Commentary

THOUGHTS: WARM WEATHER IN DECEMBER & JET STREAMS

Updated with graphic below.

This week’s weather is expected to be influenced by a flat to ridge-shaped jet flow.

I know the TV weather people like to talk about the jet stream as something that has a mind of its own. They talk about changes in the jet stream as something that just changes, without any apparent reason.

Frankly, I don’t think that is all that accurate nor is it conceptually useful.

What makes more sense to me is that the jet stream winds flow confined to the three dimensional valley created between the bulges of the two major air masses. The position of the jet stream and its configuration takes on the shape of the edge of the air masses and the valley between them.

GEFS forecast jet wind speed and position (300 mb level) for this Thursday. Air mass 1 and Air mass 2 have their own circulations. Between the two air masses bulges is a valley where the the jet stream flows. Conceptually, it makes more sense to think of the jet flow shape and position as influenced by the size of the relative air masses and their individual shape. Speed ups (“jet streaks”) and slow downs in the jet flow are result of constrictions and widening between the air masses. I have intentionally drawn air mass 1 smaller than the tropical air mass 2.

Localized increases in speed of the jet stream (called ‘jet streaks’) are related to narrowing of the valleys between the air masses, just as a garden hose nozzle creates a water jet of increased velocity.

(Indeed, the the name jet stream has little to do with jet airplanes. The stream of air comprising the jet stream displays the physics of “jets” in the field of fluid mechanics, as in the garden hose reference above.)

The movements, size, temperature and densities of the main air masses on the spinning earth determine the changes in the jet wind shape and position. While there certainly is a feedback mechanism where the jet flow reciprocally affects the air mass shape, conceptually, I feel this leads to a poorer grasp of the main determinants of jet stream position and shape.

So why am I talking about this today?

Well, the position of the jet flow this week with its northern position (for December) and bulge northward is the result of the the tropical warm air mass (2 above) being larger than the current colder denser polar air mass to the north (1 above.) Simplistically, there isn’t a larger or dense enough cold air mass to displace the very large warm tropical air mass to the south.

Why is the tropical air mass so warm? Well maybe the ocean surface temperatures, warmed by the atmosphere are maintaining their heat.

Global Sea Surface Temperature anomalies. Things are looking pretty warm out there! (Click on image for a larger view.)

People talk about climate change and global warming. This is one of the ways it manifests itself.

Eventually, very dense cold air to our north will accumulate enough that it will bulge downward and push the jet stream position to our south. How the cold air slides down and its shape will determine the new valley position and the new jet stream configuration.

There is some evidence that a pattern change may be setting up, with cold air pooling in far northwestern Canada by Christmas eve. Following a very mild upcoming Christmas weekend, the final week of this year and the first week in January may be positioning itself for a pattern change

ICON model forecast for this Friday. Cold air pooling in Canada suggests a movement southeastward, pushing the jet into a more winter-like position by the very end of December. (Click on image for a larger view.)

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Update Sat @ 5:07 PM — Sunday’s forecast (below) remains on track with clearing from west to east between 10 AM and about noon. Colder, more seasonable temperatures on the way.

Next week has a flat jet flow (shown below) with little chance of any snow here in the Philadelphia area for Christmas.

No major weather events currently predicted for us, although there are some significant differences emerging in the long range ensemble models that may throw some spice into an otherwise straightforward outlook.

Update Fri @ 10:50 PM — Tonight’s models have the rain lingering until late morning. Then dark low clouds, but there’s the possibility of drizzle, fog and mist continuing. A memorably gloomy day with the low sun angle behind a thick, low cloud deck.

The sun returns by late morning Sunday.

Update Fri @ 5:55 PM — This afternoon’s models show little change in the forecast below. Saturday forecast changes are highlighted below.

Previously Posted Fri 10:20 AM —

This early edition forecast will be updated Friday evening.

A blocked pattern with a split jet stream has affected our area and has given us the mild weather this past week, as described in the graphic caption below—

Jet Stream forecast for Sunday 10 PM. (300 mb wind speed). Blocked pattern due to major ‘kink’ (1) in the jet flow over Pacific has caused the upper scale ridge (2) to be with us for most of this past week. This ridge (2) will finally move offshore by Sunday evening, allowing more seasonably cold air to move in. However, the jet flow will be split between two streams, causing long range forecast uncertainty next week. A flat component to the jet (3) will move in for much of next week, but the southern jet may create some interesting weather.

For this weekend, a cold front will move through as the above mentioned upper ridge (2) moves east into the Atlantic.

Saturday

A departing high pressure system will bring an easterly damp, colder flow ahead of an approaching cold front. Most of the showers will move through early Saturday morning and be north of us by 9-10 AM.

The remainder of the day, dark low-level cloudiness and damp with a chilly easterly wind. Some fog and haze also expected. Showers return during the evening and after midnight ahead of the actual cold front. High temp 47.4º ± 3.0º NBM model Blue Bell.

Sunday

The front slowly moves through after midnight Saturday and Sunday will start cloudy, but clouds should break for sunshine about 10-11 AM. Somewhat windy. High temp 42.7º ± 2.1º NBM model, Blue Bell.

Winds
NBM Wind forecast Blue Bell PA

Weather… and Other Things 'Up in the Air'