Posted January 5, 2009
We kicked off the first work week of the new year today with another morning of low temperatures that exceed our normal daily highs for the date, something we've done a few times recently. Today our low so far has been 55 degrees, while our normal high is 50 and we're headed for a high of 63 or so, if not a little warmer.
Beyond today, just about everything in the forecast becomes a bit of a head-scratcher through mid-week as a form of Cold Air Damming (or "CAD," also called a "wedge" or sometimes in the past a "looping warm front") appears likely to set in late tonight and Tuesday, with big question marks regarding its intensity and duration, and therefore its impacts on our temperatures Tuesday and Wednesday.
A damming pattern of this sort in general is one in which a shallow, cool and relatively dense airmass becomes lodged in place across the Piedmont and parts of the Coastal plain east of the Appalachians. In many cases, warmer, moisture-laden air flows in above the cool air to provide cloud cover and periods of precipitation, and depending on the particulars of the pattern and all the elements that comprise it, the cool air may be locked in for a day or two (sometimes more) becoming a major forecast headache when it comes to the distribution and evolution of clouds, rain and temperatures in its vicinity.
Tuesday could be an interesting day in this respect because we have what appears to be a fairly weak CAD pattern, of a variety called a "hybrid" that involves a relatively weak and transient high pressure center north of us to feed cool air our way at the surface, and a warm front surrounding the cooler air that will advance northward (and perhaps inland across the coastal plain toward the northwest) at a rate that will depend heavily on the intensity and placement of precipitation into the cooler air. Computer models help us to project the onset and evolution of the pattern, but they have some notable difficulties with forecasting some of the details, and especially the erosion of such cool air wedges, and as a result there is a large spread of potential outcomes suggested by models for the next day or two.
Our best estimate for now is that the cool air may be pretty slow to settle into the area tonight, and our official high temperature for Tuesday may be rather mild, in the upper 40s or even low 50s, just after midnight. On our 7-day forecast, you will see 44 (as I write this) which was chosen as about the highest temperature for the Triangle area during the daytime tomorrow when most folks are out and about, as we may see temperatures fall through daybreak and maybe even into mid-day, perhaps into the upper 30s to low 40s by early Tuesday afternoon. On the other hand, there may be a pronounced gradient of temperatures tomorrow, so that some places in the southeast corner of our viewing area end up reaching 60 or better.
Later tomorrow afternoon and tomorrow night, temperatures could run against the grain and climb through the night instead of fall, with some models suggesting we could be near 60 in the Triangle by Wednesday morning. However, it may be difficult to scour out the cooler air that fast and we're thinking temperatures may climb through the 40s to around 50 early Wednesday. Likewise, on Wednesday a strong cold front will sweep in from the west and likely generate a band of showers, and maybe a thunderstorm or two. Some models suggest we'll climb well into the 60s in advance of the front, but sometimes the frontal passage itself is required to eliminate the final vestiges of the cool air wedge left behind from the CAD pattern, and this can hold temperatures down. For now, we're going with 57 for Wednesday's high, but it could certainly change a bit should the cold front move faster or slower than current projections.
Once that front passes, we should see blustery, colder and drier conditions for a couple of days, although even then, there is a potential fly in the ointment that makes the forecast less than totally straightforward. On Thursday, a strong upper level trough will cross the area, and while temperatures at the surface turn colder than previous days, temperatures aloft will turn much colder. This can lead to variably cloudy skies once the sun heats the surface a little and enhances instability, and if there is enough moisture in place an odd sprinkle or two (and maybe an isolated flurry) would not be out of the question. Right now the air looks a little dry for many of either to occur, and a partly cloudy, windy and seasonably cold day looks like the best bet. Something to keep an eye on though, in case there's a bit of lingering low-level moisture around at that time.
To get an idea of how much computer models can vary and what kind of impacts small differences in their formulation and small measurement errors in the observations that initialize them can have, you might be interested in looking over a product from the Penn State weather page that shows NOAA's Short Range Ensemble Forecast, or SREF. SREF uses, in essence, 4 different computer models, with some "perturbations" of initial data in the models intended to simulate realistic atmospheric measurement and analysis errors, for a total of 21 different model runs that can be considered individually, can be eyeballed in a group for a "consensus" solution, or can be averaged all together for a "mean" solution. The Penn State site shows small images of all 21 members, and a larger panel with the averaged results. You can change the time of the model projection by simply mousing over the green times above or below the image. If you step through the next two or three days and looks closely, you'll see an interesting range of possible outcomes for the storm system that's developing. The two addresses below show a pressure pattern with estimated precipitation in the first link and predicted surface temperatures in the second. Sample images of each are shown above.