Day 23: Is the Cold Air Returning?

Posted by pmarsh on 23 Jan 2010 | Comments Off

The National Weather Service’s Climate Prediction Center (CPC) produces “climate” forecasts on various time scales ranging from 6-10 days all the way to the next three months.  These predictions are rather generic in nature, essentially providing a prediction as to whether temperatures will be above normal, equal chances of above or below, or below normal.  They do the same for precipitation.

The forecast below was issued by the CPC on 23 January and is valid for the time period of 29 January – 2 February.  It is an example of their 6-10 day outlooks.  What is impressive about this forecast is the vast area that is expected to have at least a 33% chance of having colder than normal temperatures in that time frame (this is the blue shading).  Most of this same area is expected to have greater than 50% chance of having below normal temperatures, and the south central United States (Texas area) is forecast to have a greater than 60% chance of having below normal temperatures.  With values like this forecast, there must be a pretty strong signal that cold air is coming.

So how can meteorologists offer such a high probability forecast so far in advance.  Well, for one, the numerical weather prediction models (NWP) must be in fairly good agreement that something like this is possible.  Another technique, and the one I advocate strongly to my students, is to take a look at observations and see if the signal is already there.

The image above is the current 00Z (6 PM CST) North American surface map.  A couple of things stand out.  First, there are several big red “L”s in the middle of the map connected by a blue line with barbs on it.  These big red “L”s are where the surface pressure is relative minimum, or, in other words, lower than all surrounding areas.  This is what meteorologists are referring to when they say “low pressure”.  These low pressures and the associated cold front(s) (depicted by the blue lines with blue barbs) are responsible for the precipitation in last night’s image.

Of interest for this post is the blue H (a relative maximum in surface pressure, or, in other words, the point where the pressure is higher than all surrounding areas) in northwest Canada, near the Alaskan border.  This high pressure is associated with cold air in northwest Canada, that is poised to filter southward.  How do I know this?

Well, take a look at the reddish, brown lines that encircle the L’s and H’s through the map.  These lines are known as “isobars”, or lines of constant pressure.  If you look at the lines encompassing the high pressure in northwest Canada, you can see how there are distinct “kinks” in the circle that stretch from the H, all the way southward to the cold front (blue line with barbs). (At this point I should mention that the image below is identical to the image above.  The difference is that I’ve circled the high pressure in yellow and drawn an arrow through all the kinks in the isobars in pink.)  Believe it or not, but the high pressure in Canada is affecting the cold front that is moving through Kansas, right now!  Over the next week, this cold airmass in Canada will initially filter southward along the path laid out by the kinks in the isobars to the south of the high pressure.

Why am I so confident this will happen?  One reason is that there are no fronts (cold, warm, or stationary) between the cold front in Kansas and the high pressure in Canada.  Fronts are boundaries in the atmosphere.  In fact, the reason meteorologists call these boundaries “fronts” is because in warfare the boundary where two armies fought is known as a front.  Well, in the atmosphere, when warm air and cold air meet, the boundary is known as a front.  Because there is no front between the high pressure in Canada (associated with arctic air) and the front in Kansas, there isn’t really anything in the atmosphere to stop the cold air from moving southward – especially on the east side of the high pressure.

It will take several days for the brunt of the cold air to make it into the United States.  However, as it does, NWP models indicate that a storm will move out of the southwest United States into the central plains.  Depending on how long it takes the cold air to become entrenched in the US (and how far south it makes it), portions of the central United States could be looking at a major winter storm middle-to-late this week as far south as Oklahoma.  Freezing rain, sleet, and snow will be possible in the cold sector (cold air), and severe thunderstorms will be possible in the warm sector (warm air).  I’m sure I will be blogging about this more as the event gets closer.

The two images below are of the same mentality as the pictures above.  The only difference is the images are much larger and have more surface observations on them.  I encourage you to click on the images and take a look at the cold air near the high in Canada.  That’s the cold air that will be coming south in the next week or two.

(Note, once you click on an image, if you click on the green arrow in the bottom center, the image will appear in full resolution.)