Two Brushes with Old Man Winter...

After lots of concern that winter might never appear given the long stretch of unseasonably warm weather in December and early January, we finally saw a shift in the mean position of a trough in the polar jet stream last week, one that made it easier for cold airmasses to make their way out of Canada and deeper into the central and eastern U.S. than they had before. It didn't take long at all before this shift made an impact here in central Carolina, with two rounds of wintry weather in the past five days, although for snow lovers both episodes were probably a bit disappointing.

In both cases, we ended up with a progression of weather elements that followed the forecasts we had out rather closely, but in each case with a small deviation (in the overall scheme of things) that illustrated how much impact relatively minor errors in conditions projected by forecast models, and the associated interpretations and modifications by humans, can have in terms of the actual observed weather. In each case, we had a somewhat classic configuration involving a cold center of high pressure northeast of our state, with a shallow ridge of high pressure and associated cold, dry layer near the surface trapped along the east side of the Appalachian mountain, a pattern known as cold air damming. In each case, air flowing into the region from the south, southeast and southwest above that cold layer brought a fairly rapid warming trend at altitudes of about 1500 - 6000 feet above the ground, and our forecast puzzle was to determine the evolving shape of the vertical temperature and moisture profile, and what type of precipitation it would support at the surface as the events progressed.

One problem in all this is that the shape of that profile can affect the timing and intensity of precipitation by altering the rate of vertical motion of moist air flowing into the area and the amount of precipitation that is able to reach the surface without evaporating first, but the timing and intensity of precipitation can also impact the shape of the profile through processes like evaporative cooling and cooling due to the melting of snowflakes, or if an especially warm layer develops aloft, raindrops can absorb some of that heat and warm some of the colder air near the surface. We have computer models that are designed to account for as many of these processes as possible, but the models themselves are built upon good but imperfect mathematical descriptions of the atmosphere, and on top of that they depend for their initial conditions upon thousands of temperature, wind, humidity, and pressure observations taken at widely separated locations (leaving a lot of air unmeasured in between) and taken with instruments that have some level of inherent measurement error, not to mention a few that have drifted from optimum calibration. Given all of that, I'm often amazed at how close to reality the model forecasts actually are a good percentage of the time.

On Thursday morning, models indicated a slowly developing precipitation shield encroaching from the south, with rather light precipitation rates early in the day and temperature profiles just barely too warm for snow, with a better potential for mixed sleet and light rain. However, in the early stages of the event, several patches of moderate intensity precipitation developed, and this increased the cooling rate and modification of the low-level cold airmass though evaporative and melt-driven cooling. The warmer portions of the temperature profiles (about 2-4000 feet up) in these areas fell to a couple degrees below the forecast levels, and suddenly we had light to moderate snow in many locations, enough to leave anything from a dusting to near two inches of snow over parts of the Triangle and north Central NC. Snowfall rates were even strong enough for an hour or so to overcome very warm soil temperatures and allow for a slushy buildup on some roads, something that never would have occurred with a sleet/rain mixture. As soon as the snow lightened up and turned back to a sleet/rain mix, heat welling up from the lower portions of the road and the ground below quickly melted the snow and slush, and we returned to simply wet roadways. From that point, temperatures aloft continued to warm as originally forecast, and we progressed from a sleet/rain mix to all rain by midday.

On Sunday, we had a slightly different though related setup, with most of the moisture approaching in a plume extending from the Gulf coast into the mid-Mississippi Valley and sliding toward NC from the west. Again, a wedge of cold, dry high pressure air waited on the east side of the Appalachians, this time with a slight weaker high pressure system attempting to lock the cold air in. Computer models again indicated that the chance for snow would be minimal, with our concerns more along the lines of sleet and freezing rain, mainly over northern parts of our area. In general, this scenario played out nicely, but with one significant deviation. Models indicated that the depth of dry air near the surface would cause measurable precipitation to hold off in our area until around 2-4 pm Sunday afternoon (some were even later than that). Given that timing, and the fact that we had a few breaks in the clouds as the day began, most of us forecast high temperatures around 40 degrees or a little above, with an expectation that temperatures would drop rapidly late in the day as evaporative cooling kicked in, making some freezing rain a possibility by late afternoon and evening. As it turned out, a plume of precipitation, very scattered at first but later filling in and broadening into a band 20-40 miles wide, formed during the mid to late morning hours out ahead of the main shield that was expected at mid-afternoon.

This band caused precipitation to overcome the dry air at the surface about 3-4 hours earlier than forecast over central NC. It didn't have a big impact on the wintry aspects of the precipitation, as we still saw the expected rain/sleet mix, and later some freezing rain from about the Triangle north and west, but it did lock down our temperatures by late morning, so that we never got past 35 degrees at the RDU airport, notably short of our forecast high of 41. In the end, the late afternoon and evening saw a thin glaze of ice form on elevated surfaces, along and northwest of a line from about Warren county in the northeast to Chatham county in the southwest. Soil temperatures remained in the upper 30s to around 40, so there were no major travel problems associated with the ice, and since it usually takes about 2 tenths of inch of ice to begin causing isolated power problems, about 3 tenths for scattered outages, and around a half inch for widespread failures, we thankfully had few if any problems in that respect.

You can find text reports detailing ice and snow reports from around the region for these two events at

http://www.srh.noaa.gov/fwd/productviewnation.php?pil=RAHPNSRAH&version=0

Just use the "Previous Versions" link there to step backward in time until you find the rundown that covers the event you're interested in.