Friday, February 24, 2012

Contrarian Climatic View On Cancelling Winter

Philadelphia's likely ending up with a top five warmest winter when all is said and done -- the devil is in the details whether the city can end up in the #4 spot or in the #5 spot as we're battling it out with the infamous winter of 1997-1998 for that four seed in the tournament of cold monger discontent.   Does a warm winter mean a warm March?   Matt on Wednesday provided modeling analysis that showed this warm winter may yield a warm March; however, past winters have had some rather fickle ends to their cold season.

The above chart is the five warmest winters in Philadelphia (December-February timeframe) and their snowfall, included with a climatic snapshot of their following March and its snowfall.  Note that each of the five warmest winters did pick up at least a trace of snowfall in March...and in the case of 1932 and 1890, March was not only colder than the winter average, both Marches picked up more snow than December through February combined.  March was the coldest month in both of those winters as well as temperatures were a good three to four degrees below the climatic norms for their era.

Looking at a slightly larger data set (ten warmest winters) and using a 140 year temperature "norm" as the baseline (42.0 degrees for all Marches since 1872, 3.5" norm on snow for all Marches since 1885), the average March of the ten warmest winters was 42.7 degrees with 2.4" of snow...slight lean towards milder conditions, nothing too unusual on snowfall although it varies from a trace (three times) to 7.8" in March 1937. Of the ten warmest winters, five had a milder than the 140 year "norm" March and five were colder.  Flip a coin and get a similar result.

Our normal March (1981-2010 standards of normal) has an average temperature of 43.5 degrees and picks up 2.8 inches of snow.  Of course, March being a notoriously fickle and wild month (I love to use the warm "beware the wilds of March" as the weather here can turn from 80 to icy on a near dime...been there, done that a few years ago) the snowfall aspect can vary from year to year...we picked up nine inches of snow three years ago and in the same breath end up with trace amounts in March -- however, we have picked up at least a trace of snow in each March since 1946 except for March 2000.

It's tough to get a March that's both warm and "snowy" -- only five times since 1885 have we yielded both temperatures more than two degrees above the 140 year mean (meaning the temperature that March was 44.0 or warmer) and snowfall of more than four inches and the last time we turned that trick was 1997.   That would make sense as average daily temperatures during the month climb into the 50's for highs and through the 30's for lows...meaning that the average departure on a snow day would be significantly below normal on a snow day and help "cool" things.

All that said, while winter is waning and yes, the odds of big snow are diminishing, it's not quite over given our warm winter status.  We could still score a bootleg late season snowfall or a chilled March (even if modeling data shows contrary) but the odds of the snow will diminish as the month progresses.