It hit 36 degrees last night... I'm overwhelmed by anticipation for the coming winter! Hopefully I can drag myself out of bed early enough tomorrow morning to see if we get any frost. Now the leaves will start changing colors and everything! I love this time of year!
As for this winter, the NE is still uncertain but almost everybody is calling for a wet and wild season for the Northern Plains region.
I'm going to try to explain what I know about winter weather patterns, so you all can make your own forecasts. It's always best to know why we predict that something will happen; otherwise, you're just taking our word for it. Here goes:
Everybody knows the terms "la nina" and "el nino". But what do they actually mean? This year will be another La Nina, the second in a row. According to NOAA, a La Nina occurs when the Eastern Equatorial Pacific Ocean is colder than normal. Here's a link to the NOAA site:
http://www.publicaffairs.noaa.gov/lanina.html La Ninas and El Ninos come in three different forms for the U.S.: West-based, Central-based, and East-based. For La Ninas, West-based means hot for the SE and normal to cool for the NE, Central-based means cool for the NE and warm to normal for the SE, and East-based means cold for the NE and cool to normal for the SE. This year will be a Central- to East-based La Nina. Last year's was East-based.
So, that's what they do to temperatures. What about precipitation? In order to explain their affects on snowstorms, I must explain first the mechanisms of the Jet Stream.
The Jet Stream is the allmighty being that controls all weather. There are two Jet Streams that affect the U.S., and these are the Polar Vortex (that's good) and the Southern Jet (that's bad).
The Polar Vort is made of really cold air, and is what causes our cold outbreaks. You can think of it like a giant river, flowing East across Canada, Greenland, the U.K., Siberia, and Alaska. It is really high off the ground, so you don't feel the hurricane force winds in it unless you're standing on top of Mount Washington. The path that it takes depends on surrounding airmasses- just as streams part around boulders and pour into valleys, the Polar Vort shifts around high-pressure air masses and shifts towards low-pressure systems.
Last year, the Polar Vortex ran right through the U.S. for most of December-January, and as a result, it was really cold and stormy. In fact, the jet stream went as far south as Texas, blowing all of those nor'Easters and coastal blizzards into the East Coast. This was because there was a large high-pressure air mass over Greenland, known as the Greenland Block, that forced the Jet Stream to move around it- so it did, and it moved South. It will also be pulled south behind large storms such as Nor'Easters, resulting in rain for the first half of the storm and then snow for the second.
The Southern Jet is what kills our storms. Normally, it stays South of us, keeping the warm and dry air down in Mexico. However, if the SouthEast Ridge is present, it shifts North and ruins our winter. The SE Ridge is a large warm airmass centered between Bermuda and the Caribbean.
Most of our exciting storms along the East Coast last year formed in the Gulf of Mexico and then were pulled North by the Polar Vortex. Because they came from the Gulf, they had plenty of moisture to work with. Other storms form from packets of energy originating in the Pacific Ocean and then sweeping East. I'll explain these in more detail later.
Back to La Nina/El Nino, in a La Nina year the Polar Vortex tends to be bigger and more powerful, and sometimes farther south, but the SE ridge tends to be dominant. This means that warm and dry air dominates the SouthEast while the North East gets blasted by the Polar Vort. However, East Based La Ninas typically push the SE Ridge farther East, so it doesn't affect us as much. This winter, it is looking like the first couple months will be cold and exciting before it switches to a Central based La Nina and the SE Ridge comes back. A similar scenario happened last year.
That's basically all there is to the La Nina. I learned everything last year, so I don't really know what an El Nino does.
Next topic: Computer models.
Computer models are what forecasters use to predict the weather. There are hundreds of them out there, but here is a list of the most important ones and their quirks, as well as a link to their data:
GFS (Global Forecast System): one of the "big three" models, the GFS is a versatile model that is relatively accurate for all kinds of weather events. It forecasts up to 380 hours from initialization. The GFS often shows "fantasy storms" in its extended range, storms which are so powerful and intense that they could never really happen. Last year, it showed a giant Nor'Easter equivalent to a category 4 hurricane covering all 48 contiguous states sometime in December. Obviously, it didn't happen, but it will pick up on possible storms and time frames.
NAM (North American Model): Another one of the "Big Three", the NAM is a shorter-range model (74 hrs) that is usually pretty accurate.
ECMWF (European model or Euro): The most highly regarded computer model, and the last of the "Big Three", the ECMWF is more accurate than the GFS but goes to 180 days, more than twice the range of the NAM. It was always really hard to find any data for the ECMWF, but now Wunderground is providing it on the wundermap for free.
Other models: The CMC (Canadian), Ensemble, NOGAPS, and others. These are usually only looked at to see if they agree with the NAM, GFS, or ECMWF.
About models:
-The Operational run (OP run) is the 12z or 00z (Zulu time, subtract 4 for Eastern time) run. These runs are important because they are when the computers get new data on current conditions.
-Types of model data:
-MSLP: Mean Sea Level Pressure
-850 mb temp: temperature at 850 mb (the elevation at which the atmospheric pressure is 850 mb). It has to be below 0 C to snow.
-__ mb Heights (hgts): how high up it is when the atmospheric pressure is __ mb. Colder air=lower heights, warmer airmass=higher heights.
-Vorticity: A measure of the ability of a front to produce storms (instability).
-QPF: Quantity of Precipitation Forecast. The amount of liquid water that is supposed to fall from the sky. Usually, 1 inch of water equals 10 inches of snow.
-500 mb temp: Not as important as 850 mb temp. It is more useful in determining the type of snowflake/ice that will fall from the sky. When this is below 0 and the 850mb isn't, ice storm.
That's all the important ones. I'm thinking about making a video tutorial on model reading.
Next topic: Resources for current conditions
I'll get to that later. It's a huge topic, and I'll have some pretty pictures to go with it.
What do you think? Can you guys follow that, or do I need to break it up with pictures and/or video?