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Maybe, Just Maybe, a White Christmas
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20 December 2016   Dr. Curtis James

We just might have a white Christmas this year.

The figure above indicates the forecasted high in orange (with error bar denoting the range of ensemble forecast members), average high for this time of year in orange (dashed line), forecasted low in blue (with error bar), average low this time of year in blue (dashed line), and forecasted wind speed in purple. Note that the wind speed forecast is sustained wind (not gusts), and it is based on only one model (not an ensemble).

Weather Discussion:

Last Friday night at around 10 pm, wind gusts associated with the passage of the cold front rose perhaps 20 mph or more above forecasted values around the Prescott area. The National Weather Service was forecasting maximum gusts for prescott of 48 mph, and I had no reason to believe that the gusts would be any stronger. However, it turned out that that atmosphere became more unstable than previously forecast that evening and there were scattered heavy rain showers and isolated weak thunderstorms across Yavapai County. This instability promoted deeper mixing in the atmosphere, and I suspect that it led to the unusually strong wind gusts, due to the downward mixing of stronger winds from higher up in the atmosphere down to the surface. Peak wind gusts up to 64 mph at the airport (at 10:16 pm) and up to 78 mph on the roof of the ERAU Academic Complex were observed. There was considerable impact from the event.

To my knowledge, this was the second strongest wind event ever experienced at the Prescott Airport since peak winds have been recorded (starting in July 1996). The record event was 74 mph at 10:23 pm on Dec. 7, 2009. Ironically, on the 2009 event, the peak wind gust on the roof of our building was also 78 mph, just like last Friday’s event. Nevertheless, the damage back in 2009 was probably more extensive. The third place holder was 61 mph on July 3, 2002.

For this week, a complicated weather pattern is setting up and there is considerable spread in the forecasts. A weak upper-level low pressure system is becoming cutoff from the jet stream and will circulate in a counter-clockwise fashion along the west coast of the Baja of California for a few days. This circulation will help to usher moist and warmer air northward towards Arizona. Some clouds will move into Arizona on Thursday, but precipitation is not expected until Friday when a colder trough dives down the West Coast. This second trough will likely tap into some of the moist air from the previous system. The timing of the second system moving through Arizona and exact positioning are still quite uncertain…thus making rain/snow probabilities and forecast temperatures uncertain (as shown by the large error bars on the forecasted temperatures and low precipitation probabilities in the attached forecast graphic).

The second trough will bring a surface cold front across the state bringing rain showers changing to snow showers. The most likely timing of the precipitation is Friday night – Saturday night, but there is fairly good chance of having snow on the ground on Christmas Day, and a slight possibility of falling snow on Christmas. Having falling snow on Christmas happens less than 10% of the time…so if it happens…enjoy it!

C. James

 

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Met Mail is an unofficial weather discussion and forecast transmitted once or twice a week via e-mail by the Embry-Riddle Department of Meteorology (http://meteo.pr.erau.edu/). Embry-Riddle offers an undergraduate bachelor-of-science degree program in Applied Meteorology. Please spread the word to all potential qualified candidates!

Further Information:

ERAU Applied Meteorology degree program

Official National Weather Service forecast

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