Thursday, December 13, 2007

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


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.

Wednesday, December 12, 2007

Cold Rain Gonna Fall
Whither Wintry Weekend?

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


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.

Monday, December 10, 2007

DC: Dreary, Chilly


Cloudy, damp, cool. While temperatures in the Washington metro area moderated somewhat from yesterday's damp chill, within about 100 miles, spring-like conditions prevailed from central Virginia southward. Daily temperature records were set at Lynchburg, Roanoke, Blacksburg, Richmond, Norfolk, and Wallops Island.

Temperatures in the low and mid 50s were accompanied by dewpoints in the mid and upper 40s, but despite the humidity, mist and haze were much more prevalent than rain showers, which were mainly light. Highs were: National 54°, Dulles 48°, BWI 47°.

Clouds and humidity will persist into tomorrow with at least a chance of some showers. The models frequently underestimate the tendency of this type of cold air wedge to hang on in the area, but some warmer air is likely to make it this far northward in the next couple of days before everything gets swept eastward toward the end of the week.

Real-time Mesoscale Analysis (RTMA) from NWS/NCEP at noon today shows a 40° temperature range in about 100 miles from near DC to central Virginia.

Tonight and Tomorrow

Mostly cloudy, chance of showers, possibly milder. There's a 30% chance of light showers tonight with lows 41-45°. Some patchy fog is possible in the early morning hours. Tomorrow will be mostly cloudy with a 30% chance of showers, but some sun is possible, especially in the afternoon. Highs will be anywhere from 52-61°, depending on how much of the warm air can move northward.

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

Tropical Topics

The National Hurricane Center issued an updated Special Tropical Disturbance Statement at 11 this morning on the low pressure area about 100 miles east of Puerto Rico. Although a tropical or subtropical cyclone could still form in the next day or so, upper level winds are now expected to become less favorable for development. Nevertheless, near gale-force winds and heavy rains are expected for Puerto Rico, the Virgin Islands, and Hispaniola.

Conference Report

The Fall Meeting of the American Geophysical Union (AGU) got underway today in San Francisco. Over 15,000 geophysicists from all over the world are expected to attend sessions in all disciplines of earth and space science. Several major lectures are available as live webcasts, including "Abrupt Climate Change and Our Future" at 9:15pm Washington time on Wednesday.

Seasonal Outlook

Latest seasonal forecast: Click here.

Latest 3-month temperature outlook from Climate Prediction Center/NWS/NOAA.