Saturday, April 1, 2017

Certain as the Sun...

1 comment:
"Certain as the Sun, rising in the East..."

Sunrise at the Neuschwanstein Castle in Bavaria, Germany

8:00 AM 4/1/17 Storms come. Forecasts go. Seasons change, then we look back on the Winter that was, and wonder amid a tale as old as time, "will next year be different?

While we cannot promise unbelievable skies on a magic carpet ride, it can be said the seasons ahead are revealing early clues about what dreams may come. 

Summary of this report:
  • Lessons learned from the March 2017 failed Big Kahuna, and what it tells us about storm forecasting in La Nina Winters.
  • Growing potential of El Nino returning by this summer, with implications for winter storm forecasting in Winter 2017-18. 

"Something there, that wasn't there before." 

Part 1 - Lessons Learned from March 2017 Big Kahuna

When evaluating present day patterns coming off last month's semi-failed Big Kahuna, we think Mrs. Potts best captures the theme with that one line.

A) What we saw in the data:
  • 1) Wavering lines. Multi-day trend in the models showing the rain/snow line from this storm would waver back and forth

  • 2) Big snow, then no. General agreement in the models of significant snow for most areas, until about 24 hours before, when the rain/snow line wavering scenario returned.  
  • 3) Late stage changes in the short range. Late Monday night into early Tuesday, a few hours before the infamous sleet beast exploded across the region, the NOAA High Resolution Rapid Refresh model (the HRRR). This model has a consistent record of accuracy, and in past storms, failure to consider the short range data had major impact on forecast accuracy. Around 10:30 PM, this model began showing a more defined westward movement of sleet than had been previously depicted. 
  • As shown above, the HRRR projected sleet initially staying south of the PA line, and lasting 3-6 hours. By 3 AM Tuesday, the situation had changed drastically, with sleet forecast to continue through 10 AM and extend well north of I-70 into southern & southeast PA. In some places, sleet began 2 hours earlier than forecast and before radar returns had even identified a change in precipitation type. By the time many readers awoke, expectations for 6-12" had been dashed by the window pinging of unwelcome sleet.
B) What we should have seen: Climate indicators and past storms as prologue. Having studied the intricacies of winters influenced by El Nino & La Nina, and knowing the patterns that usually play out when a strong signal exists from either phenomenon, we should have seen the evidence refuting a large snowfall accumulation in the southern Mid-Atlantic (south of the PA line) from this data.
  • 1) Are big Mid-Atlantic snowstorms likely during moderate La Nina years? Answer: Not really, backed by analysis of 50+ years of Nino/Nina data correlated with winter storm outcomes in the Mid-Atlantic in those years. An excellent overview of the data is published at this link to a study by the Sterling VA NWS Office.
  • 2) Have major snow forecast busts occurred in La Nina winters? Yes. 
March 6, 2013 was the most significant bust in Foot's Forecast team history, as calls for 6-8" of snow across the region by us, the NWS and most other media outlets resulted in virtually nothing. 
March 5, 2001, though predating FF, was perhaps the single greatest storm forecast bust in the modern digital era. Official and private forecasters alike called for up to 24-36" from Washington to Boston. The result: Schools and governments closed a day early to prepare, then the storm shifted east and the metro areas, including Baltimore and Philly received 1-3" of slushy wet snow.  
What we will do different next time? 
  • We will simply ask the question: Did a similar storm occur in a previous winter under conditions that resemble present day? If not, why not and what happened.
Part 2 - Next Winter: Early signs & signals

  • Analysis in progress, article coming soon. Until then, enjoy the sunrise and the welcome arrival of April heralding our true change into the Spring season.
Sunrise over Stonehenge