A CLASSIC SET-UP
So far, we’ve had a practically snowless winter. It took until Jan. 17th for the first measurable snow to fall in Philadelphia, and that was just ½ inch. But now we are looking at a chance of a snowstorm in much of the area by the weekend. A lot of ingredients are in place to give us more confidence than normal a full FOUR days ahead of the start of the storm.
#1 IT’S COLD!
Arctic air moved in Sunday night, and will hold firm for much of the week. We often lack enough cold air for snow in strong El Nino winters, and the current one is no exception. But this time it’s clearly cold enough for the storm to at least start as snow through just about the entire area. And, as long as the storm tracks south of us, it’s going to be hard for all of us to see a change to a wintry mix and/or rain. Some area is going to see all snow-and probably a lot of it.
#2 THIS ONE IS A “TYPE A” STORM: FEWER SURPRISES
Meteorologists have been talking about “Type A” and “Type B” winter storms for decades. To give credit where credit is due, they are called “Miller-Type A” and “Miller-Type B," first published in 1946!
Type A storms have the fewest surprises, and tend to give more widespread heavy snow, if it’s cold enough. These storms track near or north of the Gulf of Mexico and already have plenty of moisture with them by the time they hit the Atlantic coast. The “Blizzard of ’96," and what is known as “The Storm of the Century” in March ’93 are examples. We generally have more confidence predicting these types of storms days ahead of time.
#3 A SIMILAR SET-UP: FEBRUARY 1983
It’s sometimes hard to get a snowstorm around here during a strong El Nino year. The winters of 1997-98 and 1972-73 were virtually snowless. But in 1983, after a relatively snowless winter, a Type A storm developed and led to 21” of snow in Philadelphia. At the time, it was the biggest snowstorm ever recorded here. So, it has happened before.
#4 A VERY NEGATIVE AO OR NAO
We’ve often discussed the importance of negative phases of the AO (Arctic Oscillation) and the NAO (North Atlantic Oscillation) in leading to big winter storms in this part of the country. In this case, the AO is just about as negative as it can get:
#5 THE COMPUTER MODELS STILL AGREE
The more often we can say this, the more confidence we have that the storm will hit as predicted. All of the computer models are still showing LOW pressure tracking south of us, with plenty of moisture. Some bring more warm air in from the ocean, which would cause a change-over to rain. But the European model has now shown virtually the same thing for FOUR different “runs” (the model is run every 12 hours). When the best model overall is that consistent, our confidence grows.
Below is the latest forecast map from the EURO. The LOW is in a nearly ideal position for a major winter storm in our area.
We’ve also mentioned “ensembles” before. The EURO is run 51 times each 12 hours, with slightly different initial conditions. The average of the 51 runs is considered by many as “the best of the best." It has the LOW taking almost the exact track of the previous EURO solutions.
THE BOTTOM LINE
It’s still too early for specifics, but there is increasing potential for a major winter storm to affect much of our area during the Friday afternoon to Saturday period. We will provide updates with the latest thinking each day as we go through the week. Plans don’t need to be changed just yet, but by now, the forecast should be something at least in the back of your mind that plans might need to change.
The area most likely to get ALL SNOW is our far Northern & Western Suburbs (Lehigh Valley, Berks, Upper Bucks/Montgomery, and North & West Chester Counties.) The least snow should fall at the shore, since there should be more hours of rain as warmer air moves in. The possibility of a mix with or change to rain makes the rest of the area trickier to forecast with precision. We should know more details and have more confidence each day.