Frost advisory in Allegany Co.

Pretty sure this is the first mention of frost in the NWS forecasts for Maryland this season. They've issued a frost advisory for the high country out in Allegany County, including the cities of Frostburg

and Cumberland, where overnight lows are headed for the 30s.

We won't see anything like that down here in the lowlands. But after a patch of showers passes by later today, we should see some rapid clearing as high pressure moves into the area. The clear skies and diminishing winds open us up for some radiational cooling tonight. And that will send our overnight temperatures into the 40s, for what will likely be the coldest night of the season so far. 

So by Thursday morning it will really feel like October.

It's good news for stargazers, of course. Clear, cool, still skies are great for looking out at the universe. The moon has now moved past Jupiter, but the big planet is still big and bright in the southeast in the evening. With a good pair of binoculars, you should be able to make out as many as four of the Galilean moons, lined up in a row on either side of the planet's disk.

If you're up before dawn Thursday, look due east for a view of bright Venus low on the horizon. This is also a good time to catch a glimpse of elusive Mercury, rising just below (and just after) Venus. Mercury is followed by Saturn a short while later, but the brightening dawn will likely begin to fade the show. For a sky map, click here, and scroll down to "This Week's Planet Roundup."

Mercury, as you may have read, was visited Tuesday evening by NASA's Messenger spacecraft, which flew within 142 miles of the tiny planet just before 6 p.m. Scientists at the Johns Hopkins University Applied Physics Lab have been downloading science data from the flyby this morning. Messenger is due to enter orbit around the planet in March 2011.   

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