Vinnie’s Bufkit Tutorial III

Snow Growth Ratios and Precipitation Amounts

Forecasting Precipitation with Bufkit:

1. •  The first thing I do is click Omega in the Contour panel and compare that to RH.
   

2. •  In areas of high RH (it is color coded) with negative Omega I expect precipitation.
   

3. •  For omega the red circles are negative and indicate rising motion.
   

4. •  This combination of water vapor and rising air leads naturally to precipitation.
   

*TIP* You can click the colors in the RH tab to only plot values from that percentage up. Click on the 70% tab and only RH values at or greater than 70% will be listed. This makes the graph easier to use. 

I performed a forecast for the WX Challenge for Birmingham, Al in Feb of 09. GFS was calling 2.67, the NAM for 2.3 and the USL12 and 22z 1.87 and 2.12 with NWS going for 2.82”. Seeing the Bufkit profile of a very stacked omega pushed me to go with 2.9 and the total for that day was 2.99. Not bad though it doesn’t always work like this.

The omega values were stacked, at -60. I usually get -20 in CT for strong winter storms.

Convective Tab and Thermodynamic Indices

Obviously if you are forecasting precipitation you must consider the possibility of convective instability. Thunderstorms will drop large amounts of precipitation in short time periods and are generally dangerous. See Tutorial number two for information on thermodynamic indices and using them to forecast potential severe weather outbreaks.

Forecasting Snow Growth with Bufkit

Dendritic snow grows best at certain temperatures in our atmosphere. These are the light, fluffy flakes that accumulate more significantly than other types.  Therefore, their formation is indicative of the amount of snowfall we will receive. 

Open up the overview in Bufkit. Leave RH plotted. Plot “Omega” and “Snow Growth” from the Contour panel. Snow Growth will appear as a serieso f double purple and double yellow lines. These represent the -14 and -18 degree C isotherms which are optimal for dendritic growth in clouds. When high RH and Maximum Omega occur in the purple and yellow Snow Growth lines maximum snow ratios will occur.  You can un-plot RH if it makes it easier to read. Just realize that it is needed as well or simply click the 70 or 80% tab on the top left to only plot values at this level or greater. You can make sure temperatures are cold enough throughout the profile (Plot Temperature on the Contour tab) and also 2-m Temp under the temperature tab. The picture blow plots omega with snow growth:

Notice the best region for snow growth (inside the yellow lines does not perfectly line up with the maximum region of omega. This will still give a good snow ratio as the plotted blue line indicates. Better than the 10:1 average. Under the Precip tab you can plot the Snow ratio as I have done using three different techniques (just average them if you aren’t sure what they mean like me). You can also plot “Precipitation and Snowfall”. To measure the amount of the precip you have to right click and drag the mouse over the precipitation plots. It will measure in inches of precipitable water. You can use the control tab to change this setting to snow rather than precipitable water. Go from right to left as Time = 0 on the right. 

        Bufkit was off by several inches, as were most meteorologists on this infamous March 09 Snow storm that was predicted on the upper end to drop 12-18 inches in our region. I put it at 9-15 and we received about 6-12 across the state. It was obvious all the forecasts busted within 6 hours of the storm looking at the radar. But the snow ratio techniques in Bufkit give forecasters an advantage in forecasting the amount of snowfall for a given region. They are not perfect and snowfall amounts are difficult so any advantage is a huge addition to a forecaster’s repertoire in this case.