Bust Analysis of the November 2nd-3rd, 2000 Northwest Flow Snow Advisory

By Bryan McAvoy

WFO Greer, SC

This web page records data form a forecast "northwest flow snow event" which never occurred.  On this occasion, I issued a snow advisory for the spine of the western North Carolina Mountains, which encompassed  the counties of Graham, Swain, Madison, Mitchell, Yancey and Avery. However, only a dusting of snow was reported in a few locations in the advisory area, mainly from Madison county and points north. Actually, I inherited the advisory, but I chose to extend it south, as the original advisory only included the 4 northern most counties listed above. In the past I have had a fair amount of success with northwest flow, mountain snow  forecasts. However, many of the previous events shared common features, such as an ample pool of low level moisture and strong 850 mb flow, two elements (among others) lacking in this event.

For this review, I've constructed a series of 4-panels which follow the event in time. I explain what I thought each level represented for snow potential before the event, and what I learned about the importance of these fields as predictors thereafter. Many of these images are "thumbnails". This means you must click on the image for a larger version. The model I used for this bust analysis was the 12 UTC run of the 22 km Eta model. This was the run I used to decide to issue the snow advisory. Most of the these images start 18 hours into the Eta run, or 06 UTC on Tuesday. This was about the time I thought the best snow would develop.

To see our forecast area (helpful in understanding my perspective on this event), you should briefly visit this link to see the WFO Greenville-Spartanburg County Warning Area

 

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Working down through the atmosphere, I'll start with the 500 mb level. Included here are contours of 500 mb geopotential height (solid green) and vorticity (dashed blue) valid at 18, 24 and 36 hours into the 22 km Eta model run (30 hrs was not available). This represents 6 UTC and 12 UTC on Tuesday and 00 UTC Wednesday.

I never thought that vorticity advection would be much of a contributor to this event. However, a sharp, short wave trough, accompanied by significant positive vorticity advection, is a well known contributor to northwest flow snow, and certainly would have helped. In retrospect, seeing that most of our mountain counties were on the south side of a channeled shear axis, I might have wanted to weigh the 500 mb features a little more in my thinking, as this is quite unfavorable for northwest flow snow (as there is usually strong drying and subsidence aloft). I did not plot relative humidity as 1000-500 mb layer RH's are all but meaningless in these events. Apparently the wave was progged to be further south when the previous shift initially issued the advisory

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Surface plots from 18, 24, 30 and 36 hours into the 22 km Eta model run, or  6 UTC, 12 UTC  and 18 UTC Tuesday and 00 UTC Wednesday. Included are sea-level pressure in millibars and surface dewpoint in degrees Fahrenheit. This map is better complimented by the actual surface charts from the event, as well as satellite data. What they show are very low dew points upstream from the mountains. While temperatures aloft were also cold, the low levels are the atmosphere were quite warm and dry, leading to large dew point depressions. This is evident by the next figure, satellite data from the event.

Here is possibly the best indicator of the entire event. The decision to issue a winter weather advisory had actually been made the night before. However, during the afternoon of the 20th, we decided to expand to aerial extent of the advisory by a few counties. Looking at the satellite data during the day, I lost track of one of the best rules of thumb that we have. Typically, if there is not a big upstream reservoir of low clouds 6 to 12 hours before an event is forecast to take place, snowfall totals will not be very high. Note the lack of low clouds of most of Tennessee and Kentucky during the day of the 20th (Monday). Monday night was not much better, with just a narrow band of clouds forming at the highest elevations.

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The above images are 18, 24, 30 and 36 hour plots of 850 mb heights and 1000-850 mb layer average moisture. Plots 1 and 4 have 850 mb winds in knots, while 2 and 3 have 850 mb temperature in degrees Fahrenheit . Note that the layer average relative humidity never rises above about 80%. Despite strong cold advection, the 850 mb winds were only 25 to 30 knots. It appears that the strength of the wind and the projected layer average relative humidity is a much better predictor than low level thermal advection. However, our best snow in these events still needs to occur with 850 mb temps of less than -12 deg C.

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This surface map is from 18Z Monday the 20th. Notice the 25 to 30 degree Fahrenheit dew point depressions behind the surface cold front ( blue penciled in numbers). Another interesting thing is the strong winds in the upstate. Winds gusted to 30 knots from the southwest this day. There was even some minor damage -- more than was reported with northwest winds behind the front! 

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This series of images, valid for 18, 24 and 30 hours into the Eta model, show orograhic omega, or the component of lift in the low levels of the atmosphere owing to terrain effects. The orographic omega values were quite respectable for this event, anther thing which had me going. I figured that the very cold 850 mb temperatures, coupled with the reasonably strong upslope would lead to respectable snowfall at the higher elevations. For reasons previously demonstrated, this was not the case.

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In the same vein as the previous 4-panel. This time series represents total vertical velocities from 1000 to 850 mb. What I find interesting about this series is that the lift seen on the upshear side of the mountains is much stronger in West Virginia. This is from the combination of stronger 850 mb winds and being on the cyclonic side of the 500 mb jet. I may start using this field in lue of orographic omega fields in the future. As I do not have any other cases posted, I do not yet know if this will work.

 

In conclusion, and based on better luck last year, here are a few cook-book type ingredients that should be useful when forecasting a northwest flow snow event:

  • 850 mb wind speed of 30-40 knots.
  • considerable upstream low level clouds.
  • 1000-850 mb relative humidity of >85%.
  • 850 mb temperatures of -12 to -14 deg C (but only w/high layer average relative humidity).
  • surface dew point depressions of 15 degrees or less during the maximum daily mixing.
  • located north of the 500 mb shear axis; even better if there is ample vorticity advection.
  • total 1000-850 mb omega values bulls-eyed along the western NC mountains.