The recent winter storm will definitely be a memorable on, just probably not in the way we normally think about “memorable storms.” The storm was an everything storm – a snow storm, sleet storm, ice storm, and rain storm – just depending on where you live. There was a lot of uncertainty leading up to this event, and every forecaster in the area stressed this point – and that totals were very uncertain. Let’s take a look at how the forecast did, and look at why the errors occurred.
I typically issue three forecasts leading up to winter storms – my preliminary forecast, first call forecast, and final call forecast. The final call is my official forecast for the event, and what is/will be assessed.
What my forecast got correct:
Timing: My forecast nailed the timing of this event at the onset, and the precipitation changeover. I said snow would move in between 6 and 9 PM – and that was right on the money. I also expected sleet to start making an appearance between 10PM and Midnight from SE to NW – which was nearly perfect.
I also expected the heaviest of the precipitation to occur between midnight and 6 AM – which was the case. Some heavier precipitation occurred during the morning as well, but the bulk of the activity was during the time frame I had expected.
Temperatures: I fully expected everyone except lower southern Maryland to stay at or below freezing during the entire event – and that’s exactly what happened. I highlighted in my final call forecast the areas east of I-95 that are always colder than the models show, and that verified once again. This is a rule of thumb for me – and its why other forecasters missed the Freezing Rain.
Freezing Rain: Most forecasters missed the freezing rain. To my knowledge, I was really the only one highlighting the potential for freezing rain, especially east of I-95 in Charles, PG, and Anne Arundel counties where others were saying rain. However, I do wish I had put more emphasis on this, and say that ice accretion could reach levels that could lead to tree damage/power outages.
Snow Forecast: I’m giving my snow forecast partial credit. I took the <1″ & 1″ – 3″ bands too far SE. Because of the sleet, the 6″ – 9″, and 10″ – 15″ regions were also too far east. I also underestimated the snow in Allegany County. Other than that, the snow forecast was actually really good.
Back End Snow: I had expected the cold air to wrap behind the low and erase the warm layer responsible for the sleet/rain. The back-end snow did occur, however, I was expecting the return to snow to happen during the mid morning – which it took a little longer to occur. The snow also was focused more north than I had anticipated too.
What my forecast missed:
The Sleet: I, and other forecasters, never expected the sleet to push as far west as it did. The sleet pushed to the Blue Ridge, and areas I expected to stay all snow saw a significant duration of sleet which cut down snow totals.
Overall, I give the forecast an A-.
Post-Storm Analysis: Why the Sleet was a bigger issue
In order to understand why the sleet was a big issue, we need to understand the warm layer that existed in the low levels of the atmosphere and why that was the case.
Take a look at the image below. This is what’s called a sounding, which is a vertical profile of the atmosphere. I’ve highlighted a few things. The Green Line is the Temperature/Dew Point of the air in the atmosphere. The light blue line is the freezing line. If the green line is to the left of the blue line, the air is below freezing, to the right, above freezing. I’ve highlighted the warm layer, about 3500ft-6500ft above the ground (estimated). Notice how the green line goes to the right of the blue. That melts the snow, and the layer is large enough complete melting occurs. Notice just below the deep layer that returns below freezing. That’s where the refreezing into sleet occurs. Also notice, the winds above the surface are southerly – bringing up the warm air. The surface winds were NE, locking in the cold air near the surface.
That’s where the warm layer was, now why does it exist?
The type of storm we had was called a Miller-B storm. It’s a classification of a type of storm that takes a certain track. What happens is an initial upper low tracks over mid-west and dies out. The contrast of warm ocean/cold land, along with other upper level dynamics become extremely supportive off the east coast, usually somewhere off the Carolinas. The initial upper low over the Midwest transfers it’s energy to the coast, and a new low forms along the coast.
Because we initially have a low in the Midwest, we are initially on the eastern side of the low, experiencing strong southerly flow. This drags a lot of warm, moist air off the Atlantic inland, usually leading to a warm layer of air that creates the rain/mix/snow lines our area so commonly sees.
Now, that track of that initial low determines how much warm air we see dragged inland. The blizzard last year was a Miller B storm, but the upper low dropped farther south, and there wasn’t as much warm air pulled north – plus it was incredibly cold leading up to and during the event. In the case of the storm we just saw, the low tracked over Ohio, which is WAY too far north. This pushed the warm layer way inland, and led to the under forecasting of sleet.
Watch this take place in the animation below. 850 mb (millibars) is a pressure level in the atmosphere, roughly 5000 feet above sea level.
So now you know…
This is probably the last winter storm of the season, and I think it’s fair to say winter is trying to go out with a bang!