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Washington’s weekend forecast has taken a soggy turn

A damp day in Washington last month. ( <a href="">John J Young/Flickr</a> )

Washington’s prize for enduring the season’s first heat wave was supposed to be a beautiful weekend with partly sunny skies and highs in the 70s. Not anymore.

The cold front providing relief from the heat is set to stall over the region, bringing cloudy skies and on-and-off rain.

Models differ on exactly how much rain will fall and when, so it’s premature to cancel outdoor plans. Some shower activity seems unavoidable, but we should have dry intervals, too. Our best advice is to stay flexible and have a Plan B for anything outside.

Saturday afternoon into Sunday afternoon appears to be the window when showers are most likely. The occasional rumble of thunder and heavy downpours are possible, too. We have a chance to stay dry Saturday morning, and rain should diminish late Sunday, but this timing is fluid and subject to change.

Models generally suggest a slow-moving area of storminess will develop along the front draped over our region Saturday, drawing moisture northward from the Gulf of Mexico and westward from the Atlantic Ocean.

Both the American and European models project about half an inch to one inch of rain over the weekend as the disturbed weather crawls north. The clouds and showers should hold temperatures down, with highs of 70 to 75 on Saturday and probably only in the 60s on Sunday.

This forecast could still change. If the front positions itself far enough south, then the bulk of the rain may remain to our southeast, inviting skies to brighten. If it shifts more to the northwest, it would be warmer and we’d probably be in more of a hit-or-miss thundershower situation, mainly in the afternoon and evening hours both weekend days. We’ll work to refine the forecast on Friday.

Why are we in this challenging forecast situation?

The problem with cold fronts, especially in the late spring, is that they can run out of steam as they push southward and eastward. They move fastest when the division between the warm air ahead of them and the cold air in their wake is sharp. But when this transition zone becomes more diffuse, they tend to slow and ultimately stall.

So, as forecasters and informed weather consumers, whenever a front enters our vicinity at this time of year, it’s wise not to assume it will make a clean pass. Often, they don’t.