In need of Weather Systems 101

[JBC]

Where do weather systems in various parts of the world originate? My understanding is that in the USA weather comes from the west to the east, except for hurricanes. Why does it do that and do those systems ever make it to Europe? Where would Europe weather originate? Asia, the Middle East, southern hemisphere Africa and South America?

[Mr. Rainman]

I can't answer every part of that question, but I can tell you what I do know. In the United States, weather generally moves west to east. If I had to guess, it would probably be because of the movement of the jet stream. That would probably carry systems from the west to the east. Europe would probably see something similar, but not the same. I can't say for sure what happens over there.

With hurricanes, storm systems buck the trend and move mostly from east to west. This is because of easterly winds, which blow westward. The storms are pushed by these winds to the west. The usual turn to the Northeast is thanks to the Coriolis effect, which is a deflection of an object on a rotating plane. This deflects hurricanes to the "right," relatively speaking. And yes, these storms can and do affect Europe. The remnants of Hurricane Maria in 2005 caused significant rainfall in Norway that killed three. Hurricane Vince, in the same year, impacted Spain and Portugal.

In the Southern Hemisphere, I would assume that everything is switched over. Weather systems move from east to west in the land Down Under. Hurricanes (or Willis Willis, as they are called by Australians. The country's weather service calls them cyclones.) down there move from west to east, carried by the westerlies, and deflect to the "left," turning back to the west and following regular patterns.

This is just a guess. Patrick, am I right on this?

[phw115wvwx]

What you're asking about can be answered with global circulation patterns.  It's not as simple as you think.  There are actually three belts of winds in each hemisphere, two that go easterly, and one that goes westerly.  Here's a picture to help you see how our flow is across the globe in general:



We live in the mid-latitude westerly belt for the Northern Hemisphere, so our weather goes from west to east thanks to the westerly winds of the jet stream.  Our weather does eventually push over to Europe and Asia, and it'll eventually come back around to us again.  However, the tropics have easterly flow due to the trade winds in both hemispheres.  Near the polar regions of both hemispheres, easterly flow again is found.  You can also see the 3-D circulations that we get as a result of this global pattern as air is converging together or diverging apart depending on the latitude.  Ever notice that the tropics are always wet, but the subtropical parts of the world feature most of our deserts?  Those cells explain why.

In short, it depends on where you live as to where your weather originates.  Keep in mind that these belts are not always perfectly straight as they can bend, compress, or stretch out with time.  Thanks to our rotating planet and the unequal heating between the tropics and the polar regions, we get this complicated global circulation pattern for the flow aloft in the atmosphere.  It's even more complex when we deal with flow near the surface, but hope this explanation helps answer your question.

[JBC]

Thanks, and it does help though it is still somewhat beyond me.

What do the forecasters in the Seattle or Anchorage areas look for in making their forecasts. It is harder for them than the east coast forecasters? Maybe those Alberta clippers make it difficult to forecast in the winters. 

One last simplistic item. How do the Farmer's Almanac people seem so accurate? I'm sure they do not use meteorology science in their predictions. 

[phw115wvwx]

The Farmer's Almanac goes by long-term climate patterns, but you obviously haven't seen when they bust badly.  I'd never consider that as a great source for daily weather conditions.  There are models that cover the whole globe, but it's difficult for forecasters on the West Coast as their weather is coming from the Pacific Ocean.  As you can imagine, data is definitely more sparse over ocean than land.  Satellites are trying to help us there by indirectly measuring conditions and relaying the data into the models, but we still have a ways to go.

[P71nnacle]

Most of the cyclogenesis (low pressure system formation) in the United States forms on the lee (in this case, east) side of the Rocky Mountains during the spring (tornado season in the Midwest), and usually, nor'easters form anywhere from Texas to Louisiana, with a second low often establishing itself off the East Coast once that low tracks eastward. (Interestingly, cold air damming can cause a hydrostatic pressure gradient; essentially meaning the pressure drops itself without any influence.)

As for Alaska's forecasting, it's actually very well covered. Geostationary satellites are actually able to view well up into the 70s in latitude, after which the view becomes too skewed. Polar orbiting satellites have good resolution, but are often unable to view the same place more than twice a day, limiting its potential as a weather observation and forecasting source.