I try and make my campaign worlds as realistic and plausible, as possible.

One of the big stumbling blocks I keep hitting is weather systems.

I'm curious who among us simulates weather ( other than: winter = cold, summer = hot ), and what methods you use to do it?

The system I'm thinking of shifting to, is simply to pick a point on Earth which approximates the location the party is, and blatantly steal the real world weather data off the Internet. The party is on Ravenna? Well, that's likely closest to Cyprus in the real world, so the weather in the city of Celenae coincidentally matches that of Paphos. Sites like www.worldweatheronline.com even have historical weather data, so that when real-world progression of time, and game-world progression of time get out of sync, you can just pull up the weather in that location on an appropriate day. All one would need to do is have a way of mapping your fantasy-world calendar dates to dates on the real-world calander.

What do you use - if anything?

I don't use anything yet, but I'd pretty much do the same; steal weather data from records and apply to the game. My world is based on the real one so it shouldn't be too hard.  

Really, there are three variables to weather: precipitation/fog, winds, temperature. For my campaign, I set up a little table for each and rolled some percentile dice every adventuring day or so. My campaign location is Forgotten Realms's Chult so my weather was pretty consistent (it's a jungle, it rains every day) but the table helped me mix things up a bit and nor have to worry about if it felt too repetitive.

I'm not a fan of random tables, as they have the possibility of causing disjoint weather if they're to simple: storm, sunny day, storm.

Make them overly complex - like where you shift one state up or down on wind/precipitation/temperature scales based on the current day, and also take into consideration seasonal shifting - and you get consistency, but  it becomes a pain-in-ass to track. Plus it's tough to do time jumps with that system.

I've tried both, and if either approach works well enough for you and your players, go with it. I'm being too picky :p

I just like the idea of being able to have realistic ( because it is/was real ) and consistent weather without having to do a lot of mechanics fiddling about. 

I'm tinkering with writing some code, and using a historical weather web API, and just churning out a "weather almanac", with a few years of daily ( even hourly ) weather data for a list of representative climates. Then it's just a case of looking up the day-of-year and the climate type, and presto - daily weather. Having several years of historical climate data, and it's less likely to be noticeably repetitive ( Hey, ever notice how it storms every August 11th? )

I think you've already hit on the best method for modeling realistic weather, which is using weather forecasts for a real world place that roughly matches up to the in-game location. This is a way I've handled it in the past as well. I didn't go so far as to create an almanac, but you can use the climate data for a specific real world locale to get a sense of the averages and the upper and lower limits of what's reasonable. Beyond that general framework, I tend to just go with whatever weather seems thematically appropriate for a scene. Weather can play an important role in setting tone.

This is something I tend to care about more when the PCs are out in the wilderness, especially if they're trying to stretch their supplies over a long, perilous journey. In the city, it's generally going to matter less. But I am running a campaign now in a London-like city, where the constant rain and fog help add to the mystery and noir-like feeling. It's also a boon for all the skulking around they've been having to do, though being constantly drenched has been pretty unpleasant for the PCs.

I definitely understand the desire to do this just for realism's sake, but weather is only going to really matter at the extremes, when it might end up having some noticeable mechanical effect. I don't think players notice much more than that, unless the weather is really incongruous for the place the PCs are in. I'd suggest that if you're going to go to this much trouble, you might want to come up with some robust mechanics around weather and think more about how it affects encounters and other situations. Ultimately, if the PCs are actually feeling the effects of the weather, they're more likely to take notice of it and care about it in the long run.

One other thing to consider is whether the weather in your campaign setting actually does work the same way as weather in the real world. It's possible it doesn't adhere to quite the same patterns and principles, and magic might play a role in determining the weather from day to day. Also, the weather could be a source of occasional supernatural phenomena, whether that be something dangerous, beneficial, or just interesting.

At the moment I create the area and do take the kind of climate into consideration. For example we're currently in the Elsir Vale which is tropical. It takes place during the summer months so its constantly 38-42 celsius. Really hot and the ones in heavy armor get described as baking and sweating profusely in their armor. However there can still be an occasional moment of rain or extremely heavy thunderstorms with a rare occasion of a tornado.

I combine it with a randomized table of travel. Where I created 20 days and each day broken up in 4 hour segments. Each day has a theme like running into a dog. Depending how the the group treats it he'll follow and bring something he finds or runs away. Later on they might encounter a hermit that asks for his lost dog. Some days they run into an old idol next to a road. Or bump into a traveling merchant. Or a pair of wyverns doing a mating dance in the sky. Most days are fairly uneventful like that. However some incorporate having the wind in their backs so they travel a bit more then usual. Some days mention a sky darkening giving a provision gathering and survival bonus check to find a place to camp with food aplenty. During the evening and  night there is heavy rain that interrupts their nights rest. Having to get outside to make sure the tent is still secured. Meaning they don't get the benefits of a long rest. At another time they see a small tornado funnel. So I basically incorporate weather and its events into the travel table. If x event happens too closely to another I just re-roll. Otherwise you indeed get the disconnect from the randomness. But the chance is small if you break it up with other uneventful days or stumbling into a small old burial ground or some event. And of course in case of real heavy rain and winds, or even tornado's, I even have the rules for vision and movement for such events just in case. Each climate can have 5-6 different weather types that you can incorporate in such a way.

If you don't travel you can still roll on the table and see if one day shows up with a weather event that might be fun to use when in town or a dungeon.

jreggers

Some excellent points there.

I have always felt comfortable overriding what the weather system/almanac tells me if the story requires the weather to be different - so long as I'm not creating too jarring an effect without there being a good in-story reason ( it was 40 C yesterday, why is is snowing today?! ).

I think that if weather is purely "fluff", then it tends to get ignored.

I think you're correct though that at times it's going to have a mechanical effect: it's dark and stomy? OK, stealth checks are at +1, but climbing might be -1. Clear calm night, and 2 of the 3 moons are full? That might be -1 to stealth. Blizzards on the road? If you're not doing anything to take shelter and wait it out, then roll Survival not to get lost, and make a Con save not to get exhausted ( or some other effect ). Heavy thunderstorms on the road? Travel is slowed.

It occurs to me that it would be possible to map some of those effects out in the almanac idea so that they're automatically listed for DM based on pre-defined weather conditions: Cloud cover was > X, and Wind was > Y, then +1 to stealth at night. But these have to be fairy minor effects, used sparingly, or the party is going to feel at the mercy of the weather all the time. That doesn't sound fun.

I think weather can be a direct actor in a lot of what the party does - it can have lots of minor effects, and those little things bring them to player attention. And I think that if players are aware of the world around them, it feels more real. But yes, there have to be - minor - mechanical influence for it to be noticed.

I think the almanac idea is a ton of work - probably overkill - but it's an easy way to get seasonal variation into the weather cycle as well. If it's summer, it's probably safe for the party to travel the Rala Gap between Coria and Vectris, but if it's midway through the Winterspan, then the chances of blizzards and the possibility of mountain avalanches are just too high; best take the extra time and hire a ship.

I think you raise a really good point though that fantasy world weather need not behave as real world weather does, at all! Look at the seasonal variation in A Song of Ice and Fire / Game of Thrones . I'm not sure what governs the seasonal progression in Westeros, and that definitely gives the world part of its unique feel.

The almanac does sound like a lot of work, but I think it's a very cool idea. If something like that existed, and especially if it laid out some interesting mechanical effects, weather-related challenges, and plot hooks, I would definitely be inclined to use it. A quick search on DM's Guild seems to indicate that there's not much out there in terms of weather-related supplements. Of course, D&D 3.5 had Sandstorm and Frostburn, but I haven't seen anything equivalent for 5e just yet.

I think Westeros is a very good example of how you can not just incorporate weather into a campaign but also use it as a central theme. This is also true of any campaign set in an extreme environment or any place with very specific climate features (a desert, the arctic tundra, a rainforest, tropical islands, etc). I've always thought it would be fun to have a very volatile campaign world rooted deeply in the elements, where varying weather and climate (both natural and supernatural) present a constant challenge to the PCs.

I mostly do what you do, that is pick a real-life location with equivalent geography and go off its average weather patterns. (I base the societies in my games off of real life countries, too, so this is just more of that.)

However, I also found this weather generator that I've used a couple times, and it's fairly useful. It has a few variables, including what season it is, and whether you want magical weather, so you can tweak it to your liking. I use it in combination with the above method, so if the generator spits out something that I know doesn't work for the location I can adjust the temp up or down as needed.

One thing is to decide what roll do you want weather to play.

Most weather conditions are pretty regular.  I live in Boston MA and previously lived in Kansas and the Middle East.

I can tell you Boston's weather changes a lot (90s one day and 60s two days later then back).
We occasionally get Thunder Snow.  It's a snow storm which is so bad the water vapor creates thunder...  also the lightning it makes can be purple. PURPLE.
We're also famous for "Noreasters"

To quote Lewis Black "That kind of weather wasn't even in the Bible!"  Two years ago we got 7 feet of snow in a month, after it was 60' on Christmas day.
New England has rolling hills so line of sight is pretty terrible.  So weather patterns feel like they just "appear".  You might have an hour before you see the clouds and the rain hits.

In Kansas you see 3mi, because that's when the curvature of the earth runs out.  You can watch the storms slowly rolling in, lightning flashing and dark clouds filling the sky.

Abu Dhabi is hot and humid.  Arizona is hot, but to quote Pvt. Hudson "Yeah but it's a dry heat".  They have dust storms... it's like a dry blizzard.

I use a fantastic weather generating spreadsheet that I found (I believe) on /r/dnd. You can switch around months and days, set climate, seasons, and even add in new things (like arcane-affected weather in highly magical locations), and then it will generate weather for every day of the year. If I can find the original (I just have my edited version) I'll edit it in here. It's a pretty useful little thing, and I do one for each campaign I run...

Here it is!

Though I still throw in differing weather, in the case of drama or challenge. (Have this airship battle in a lightning storm! Walk a treacherous pass in a blizzard! etc.)

I have seen this idea of using real world weather before and it is fact what I did. Being old school I bought an old copy of the Farmer's Almanac from a used book store for like a quarter and I just take the weather from there. Being an almanac it was more of prediction than a record but your use of this new fangled technology would probably work wonders. And there is a wonderful campaign weather calendar template out there by Gnome Stew that you can download and just drop in the appropriate data.