Our weather will turn cooler and more unsettled toward mid-month


March featured plenty of dry days and unseasonable warmth in Pennsylvania. In fact, we finished with our 12th warmest March on record in Harrisburg. April started off with a quick blast of cold air, but the past few days saw a return of dry, warm weather. Highs are likely to approach 70 degrees again Wednesday, but the return of the “Greenland block” will usher another change in our weather soon that could last deep into the month.

Over the next 10 days, The North Atlantic Oscillation (NAO) will tank, which happens when a blocking ridge of high pressure sets up over eastern Canada and southern Greenland. The block keeps most cold and storminess from leaving the eastern US as westerly winds over the mid-latitudes slow down. This is a similar pattern that took place for much of December, January, and early February, the same time winter weather got very active in Pennsylvania. At this point, we’re not expecting snow, but a return to average and below average temperatures is expected, with increased chances for rain through mid-month. Cloud cover will be abundant, and any warm, sunny days (at or above 70 degrees) look few and far between.

The forecast calls for the NAO to sharply dip through mid-month, with a slow recovery after April 15th

The opposite is true during a positive NAO. When low pressure is centered over Greenland, west winds blow faster and keep most of the cold bottled up north of Pennsylvania. This was true in April of 2017, which resulted in Harrisburg’s warmest April on record.

April 2017 featured below normal heights over eastern Canada and Greenland, with well above normal heights over the northeast indicating strong high pressure overhead.

However, much like the last few years, the cold is not so quick to permanently exit this spring. Last year, we saw snow on May 9th, along with our coldest May temperature on record the following morning. Disruptions of the polar vortex can have effects that last months down the road, and it appears once more this year we’re seeing a late-season round of chill setting up. The expectation for this month should be most daily high temperatures in the 60s, but with several days only in the 50s as we approach April 15-20th. One or two more frosty nights are not out of the question before we pull out of the cooler pattern by late-month. Heavy rain events are not expected, but we’ll see plenty of cloudy and showery days that will aid in keeping temperatures down.

A bit of good news though: The below normal heights combined with cloud cover and numerous showers means any widespread severe weather is going to be tempered through mid-month. Much like last year, it may take until May or even June for storms to really get going locally as the warmest and more unstable air is held off.

Historically, during a -NAO in April, there is a moderate correlation with below normal temperatures across much of the continental US

-Meteorologist Adis Juklo

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