Loudoun Weather

What is up with Sunday/Monday.

My current confidence levels:

85% we see some snow
75% we see an inch or more
60% we see 4 inches or more
55% we see 6 inches or more
45% we see 8 inches or more
30% we see 10 inches or more
25% we see 12 inches or more

I am very torn on this system

The American Model shows about 4 to 8 inches.

The European still shows a big storm, especially for the western areas.

What is so different about these two models?

I will summarize.

To the north, we have some “blocking” occurring that is causing storms to slow down as they head north. The blocking stops the storminess and the energy of the storm from going further north when the storm is west of Virginia. The blocking also keeps colder air in place. If the storm gets too far north (like the American Model shows) then we get into southerly winds since the low is due west or northwest of our area, and the colder air is eroded out of the area when the precipitation comes. In this setup, on the American Model, we have had about 4 to 8 inches of snow. The point where the winds change from the north to the south is a warm front (Warm air advancing into colder air). Once the warm front is north of you, you will change to rain or a mix. We do have the added help of Cold Air Damming in the region, so the warm air can push overtop the colder air and we will mix with sleet and freezing rain before seeing just plain rain. One note, however, is that once the warm front pushed through the precipitation will often cut off as the main low is way west of the area. The moist air overrunning the warm front and being pushed into our area is not uncommon to cause snow. Our failure earlier this week was related to this setup. We would have had more snow if the ground temperatures were colder. We will have cold air in place as we head into this Sunday’s event, so we will not waste the precipitation on getting the air cold. So we get the warm front assisted snow. Since the low and energy cannot push further north, they transfer this energy to the coastal front that is along or near the East coast. This Low-pressure system death to our west and transfer is known as a Miller B type set up.

Finally I can explain the potential and the potential busts for this forecast.
THE KEY is “WHERE and when does that transfer take place?”

Scenarios that would cause a “bust”

IF the transfer is at our same latitude or further north, we may get the front end snow, but not the snow from the new low pressure that formed off the coast.
If the Primary low remain strong and transfers the energy slowly, we get into southerly winds and get too warm for snow
If the primary transfer is just slow and we stay between the heavier precipitation from the primary low, and too far west of the coastal low.
Another potential impact to lesser amounts of snow will be how close to the coast the new low-pressure tracks. IF it gets too far inland the warmer ocean water could turn the snow to sleet. This does have some possibility and will need to be watched (especially eastern areas)

Now, we do pretty well in the American Model setup on Sunday, and we get into the warmer sector of the storm, but we may not have too much precipitation after the warm front aided snow so we get some drizzle. Then the new storm takes over, but it is too far north to get us any of the coastal aided snow. THIS has been the setup on the GFS for days, but even now it has upped the totals to more than 6 inches from this front end snow!

BUT, THE EUROPEAN model is different! The Primary low on the European does not make it as far north to get us into southerly winds, and transfers the energy to a Low along the front down in North Carolina! The low begins to strengthen and organize which is sending precipitation over us in light to, to increasingly moderate, and then heavy snow. Because it is organizing, it slows down its forward speed for almost 12 hours.. so we are getting snow the entire time…then, as it strengthens and lifts northwards, we get in very cold and heavy snow. This is the snow on the Northwestward side of the system that has the moisture from the ocean, but is also pulling in the cold from the north! It is referred to as the Cold Conveyer Belt (or CCB). We get real heavy snow from this, and, sometimes, higher ratio fluffy snow which makes more snow with less moisture! We appear to get into this snow for a while on the European!