Yesterday, we noted that a southern stream system with lots of moisture was likely to bring a protracted period of precipitation that could start as early as late Sunday afternoon and could last well into Monday night. Not much has changed.
Most models are predicting some type of mixed precipitation event. However, they still differ significantly on the evolution and on how much of the precipitation will fall as rain, freezing, rain, sleet and snow and on how much of an impact the storm might have on the area. The track of the initial surface wave will have a big impact on how much rain we get versus other types of precipitation and on how quickly the low level cold air settles southward into the area.
Today’s models – generally speaking – have shifted to an icier/snowier look compared to yesterday, when some suggested the possibility of a mostly rain event. We remain close enough to the line that a mostly rain event is still possible but we now need to come to grips with the possibility another significant winter weather event could be in the cards.
The morning’s NAM tracks the first surface low almost right over us which would lead to the precipitation starting as rain. It’s in the same camp as last night’s European model which tracks the initial low far enough north so that the majority of the precipitation would fall as rain before the precipitation eventually transitioned to freezing rain, sleet and then a brief period of snow. Yesterday’s European model and the NAM suggest the event would not have a major impact on the area roads.
On the other hand, today’s European model joined the GFS and Canadian models which are much more bullish on the storm’s winter potential. They predict an extended period of frozen precipitation in some form or another with the precipitation ending as snow. The bottom line is we’re again very near the rain/snow line. Because there is so much cold air around, the north to south variation of temperatures great. Times when the temperature gradients are larger than normal make forecasting precipitation type a nightmare.
This morning’s 06Z GEFS ensemble member forecasts of the temperatures at Dulles Airport (below) give an idea of how difficult the precipitation type evolution is likely to be with this storm.
Each line represents a model forecast where the physics are identical but the initial conditions have been tweaked a bit. However, the black and orange lines are not individual members but are the mean and median. The temperatures near the ground (2 meters, top image) fall much quicker than around 5,000 feet aloft (850 mb temperatures) By 7 p.m. Sunday night (00 GMT Monday, March 3) the ensemble mean temperature near the surface has fallen below freezing. However, a few members remain well above freezing reflecting the idea on those runs that the surface low tracks over us or just to our north putting us on the warm side of the storm. However, most members have the low just to our south and the shallow cold air feeds into the area dropping the temperatures to below freezing.
The ensemble forecasts at around 5,000 ft (see above) are even more ambiguous in terms of precipitation type. Note that on 7 p.m. Sunday night (00 GMT Monday, March 3) the forecast temperatures range from around -5 C to around +6 C. The former would probably support snow while the latter would definitely result in rain. However, both the median (orange line) and mean (black) of all members is around +1 C. Each would favor sleet.
With time, the 2 meter and surface temperatures drop well below freezing while the temperatures aloft struggle to fall below freezing suggestive of the dreaded mix scenario.
The GEFS ensemble precipitation amount and type plots (below) argue strongly for a mix event probably ending as snow. Remember the steeper the slope of the line, the heavier the precipitation. Almost all eventually turn the precipitation to snow but they differ significantly on how quickly the transition might occur.
Almost all the GEFS members indicate the precipitation will start late Sunday afternoon or during Sunday evening. Given the temperature profiles indicated by the median and mean of the ensemble members, it’s not surprising that most members predict a significant period of sleet.
Sleet is somewhat of a wild card as it occurs with a very narrow range of temperatures within the warm layer in the range of 1 to 3 C range. Warmer temperatures within that layer usually leads to rain or freezing rain. As long as the surface temperatures remain below freezing, you’re always flirting with transitioning to freezing rain or snow .
Sleet will accumulate on roads this time of year but freezing rain is a tougher call as the sun is now getting strong so temperature need to fall well below freezing to have much of an impact on the roads during the daylight hours. How much of an impact the storm has on roads will depend heavily on how quickly the temperatures drop and on which of the model camps ends up being more right.
The bottom line is that this does not look like a major snowstorm (although the European hints at the possibility for our northwest suburbs) but does have the potential to cause problems if the colder model solutions end up right. What we do know is there will be lots of precipitation that will probably start as rain sometime late Sunday afternoon or early Sunday evening.
This is another one of those events where the biggest threat of winter weather will be north and west of the city. However, most of us will probably eventually see at least a brief period of winter weather. How much and how big an impact the storm might have across the area is the giant question mark. Once the storm exits the region, another jolt of cold air will arrive.