E-Portfolio #2 - MD 95 - January 23, 2007
In the early evening of January 23, 2007, conditions were ripe in upstate New York for heavy lake effect snow (LES). At 6:45 pm, the NWS's Storm Prediction Center (SPC) issued Mesoscale Discussion 95 (link opens the text of the MD) concerning the possibility of LES developing on the Tug Hill Plateau of upstate New York.
Mesoscale disccussions (MD) are issued by the SPC when conditions are appropriate for severe weather and the need for a watch or warning is expected, as well as for significant short range winter weather events. The MD provides a discussion of the current situation and the reasoning behind the anticipated severe winter weather.
In analyzing MD95, we will first look at the synoptic scale weather patterns affecting the area. We will then turn to a mesoscale analysis of the weather in the area covered by the MD.
LES is snow showers (that may be heavy) created when cold dry air passes over a relatively warm large lake, picking up moisture and heat. The resultant convection causes snow to fall to the lee of the lake. In forecasting for LES, a forecaster must look to the following:
- Surface isobars becoming cyclonic in the wake of a cold front
- Heavy LES typically has a 500mb vort max trailing the cold front helping make the surface isobars cyclonic
- Temperature difference between water and 850mb at least 13C
- Temperature inversion is higher than 850mb
- Temperature at 850mb is -7C or lower
- Weak vertical wind shear
Let's examine each of these factors, starting with the synoptic scale, to analyze the MD, and determine why heavy snow was forecast and how each factor aids in that forecast. I will highlight the appropriate language of the MD in the caption to each image (copied verbatim, with typoes and all) to show how the forecasters approached this storm.
The Big Picture
In examining a mesoscale system such as LES, we should first start by looking at the synoptic pattern affecting the area. Early that evening, a cold front was approaching Lake Ontario out of the northwest, trailing off of a low centered over northern Quebec.
The small surface trough centered over the northern tip of New York caused the surface isobars over Lake Ontario and southern Ontario province to become cyclonically curved. The cyclonic curvature corresponds to surface-based convergence, which we know can help initiate convection. Now let's turn to 500mb.
There is a subtle 500mb shortwave trough over southern Ontario and upstate New York, which would put the associated vort max over the area of interest. This helped develop the cyclonic curvature mentioned above, having the effect of enhancing surface convergence. Upper-air divergence is associated with surface convergence, enhancing the likelihood of convection.
Turning to the mesocale, MD addresses several of our bullet points and raises a few other factors favorable for development of LES. The 00Z sounding at KBUF will provide us with much of the data needed to analyze this LES, and indeed was relied upon by the SPC forecasters.
The Skew-T shows that 850mb temperatures were approximately -12C, . As the NOAA buoys are pulled in winter months, no accurate reading of lake surface temperature is available, but we can look to the average surface temperature for Lake Ontario to make an estimate:
For late January, average lake surface temperature is approximately 2.5C. The temperature difference between 850mb and lake surface temperature is therefore close to >13C. This allows for a sufficiently deep layer of atmosphere to allow cumulus clouds to develop, as this approaches the typical dry adibiatic lapse rate. The LES season in
Turning again to the Buffalo sounding, the temperature inversion is located at approximately 760mb, above our threshold of 850mb. As was noted in the MD, this equates to about 6000 feet, and the aforementioned shortwave trough (noted in the 500mb discussion above) favored increasing the inversion height. This inversion above the well-mixed boundary layer serves to cap the convection at this height. Therefore we need to make sure it is sufficiently high to ensure a reasonably deep layer for convergence to occur.
For LES to develop, we need cold air blowing across relatively warm lake water, causing heat and moisture mixing in the boundary layer. With sufficient fetch, convection can intensify, so we need to look to the wind direction and resulting fetch over the lake.
We can see from the 850mb streamlines that winds were westerly and had the entire fetch of Lake Ontario from which to draw moisture and warmth. This also suggests that there will be a single band of LES; multiple bands are favored when the winds blow perpendicular to the long axis of the lake, but here we have winds flowing parallel to the long axis.
Looking at wind shear, the MD references low-level directional shear of less than 30kts. The sfc-1km shear vector map bears this out.
Weak low-level shear is one factor forecasters look to in forecasting LES. Strong vertical shear conditions tend to cause the band(s) of LES to collapse. Indeed, this is important enough that the SPC has RUC analyses which show this criterion.
Finally, we need to turn to the topography of the Tug Hill Plateau.
The land slopes upwards from the lake surface to the top of the plateau, which tops out at about 2000 feet. As mentioned in the MD, the upslope flow off of Lake Ontario onto the plateau favors LES; the orographic lift can enhance the amount of snowfall associated with LES.
In the end the forecast proved out. The radar loop shows the single band of LES coming off of Lake Ontario, and the meteogram from Fr. Drum, NY (KTGB) shows the precipitation falling as forecast.
A prudent forecaster always looks to the conditions and patterns of the atmosphere above the surface when trying to understand conditions at the surface. This is as true in LES events as it is in a thunderstorm. Given that LES events are inherently lower in the atmosphere than thunderstorms, we need to consider 'lower' upper air levels in analyzing them.
On the synoptic scale, the upper air (500mb) environment provided the needed divergence aloft and vorticity to allow convergence to occur at the surface. The short wave trough over upstate New York allowed the streamlines to bend cyclonically and helped permit the surface convergence to occur.
On the mesoscale, a lot is happening above the surface of the earth up to 850mb or so. For LES to occur, we need essentially a dry adibiatic lapse rate; a capping inversion of sufficient height to allow cumulus clouds to form; and sufficient cold temperatures to allow snow to fall rather than rain. In the layer between 850mb and the surface we need weak wind shear or the snow bands will be destroyed.
The connection to from the surface to the air above it, particularly the boundary layer in the case of LES, could not be more clear. Without favorable conditions above the surface, LES could not occur. MD 95 essentially ran through our checklist of above-surface conditions to accurately forecast this LES event.