Dropping The Snowless Winter of Our (Dis)content

Tomorrow marks the "official" release of the newest average temperatures for Philadelphia from the good folks at NOAA and the National Weather Service. This new data incorporates the 2000's while discarding the 1970's from the mix as average temperatures are calculated on a 30 year basis locally. One of the things you will notice with average daily highs and lows is that they are smoothed out over the course of the year since the average high and low on a given day can change by a couple of degrees. The data smoothing process takes out that "jumpy" nature so the average high doesn't go from say, 80 on June 4th to 84 on June 5th back to 81 on June 6th. Drat that dreaded quality control process...just kidding!

Some of the data, specifically monthly averages, can already be used since it is based on raw data and is generally not subject to smoothing. There are a few trends, some not surprising in the grand scheme of things.

1) The average temperature in Philadelphia on an annual basis has increased by a half degree, from 55.3 to 55.8.   The 1970's did feature several years of coolish temperatures (averages in the 53 or low 54 degree range) which are below the historical 1872-2010 average of 54.8.   Discarding those years will naturally account for some temperature hike...as will the warm stretch in the 2000's.   No single month saw their average temperature drop although December's average looks to remain steady at 37.4.   All other months saw the needle move higher by at least 0.3 degrees, with February, April, and June gaining 0.9 degrees on average.

2) Winter snowfall average has increased from 20.2" to 21.8" -- the 1971-1980 time frame that's being dropped averaged 21.8" while the 00's averaged 26.8" so the increase in snows in this decade -- thank you three winters of 30"+ and December 2010 for the increase.  While the late 70's were relatively snowy, the early parts of the decade were not and the snowless winter of 72-73 is no longer factored in the average snowfall...the snow starved should rejoice knowing the measuring stick of average is a full 1.6" higher.

3) Rainfall averages dropped about a half inch for the year -- winter averages trended a bit more dry while the rest of the year didn't see much major change in the rainfall department.   July is still the wettest month locally, February the driest.  October and November flipped spots as the 'dry' month in the Fall season.

In 2001, the average temperatures were bumped by a full degree as the 1960's were discarded.  This decade's average climb was a half degree.   I'll post more info this weekend about the temperature changes on a daily basis but figured I'd throw the first parcels of data out there for consumption.

Snow Potential Not Quite Over....But...

It's been a rather nice nine week run for cold lovers. Since December 1st, temperatures in the Delaware Valley been average or below with the exception of a small number of days. Included in that run of cold was a five week run of snow where the Delaware Valley just couldn't shake out of the fits of occasional modest to larger snowfall.   That run came to a close this weekend as temperatures are nudging higher...however, winter is not quite over.

Going back to 1981, February has featured 22 years where the thermometer has cracked 60 during the shortest month on the calendar so this upcoming warm wave that is approaching should not be surprising, overly unusual, or a sign of impending climate apocalypse.   Warmth happens in February...sometimes more often than we like to admit.  It's the beauty of our location and geography that we can get such volatility and warmth in the last month of winter.  Heck, it's been in the 70's in four years so early season warmth is not out of the ordinary.

Just because there's warmth doesn't mean there's going to be no more snow.   You can see that in each of those 22 years we've had at least a trace of snow and in 18 of the 22 we've picked up at least a tenth of an inch (measurable).   As a result, snow can happen and still does...even into April in some of these years (2004 to mention one).  However, the potential of big snow seems to be diminishing a fair bit.  The largest snowstorm in these 22 years is nine inches, the March 1st-2nd, 2009 event in Philadelphia, with only 1984 bringing us more than 10" of snow in March and April in the last 30 years.   While the "big snowstorm" could still happen, climatology (longer days, stronger sun, temperatures on average increasing) is suggesting that those odds are closing somewhat.  Warmth is a sign of Spring's impending volatility...and also a sign that those who love their snow may be snow starved for several months to come.

Margins, Margins!

If you don't catch the subtlety of the title, it comes from a movie that runs for 24 hours on December 24th and 25th with regards to a "theme" paper that was turned in by one Red Rider toting youngster.   It's also appropriate for the upcoming Saturday event because it's a marginal event for snow in the Delaware Valley but things could come together for a snowy end thump across parts of the region, especially the farther north one resides.

Computer guidance has been relatively consistent in bringing energy from the Deep South on northeast along a frontal zone and into our region early on Saturday.   While the event will be a mixed bag event as milder air surges northeast aloft and somewhat at the surface, the energy will intensify and develop into a low pressure center over the Mid Atlantic as it continues its northeast trek.   Some of the computer guidance is trying to spit out a rapid intensification and as a result the stronger solution could yield more snow...and snow a bit closer to the region...on Saturday afternoon and evening.

For now, precipitation starts as a mixed bag of snow and freezing rain before temperatures rise above 32 and we transition into a period of plain rain for much of the region except perhaps the Lehigh Valley and the 222 corridor, which may have issues getting above 32.  The Poconos may transition to a mixed bag of sleet and freezing rain for a time during the event as temperatures at the surface stay cold enough to avoid going above 32 degrees.

As that low develops and intensifies, colder air drains back in on the west side of the low track and the intensification allows for enhancement of precipitation, especially on the north/west side of the low track.   The NAM above hints at the development of precipitation across Central Pennsylvania and quickly enhances as it lifts northeast.  

One thing we'll need to watch is the trend in guidance over the next 36 hours -- guidance has trended a bit colder and off runs of the NAM have been trying to blow up the snow a bit more suddenly.   This could catch some folks north/west a bit off guard if the faster development does take hold -- especially since this hasn't been talked up much thanks to the big icing event.   We'll have to see how that interaction between upper level energy coming in from the west and the surface wave play out.  

For now, we're projecting a coating to an inch of snow and slop along I-95, with 1-3" of slop and snow to the north/west of the city.   Around town, we're projecting a brief period of snow and mix before a transition to rain takes hold.   It's also possible we skip snow and go directly to a mixed bag of sleet and freezing rain for a time before temperatures rise above 32.   North and west should see a bit longer period of snow and I've also accounted for the potential that snow could be on the backside of the storm.   Farther north, 3-6" of snow is currently forecast.

Things could change -- perhaps for the snowier to the north of town.