The table of maps below shows the predicted evolution of the 500 mb height (dashed lines) and the 850 mb temperature anomaly (departure from normal, shaded) from yesterday afternoon's GFS and ECMWF model runs. The maps are valid at 24-hour intervals from Tuesday afternoon in the top pair to Friday afternoon in the bottom pair. It's clear that the model agreement is excellent and therefore confidence is high. Remarkably, the models show 850 mb temperatures reaching more than 20 °C above normal over the eastern North Slope - I've created these maps before, but this time I had to adjust the color scale to accommodate the magnitude of the anomaly.
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The good agreement between the models provides a nice opportunity to test my recent hypothesis that the National Weather Service temperature forecasts for Fairbanks are sometimes too conservative at the end of the short-range forecast period when the models agree in predicting a major anomaly. The 850 mb temperature anomalies predicted for the closest gridpoint to Fairbanks are shown below, and the current NWS forecast for the high temperature each day is also shown.
| Date (AKST) | GFS | ECMWF | Mean | NWS High Temperature |
| 3pm Nov 11 | +11.3C | +13.5C | +12.4C | 31F |
| 3pm Nov 12 | +17.2C | +15.3C | +16.2C | 34F |
| 3pm Nov 13 | +15.8C | +17.7C | +16.7C | 34F |
| 3pm Nov 14 | +13.6C | +14.3C | +13.9C | 26F |
We see that the NWS forecast is already calling for well above-normal temperatures, and in fact the forecast doesn't look conservative at all; the normal high temperature in Fairbanks on November 13 is 11 °F. However, the record high temperatures for this time of year are in the 40s, and the upcoming event looks like it intends to set some records. Could the thin (2-inch) snow pack be in danger in Fairbanks? We'll find out soon.
Why not for Alaska? Maybe it's our turn for a mild winter.
ReplyDeleteGary
Question please. How does an extra-tropical Low build atypical 500 mb heights?
ReplyDeleteGary
Great question, Gary. A complete answer would require a thorough revisiting of graduate-level dynamics and a lot of math. One of the basic ideas is that northward transport of low-level warm air occurs east of the low pressure center, owing to the cyclonic circulation. The warm air is less dense than the air it replaces, and so there is an increase in "thickness", or the height difference between two pressure levels, and this causes the 500mb height to rise. The reverse occurs to the west of the low pressure center, of course, causing the trough to intensify (until negative feedbacks kick in).
ReplyDeleteThe next step is that air flowing through the 500mb ridge induces convergence downstream of the ridge axis, and this adds mass to the column and builds the surface anticyclone. If you refer to the upper-level charts, you'll see that the ridge and trough axes are always tilted westward with height; for example, today's upper ridge axis is over southeast Alaska, but the surface high is over northern Alberta.
The set of concepts and equations describing these phenomena is referred to as "quasi-geostrophic theory"; it's a wonderfully successful framework for understanding and predicting large-scale atmospheric circulations in the mid-latitudes.
Excellent, and thank you for the easily understood explanation. I'd heard/seen some of the terms and relationship before when such storms move north, and wondered why the potential for cause and effect. Now Canada and the Lower 48 are exposed to the descending flow of the anticyclone (H).
DeleteMore of these brief explanations would be welcome when possible as a bridge between what's happening or forecast and why. Nothing wrong with the challenge of learning something new beyond the basic forecast.
Gary