Friday, April 20, 2012

Weather History: 90's In April

April has featured 33 days of 90 plus temperatures.  It's not unusual for April to get hot...rare, yes, but it isn't unusual in the sense that every few years the region gets a chance to bake with a stretch of heat.  It is rare to get an April heat wave -- it's only occurred five times since 1874 in Philadelphia, last occurring in 2009.

April heat can occur at any time in the month but is more common in the latter half of the month...only four times has the thermometer hit 90 before April 14th...none before April 7th.   This isn't surprising given the trend for the surface to warm further as the month progresses and given extended daylight into late April...but one can get to 90 or close to it at just about any point in April.   The longest April heat wave occurred back in the mid 70's -- four day stretch  that topped out at a then-record for the month of 94 degrees on April 18th (since broken in 2002).

The supporting pattern of April heat is worth pointing out. The graphic on the right shows the mid level ridge/trough anomaly of the atmosphere on those heat wave days going back to 1976.   The ridge placement and axis of strongest ridging is quite similar to that we found this past winter over the East Coast.  In essence, strong ridge east...strong trough west.  No real surprises is interesting to note how close it matches what we went through this past winter.   These type of patterns don't always correlate into summer though!

Heat waves in April aren't always going to equal hot summers.  There are exceptions -- 2002 had 46 90 degree days but most years with an April 90 don't see that many days of 90+...2009, for instance, had just 15 for the entire year and the peak heat day of 93 degrees was the third warmest day for the entire year.  Hot days in April correlate about as well to the summer as fluke snowstorms in October (hint hint) and don't necessarily indicate a trend one way or the other in terms of how the summer pans out.  The graphic above and left shows the summer temperature in years following an April heat wave of three plus 90 degree days in a row.  The tendency is slightly towards warmer than average for the summer compared to current averages -- 2002 was a rather warm summer and helped sway the needle on the warmer side of the fence.  If we threw out 2002, the average summer temperature for the other four years was a rather pedestrian 75.1 degrees, slightly below the 30 year tally of 75.9.  With 2002, the average bumps to 75.6, much closer to the norm.

For those wanting heat this summer (I'm not sure who does), take solace in the fact we missed out on an April heat wave!