|Fig 1 -- 0.5 degree base reflectivity image from KBUF at 2113Z, Dec 1, 2010|
SNOW ACCUMULATIONS: 14 TO 20 INCHES IN THE MOST PERSISTENT BANDS. SNOWFALL RATES OF 1 TO 2 INCHES PER HOUR LIKELY.
So what goes into a lake effect snow event? What conditions have to be met to see extraordinary snow bands like this one?
There are at least five main qualifications that usually must be met to get a good lake effect snow event. This list is adapted from a presentation in Dr. Fred Carr's mesoscale meteorology class at the University of Oklahoma (though I believe the actual presentation was given by Todd Kluber). But here are the five:
- Lake-air temperature difference --> Generally, the lake surface water temperature should be at least 13 degrees Celsius warmer than the 850mb temperature.
- Height of the capping inversion --> To get any real snow at all, the capping inversion needs to be at least 1.9 km above the ground. For heavier snows, the capping inversion should be more than 2.3 km above the ground.
- Wind direction over lake fetch length --> The winds must be blowing in a direction such that the length of open water over which they pass is at least 80 km for light flurries and at least 160 km for vigorous banded snow.
- Vertical wind shear --> The best banding occurs with little to no directional shear of the wind underneath the capping inversion. If there is more than 30 degrees of directional shear, the snow band location becomes difficult to pin down and with more than 60 degrees of directional shear bands are unlikely.
- Lake-land temperature difference --> In some cases, "lake breeze" convergence zones can set up if you get offshore (or onshore) flow in association with a lake-land temperature difference (like the sea breeze near the ocean). This convergence can enhance bands of lake effect snow.
First, the lake surface water temperature should be at least 13 degrees Celsius warmer than the 850mb temperature. This should be easy enough to check. NOAA's Great Lakes CoastWatch division publishes daily estimates of lake surface water temperature. Here's yesterday's image (today's isn't out yet, but lake temperatures don't change as fast as air temperatures...):
|Fig 2 -- Analysis of Great Lake surface water temperature for Nov 30, 2010. From NOAA CoastWatch and the Great Lakes Environmental Research Laboratory.|
|Fig 3 -- 12 Z sounding from Buffalo, NY on Dec 1, 2010. From the HOOT website.|
|Fig 4 -- RUC 21Z Analysis sounding for Buffalo on Dec 1, 2010. From the twisterdata.com page.|
Next, we need the height of the capping inversion to be at least 1.9 km above the ground and over 2.3 km above the ground for very heavy snow. Fortunately in the forecast sounding in figure 4, the capping inversion starts at right around 700mb and on the left they've conveniently labeled the height of the 700mb level at 2838 meters. That's about 2.8 km, which is greater than 2.3 km--enough for heavy snow. Since lake effect snow is essentially convection driven by cold air over a much warmer lake, like all convection in ther atmosphere it is going to be inhibited by any inversions aloft (unless the lake is much, much, much warmer than the air). As such, the capping inversion essentially represents the maximum height the lake effect clouds will be able to reach. Deeper clouds tend to produce more snowfall. This is why we need such a high capping inversion for heavy snow.
For our third condition, we need to have winds beneath the capping inversion blowing across at least 160km of open lake for heavy snow. Well, according to the forecast sounding above, the winds beneath the capping inversion are generally out of the west southwest. So, let's draw a line from just south of Buffalo (the middle of the snow band) to the west-southwest (actually, if you go into the numerical values for the sounding, it's more west by south, or around 250 degrees) and see how much open water that covers.
|Fig 5 -- Fetch length from a point south of Buffalo west by south across Lake Erie. The fetch distance is about 174 kilometers. Image from Google Earth.|
For the fourth condition, we want as little wind directional wind shear as possible beneath our capping inversion. Once again, in our forecast sounding above we see almost no directional wind shear beneath the capping inversion--all the winds are generally out of the west-southwest. Strong directional wind shear would limit the ability for convection to occur over the long fetch of the lake as winds at different levels would be advected over different lengths of warm water. This could create spurious inversions and other inhomogeneities that would disrupted the banded structure of the snow and perhaps inhibit it all together. To get a strong band of snow, you need uniform wind directions through the cloud layer and we have that.
The fifth condition is a bit more tricky to apply, as it only describes enhancement of precipitation due to lake breeze convergence. It's possible to look into this more, but we've already more than satisfied our other lake effect snow conditions. So, there may be some enhancement due to convergence of winds over the lake, but it's very difficult to quantify that.
And there you have it. Lake effect snow in Buffalo that should be and is happening, with all of our forecasting rule-of-thumb guidelines met. Note how a lot of this analysis was simply based on looking at a sounding (or rather, a forecast sounding) over Buffalo--and that's it. This is why the forecasters at the National Weather Service--Buffalo forecast office developed the now widely-used forecast sounding analysis tool called BufKit. It was originally designed to predict lake effect snow, but has now been expanded to all sorts of uses. I encourage you to look into the software if you want to experiment with some fun sounding analysis tools. And best of all, it's free...