This will be a storm track and low pressure interaction pattern known as Miller B, the hallmark of which is two low pressure centers interacting - one from the west over land and one from the southeast.
Our first two big storms (December 19-20 and February 5-6) last season were both Miller B storms - so we know they can produce large amounts of snow. However, it is the timing and location of the interaction between the two lows and their original intensity (especially of the inland storm) that are crucial components.
Last year, in both cases, the low from the west passed close to our area before transferring its energy to the developing offshore low well to our southeast. As the storm moved up the coast this infusion of energy and time over the Gulf Stream caused the "bombing out effect" that buried our area in snowfall.
This time will be a bit different.
So let's look at some near-term model maps to see where this transfer might take place. This close to the storm, I prefer to look at the NAM which is usually a very good short term model . However, I should note that for this storm - at least so far - the NAM is an outlier. It is slower with timing overall and stronger with the inland low. Making it even more suspect is the fact that, generally, there IS consensus between the Euro, GFS and UKMET models and all of them are a little faster and a little weaker with the storm for this area.
However, the NAM essentially agrees with the other models for the time period leading up to early morning Wednesday. So, I'll show you the NAM because it represents the strong side of the guidance available - but my actual forecast rationale will be like a buffet: an entree of Euro/GFS/UKMET and a NAM dessert.
That darker-green area in Illinois is our inland storm moving off to the northeast. Our coastal storm is developing off the Carolinas. This would be about 15 UTC or around 10 AM tomorrow. As you can see, not much going on around here according to the NAM. I would agree. Maybe some flurries or light snow showers but I think most of the action will be later in the afternoon. NAM is a little slower than the other models but not significantly.
Six hours later and the NAM is forecasting the edge of the steadier light snow in our area. This is late afternoon tomorrow - around 4 PM. The low to the west is a bit better organized as is the one off the coast. These will continue to function semi-independently for another few hours before the energy transfer and phase begin to take place. I expect the light snow to begin falling steadily sometime tomorrow mid to late afternoon. This rationale is supported by the other models.
Here's where the models disagree - or really where most of the other models start looking at the NAM with a suspicious glare.
In the NAM map above (9 hours later - around 1 AM Wednesday morning) you can see the two low pressure areas characteristic of a classic Miller B presentation - one in western Pennsylvania and one just off the Delmarva coast. This is where the NAM forecasts the transfer and phase to really get going. The NAM is slower to make this transfer and, because it's also forecasting a stronger inland low, our region experiences a more extended period of moderate snow before the transfer is complete and the storm off the coast takes over.
Compare the NAM to the GFS forecast for the same hour:
The GFS/Euro/UKMET gang advocate a weaker inland low (it's not even rendered with an "L" in western PA unlike the NAM), a quicker energy transfer and a faster end to the snow in our area.
One other thing to worry about: when the 850 MB low passes to the north of our area in a Miller B scenario, there is a high likelihood of being dryslotted around here. We essentially end up in a "hole" between the storms as the phase takes place. Check out the precipitation totals by the time the storm is off the coast of Maine Wednesday night:
See the hole in the precipitation field over our region and to the west and south? Dryslotting.
It's worth noting that even the NAM - on the strong side of the guidance - forecasts the coastal storm takeover to be too far north (off the Delmarva coast) to affect us much.
Here's what I expect:
Based on the previously-discussed issues and scenarios and a few glances at upper level data which suggests that there will be a fair amount of precipitation ahead of that coastal low even before the Miller B transfer begins, I would say a chance of flurries or light snow beginning late morning Tuesday but especially by the afternoon. By the late afternoon/early evening I think the light snow becomes steadier and and more widespread and continues into the night. There's a good chance that the snow becomes moderate for a time Tuesday night (especially the further east you are) before it ends from west to east overnight on Tuesday into early Wednesday morning.
This is another storm where the lightest amounts will fall to the west (except once you get over the mountains) and the heaviest amounts to the east (except possibly along the coast and in southern Maryland where mixing of snow with sleet and rain could occur).
Generally in the DC Metro I'd expect 2-4 inches in a southwest (low end) to northeast (high end) gradient up to about Baltimore. North and east of Baltimore things could get interesting quickly with 5+ inch amounts. Similarly, towards the coast amounts could reach 5+ inches.
By Wednesday morning this is all out of here and in the wake we get more of the same so far this winter: cold and windy into the weekend.
What could change things?
- The NAM could be the start of a trend rather than an outlier. Slower and stronger means more snow for us.
- The phase could happen earlier and further to the south. The more time we spend under the influence of the coastal low, the better. If the phase has already happened at a time when precipitation is still falling from the coastal low, we'll get more snow.
- The UKMET model could be right. It represents the weakest edge of the guidance. The UKMET solution is for a storm further out to sea. We get even less of an impact from the coastal low than is already forecast by most of the other models and, as a result, less overall snow.
Why is forecasting winter weather so complicated and so often inaccurate?
I could write 6 posts about this and not be done but there are a number of reasons for it:
- The guidance is, for the most part, forecasting massive-scale events and we try to translate them to the small-scale. Take the storm after Christmas, for example. The distance between Ocean City and Washington, DC is about 100 miles (as the crow flies). You'll remember that we received a dusting of snow from that storm here in the DC area. Ocean City? 14 inches. We were around half that distance away from 6+ inches. 50 lousy miles in a storm hundreds of miles in diameter.
- The psychology of snow around here. Let's take last season's largest snowfall for example. Midweek forecasts that week were for between 24-32 inches of snow. Some had higher amounts up to three feet. By the time the snow started falling, forecasts were trimmed back by about 4 inches to allow for the reduced snowfall during the Miller B transfer. Do you think anybody cared that their snow forecast was now 20-27 inches instead of 24-32?
But now let's say I'm 4 inches off in my forecast for tomorrow. That's the difference between snow and no snow at all.
- Storm track is almost the whole ballgame. In our case for tomorrow it's actually TWO storms we have to track. What if the one from the west actually gets closer to us? What if the track of the storm from the south is too far east? Everything changes.
- Storm intensity is the rest of the ballgame. Our storms for tomorrow are just getting organized. How strong will they be? You can see the models already disagree about that with the NAM having a stronger inland storm than the rest of the models.
- Small-scale features. Things like "mesoscale banding" can cause big differences in snow totals. Check out this radar image from the December 26th storm in New Jersey/New York. You can see how a forecast of 12-14 inches ends up being 20+. Those yellow bands of snow are almost impossible to forecast in duration and extent. But if you get under one, it's like the snowpocalypse all over again.
Hey - I'm not making excuses - sometimes winter weather forecasts around here can be wretchedly bad. Sometimes meteorologists ignore their better judgement and their intuition in favor of computers. But it's a crazy science and even in this age of incredible technology advancement, there's still a lot of guessing to be done.
I'll be watching the afternoon model runs. If anything changes, I'll make a quick post to let you know.
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