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 (10 answers total) 14 users marked this as a favorite
Best answer: 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 [1 favorite]

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

Best answer: Seconding Capital Weather Gang.
posted by Johnny Wallflower at 11:17 AM on October 4, 2015 [1 favorite]

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

Best answer: 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]

Best answer: 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]

Response by poster: 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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