ranges from 1 to 5, and the higher the G index, the stronger the storm (and the lights!). Predicted G values are in parentheses eg. “(G1)”.


Going back to our example forecast, the first column represents 3-hour time slots for each row, and the top row indicates the forecast day:

The decimal numbers in columns 2, 3, and 4 are the forecasted Kp values on the day at the top of the column, during the time indicated for that row. For example, On July 19th, from 00:00 to 03:00 UTC, a Kp value of 3.67 is predicted.  A (slightly) better way to visualize the forecast is below:




Once you know how to read these, it’s pretty straightforward to figure out when the aurora is predicted to be strong, and when conditions will be quiet. The forecasts are updated at 7:30PM EST and 7:30AM EST, and it is always worth looking at this forecast if you’re planning a chase. That said, aurora forecasting is incredibly difficult and is often wildly off in both timing and strength predictions. However, a good understanding of the forecast can inspire some confidence in your decision to play the odds and chase the lights, or stay home and sleep to chase another day.



What a forecast means for us.


Now that you know how to tell what the Kp forecast is and when, what do we actually do with this information? Kp isn’t a very descriptive value in itself, but the higher the Kp, the better our chances are of seeing or photographing the lights. I have found that Kp values at or above 6 create interesting conditions for aurora hunters in New England, with the best hours between 03:00 and 08:00 UTC (two hours on either side of our midnight).


The table below is based on my personal experience chasing the lights; many factors will affect your experience, but here’s what I’ve noticed on my chases:





There’s much, much more to determining what you’ll actually see on even the best of days, and forecasts rarely work out exactly as predicted. Suffice it to say that aurora forecasting is hard, and sometimes a solar storm never arrives or takes us by surprise. In any case, I start paying attention to the space weather when I see a forecast of Kp 6+, and if it doesn’t work out, one can still have a beautiful night out under the stars, enjoying New England’s dark and quiet places.



The case for SWPC


Most Aurora enthusiasts already have their own forecasting tools, or wait until the local news or social media mentions something about the northern lights. The main reason I suggest using SWPC is that they are the first and best at creating a semi-reliable forecast, with observers and forecasters working 24/7 to provide accurate information. Beyond that, there’s some practical considerations as to why I choose SWPC:


  1. No Hype. The SWPC is not interested in clicks, shares, or likes, and it’s unlikely to whip up a frenzy based on false information. It is in fact a boring government agency, concerned with just the facts.

  2. Forecast Discussion and Science. Along with the SWPC 3-day forecast, the forecasters include a scientific rationale for why they predicted what they did. Here you can learn about the solar events and observational trends that lead to that prediction, as well as insight into their overall confidence in the forecast. In a separate product on the SWPC website, there is a page called the “Forecast Discussion” which goes into further detail about the forecast, and can hint about possible changes ahead.

  3. Location-Neutral Advice. The SWPC doesn’t advise on whether or not to go out and chase the lights based on where you are, which a lot of basic apps do. Instead it gives a forecast for the whole planet and lets you draw your own conclusions. Some aurora apps are designed for arctic vacationers in Iceland or Norway, and will advise you to go out searching for the lights with a Kp value of 2. As we learned earlier, this isn’t useful for New England, so it’s best to use the latitude-agnostic predictions of the SWPC.

  4. Your taxes pay for it. SWPC is part of the National Oceanic and Atmospheric Administration, and it is truly a marvel of American science and engineering expertise (along with significant international collaboration)! It’s also the primary source for lots of other Aurora predictions which repackage the SWPC forecast into human-readable language. Before you pay for an aurora app, remember that you can get the data for free at swpc.noaa.gov!


Conclusions


You have now learned the bizarre alien language of SWPC forecasts, a valuable resource that's free and open to the public. Forecasts are one part of the constellation of skills one can learn to understand the aurora, and it's a good place to start, especially when you're learning to filter out the noise of aurora-mongering that the media loves to engage in. One last time I'll note how hard it is to get an accurate forecast, but hopefully it will inform the most important decision you'll make on your next chase: whether or not to gamble on the weather and get out there!



One last thing: UTC is the One True Time Zone


If you're as annoyed as I am about the concept of time zones, it's a real pain to convert the difference between UTC and EST (Or is it EDT?) on-the-fly. One solution is to wear a GMT watch at all times, but for more practical purposes, I made an annotated forecast that may or may not be helpful. See the Eastern Time conversions in red:



Ask me more about why time zones are stupid

The easiest way to deal with this when reading a forecast is really to focus on the first two or three rows, as these rows represent the most likely times for auroras to appear in New England.


Additionally, when SWPC calls for a G2 storm watch for "July 10th" for example, remember that they're talking about the July 10th UTC day, which spans from 8pm on July 9th in New England, to 8pm on July 10th. Therefore, if you're looking to chase a storm predicted on the 10th, you'll actually want to set out on the 9th, so you're in position if that storm hits when the UTC clock strikes midnight.

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