Dwight Silverman

Want to know what a storm is likely to do where you are? Dig into your phone’s weather app

In brief: We’re happy to have Dwight Silverman, a longtime Houston tech journalist who is working with us behind the scenes, contribute this post on how to find hyper-local forecast information from the powerful computer in your pocket. Dwight explores some features and data in your phone’s weather apps you may not know existed.

Whenever a storm is bearing down on the Houston area, Eric and Matt are swamped with questions about conditions at individual readers’ locations. They have time to answer a few of them, but given what needs to be done and the volume of work they face, they can’t respond to all of these queries.

But there’s good news: Often, the answers you seek can be found in the palm of your hand.

Nearly everyone carries a smartphone, and many weather apps that are included with them can give you a lot more detail than just the current temperature and rain chances. Some let you drill deep into the metrics you want to know most about a major storm – wind speed and precipitation. Want to know what peak wind gusts will be when a storm is forecast to be nearest you? Or how much rain is expected to fall near you? There is, indeed, an app for that. They’re not perfect, and they don’t do a great job with uncertainty in forecasts, but they offer a solid approximation of what to expect.

This post is not intended to be a manual for weather apps, but rather to shed some light on features you may not realize are there, or are hidden a few layers deep in an app’s design. There are many third party weather apps out there, and I am not going to try to cover them all. But I will offer tips for some of the better one, and those that are included on your phone.

Space City Weather

You may have heard that we have our very own, free app? It’s true, and it’s available for both Apple products (iPhones, iPads, Macs) and Android devices. The app is primarily designed to give you quick access to Eric’s and Matt’s posts, current conditions, hourly and daily forecasts for a week out, as well as radar for any of 12 cities in our region. But one thing you may not realize is that it includes detailed, up-to-date National Weather Service forecasts and advisories on storms and hazardous conditions.

The lower right hand icon on the Space City Weather app home page takes you to the National Weather Service forecasts and current alerts.

Open the SCW app, tap the bell icon in the lower right corner of the screen to get to the Active Alerters as well as short- and long-term advisories. There are also discussions of aviation and marine weather as you scroll down. (Note: The aviation section will NOT tell if your flight has been canceled! Check with your airline, and keep an eye on the FAA’s airport status website.)

Apple Weather

After Apple bought the popular Dark Sky app, it soon redesigned the app that came with iPhones, iPads and Macs to incorporate the former’s hyperlocal forecast features. It will give you details about conditions – both current and future – at relatively precise locations.

However, this integration has been, umm, less than smooth. For example, last year the app was not loading data, and in other instances it was forecasting rain when there was none. The patient seems to have stabilized lately, but it’s a good idea to double-check what you see in Apple’s Weather app with other sources. (You can find where Apple gets its weather data here.)

That said, the app makes it easy to see forecasts for specifc conditions, particularly wind and precipitation, at your location. On the iPhone, iPad and Mac, it shows you conditions in individual tiles, and tapping any of them takes you to a screen that lets you move forward in time to check the the forecast for a specific condition.

Tapping on any of the tiles in the Apple Weather app gets you more detail, including forecast for specific conditions.

For example, if you scroll down on the iPhone to the Wind or Precipitation tiles and tap one, you’ll get the current conditions for the day in a chart. Scroll to the left to move to the next day, or tap the calendar date near the top.

Traveling and want conditions and forecasts for a different location? Enter the ZIP code in the search field. You can save any of the locations for future use. You’ll also see forecast alerts, warnings and advisories for those locations.

Android devices

Unlike Apple devices, which all have the same detailed Weather app, Android smartphones and tablets use different apps and services, depending on who makes the hardware. For example, Google’s Pixel phones use Google’s own weather service for much of its onscreen information, while Samsung and OnePlus phones will kick you out to a weather website, typically Weather.com.

But there is a general strategy you can use: In the five- or seven-day forecast list, choose the day you want to explore and tap it, then look for the metric – wind or precipitation – for the target location.

On a Google Pixel 6 Pro, tapping the weather widget on the home screen goes to Google’s weather information, and from there you can get to specific forecast metrics.

In the image above, tapping on the weather widget that’s the default on a Pixel phone’s home screen, you’re taken to a general weather page. From there, tap the day for which you want details, and scroll down for wind and precipitation forecasts for that day.

Experiment with the app on your Android phone to find the best route to the information you need.

Next, let’s look at a couple of third-party apps that, while not free for all features, may be well worth buying if you live on the Texas Gulf Coast.

RadarScope

RadarScope is one of the best apps for looking at weather radar data that’s available. It’s a favorite of Eric, Matt and mine, and while there’s a limited-capability free tier, it’s worth the $9.99 annual subscription for the Pro 1 level. (There’s a Pro 2 tier, but most folks will only need Pro 1.) It’s available for Apple devices – and when you pay for it once, you can use it on all Apple products, including the Apple TV 4K streaming box. It’s also available for Android devices, as well as Windows-based PCs.

This is indeed a professional-grade app, giving you access to different kinds of radar technologies and views. Once you unlock one of the Pro tiers, you’ll be able to see the direction storm cells are heading, the location of lightning strikes and local storm and damage reports from the NWS’s crowdsourcing MPing app.

RadarScope shows lots of details about storms on radar, here displaying the initial bands from Tropical Storm Beryl as they move toward Houston on the morning of Sunday, July 7, 2024.

You can pick and choose specific radar stations, both in the United States and abroad, including 13 in Texas. It’s easier to use and navigate on a traditional PC, but still extremely useful on your phone. The help files in the desktop version are helpful, and there’s even a Facebook group you can join to learn from other users.

Windy

Windy is available as a website and an iOS/iPadOS and Android app. As the name implies, its initial focus is on wind conditions, but it’s really a full-featured weather app, with one very interesting features: You can track storms based on computer weather models. Choose the model you want, along with the date and time for a forecast, and it animates the storm on a beautifully rendered map.

Windy lets you choose your weather model, then animates its resulting storm forecast on a map.

From the home screen, press and hold the Home icon. Tap the Basic label at the bottom to see the various forecasting models that are available. Choose one to see the storm at the current location, then scroll through the timeline to see where the storm is expected to be based on that model’s forecast.

While most of Windy’s features are available for free, some – like a 15-day forecast – require a subscription, starting at $2.99 a month up to $29.99 a year. But most folks can get along just fine with the free version.

There are many, many other weather apps and websites that can help you keep track of storm developments, and as I said at the outset this is not meant to be an exhaustive list of resources. (I didn’t even mention one of my favorites, Merry Sky, which was developed for those who miss the look and feel of the old Dark Sky app. Oh, wait, I just did!) If there’s an app you like, feel free to let others know in the comments.

The SCW Q&A: Physics vs. AI; hype or reality; potential cyclones; tornado or not; our day jobs are safe

Houston may be settling into its summer weather doldrums, but let’s face it: In June, the skies over the region were a happenin’ place. As happens this time of the month, Eric and Matt have cracked open their inboxes and answered your queries.

Got a burning question of your own? Smash the Feedback button in the blog’s sidebar, respond here with a comment or talk to us on our socials: X, Facebook, Instagram, Threads, Mastodon, Bluesky and LinkedIn.

Q. So I’m curious which model actually panned out (regarding TS Alberto). Was it the regular models or the AI models? I remember it being mentioned that there were some differences between them a few days ago.

Thanks for the question. We took a look at this in the immediate aftermath of Alberto, and in terms of track the physics-based models significantly outperformed most of the AI-models. Essentially, some of the AI models wanted to bring the tropical system much further north into the Gulf of Mexico (toward Texas) whereas the traditional models kept it bottled up in the Southern Gulf, which is what ultimately happened.

I’ve also been looking at the AI guidance more frequently for non-storm events and so far I’m not convinced it’s any better (or even as good as) the physics-based models. Of course this is just my personal experience. The best way to determine the value of these new models will be using rigorous, comparative studies of what the models predicted across a range of variables against what ultimately happened. Now that the AI forecasts are coming out in real time, those kinds of studies can be done. I look forward to seeing the results in the coming months and we’ll be sure and share any interesting findings here.

–Eric

Q: Is the news media hurricane prediction for 2024 hype or more on the real side? I am debating getting a generator, but don’t want to buy into the hype if that’s what it is.

A: It’s certainly not hype. But you have to look at it logically. The data almost unanimously suggests that this should about as active a hurricane season as we could have. Thus, the seasonal forecasts are all quite bullish on hurricane development.

Colorado State has a generally good track record with seasonal forecast predictions, so their bullish outlook for 2024 has raised eyebrows and alarm in some circles. (Colorado State)

In fact, one thing we did at our companion site The Eyewall back in early June was to expand on ways that the active hurricane season forecasts could bust. The reality is that it could bust. Absolutely. Would that mean the preseason forecasts were all hype? As a scientist, I will tell you certainly not. As a member of the public, I get why you’d have that opinion though. So we encourage people to look at the decisions they make with regard to seasonal hurricane forecasts with nuance.

Were you probably going to get a generator anyway? We’ve had far more events non-tropics related that have caused extensive power outages in our area in recent years. Just consider the whole picture. And in this case consider a busted forecast a good thing if it happens.

–Matt

Q. Why is Potential Tropical Cyclone One not called “Tropical Depression One”? I thought a storm was called a tropical depression before it became a tropical storm?

It’s all a bit confusing, isn’t it? But the intent here by the National Hurricane Center is good. A “Potential Tropical Cyclone” is sometimes issued before a tropical system becomes a ‘depression’ or a ‘tropical storm.’ This happens when a system could bring tropical storm or hurricane conditions to land within 48 hours.

Essentially, forecasters from the hurricane center do this when they want to highlight the threat of a tropical system to land, but that system has not yet formed. Why do this? Because sometimes a tropical storm does not intensify until very shortly before landfall, at which point formal warnings are often too late to be actionable. The hurricane center started doing this in 2017, and you can find more information here.

–Eric

Q: Curious to know how the NWS determines if a tornado occurred and categorizes it after the event? Is it based on eyewitness accounts, damage, etc?

A: There are a few ways to confirm a tornado has occurred. Some larger tornadoes have “tornadic debris signatures” on radar, where you can see the radar showing you lofted debris from tornado damage. The most obvious and easiest way to suspect a tornado is eyewitness accounts.

But sometimes well-meaning people think something is a tornado when it is not. That’s why the NWS usually sends out a survey team to go investigate damage after a storm if a tornado is suspected. They can combine spotter or eyewitness accounts with radar data to determine exactly where to look for tornado damage. Generally speaking, if the damage is all pointed in one direction, that’s a tell-tale sign of straight line winds, or a microburst/downburst, not a tornado.

But if the damage is more chaotic or pointed down in different directions, that’s a probable sign of a tornado. This page from the NWS in Binghamton, NY shows sort of how they go about this process.

Rating the tornado’s intensity is another matter. There is some subjectivity involved, but in general, the damage indicators as provided in the Enhanced Fujita scale provide a decent categorization for a tornado.

–Matt

Q. You are all hype. I thought your website was dedicated to providing weather insight for Houston without the hype but your Stage 2 flooding article was all hype!! Don’t quit your day job!

A. Let me put your weary and worried mind at ease: we won’t be quitting our day jobs. My day job is writing about space exploration for Ars Technica. Matt is a meteorologist for an energy company in Houston. Would you believe that Space City Weather is hobby for both of us, a second job we do to try and help the community?

I do think it is useful to revisit our thinking about issuing a Stage 2 flood alert on Sunday, June 16. Here’s the post, which we published at 9:53 a.m. Note that this was Father’s Day, and Matt and I spent a couple of hours that morning strategizing, thinking, and writing about the forecast. I mention that just so you understand we’re pretty dedicated to this stuff.

That morning we actually gave some thought to issuing a Stage 3 warning for coastal areas, but I decided it was best to hold back for a time. At the time some of the modeling was rather dire (showing upwards of 20 inches of rain across parts of Houston in some instances). The ‘official’ precipitation forecast from NOAA showed a bullseye of 12 inches of rain just south of Houston, and the flood threat was very real for coastal areas. In any case, we felt it was time to raise the alarm about the potential for significant flooding in the Houston region for the coming week. So we pulled the trigger.

NOAA rainfall accumulation forecast issued on Father’s Day morning. (Weather Bell)

Some uncertainty definitely remained—we caveated our forecast with the following statement, “Our forecast modeling remains a bit split as to whether the heaviest rain will fall near Galveston, or further south closer to the Coastal Bend.” The latter is precisely what happened.

In forecasting, you win some, and you lose some. All we can really do is to try and do our best, and that’s just what we do here.

–Eric

The SCW Q&A: Underwhelmed; micro/macrobursts; hurricane prep; hot summer; tornado terms; apple of our eye

If it’s almost the end of the month, that means Eric and Matt have once again scrambled to answer your most urgent questions. And given the busy weather month we’ve had in May, this edition of our Q&A is particularly timely. Got your own queries? Hit the Feedback button in the blog’s sidebar, respond here with a comment on hit us up on our many, many social network feeds: X, Facebook, Instagram, Threads, Mastodon and Bluesky. We’re everywhere, and now that includes LinkedIn!

–Dwight

Q. I feel like y’all are being TOO underwhelming. I definitely get not overhyping the weather. Which is absolutely why I started following you. But the last three storms you’ve underhyped and I was not prepared. I’d rather be over prepared than under. I didn’t at all expect the insanity that Oak Forest got on [May 16]. Thank goodness my cousin texted me to take shelter minutes before the storm hit. Please consider a middle ground.

A. This isn’t a question. Next one, please.

Just kidding. It’s a fair comment. The reality is that just as we don’t ‘over-hype’ the weather we also try not to ‘under-hype’ the forecast. Matt and I really are trying to simply be as right as we can be. You mentioned under-hyping the last three storms. That’s not true. On May 13, we predicted significant hail could accompany strong thunderstorms as they moved through the Houston area. Some areas, we said, may see hail as “large as an apple.” In reality, no one saw much, if any hail that day. I realize that pointing out how we missed on another forecast is perhaps not the best defense of our record. But my point is that we do not actively seek to under-hype storms. We are striving, always, for the middle ground you espouse. Some days, however, it’s damn hard to find.

I’m also sorry you felt unprepared for the derecho that slammed Oak Forest and other parts of Houston. We wrote about that recently, and some of the learnings we are taking away from it.

–Eric

Q: During the recent extraordinary Houston area (and beyond) weather phenomenon, is there any consideration that “microbursts” were present?

A: Yes, in our post about derechos, we noted that basically, they’re gigantic bursts of straight-line winds. In a sense, you could probably refer to a derecho event almost like a macroburst, which is wind damage on a scale greater than two and a half miles long. In reality, within this complex storm, there were probably a mix of macrobursts and smaller-scale microbursts, in addition to a couple tornadoes. We did not exactly see uniform damage across the area, but clearly some neighborhoods were harder hit than others. The more meaningful answer is that it’s all semantics in a situation like that, and the widespread damage is damage.

–Matt

Q. This storm has sobered us for what we are in for this hurricane season. How do you prepare? What do we need to keep in stock? What food should we store up?

A. Whoa, hurricane season officially starts Saturday! Fortunately, it doesn’t look like things will be insane right out of the gate. But there’s no doubt it’s going to be a busy season. The most important thing you can do right now is understand your vulnerabilities. Is your home at risk of storm surge because you live near the coast, and at a low elevation? Are you at risk of inland flooding? Is your home built to withstand hurricane-force winds, i.e. does your roof have hurricane clips? Are you willing to go without electricity for a week or three in September? All of these questions are important to determine in what circumstances you would evacuate. And after you understand when you would evacuate, you should make a plan for where you would go, what you would take, and how you will get there.

More generally, I find the preparation section on the Ready.gov website to be useful in making plans for an evacuation or to ride out the storm.

–Eric

Q: Is there any connection between the kind of intense spring weather we’ve been having and how hot the summer will be? Or are we most apt to have a drought when spring weather is severe?

A: The simple answer is not really. If you look at enough meteorological variables, you can find loose correlations among a lot of things. But correlation does not always (or often) mean causation. The best proxy to an “intense” spring may be storm reports in Harris County. If I take the five busiest years prior to this one from March through May, I can find 2007, 2019, 2020, 2023, and 2009 as my analogs. Combined, you get the map here:

Yes, active springs in Houston since 2000 have ended up with a net hot summer outcome in Texas, but correlation does not mean causation. (NOAA NCEI)

So yes, those active springs have led to hot summers on net. 2023 was our hottest summer on record, 2019 was the fifth hottest, 2009 was the seventh hottest, and 2020 was the tenth hottest. But years like 2011, 2022, and 2010 are on the top ten list as well and saw few severe reports in spring. In terms of drought, it gets a little noisier with some dry years mixed in with wetter ones. So no real solid signal.

We expect a hot summer this year, but it’s not because of the intense spring thunderstorms. The combination of a very warm Gulf, a post El Niño warm hangover, climate change, and other factors buoy confidence in the hot summer expectation. Severe weather does not. But it does make for a fun exercise.

–Matt

Q: I’m a bit confused about the tornado warning terms used here in Texas. It seems like there is a different meaning to the terms than what I am accustomed to. Could you clarify the Texas tornado warning system in one of your posts? ( As a Midwestern Indiana girl, I learned that a tornado watch was when the conditions were likely for a tornado and a warning was when a tornado was spotted, heading your way and you needed to take cover immediately.)

A: The tornado watch vs. warning system is the same anywhere in America. A tornado watch means that conditions are favorable for possible tornadoes. A tornado warning means that forecasters have identified a likely tornado or an in-progress tornado via radar or visual observations from people in the field. In a nutshell, a watch means you should pay attention while a warning means you should take immediate action.

Every tornado warning issued in the Houston area since 2020. (Iowa State Mesonet)

We get a lot of tornado warnings in the Houston area. Since 2020, there have been 136 of them issued by the NWS Houston office. The typical tornado we get here is a little different than what you see in the Plains or Midwest. Our tornadoes here are most often quick spin ups in squall lines (what we call QLCS tornadoes) or brief south to north moving supercells in the spring or fall ahead of cold fronts. Also, tropical systems. Harvey alone in 2017 led to over 140 tornado warnings. The QLCS and tropical tornadoes are a bit like playing whack-a-mole because they usually flare up and weaken rather quickly or sometimes look like they’re about to produce a tornado and do not. In the Midwest, you often have a little time to see them coming. Here, it’s just a different reality.

That said, after the derecho this month, I think that it’s apparent that folks should take these warnings seriously, even if they are inconvenient at times.

–Matt

Q. Can you please not use an apple as a comparison tool for the size of hail possible? Is it an gala apple? Is it a red apple? Is it a green apple? Is it a honeycrisp apple? Is it a pink lady apple? Is it some random miniature apple? Or is it ping pong to baseball sized?? 1.5 to 3.5 inches wide or 3.8 to 8.9 centimeters wide is easily accessible to most people. And fun fact the National Geodetic Survey (NGS) has updated their definition of a foot as of 2022 to better match international standards. An apple is completely subjective.

A. You forgot the most important argument in favor of apples. They’re delicious.

But just like my Granny Smith, you make a good point. The next time I’m writing a post on my McIntosh computer, and we’re expecting hail, I’ll try not to be so confuji in our terminology. Honey, I’ll try to be crisp in our language. Hope you find that apeeling.

–Eric

The SCW Q&A: First, the bad news; obscured by clouds; shuttered; humidity is relative; predicting it.

Welcome to the “just in the nick of time” April edition of the SCW Q&A. Each month Eric and Matt tackle the most interesting questions you’ve thrown at them. You can leave more questions in the comments here, or hit the Feedback link in the blog’s sidebar. We’ll also scan our social media channels (Facebook, Instagram, Threads, X/Twitter, Mastodon, Bluesky) for queries. And we’ll try to get the Q&A posted sooner in May! 

– Dwight

Q. Could this be is our last front until Sept/Oct? April is coming to a close soon, and I can’t recall many pronounced cooldowns in May.

A. Let’s get the bad news out of the way first. We are very near the end of the road when it comes to cool fronts in Houston. However, it is not unheard of to see cooler nights in May. That is not to say cold. If I squint into the distance, I can see a decent chance of a front around the May 11-13 period, so a little less than two weeks from now. That’s a long way out to have some confidence, but I’m modestly hopeful we may have one final fling with drier air before sultry summer settles upon us for months on end.

–Eric

Q.  I had read before the event yesterday that clouds sometimes clear during an eclipse due to the cooldown. Is that what happened yesterday in Houston when we had a thinning of clouds enough to see the crescent?

A. This is a great question. The answer is yes. During the eclipse in Houston, we had an issue with various sorts of clouds. We had higher level cirrus clouds, mid-level clouds, and low-level clouds. Most of the low-level clouds were cumulus clouds. What usually causes cumulus clouds to form? Heating. You heat the ground during the daytime, the heated air parcels rise, cool, condense, and you get clouds. Well, as you begin to block out the sun during an eclipse, you begin to reduce that heating mechanism. And in time, a lot of those cumulus clouds (or what we often call “fair weather clouds”) will dissipate. The animation below is from the 2017 eclipse, and you can see how the cumulus cloud field over Missouri and Illinois really dissipated as the eclipse unfolded.

A satellite loop of the 2017 total eclipse over Missouri and Illinois. (University of Wisconsin CIMSS)

This is likely what “saved” much of Texas this year. There were still plenty of clouds higher up, which are less impacted by daytime heating and more driven by physical storm systems. But by virtually eliminating a layer of lower cumulus clouds, you had one less obstruction to view the sun, which periodically broke through the higher clouds.

–Matt

Q. How far inland would you recommend having hurricane shutters to board up if a storm comes inland? I find myself trying to calculate what would be the strongest type of hurricane that we could feel in order to decide if I want to invest in shutters, but can’t find any info. Would you guys know?

A. We’re not storm damage experts, but whether you should have hurricane shutters depends upon several factors. First of all, the most important factor is whether you’re close to the coast, as winds do die down significantly as a storm moves inland and encounters friction with trees, homes, high rises, and other structures.. The second factor is the proximity of stuff, like trees and unattended deck chairs and you name it. The most common reason windows break is due to flying debris. So if there’s lots of things around that you can imagine flying into your windows, that’s a risk factor.

Maximum sustained winds from Hurricane Ike in 2008. (NOAA)

I’ll be honest with you. I’ve lived through flood storms in Houston. I’ve lived through surge storms. But I’ve not really experienced a wind storm. Hurricane Ike was the closest thing, but it barely brought Category 1 winds to parts of Houston. This was bad enough, knocking power out to the city for up to two weeks. At the time I lived in Clear Lake and we had to get a new roof. I guess what I’m saying is that it’s not just your windows, it’s also your roof. And a prolonged period without power.

I guess what I would advise is that, if you live south of Interstate 10, and can afford it, hurricane shutters are not a bad idea. You will probably never need them, but if it proves peace of mind, that is not a bad investment in an uncertain world. Make sure they’re easy to put up on short notice.

–Eric

Q. On your weather app I notice that the humidity (sorry I mean HOUmidity) can be low in the evening and then go way up in the morning, coming back down again as the day goes on. Why?

A. Relative humidity (RH) is just that, relative. Dewpoint temperature is the temperature to which you’d need to cool the air to saturate it. In other words, the higher the dewpoint, the damper the air is. Dewpoint can fluctuate, but it’s usually more stable than temperature. So, let’s say you have a dewpoint of 72°. The morning low is 75° in this example. That means your morning relative humidity at 75/72 would be 90 percent. Obviously, you warm up during the day, so with a high of 88 degrees but a steady dewpoint still sitting at 72, your RH at 88/72 has dropped to 59 percent. As the air cools around sunset bringing the temperature relatively closer to the dewpoint, the humidity rises again. It limits the usefulness of RH. This is why we flout other metrics like dewpoint to drive home how humid it actually feels.

–Matt

Q. After your earlier answer, I’m confused by the description of what “percent chance of rain” represents. I was taught in my meteorology classes that probability of precipitation in weather forecasts means the probabilistic chance that it will rain at the point specified in a given time period. For example, if I enter my zip code to get a National Weather Service forecast for my area today, a 40 percent chance of rain means each point within my zip code has a 40 percent chance of seeing rain, not that 40 percent of the land area within my zip has a chance of seeing rain. Insights? Or is this just splitting hairs?

A. It’s more or less splitting hairs. But this is a fun topic so why don’t we dive a little deeper. Let’s go directly to the National Weather Service, where we find this delightful introduction to its “probability of precipitation” explainer:

The probability of precipitation forecast is one of the most least understood elements of the
weather forecast.

I just love the “most least” phrasing. A perfectly confusing introduction to a confusing topic. Anyway, fundamentally you are correct that it is a point forecast rather than an areal forecast. But it is a point forecast based on an areal forecast. If that makes sense. Which it probably doesn’t. And that’s ok.

There are two factors that go into making a “probability of precipitation” forecast. First there is the forecaster’s certainty that precipitation will form or move into the area, and this is multiplied by the areal coverage of the precipitation that is expected. Here are two examples for how one might derive a 40 percent chance of rain:

(1) If the forecaster was 80 percent certain that rain would develop but only expected to cover 50% of the forecast area, then the forecast would read “a 40 percent chance of rain” for any given location.

(2) If the forecaster expected a widespread area of precipitation with 100 percent coverage to approach, but he/she was only 40 percent certain that it would reach the forecast area, this would, as well, result in a “40 percent chance of rain” at any given location in the forecast area.

Does that clear things up? Probably not.

But given the general inaccuracy of precipitation forecasts, I think it’s fine to think of a “40 percent chance of rain” as either a point forecast or, more generally, that about 40 percent of an area will receive rainfall during a given period of time. This is not an exact science, it’s just a forecaster making a most best guess of things.

– Eric