Long range forecast hinted at storm potential
Last Thursday, Capital Weather Gang’s winter weather expert Wes Junker correctly noted that snow chances were increasing and higher than normal in the last two weeks of January. He favored light snowfalls in his assessment, but said the pattern could produce a moderate snowstorm noting examples from the past which had strong similarities to Tuesday’s storm.
Storm snuck up on us in the short range
Despite noting a favorable pattern for snow, this storm was not one we saw coming for days. Through Saturday and Sunday morning, we were only forecasting a chance of light snow showers Tuesday. Up to the point, models were only simulating a weak disturbance slipping by in the fast-moving flow. But Sunday afternoon, after viewing the morning model runs which suggested the storm might tap some Atlantic moisture, we posted an update noting the possibility of accumulating snow Tuesday.
Adjustments in 24 hours leading up to the storm were good
By Monday morning, we were forecasting at least an inch or two and, while restraining from putting out a specific accumulation forecast, said 2-4 inches was a reasonable first estimate. Our first accumulation map issued at 12:50 p.m. Monday called for 4-7 inches area-wide. At that point, it was too soon to provide a lot of geographic detail about where the heaviest snow would fall.
Monday night, after reviewing the latest models at 11 p.m., we bumped totals up to 5-8″ around D.C. and 6-10″ in our northern suburbs, but dropped totals in the southern parts of our area (around Fredericksburg) to 2-5″ – noting a slight northern shift in heavy precipitation.
Early Tuesday morning, at 6:25 a.m., after seeing models shift some of the heavier precipitation even farther north and noting temperatures around the city warmer than we expected, we made one final adjustment, decreasing totals in the immediate metro area to 4-7″, and reducing the confidence of 2-5″ in our southern areas to “medium” from “medium-high” and also increasing the probability of a bust.
Nowcasting was steady
It would have been easy given above freezing temperatures Monday morning and lack of accumulation in the initial hours to throw in the towel and start reducing snow totals. But we stuck with our basic story – that temperatures would fall, conditions would worsen as the afternoon wore on, and that snow totals would eventually get to forecast levels, even if at the low end.
I did say southern suburbs looked like they could bust low on snow totals and that was the case in some areas. But a moderate band of snow towards the tail-end of the storm expanded surprisingly far south, and brought many of the southern areas into the predicted 2-5″ range.
Some lessons learned
On Monday, when we realized this was going to be a significant storm for the region, we initially dismissed above freezing temperatures from being an issue that could reduce accumulations as models showed temperatures crashing to below freezing by Tuesday morning and then falling throughout the day. They did the latter, but not the former, and that resulted in less snow accumulation and fewer impacts early in the storm. Unless temperatures are subfreezing in the day prior to the storm, we should simply go in with the expectation that temperatures – especially around the city – will be an issue that could limit accumulations.
Also, even as temperatures fell into the 20s around the region by midday Tuesday, snow still had a hard time accumulating on streets, especially in urban areas. We need to remember that when snow falls during the day time, it needs to fall moderately to heavily to accumulate and have high impacts on travel. Light snow falling in the 20s during the middle of day does not meaningfully accumulate on treated and/or heavily traveled roads. Of course, as the snow intensity picked up later in the afternoon as the sun was going down, snow began to accumulate much more readily throughout the region. And, in our defense, we did say we expected the worst impacts in the late afternoon and early evening.
Precious few winter storms behave exactly as we expect around the region as so many factors and variables influence their evolution. This was not the most challenging forecast we’ve faced, but also not the easiest. Overall, I’d give us a B+. What do you think?