DC: Dry Christmas?

Now

Cloudy, seasonably cool. Despite the impressiveness of west-of-the mountains radar echoes inspiring some model maligning in Comment-ville last night, the Washington metro area remains mostly cloudy, but dry, on this pre-holiday-weekend Friday. Temperatures have been very close to seasonable levels, in the low to mid 40s. Highs were: National 46°, Dulles 45°, BWI 43°.

A weak low-pressure area now off the Georgia/South Carolina coast will continue eastward, but easterly wind flow will keep clouds in over us as an area of rain approaches with a cold front from the west by the second half of the weekend.

Tonight and Tomorrow

Mostly cloudy, cool, some drizzle or showers. Lows tonight under mostly cloudy skies will be generally in the mid to upper 30s. Some drizzle or light rain is possible, along with patchy fog. Tomorrow will remain cloudy with some drizzle and a 40% chance of measurable showers. Highs will be 45-49°.

Scroll down for Camden's outlook through the rest of the weekend and into next week, including the vanishing chances for a white Christmas.

Christmas Climatology: Whitest and Wettest

Climatologically, there is only about a 10% chance of 1" or more of snow on the ground on Christmas in Washington. Since 1929, there have been only 8 such occurrences. The largest amount was 7" in 1966, with 5" and 4" in 1962 and 1963, respectively. That was the only time it happened in consecutive years. The most recent was 1989, with 2". The wettest was 1.41" of rain in 1945.

Blast From the Past: Meteorological Mystery

The winner of the virtual T-shirt in yesterday's Meteorological Mystery contest is Augusta Jim, who recognized that the low temperature can occur any time during the day, including right before midnight, so the Dec. 20 record low of 2° and the Dec. 21 record of 1° were in fact one event. You can read his explanation in yesterday's Comments section.

An Honorable Mention for creativity goes to Havoc.

Mediaorology

The dcrtv blog is reporting today that meteorologist Steve Rudin, formerly of channels 9 and 5, was spotted on News Channel 8 this morning.

In other weather media news, dcrtv reports that news station WTOP's audio has replaced classical music as background on WJLA-7's Doug Hill Weather Now digital channel (Comcast 204), and WASH's adult contemporary sound is accompanying WUSA-9's Live Doppler 9000 HD (Comcast 203).

And, in case you missed it, the results are in for the "Battle Of The Local Media Hotties": Channel 4's Chuck Bell beat out Channel 7's Adam Caskey by 1 vote. If we had only known earlier that nerdiness was so hot!

Best wishes for a happy and healthy holiday to all of our site visitors and their families from Update Central.

DC: Developing Cloudiness

Now

Sunny, seasonable. Despite light northwesterly breezes behind a weak cold front, bright sun has helped push temperatures to seasonable levels in the Washington metro area this afternoon. Highs were a little warmer than originally expected: National 49°, Dulles 46°, BWI 47°.

If you didn't get out and enjoy the sunshine today, you'll have to wait several days as more unsettled conditions develop into the weekend.

Tonight and Tomorrow

Increasing clouds, cool. Clouds will increase tonight with lows 32-35° in town ranging down to the upper 20s in the 'burbosphere. Tomorrow will be mostly cloudy with highs 42-47° and a 20% chance of light showers or drizzle developing, especially westward toward the Blue Ridge, as a storm system scoots by to our south.

Scroll down for Josh's outlook through the the weekend and into next week, including the prospects for a white Christmas.

Blast From the Past: Meteorological Mystery

As noted in "Today in Weather History" to the right, Dec. 20, 1942 was the beginning of a cold spell in which the modern Washington low temperature record was set for December, exceeded only by the -7° and -13° on Dec. 30 and 31, respectively, in 1880. The month as a whole went on to be about 5° below the long-term average, but it barely made it into the top 30 coldest Decembers, averaging 6.5° warmer than 1989, which is tied for second place. By comparison, the monthly low in 1989 was 5°.

An excerpt from the early-morning (1:30am) weather map for Dec. 20, 1942 is shown above. The temperature at Washington is 21° with light snow ahead of a low pressure area centered in the Tennessee Valley. Moderate snow extends back into the Midwest, with heavy snow at Chicago. Washington went on to receive 3.7" of snow for the day.

The Meteorological Mystery is: How could the reported low temperature for the day drop to 2°? The original map notes, "This map was not released until 7 days after above date." Was it a ruse to confuse the enemy in war time? Was it an "urban cold island" at then brand-new National Airport? Was it a conspiracy by the World-Wide Global Warming Gang? The first correct and complete explanation wins a CapitalWeather.com T-shirt (IF we ever have any CapitalWeather.com T-shirts!). Members of CapitalWeather.com and professional meteorologists not eligible. The answer will appear in a future PM Update.

DC: Dry Clouds DeCreasing

Now

Cloudy, cold. Following some very light sprinkles and flurries this morning, most of which did not reach the ground, persistent overcast has kept afternoon temperatures from varying more than a degree or so from yesterday's. Daily highs were: National 42°, Dulles 41°, BWI 41°.

More sun but similarly cool temperatures are likely tomorrow after a weak cold frontal passage, with the next precipitation probably not on tap until the weekend.

Tonight and Tomorrow

Decreasing clouds, cold. Clouds will decrease overnight with lows in the low to mid 30s in the city and the mid to upper 20s in the 'burbosphere. Tomorrow will be mostly sunny with highs in the mid 40s.

Scroll down for Dan's outlook through the rest of the week and into the weekend.

Climate Corner

NOAA's National Climatic Data Center issued its preliminary annual report on the climate of 2007 last week. It states that "2007 is on pace to become one of the 10 warmest years for the contiguous U.S., since national records began in 1895 . . ." and that

The global annual temperature for combined land and ocean surfaces for 2007 is expected to be near 58.0°F and would be the fifth warmest since records began in 1880. Some of the largest and most widespread warm anomalies occurred from eastern Europe to central Asia.

Closer to home, unofficial CapitalWeather.com estimates show the 2007 Washington average through the first half of December of 58.9° to be in the top 10% of warmest years, tied with 1953, 1959, and 1999 for 12th place in 137 years of records. Before getting too anthropocentric about that statistic, however, it's important to note that DC's 61.4 square miles are only 0.0017% of the total area of the U.S., which in turn is a small fraction of Earth's overall land area. Total land area itself is only about 30% of the total planetary surface. Interestingly, however, DC's annual average is within about 1° of the worldwide average, making Washington in some strictly symbolic sense representative of the entire planet's temperature.

DC: December Chill, Dueling Climate

Now

Sunny, cold. After reaching morning lows of 27° at National and 20° at Dulles, temperatures are again barely reaching the 40s in the Washington metro area this afternoon, in spite of light southerly breezes. The air remains dry, with dewpoints within a couple of degrees either side of 20°. Last year's record of 74° was clearly in no danger of being broken, with identical highs of 41° at National, Dulles, and BWI.

Some high clouds overrunning the cold high pressure area sliding out to the east have dimmed the weak December sunshine. More clouds will be rolling in tomorrow, but with only a slight chance of precipitation.

CapitalWeather.com chart from NWS data, photo © Kevin Ambrose

Tonight and Tomorrow

Increasing clouds, cold. Lows tonight under partly cloudy skies will be not as cold as last night, 30-34° in the city and 24-28° in 'burbistan. Tomorrow will be mostly cloudy with highs 43-48°.

Scroll down for Matt's outlook through the rest of the week and into the weekend.

Political Science: "Stalling in Bali"

The performance of the U.S. at the just-concluded U.N. climate conference in Bali gets some serious criticism in at least one major newspaper today. The Dot Earth blog at the NYTi had a conference review over the weekend.

Last night's PBS NewsHour featured dueling climate negotiators, past and present. Transcript, podcasts (real and mp3 formats) and streaming video are all available at the show's web site.

Wild, Winning Weekend Winds Winding Down

Now

Sunny, windy, cold. After causing as many as 100,000 customers to lose power and aiding the Redskins' win last night at Giants Stadium, the winds from the weekend storm were gradually diminishing this afternoon. Some gusts were still as high as 30 mph, however, making the temperatures in the upper 30s to low 40s feel even colder, despite bright, sunny skies. By 5pm, National was down to 16 mph, gusting to 23, and Dulles was at 15 mph.

The air is also very dry with dewpoints in the mid teens. The afternoon highs were: National 41°, Dulles 39°, BWI 39°.

With high pressure now dominating virtually the entire country from coast to coast, we can expect several days of more tranquil weather before the next storm system develops later in the week.

Tonight and Tomorrow

Mostly clear, cold. Under clear skies and diminishing winds, lows tonight will be in the upper 20s downtown to the upper teens in the cooler 'burbosphere. Tomorrow will be sunny with a few afternoon clouds and not as cold, highs 44-47°.

Scroll down for Jason's outlook through the rest of the week.

Weekend Storm Review

Although they were a bit iffy on precipitation type, as they usually are in borderline situations, the models were quite accurate in their prediction as much as 7 days ahead that a "bombogenesis" (rapid low pressure area development) would occur along the Atlantic Coast this past weekend. The causes of bombogenesis are rather complex, but these storms gain much of their energy from strong temperature contrasts along the coast in the winter. Probably the most definitive study on the subject was published in 1986 by Prof. Sanders of MIT, who is generally credited with inventing the term.

The surface weather maps above from HPC/NCEP/NWS show that a 1008 mb secondary low near the Outer Banks at 11pm Saturday developed into a 969 mb "bomb" along the Maine coast 24 hours later. Snowfall amounts in northern Maine by early this afternoon ranged as high as 17.2" at the NWS office in Caribou, where the month-to-date total is now triple the long-term average.

Stu Ostro has an excellent illustrated commentary on the storm through Sunday afternoon at The Weather Channel blog.

Fairly Fine Friday Saturday, Sunday: Snowy/Sleety/Soggy/Sloppy?

Now

Drizzle, cold. What had been expected to be a cold rain today has amounted to mostly an annoying light to moderate drizzle in the Washington metro area. By 4:00, National and Dulles had collected a mere 0.02" each, while the jackpot winner BWI had 0.04". Radar shows the bulk of the precipitation from this system from the Mason-Dixon line north and east, with moderate to heavy snow expected across portions of interior southern New York and New England.

The cold part of the forecast has materialized, however, as yesterday's midnight low of 47° is turning into today's high; temperatures are now generally in the upper 30s through the immediate area. After a drier and milder day tomorrow, a strong storm system will affect the Mid Atlantic region over the weekend.

Tonight and Tomorrow

Clearing, chilly. After any residual drizzle or light showers this evening, tonight's lows will be from the mid 30s downtown to the lows 30s in the 'burbosphere under decreasing clouds. Tomorrow will be mostly sunny and milder with highs 51-54°

Scroll down for Josh's outlook through the weekend and into next week.

Weekend Storm Outlook

The answer to the sub-headline's question is, "All of the above, the combination depending on your location and elevation." The models continue to paint a picture of a very vigorous storm emerging out of the Southern Plains on Saturday and bringing a wintry mix to the Mid Atlantic area by Saturday night and into Sunday. Although the latest runs from this morning and early this afternoon show some warming over the DC area, they also show a tendency for a strong secondary low to develop off the coast, which would keep more cold air in place. The complex dynamics of this interaction between the atmosphere at multiple levels and the land/ocean surface are not very well represented even by today's highest resolution models, so there is still plenty of room for variability in the final mix.

At this point, the best that can be said of frozen precipitation forecasts for this system is, "At least some of them will be wrong."

See Josh's Snow Lover's Crystal Ball below for more details on possible scenarios.

Historical Note

It might be just nostalgia kicking in, but the predicted evolution of this system reminds the Updater in some ways of his first experience with Washington snow, 50 years ago this month. The storm of Dec. 4, 1957 brought 11.1" of snow from a strong secondary development off the coast as the main system stalled over the upper Ohio Valley. That is still the second highest daily total ever for the month of December, but the temperature stayed at or above freezing; the daily high and low were 33° and 32°. The high 2 days before? 58°. That storm had a little more northerly course, however, and there was a very strong low over northern Quebec and Labrador.

Here's the forecast for DC and vicinity made the morning of Dec. 4, 1957:
"Today . . . wet snow and some rain ending this morning, clearing and windy this afternoon, highest 40°."

Weather map at 1am Dec. 4, 1957, from NOAA Library.

Cold Rain Gonna Fall Whither Wintry Weekend?

* Winter Weather Advisory in effect 3am to noon for northern and western areas *

Now

Mostly cloudy, damp, milder. Yesterday's high of 51° at midnight was enough to keep the temperature forecast from being a complete bust, but only technically, after the raw 40s in the afternoon. Today is somewhat milder, with afternoon readings mainly in the low and mid 50s, although by noon, both National and Dulles had reached the low 60s. Earlier mist and haze have given way to just mostly overcast skies.

Cooler and drier conditions are on tap for this evening while a storm system brings precipitation into the area from the southwest tomorrow. Temperatures are likely to be cold enough for the threat of frozen precipitation from about the Mason-Dixon line northward during the day, and closer to the metro area if precipitation begins earlier. A Winter Weather Advisory is in effect for the northern and western suburbs from 3am tonight to noon tomorrow for the possibility of sleet or freezing rain. Meanwhile, yet another storm system will be approaching the area over the weekend.

Tonight and Tomorrow

Cooler, rain likely. Lows tonight will be 36-41° under cloudy skies. Tomorrow will be mostly cloudy with a cold rain likely (80% chance) developing from southwest to northeast by late morning, possibly beginning as sleet or freezing rain in the northern and western areas and highs 41-45°.

Scroll down for Dan's outlook through the rest of the week and a weekend peek.

Weekend Storm Outlook

"You might have even seen an accumulation map on the Internet. I think that's nuts at this point." (Howard B on WUSA-TV 9 news at noon today). Bravo, Mr. B! Speculation on the specifics of the storm expected to develop this weekend is just that at midweek: speculation. Nevertheless, the model gnomes in the basement of Momma Nature's Weather Grill are bound and determined to cook up a wintry treat (or trick, depending on your point of view) for the Mid Atlantic region from Saturday into Sunday.

The main U.S. model run this morning continues the trend of putting the DC area in the traditional transition zone between frozen and liquid precipitation. Even though there is a respectable pool of cold air anchored over eastern Canada and New England in the model forecast, temperatures in the immediate DC area would favor mostly rain. The models do have a tendency to underestimate the ability of cold air to persist near the surface in this type of situation, however, so it definitely bears continued watching.

The storm is just now coming into the sights of the shorter-range model known as the NAM, which runs out to 84 hr. (1am Sunday morning). The run from this afternoon's data was being output as this was written. It shows a much less vigorous development, but temperatures cold enough to favor frozen precipitation.

See Dan's Snow Lover's Crystal Ball below for more details on possible scenarios.

Conference Report

The RealClimate blog is reporting near real-time from the Fall Meeting of the American Geophysical Union (AGU) currently underway in San Francisco. The latest post a few hours ago covers the extension of the Antarctic ice core CO₂ record back to 800,000 years ago. As noted on Monday, the lecture "Abrupt Climate Change and Our Future" is available via webcast at 9:15pm Washington time tonight.