Blocking sends disturbance south from Greenland

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High latitude blocking, which we discussed earlier this week as an important predictor in summer temperature patterns around our area, is at it again. A large and anomalous blocking ridge, building north from Canada towards the higher latitudes into Newfoundland and Southeastern Greenland, is forcing a disturbance and upper level low to shift southward. The disturbance will track southward today — and can be seen on water vapor and visible satellite imagery early this afternoon. The feature won’t directly impact our area, nor will it cause any widespread unsettled weather on the east coast, but it could reinforce east/southeasterly winds as it passes later this week.

The track of the disturbance is the most unusual. To see an upper level low or disturbance track southward from Greenland all the way to a point just a few hundred miles off of the US East Coast is exceedingly rare. However, one glance at the blocking pattern aloft tells you the story — the disturbance had nowhere else to go, being forced into a small window between the trough over the Atlantic and the blocking ridge to the west over Canada. (Click read more for more imagery).

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Doug Simonian

What Explains the Convective Cumulus Clouds we had Yesterday?

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Convective cumulus clouds were observed throughout the area on Monday, and many areas saw snow pellets falling despite temperatures in the upper 40's to near 50. Some very interesting atmospheric processes were at work to cause this, which we detail below.

Convective cumulus clouds were observed throughout the area on Monday, and many areas saw snow pellets falling despite temperatures in the upper 40’s to near 50. Some very interesting atmospheric processes were at work to cause this, which we detail below.

Skies finally cleared somewhat on Monday behind our departing storm system. As some of you may have noticed, we observed lots of cumulus clouds today, almost as if it were summer, despite relatively chilly temperatures. Additionally, they actually had pretty good, organized vertical structure (although not towering), and some were quite dark. Isn’t this usually a summer time convection phenomenon? What explains this?

This is where it is very important to look at what we call lapse rates. Lapse rates are the difference in temperature between two given heights in the atmosphere. Thus, a steep lapse rates means that there is a great difference in temperature between the bottom of the given layer and the top of the given layer. Mathematically, they are actually the negative of the difference in temperature divided by the change in height, since temperatures generally decrease with height. Thus, when temperatures rapidly decrease as you increase in height, you have a strong, positive lapse rate. When lapse rates are strongly positive, that means the atmosphere is very unstable.

Now, let’s apply this to the atmosphere. There is the cliche that warm air rises and cold air sinks, and this is because cold air is more dense than warm air. Thus, when an air parcel (a sample of air) is heated at the surface, it will have the tendency to rise. And if that heated parcel is rising into air that is rapidly getting colder with height, that means the parcel will have a tendency to keep rising, since it is much warmer than the air around it and above it during its rise. Thus, steep lapse rates means that you have rapidly rising air.

Usually, when we evaluate the stability of the atmosphere, we like to start with the ground layer, since the ground is what traps sunlight and heats most efficiently. If we know the temperature at the ground is much warmer than the temperatures in the mid levels of the atmosphere, then we have an unstable atmosphere and rapidly rising air. Let’s take a look at the atmospheric conditions and see if we have these phenomena taking place.

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