Forecast: Warming Up, Slowly but Surely

Interesting that DCA is 10 degrees warmer than IAD and 12 degrees warmer than BWI. Is that the effect of the river?

@bjackrian

Yes...and the urban heat island effect.

why would the river make it warmer? is it like a heat sink - warming during the day etc... is dca colder in the day?

i know in summer places near rivers/streams etc are cooler, urban heat island aside, but i thought that had more to do with evaporation. i suppose in summer, water temps are almost always lower than air temps so there's a simple radiative effect. what is the temp of the potomac river these days?

@walter-in-fallschurch

Unless the river is frozen and covered by ice (which it is not), it will tend to have a warming effect on clear nights in winter (water temp is above freezing and water is a dark heat absorbing surface--as opposed to ice which is white and reflective). Also, the low elevation plays a role in addition to the heat island effect.

is the summer daytime cooling effect similar, or is it primarily due to evaporation being endothermic?

Tuesday morning was definitely colder up on the Blue Ridge than today. Tuesday morning we got down to 2.7F, but today we were above 10.

I don't fully understand the impact on elevation. I know that some grape growers like east facing mountain slopes because they don't tend to get as much frost. At 1900 ft, we're often around 5 degrees colder than the bottom of the mountain. But some mornings, cold air apparently likes to settle in the low areas (cold air sinks and warm air rises).

is it low pressure at high altitude causing low temps?

@walter-in-fallschurch

Yes, if you have a wind off (easterly component) the river in the summer, it will produce a cooling effect.

Also, re: DCA temp -- The wind never did go calm at National last night or early this morning. So, we end up with an unimpressive low of 20. Meanwhile, Dulles tanked all the way down to 8 degrees.

Generally speaking it will be colder at higher elevations due to lower pressure, but there can be inversions -- particularly on clear, calm nights, where it's colder in valleys - because cold air sinks.

I was amazed at how much snow melted yesterday, despite the sub-freezing temps. I'd say about 20 percent of my yard is bare ground now. At this rate, it will be all be gone by the weekend. I think the piles in parking lots, etc., will last longer, however.

CapWeatherGang, thanks for the simple explanation.

One other phenomenon I've noticed more than once, including yesterday morning, is that sometimes the low temperature will be recorded after dawn. It seems counterintuitive that it would get a little colder while it's already getting light outside.

I think the ground continues to radiate/lose heat until the sun hits it more directly?

DCA also reported clouds much of the night in some form which probably slowed radiational cooling. IAD did not report as much in the way of this... though not sure if that's just a reporting difference as they are relatively close.

steske-

Noticed the same in my yard. Never underestimate the power of the March sun!

Thankfully, all of this stuff will be gone by the weekend. Can't wait for the warm up coming.

Preliminary 3-7 day charts show a few changes from yesterday. Cold front moves through late Sunday; warm front is south of us Tues.-Wed. Main forecast change is warmer Sunday; possible unsettled weather next week.

Note for steske: Two factors are probably in play: The March sun is warming ground locally and evaporation from solid to gas by sublimation is occurring even at temperatures below freezing when humidities are low. In addition warmer ground could be melting some snow from the bottom up. I've noticed the snow disappearing in Arlington, too. When snow sublimes, less black ice forms at night.