WEATHER NEWS: Chilly and periodically cloudy this evening. Breezy again tomorrow.


We may still see a few peeks of sunshine before sunset today (4:48 p.m.), but overall, skies are mostly cloudy. Afternoon high temperatures are generally in the 40s, with areas northwest of town stuck in the low 40s, and a couple of spots nearing 50 degrees southeast of town. Any breeziness should calm overnight, but westerly 20 mph gusts are likely again tomorrow. The thermometer and wind chill may be about 10 degrees chillier tomorrow.

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Through Tonight: Northwesterly breezes quickly head toward nearly calm conditions, especially after midnight. Cloudiness in the evening may ease a bit with time. Low temperatures bottom out in the mid-20s in the clearest, calmest spots west and north of town. Areas near the Beltway may only drop to the mid-30s.

View the current weather at The Washington Post.

Tomorrow (Sunday): We likely run several degrees chillier than Saturday, with temperatures only reaching the upper 30s to low 40s. With west-northwesterly breezes occasionally gusting to near 25 mph, wind chills may top out in the mid-30s at best. Skies are again partly sunny, with brightest hours likelier in the morning.

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What’s behind the clouds? Blame vorticity.

Want to know what’s producing our clouds today and why we may see a few more day and nighttime moments of clouds until Monday afternoon? This color-coded map showing “vorticity” helps find the reason.

We often get strong positive vorticity on the eastern half of low pressure systems in the Northern Hemisphere. Typically the atmosphere is unstable in and around shortwaves, which can intensify the development (or at least encourage) clouds and precipitation — often found along and to the right of these shortwaves.

Currently, just northwest of us sitting over the Great Lakes, we have a strong upper level low pressure system. Surrounding this low, a series of smaller “shortwave” disturbances swirl around its broader cyclonic (counterclockwise) flow. Notice how the contour line values on the map below (which represent the height of the atmosphere) get lower and lower heading toward the Great Lakes.

Vorticity is the strength of rotation present at a given moment and is represented by the color-coding of these eddies (perturbations) swirling and spinning off from the main low pressure system. Stronger positive values represent counterclockwise rotation and become visible disturbances. These darker colors on the map the more positive, more counterclockwise and cyclonic, the rotation. Weak spin is lighter colors.

You can think of any smaller, darker color patches representing moderate to strong swirl (or disturbance) in the atmosphere. Highly positive values are often associated with storm systems. In this case, we are receiving clouds from these “shortwaves” that are rotating off the large upper-level low over the Great Lakes.

Snowy precipitation from these shortwaves should stay limited to the highest elevations. Garrett County, Md., may get a couple of inches from snow showers tonight, for instance. A light coating of snow can’t be ruled out on other taller peaks in the Appalachians as well.

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