Rules of thumb for understanding weather patterns in the US
October 4, 2015 10:10 AM   Subscribe

I want to get a better understanding of how weather works in the northeastern US. With all the talk about Hurricane Joaquin and how "this is different to a regular nor'easter", I've realized that I don't have a good gut-level understanding of basic stuff like which way weather patterns move, what a northeasterly wind means, etc. Happy to get either your explanations, or links to things I can read/watch.

For this question, I am less interested in "the data says the average temperature and rainfall in July is x", or "this is the official definition of a polar vortex" and more in qualitative explanations of how weather patterns generally work, which will help me put the more technical stuff in context.
For example, from living in a south-eastern city in Australia most of my life and watching the nightly weather forecast on TV, I know that: weather tends to move from west to east, and all else being equal will take about three days to cross the country; although summer officially runs from December to February, you will get some December days that are still cold and wet and late February and March can be stinking hot; northerly winds in summer mean bushfires; a cyclone up north often means we'll get rain a couple of days later; rain patterns are drastically different on either side of the Great Dividing Range. What are the US equivalents?
I'm in DC, so will be very happy to get specific answers for there, but also interested in info about the northeast more generally.
posted by une_heure_pleine to Science & Nature (12 answers total) 15 users marked this as a favorite
Do you read Capital Weather? They do a great job of explaining what's going on in lay language.
posted by kat518 at 10:40 AM on October 4, 2015 [2 favorites]

National Weather Service SKYWARN classes cover regional weather patterns. You can take the free classes even if you don't want to become a weather spotter.
posted by djb at 11:11 AM on October 4, 2015

Seconding Capital Weather Gang.
posted by Johnny Wallflower at 11:17 AM on October 4, 2015 [2 favorites]

This wind map may also help. Click on an area (around DC, for instance) and then double click to zoom in. You can also drag it around to show mostly the US. Then you really see the wind patterns with Joaquin, etc.
posted by Marie Mon Dieu at 11:24 AM on October 4, 2015

That wind map is one of my favorite things. If you haven't noticed you can change variables by clicking on the "earth" text in the lower right corner.

Generally, storms move from west to east in the mid-latitudes of the Northern Hemisphere, but that is a generalization, if I may be redundant.

The east coast can also be affected by tropical systems, which can come from east to west across the Atlantic, or up from the Gulf of Mexico, or some variation of the two. The system that is dumping all the rain on the Carolinas this weekend is a disturbance out of the Gulf that never quite developed into an organized storm, but is streaming lots of tropical moisture northward, and for a while was actually stealing moisture from Hurricane Joaquin.

In the winters the East Coast will see nor'easters. These develop when disturbances move across the Gulf Coast and the southeastern states and pass over the relatively warm waters of the Gulf Stream in the Atlantic. The warm waters are an energy and moisture source for the disturbance, which can quickly deepen into a strong low pressure system that moves up the coast. They are called nor'easters because the worst weather associated with the low arrives on the northeasterly winds ahead of the center of the storm. Strong nor'easters are usually followed by several days of cold weather as the storm pulls air down from Canada. Nor'easters are notoriously difficult to predict because the atmosphere has to be "just so" and the difference between heavy snow vs. rain vs. light snow vs. no precipitation at any location is often just a matter of a handful of miles in where the storm tracks.
posted by plastic_animals at 11:45 AM on October 4, 2015 [3 favorites]

Here is some 101-level stuff you should know:

Wind direction (like NE) refers to the direction the wind is coming from. So a west wind blows from west to east, and a SE wind blows from SE to NW, etc. Another basic thing to always remember is that wind blows counterclockwise around low pressure systems in the northern hemisphere and clockwise around high pressure. For DC, important low pressure systems include "noreasters" and hurricanes. Noreasters refer to low pressure systems that move up along or just off of the east coast. Imagine a low pressure system just off of the Maryland coast. The counterclockwise wind flow around this will mean NE winds for DC. This is why they are called noreasters. Such a storm may produce rain or snow depending on temperature. Hurricanes are low pressure systems with a tropical origin. They are structured differently from non-tropical low pressure systems like noreasters, but the counterclockwise wind flow still holds. In summer, DC is strongly influenced by the so called "Bermuda High". This is a large area of high pressure centered somewhere east of the Carolinas, often in the vicinity of Bermuda. Now imagine the clockwise wind flow around that high. For DC, that will mean SW winds. And those winds would obviously transport hot and humid air from the deep south into your area. So large persistent Bermuda high pressure systems in summer are usually the cause of prolonged heat waves in your area.

Weather systems like low pressure and high pressure are moved by the jet stream--which is a strong "river of air" up in the atmosphere, usually about 20,000 to 50,000 ft up. Across the USA, the jet stream moves from a general west to east direction. So most weather systems move in a general west to east direction. If the jet stream is fairly "flat" (meaning straight) it means the jet is moving in a pretty direct W to E direction. This is called "zonal flow" and it typically leads to non-extreme weather but with frequent changes every 3-4 days as low pressure and high pressure systems move into, over, and then out of a particular area. The weather gets crazy when the jet stream becomes non-zonal--meaning a highly amplified pattern with lots of big undulations across the country. Imagine flicking the end of the rope---that is what I mean here. When that happens, the overall jet is still moving air from W to E but it does so by following these big undulations--reaching far to the north and then diving far to the south. It is this type of pattern that creates extreme conditions. It could direct arctic air deep into the southern USA or it could move low pressure (like a noreaster) due north up along the east coast. This should get you started!
posted by Seymour Zamboni at 11:48 AM on October 4, 2015 [17 favorites]

plastic_animals made an important point about noreasters that I wanted to elaborate on here. When a big noreaster during the winter threatens the east coast, you will here meteorologists talking about the difficulty in predicting rain vs snow, or snow amounts. This is because the exact track of a low pressure system will have a dramatic impact on the weather you experience. So imagine a low pressure system (noreaster) getting its act together near Cape Hatteras (a typical birthing ground for these storms). What is the orientation of the jet stream and how might that orientation change over the next day or so? That is often the forecast challenge. Will the jet be less amplified (more zonal) as I described above? In that case the noreaster will be kicked further off the coast into the Atlantic and the big cities will miss the storm. Or, maybe the jet will become highly amplified, with a big dip digging down into the SE USA and then right up along the east coast. This could grab that low pressure system and send it more to the north directly up the coast. In that case-the big cities will see a storm. But will it be rain or snow? This depends on the exact track. Remember wind flows counterclockwise around low pressure. So if the storm really hugs the coast, cities like DC, and Philly will be on the west side of that low--where winds will be blowing from N to S--meaning cold air coming in. This can cause snow in those cities. But look at the geography of the coast. Cities like Boston stick way out to the east. So this same storm hugging the NJ coast and causing snow in DC might move up over Long Island and into western New England. If that low moves to the west of Boston, the wind in Boston will veer into the south and this will bring in warm air and rain. So the same storm could cause a snowstorm in DC and rain in Boston. When Boston gets big snowstorms, like last winter, these low pressure systems don't cut inland over New England. Instead, they stay just off the coast just to the SE of Cape Cod. This keeps the low close enough to Boston to keep it in the shield of precipitation, but also keeps the wind NE, which drain colder air south from Maine and Canada--and causes snow. So the weather you experience is strongly dependent on what side of a low pressure system you are on, because that dictates whether your wind is southerly (warm direction) or northerly (cold direction). So if a meteorologist gets the track of such a low wrong--even by only 50-100 miles, the forecast can be a total bust.
posted by Seymour Zamboni at 12:07 PM on October 4, 2015 [8 favorites]

Great stuff already, but I'll add a few more things:
- Mountains have a huge impact on the weather, as you noticed with the Great Dividing range. Typically the windward side gets more rain as air is forced to rise and precipitation forms, leaving the leeward side in a "rain shadow." This precipiation difference is more pronounced in the western United States, as the Appalachians are not as high. However, the east coast will see cold air damming when colder, dense air gets shoved against the Appalachian mountains.
- Winter precipitation is not only dependant on how the low pressure system tracks, as stated above, but what the temperatures above the surface are doing. If there is a layer of warmer air just above the surface with below freezing temperatures at the surface, snow will melt and then become freezing rain and coat everything with ice. Or if the warm layer is not as thick there can be the ice pellets we call sleet. So if you hear the forecasters talking about shallow cold air, beware of ice if any precipitation is expected.
- The Capital Weather Gang blog is great, but here is some additional good info on Maryland winters.

That is all I can think of since I am not east coast centric, but would be happy to answer any specific questions if you want to memail me.
posted by weathergal at 5:38 PM on October 4, 2015 [1 favorite]

Thanks folks. This is exactly the kind of background info I was looking for, and I'll read Capital Weather Gang to continue my education.
posted by une_heure_pleine at 7:15 PM on October 5, 2015

Okay, I don't know the proper meteorology terms and theory, but I moved to Boston from the mild West coast 20 years ago and have been observing the weather since then, and I'll share my basic understanding. DC is pretty far south from here, so these won't apply exactly, but maybe it'll help you with your overall picture, plus I've got a good Nor'easter story.

Like others have said, the weather we get comes from the West. It is usual moving northwest along the ocean's edge. by the time it reaches us — I don't know why but it does. Stuff that comes in from the Northwest (Saskatchewan --> Minnesota --> Boston) can be 20-30 deg F colder, though I can't tell that it's coming from that direction just by walking around. I just know that we'll have 2-3 days where it gets extra cold.

I live about 2-3 miles in from the ocean, but it's in the city so you can barely notice it. But every once in a while there will be a saltier smell in the air. If I get to an open area I can feel the breeze is out of the northeast and I'm getting some ocean air.

Now, if there's a counterclockwise system moving up the coast, it's taking wet ocean air and pushing it onto the land. When that humid air is cold it just seems to be able to pull more heat out of you than the usual dry winter air, if I understand my physics. I remember stepping outside one day a few winters ago and noticing that although the temperature wasn't too bad, say mid-30s, it just felt colder. I noticed that the breeze seemed to be coming from the East, not the West as was usual, and thought, "oh yeah, you can feel this cold in your bones, we've got a Nor'easter." I felt very New England at that moment.
posted by benito.strauss at 9:04 PM on April 26, 2016

I am confused by explanations about noreasters. If the wind is coming from the north and east, how does it move "up" the East Coast after passing over Gulf waters? Wouldn't they come down the coast from Canada and New England? Sorry, I've always struggled with this matter, kind of like my inability to explain the infield fly rule.
Thank ou.
posted by etaoin at 9:16 AM on April 30, 2016

There are two different atmospheric movements at work. What you're talking about is the movement of the storm itself. That will be guided by winds in the mid-level (18,000 - 35,000 ft above sea level) of the atmosphere. Those winds, the upper portion of which is the jet stream, make up the mid-latitude portion of the general circulation of the atmosphere and consist of swoopy ridges and troughs that grow and shrink as they generally move from west to east. Nor'easters will typically form when the leading edge of a trough, which would mean mid-level winds blowing from southwest to northeast, is over the east coast. It's that SW to NE mid-level wind flow that influences the direction a nor'easter takes.

Around the nor'easter itself, though, air is going to circulate counterclockwise and in toward the center of the storm. That is a characteristic of any low pressure system in the northern hemisphere, whether is be a low over Iowa, a nor'easter or a hurricane. That counterclockwise circulation will generally (there are variations depending on the exact path of the storm) result in winds out of the northeast while the storm is still south of your location.
posted by plastic_animals at 11:03 AM on April 30, 2016

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