Virtual Snow Machine

With the winter winding down, many snow fanboys in the Washington metro area are bitterly disappointed as what was advertised to be a major snowstorm today appears to be seriously underperforming, especially west of the I-95 corridor. If you're feeling cheated by the ongoing snow drought, or if you just want to learn more about the precipitation process, Steve Ackerman and Tom Whittaker of the University of Wisconsin have a very slick Java applet for teaching precipitation types based on the distribution of temperature and humidity. Drag around the temperature (green line) and dew point (yellow line) and see how that affects the type and amount of precipitation, if any, reaching the ground. See if you can produce all 7 possible precipitation types. If you thought precipitation forecasting was just watching radar blobs move around, this tool shows there is much more going on in the third dimension.

Image (click to enlarge): Example from precipitation type applet by University of Wisconsin

Why Don't We Have a Specific Forecast for Inauguration Day?

You may have noticed that the discussion of the Inauguration Day weather outlook, at least so far, has been expressed only in terms of average probabilities for an entire week including the day of interest. If climate predictions can be made with some confidence well into the remainder of the century, why can't we make an accurate weather forecast less than 2 weeks ahead?

Early last year, we posted an analysis of the limits of daily weather forecasting. The upper chart to the right shows that there has been a steady increase in the skill of computer weather models, but there is essentially no value in a forecast beyond about 8 days. In spite of this, some current forecast models are routinely run out as far as 16 days. Is this just a waste of computer resources at taxpayer expense? No, it serves to generate a base of experience from which the models can continue to be improved in the future.

Because the model output is available, however, snow freaks looking for the next school cancellation, commercial weather services looking to make a quick buck, and corporate media attempting to capture more eyeballs for their pop-up, animated, cycle-sucking advertising continue to peddle the notion that a forecast for a specific day at a particular location 2 weeks or more in advance makes sense. This is not only foolish, it's also unprofessional on the part of those meteorologists who enable such activity (despite their disclaimers and caveats), and it confuses a public already being bombarded by misinformation and even deliberate disinformation on the relationships among weather, climate, and policy issues.

As an example, the 4 maps to the right are the computer model predictions for 7 am, January 20, 2009 made on each of the last 4 days (most recent at the top, oldest at the bottom). If you're not familiar with reading weather maps, the solid lines are surface air pressure, the green and blue shaded areas are precipitation, and the dashed red and blue lines are an indirect representation of temperature. You don't need to be a weatherman, however, to see that these are very different patterns.

Images (click to enlarge): Weather model skill as a function of year, from The Emergence of Numerical Weather Prediction, Peter Lynch, Cambridge University Press, 2006. Chart © ECMWF. GFS model output maps for sea level pressure, 1000-500 mb thickness, and precipitation, 7 am, January 20, 2009, from NCEP/NWS/NOAA.

Model Muttering

Note: This post is rated "G", suitable for all audiences; does not contain the weather "s" word.


Weather model skill as a function of year, from The Emergence of Numerical Weather Prediction, Peter Lynch, Cambridge University Press, 2006. Chart © ECMWF. Click image to enlarge.



In the nearly 60 years since the modern era of numerical weather prediction began, many orders of magnitude improvements have been made in both computer technology and the formulation and methods of solution of the equations describing the atmosphere. As a result, huge improvements have been made in the accuracy of daily weather forecasts. As good as the models have become, however, it's important to realize what they can and cannot do.

Click here to read entire post.

Windy Cold At Bat; Wintry Mix On Deck

*Winter Storm Watch in Effect Late Saturday Night Through Sunday Night*

Now

Windy, cold. We're in the late innings, but winter isn't ready to put away the bat just yet. Winds peaking as high as 55 mph overnight have made it feel a lot colder than the near 40° temperatures this afternoon in the Washington metro area, especially after the relatively balmy experience of 3 consecutive days above average (and above freezing). Highs were 41° at National, 39° at Dulles, and 38° at BWI.

A low pressure area just now becoming organized in the central Rockies will deepen and move northeastward toward the Great Lakes, bringing with it the threat of some wintry precipitation in the Mid Atlantic region Sunday.

Tonight and Tomorrow

Clear, less windy, cold. Under clear skies and diminishing winds, lows tonight should be from the low 20s downtown to the mid and upper teens in 'burbland. Tomorrow will be mostly sunny and continued cold, less windy than today with highs 42-45°

For the outlook through the rest of the weekend, including Sunday's potential wintry mix, scroll down to Camden's post below.

Tropical Topics

Winter may be still batting, but tropical season is just over 90 days away. Here at PM Update, we're getting in the mood by starting to read Ivor van Heerden's "The Storm", a scientist's on-the-scene report of the Katrina experience. From the tone of the introduction, it appears that he will be pulling no punches in his analysis of what went on.

Meanwhile, yesterday the National Hurricane Center published their annual post mortem analysis of forecast accuracy for last year's storm season. The good news: Atlantic track forecast accuracy set new records in the range from 12-72 hr., beat each individual dynamical model, and only slightly trailed the consensus models. The bad news: Intensity forecasting continues to be a formidable problem. Intensity accuracy was close to the average of the last 5 years, but skill levels (improvement over average conditions, or climatology and persistence) were "down sharply."

Chart of hurricane track forecast accuracy in nautical miles at 24, 48, 72, 96, and 120 hr., years 1990-2006, from NHC