Monday Night Snow – First Call Forecast

As I’m sure you know, this is a very difficult forecast. Who sees what, and the amounts are very dependent on the track the storm takes – which all comes down to how the two pieces of energy for this storm come together – I’ll discuss this below. But first, here is my official First Call Forecast. The map is a blend of the 12Z/Mid-day European Ensemble, Canadian Ensemble, and GFS Ensembles, with my own judgement used to make it more reasonable (see forecast discussion below for more…)

And the following map is the dominant precipitation types I think you’re likely to see. The mixing area I’ve broken in two, because I think sleet and freezing rain may be a concern along and just east of I95, rather than just a cold rain.

 

Timing:

Precipitation moves into the area between 4 and 6 PM Monday evening and will start as snow. As the coastal low develops, we’ll start to see the entrainment of warm air due to the easterly winds around the low and the rain/mix/snow lines will start to progress northwest ward after 10PM from lower Delmarva. By midnight, it appears everyone along & east of I95 is seeing some sort of mixing. After 3AM, winds will start to become more northeast – and that will start to bring colder air back in. At this time, the rain/mix/snow lines will start to progress back to the east, and we will see snowfall return to the area. By 7AM, I expect everyone west of the bay to be back to all snow, with several hours of snow left, though the heaviest of the snow will be departing by that time.
N&W of I95, you’ll see all snow during this event. Most of the snow during the overnight will be fairly heavy, and rates could see 1-2″ per hour in some spots (though that’s nearly impossible to forecast where and when this far out)

Additional snow showers are likely through the remainder of Tuesday, and some light additional accumulations are likely. Snow showers may persist on Wednesday too.

Note on Wednesday: Wednesday will be rather brutal around here. Highs will not get above freezing, and it will be quite windy with gusts over 40MPH possible. Wind chills will be in the teens all day

Temperatures:

Monday afternoon highs will likely reach the upper 30s to around 40. The models were actually too warm with Sunday’s highs, and I’m curious to see if they’ll be too warm on Monday too. When the snow moves into the area, temps will be in this vicinity, but we’re relying on evaporational cooling to bring the temps down – because the air is not saturated, some of the snow will evaporate, which is a cooling process. This means the first couple hours of snow may be wasted getting the temps closer to freezing – though snow can still accumulate at 33, and possibly 34. Observations are taken at 2 meters, or about 6 feet, and many times the ground is actually colder than the temperature at 6 feet.

On the precipitation type map above, if you’re in the all snow areas, or Mix region 2 (I-95 and just east), temps will hold at or below 32° the rest of the event. This is why, I do not favor plain rain in these areas. I think sleet is most likely, but if rain occurs, the surface will be below freezing, so freezing rain is a concern

If you live in Mix region 1, temps will likely creep up to the mid 30s briefly as the warmer air is pulled in, but by 7AM you’ll drop back below freezing as the colder air wraps around the system.

If you live in the All / Mostly Rain region, temps will likely remain in the mid to upper 30s, and possibly low 40s over Delmarva until the colder air wraps around the system.

Temps Tuesday probably stay in the low to mid 30s for highs, but Tuesday night, areas that saw the heaviest snow could drop into the low teens and single digits.

Forecast Discussion:

If you’ve been following me on Facebook the last two days, you know I’ve been posting 3 of the major models and what they’ve been saying. I’ve only posted basic information, and to actually put the forecast together I look at a lot more – including more models. I said in my intro that my forecast comes from the Euro, Canadian, and GFS Ensembles – which I haven’t shared with you. An ensemble is a very useful forecasting tool. What happens is we take the regular model, for example the GFS, and we change the initial conditions in some way and then re-run the model. We do this many times (21 for the GFS, and 51 for the Euro, and I don’t know off the top of my head how many the Canadian has). What this gives us is a way to check if the regular model has a viable solution. If the mean or average of the Ensembles closely matches the regular model, then the solution is probably good. If they don’t match, then there’s something up.

Overall, the Ensembles today produced a much more realistic picture of what is going to happen. That’s why I went with them for the forecast.

Here’s the GFS Ensemble snow output:

And the Canadian Ensemble:

Unfortunately, I can’t show the Euro Ensemble, because it’s data is restricted. It’s similar to the GFS Ensemble map in terms of placement of the heaviest snow, but has about 3″ less snow over the entire area.

 

The track of the storm is what ultimately decides who gets what and how much, but as I said above, this is dependent on how the models handle the two pieces of energy that have to come together.

One piece of energy is in the northern branch of the jet-stream, and one piece is in the southern branch. What is going to happen is the northern energy will catch up to the southern piece, pick it up, and wrap it up and around. We call this phasing. And this is where it gets tricky – this is the simplified version:

If the phasing happens too soon, the low develops inland and will track inland or on the coastline. This would be rain well inland, and hefty mountain snows. This is possible, as some models have shown.

If the phasing happens too late, or the northern piece of energy is too slow, the low either never develops or develops too far south and is pushed out to sea. This would pull the heaviest snow far south, graze the area, or be nothing at all. This will not happen with this storm.

The “perfect track” requires that the phasing happens at exactly the right point, which is somewhere around the Mississippi River. This is the most likely scenario, but the “perfect track” is a vague statement as specifics come down to how close the low is to the coast.

 

 

As the coastal low strengthens, the dynamics in the atmosphere will likely go crazy, for lack of a better term. Specifically speaking, the upward lift of the air in the mid levels looks impressive. This is why I support 1-2″ snow fall rates at times in some areas. Thundersnow is also possible with this event as a result.

 

The NAM model, which is a short range model that I despise, has consistently had the low over Delmarva. The new evening run on Sunday saw a big east shift in the low, more in line with consensus on the other models (though possibly too far east). We’ll have to wait and see if this is a trend or a fluke run. We’re getting into the timerange where the NAM isn’t as bad, so we’ll see. I never act on one model run though, I wait for addition runs to see if it’s real or it’s a bad run.

Stay up to date with this forecast on the Facebook page. I’ll post the new models tonight as they come out, which is now an hour later thanks to the time change. If you have any questions, please ask – though if you can, please use the Facebook page to do so, and not the Q&A section below ( which is fine if you do not have Facebook).

-Andrew

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