Modest Lake-Effect Snow Event: 23-24 November 2012

A modest lake effect snow event is looking more likely after Thanksgiving across the Great Lakes. However, the mean analog pattern and impact guidance products based on the 20121120/0000 96-h GFS forecast are indicating the potential for a modest event. Here is an excerpt from last night's NWS Area Forecast Discussion from Gaylord, MI for this coming Friday.


REALITY LOOKS TO SET IN FRIDAY AS COLD FRONT QUICKLY EXITS STAGE RIGHT AND STRONG SHORTWAVE APPROACHES. DEEPEST CAA LIKELY TO HOLD OFF UNTIL LATER IN THE DAY...WITH FALLING TEMPERATURES THROUGH THE AFTERNOON INTO THE 30S. CAA LOOKS STRONG ENOUGH TO KNOCK THE COBWEBS OFF THE LAKE GUNS...ALTHOUGH WILL LIKELY HAVE TO WAIT UNTIL DEEPER MOISTURE ARRIVES LATER IN THE DAY INTO OVERNIGHT FRIDAY TO REALLY RAMP THINGS UP. THERMODYNAMICS DON`T LOOK TO SHABBY AT ALL...WITH DEEP MOISTURE AND GOOD TO EXCELLENT DELTA T/S AS H85 TEMPERATURES APPROACH -10C. BIGGEST NEGATIVE TO ANY SIGNIFICANT ACCUMULATIONS WILL BE SHIFTING WINDS...WITH INITIAL WEST FLOW QUICKLY VEERING NORTHWEST TO NORTH-NORTHWEST...DISTRIBUTING SNOW SHOWERS THROUGH THE FAVORED AREAS. PATTERN RECOGNITION SUPPORTS AT LEAST A FEW INCHES BEFORE THINGS BEGIN TO WIND DOWN SATURDAY. LATEST CIPS ANALOGS AGREE...PAINTING 2 TO LESS THAN 4 INCHES THROUGH THE TRADITIONS SNOWBELTS. JUST ENOUGH SNOW AND GUSTY WINDS TO CONTINUE A MENTION IN THE LATEST HAZARDOUS WEATHER OUTLOOK.

Confidence in the analog impact guidance can be gained through analyzing the similarities and differences between standard mean fields and the GFS forecasts. Shown below are the 500-mb height and 700-mb temperature field comparisons.

The mean 500-mb troughs line up across the Great Lakes and the 700-mb -15 C isotherms are in similar locations. This suggests that the majority of analogs have a similar strength of cold air moving across the Great Lakes. The COOP snow impact guidance (shown in the next set of images) has mean values of 4-6" and 30-40% probabilities of >6" of snow across the traditional lake effect snow belts (e.g., north of Muskegon, MI and south of Buffalo, NY). However, very low probabilities for >12" of snow which suggests the chances for a widespread high-impact event are low.

By analyzing individual analogs, the range of possible outcomes and why they occurred can be determined. For example...the following two snow events were in the top 15 most similar analogs. The first has very little snow while the second was a major lake-effect snow event for the Great Lakes where more than 18" of snow was common across the lea of lakes Superior and Michigan.

Assuming that the synoptic-scale height and wind fields for these two analogs are similar, it is easy to understand that the depth of cold air was a factor in these two events. In the December 2006 event, the low-level air was not nearly as cold as the November 2000 event...temperatures at 850 mb were almost 10 C colder across the upper Midwest and Great Lakes in November 2000. This can have a significant impact on the amount of lake-induced instability which is important for heavy lake-effect snows.

In addition, the cold-air reservoir in December 2006 was embedded in a quick moving midlevel trough that moved east across the Great Lakes within relatively zonal flow. In November 2000, a deep trough formed across the eastern U.S. that resulted in the cold air to continue to move southeast across the Great Lakes from Canada. The current GFS forecast (that these analogs are based on) suggests that the cold air will not be as extreme as November 2000 but colder than December 2006 for the upcoming event. This gives additional confidence that the impact guidance COOP snow mean and probability maps shown above are suggesting a modest lake-effect snow event.