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Wednesday, April 10, 2013
It snowed on Thursday. Then Roanoke hit 80 on Monday and 87 on Tuesday.
Weird, huh? Well, yes and no.
The unusual part is that it snowed enough to accumulate in April, the first time in 20 years at Roanoke (1 inch officially, up to 3 inches in parts of the Roanoke Valley) and the heaviest in 35 years at Blacksburg (5.4 inches officially, up to 7 inches reported locally).
The quick warmup to above normal temperatures afterward, however, seems to be what usually happens when it snows so late.
In fact, staying cold enough after this year's Palm Sunday snow for it to hang around on the ground a few days was actually more unusual than shooting up to 80 so quickly after Thursday's snow.
A quick look back shows that late March and early April accumulating snows in Roanoke are often followed by 80-degree high temperatures within a week even though mid-60s are the norm for early April.
Recent years have shown upward bursts in temperature after significant snowfall even in late February and early March.
Last year's Feb. 19 snow of 5.5 inches — the only widespread significant snowfall of the second warmest winter on record — gave way to mid-70s highs within five days, more than 20 degrees above normal.
The March 1, 2009, snow that measured 3.5 inches in Roanoke (several inches in many surrounding areas) led to a low of 13 on March 3 — which flipped to a high of 83 by March 7, extraordinarily warm for so early in the year.
Given the seasonal warming as the sun angle heightens and hours of sunshine increase, quickly rebounding temperatures and rapid melting should be expected after almost any spring snow.
But that doesn't fully explain why it is so common for temperatures to bounce well past seasonal norms shortly after spring snow.
One thing that could be in play is that the temperature contrasts present near changes from a colder pattern to a warmer one often help fuel the kind of strong storm systems that can lead to unseasonable snow.
Even with relatively low temperatures present, getting snow to reach the surface in ample amounts to accumulate after mid-March often requires a storm system with very strong lift.
That leads to heavy precipitation pouring through colder, drier air beneath — evaporational cooling, the big key to Thursday's snow — or dynamic cooling as the dropping air pressure lowers the freezing level.
Whatever the case, the moisture and nitrogen dumped into the ground by the unusual twin spring snows of 2013, combined with summerlike high temperatures continuing into a third day today, will help paint green what had been white not many days ago.
Weather Journal appears on Wednesdays.
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