Monday, August 15, 2011

A strenghening cyclone brings rain to the northeast

I don't often write about the northeast in this blog.  It's probably because I've never lived there and therefore haven't really experienced their annual weather patterns firsthand.  I've lived in the upper midwest, the southern plains, and now the Pacific northwest, so that's where I tend to focus my writings.

But today I am indeed going to talk about the northeast.  As you can see in a radar mosaic from this morning, it's a very rainy day:
NEXRAD base reflectivity composite at 1858Z, August 15, 2011.
Some flood watches and warnings have already been issued for parts of the Hudson River valley and into Massachusetts.  It even looks like a heavier line has formed and is approaching New York City, though no severe warnings have been issued yet.

This low has actually been strengthening over the past 12 hours or so as it has entered the northeast.  Even so, the surface analysis from early this afternoon still shows a very broad low pressure center:
RUC surface analysis as of 15Z with temperature (colors) mean sea-level pressure (contours) and winds (barbs) on August 15, 2011.
A rather significant cold front (for this time of year) looked to have brought some cool air down south at least as far as the Appalachians.  But, notice how weak northwesterly winds continue through the Carolinas and Georgia but the boundary between the warmer and the cooler air (between the yellows and the greens) stays back behind the mountains.  I'm guessing that since the winds shift has continued over the mountains but the cooler air has not, the cold air pool behind this front was relatively shallow.  So, when it encountered the Appalachians, the cool air couldn't push through and has just gotten blocked up on the other side.  So, not the strongest low.

However, like I mentioned, there has been some strengthening in the last few hours.  This is particularly notable on the upper-air charts.  Here's the  300mb analysis from early this morning at 12Z:
300mb analysis of heights (contours) and winds (colors and barbs) at 12Z, August 15, 2011.
Notice a rather significant shortwave trough whose axis lies over Ohio and West Virginia.  This is providing the upper-level support for the surface cyclone.  There is a relatively weak jet streak analyzed around the base of the trough with a maximum wind reading of maybe 60 knots.

Compare this to the 300mb RUC analysis from 3 hours later at 15Z:
RUC 300mb analysis of heights (contours) and winds (colors and barbs) at 15Z, August 15, 2011.
The RUC analysis shows a much deeper trough with a stronger jet streak on the eastern side.  Here, the maximum winds are estimated to be around 90-100 knots!.  So why would we expect this upper-level trough to strengthen like this as it moved out over the northeast?

I suspect it has to do with the influence of the Gulf Stream current off the east coast.  Take a look at the satellite-derived sea-surface temperature over the past few days from NOAA-NESDIS:
NOAA/NESDIS 50km SST analysis from data on August 9 through August 12, 2011.
See that warm tongue of higher sea-surface temperatures that stretches northeastward from Cape Hatteras in North Carolina out away from the northeast coast?  That's the hallmark of the Gulf Stream--a current that brings very warm water northward from the tropics along the east coast.  This can set up a very strong temperature gradient between the often cooler land and the much warmer water off shore.  Remember from my discussions on jet streak theory that temperature gradients in the lower-levels drive the winds aloft through a relation known as the thermal wind relation.  So, if we have a strong temperature gradient between the cool land and the warm Gulf Stream waters, this is going to work to enhance the wind speeds aloft directly above this gradient--exactly what we're seeing.

This sort of phenomenon happens a lot in the northeast--upper-level winds increase in response to the temperature contrasts between cooler land and the warmer jet stream.  This contrast becomes particularly pronounced in early winter when the ocean is still warm but the land really starts cooling down.  This is why those great nor'easter types of storms strike during this time--they feed off of this temperature contrast to gain their strength.

Another side effect of the warm Gulf Stream current is that it provides a nice supply of warm, moist air to fuel storms and precipitation.  If the low-level winds are out of the south, they can bring that warm, moist air up and over land, creating lots of precipitation in the process.  You can see this in the infrared satellite image from early this afternoon:
GOES-E IR satellite image from 1831Z, August 15, 2011.
The classic comma-shaped cloud pattern of a developing cyclone is becoming pretty evident in this image.  To the east, you can see a broken band of clouds stretching from south to north just off the coast.  This is that warm-air advection region--where southerly winds spinning in toward the center of the cyclone are advecting warm-moist air northward.  As they approach the coast, the warm air rises over the terrain of the Appalachians and the cooler air sitting over the land surface.  This rising motion causes the moist air to cool and the moisture to condense out.  Hence, we see lots of rain and thick clouds over land in New England.

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