Wednesday, December 28, 2005


Southerly winds ahead of a developing low pressure area in the lower Ohio Valley are bringing mild conditions to most of the East Coast this afternoon. Temperatures are above freezing everywhere except northernmost New England, and the 60s are being observed as far north as northern and western West Virginia. Aided by strong upper level support (For the geeks, that means "positive vorticity advection" and a "negative tilt" to the trough axis), squall lines broke out in early afternoon from southern Ohio through Kentucky and in the western Carolinas.

By 3pm, temperatures at all reporting locations in the Washington DC metro area were in the 50s, except for the river-contaminated readings at National and Quantico. At 4pm, however, the official temperature did make it to 51. Radar is showing some showers scattered from north of Pittsburgh through West Virginia into southwest Virginia.

Tonight and Tomorrow

Latest indications are that any serious showers won't arrive in our area until around midnight tonight, ending by mid afternoon. The heaviest rain should be well to the south, through the eastern Carolinas and far southeast Virginia. There is a 70% chance of rain by morning with low temperatures from 41 to 45. Rain probabilities will decrease to 40% by early afternoon tomorrow with highs around 50.


I've been a scifi fan ever since I discovered the Tom Swift, Jr. series not long after they were originally published. One current author who's been below my radar screen, however, is Kim Stanley Robinson, since I'm still working through the backlog of books acquired in a decade of the annual New Year's sales at the Golden Notebook. Robinson has won multiple Hugo and Nebula awards, which scifi fans will recognize as two of the most prestigious awards in the field. His book, Forty Signs of Rain, published last year, was recommended by Books from the Crypt of North Potomac in the book issue of the Bulletin of the American Meteorological Society in May. It was the first in a trilogy about the effects of climate change right here in River City.

The second book, Fifty Degrees Below, was recently published, and it was reviewed in the WaPo Style section last week. The author was interviewed by the UK Guardian back in September in connection with the publication of the latest book:
"In the wake of a tropical storm, a low-lying American city is drowning. Buildings are demolished and bridges knocked out; tens of thousands of people are without electricity or fresh water; hospitals are bursting at the seams with the sick and the dead."
The city, in this case, is not New Orleans, but Washington DC, which has been flooded at the end of the first book. The premise of the second book is a severe cooling triggered by the shutdown of the Gulf Stream. This is similar to that of "The Day After Tomorrow", but apparently much more scientifically plausible.

Tuesday, December 27, 2005

As December Goes . . .

After yesterday's official high of 48, temperatures are closer to seasonable levels for late December in the Washington DC metro area, mainly in the mid 40s under partly cloudy skies. Winds have been brisk at times, but well below yesterday's levels. Across the country, there is virtually no precipitation east of the Rockies, except for the Dakotas.

Tonight and Tomorrow

Tonight's lows will range from the mid to upper 30s. Tomorrow will see increasing cloudiness and highs around 54.

. . . So Goes January?

Someone suggested in the previous comments that this weather is so boring it should have a web site called "". The one thing that is constant about weather is that it will always change. Since things are a little slow now, I had the chance to do some homework.

Following on to Matt's earlier discussion, I looked at the historical relationship between December and January weather in Washington. The chart on the left is a plot of each year's December average temperature (x-axis) vs. the following January's average (y-axis). As you can see, there is a lot of scatter to the data. The solid sloping line is a regression line, which minimizes the collective distance to all of the plotted points. The algebraic equation for that line is shown in the upper part of the graph, along with R². R is known as the correlation coefficient. It is a measure of how well two sets of data are correlated (in this case, December and January temperatures). The value of R can vary all the way from -1 (perfect, but opposite, correlation) to +1 (directly correlated). A value of 0 indicates the relationship is completely random. In this case, the value of R is 0.4, since R² is 0.16. This shows that the quantities are related, but not very strongly. Without getting too technical about it, R² represents the amount of variability of one set of data (January temperature) which is "explained" by the variability in the other set of data (December temperature). In this case, a little over 16% of the variance is explained---not random, but not terribly strong, either.

In the case of snowfall, all bets are off. The chart on the right shows a plot of total snowfall for December vs. January (trace amounts were considered 0). Notice how the points are much more random than in the temperature chart. In fact, the R² value is much less than 0.01; about ¼ of 1% of the variability of January snowfall is explained by the December amount. This is shown by the regression line being almost completely flat. What relationship does exist is actually negative, indicated by the negative coefficient of x in the regression equation. So, there is a non-zero probability that January snowfall will be low when December's is high, but the statistical reliability of that connection is extremely low.

Seasonal Outlook

Latest seasonal forecast: Click here.

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