The beautiful holiday weekend weather will come to an end just in time for many to return to work on Tuesday. The culprit for the changeable skies is a backdoor cold front. These fronts are typical in spring (yes, it’s still spring despite the “unofficial start to summer” noted by Memorial Day).
Most cold fronts slide in from the northwest, west or southwest. A few sweep from south to north along Interstate 95 but still have their roots to the west of the Interstate 95 corridor.
A rather unique front, although well-documented in meteorology text books, will slide through the Washington-Baltimore area Tuesday and Wednesday with its roots in New England. Called a backdoor cold front, it will edge south and west from the North Atlantic to the Interstate 95 corridor before moving west towards the Shenandoah Valley.
The reason for this movement is due to a strong high pressure centered in northwestern Saskatchewan that will move east into Quebec Monday and then slide south and west through New England Tuesday through Wednesday. As it does so, warmer, more humid air will slide into the Mid-Atlantic just as a cooler, marine air mass (that will still be high in humidity) settles south.
Climatologically, backdoor cold fronts don’t tend to produce severe thunderstorms (characterized by wind gusts in excess of 57 mph and hail larger than 1” in diameter) in the Mid-Atlantic due to the lack of upper-level wind support along and ahead of the front. However, due to the influx of Atlantic moisture with the convergence of the air masses, heavy downpours usually occur. They are notorious for dropping temperatures up to 30 degrees in 12 hours though.
Looking ahead, the front will slide into eastern Pennsylvania by Tuesday afternoon and then south of the nation’s capital by Wednesday afternoon. This will translate to showers and storms, a few with downpours on both days.
As is typical behind a backdoor cold front, temperatures cool markedly but the humidity can remain high. A low-level easterly wind will draw in Atlantic moisture to the region, keeping low clouds intact with a few spots of drizzle, especially closer to the Bay and east of the Blue Ridge on Thursday. Highs on Thursday may stay in the 60s in Washington and actually be warmer in the mountains far west of town, away from the cooler Atlantic flow!
The pattern will break on Friday as drier air pushes in from the west, so more sunshine will emerge.