First Significant DC Metro Winter Precipitation Possible on Saturday-Sunday

Humm.  Good to be awake again here on the PV blog! 

That was a strange offseason.  No real summer to speak of and then a cold autumn which seems to want to transition to winter as soon as possible.

Fine then.  Let's begin.

We've had our share of cold weather but precious little wintry precipitation.  I'm loathe to credit the seventeen flakes of snow that fell here just before Thanksgiving as "wintry precipitation" but either way, it certainly wasn't significant.

Saturday night into Sunday could be a different story.

Let's set up the scenario and I'll chat about the issues and complexity of the forecast.

First off, we have a couple of storms.  The first one goes through here Thursday night through Friday as a front passes through the area and brings colder air.  Could it end as a bit of snow?  It could.  But it won't amount to much.  The majority of what falls around here will be rain so, for the purposes of this blog, we'll ignore it, no matter what the "incredibly thoughtful and clever" folks at The Weather Channel have decided to call it. 

And by "incredibly thoughtful and clever" I mean, "galactically moronic lunatics desperate for ratings".

What the hell is a "Cleon", anyway?

Onward.  The second storm (I can think of a compound "d" word the Weather Channel can use if it needs a suggestion) is the more interesting one around here.

The synoptic situation is this: 

A strong high pressure will be moving east over New England but will have enough influence on our region to pump cold air southward.  This cold air will funnel its way into our area - channeled against the mountains like a quarter pipe.  This is a classic setup for what we call "cold air damming".  Essentially, since cold air sinks, the air just sits in a pool bounded by the mountains to the west and the warmer ocean to the east. 

Now here comes the storm.  The main low pressure center is forecast to take a path well to the west of here through the upper Midwest and into the Great Lakes region.  This is normally a horrible setup for us if we want any kind of frozen precipitation.  The counterclockwise flow around the low brings air from the south which is quite moist but also quite warm.

However.  With the cold air settled in a pool, the warm air won't dislodge it right away.  In fact, initially it will ride over top of the cold dome, condensing along the way.  If the cold air is deep enough, we'll see snow or sleet for a time Saturday night into Sunday.  As the warm air erodes the cold air below it, any snow will change to sleet and any sleet will change to rain. 

Here at the surface, the temperatures might stay cold enough for some (most?) of Sunday that any liquid precipitation will freeze on contact with the surface.  That's the dreaded freezing rain which would leave a glaze on things.  No fun.

At some point on Sunday evening, the warm air will begin to win and by Monday, plain rain will be falling with much warmer temperatures as the last of the cold air is scoured out.

QUESTIONS

- How deep will the cold air be?
- How long will the cold air last?
- How much precipitation will fall while the cold air is still around?

These will make all the difference between a forecast that is just for a bit of sleet followed by rain and a forecast that includes every possible frozen precipitation type followed by rain later Sunday night and Monday.

Most of these details have yet to be resolved.


MAPS

Here's this morning's Euro model output for total snowfall from this storm:

as you can see, the DC metro falls between the 3-6 inch contour.  This represents what I believe to be fairly ideal conditions.  Deep cold air, lots of precipitation on the front end of the storm.

I'd say, at this point, there's a 20% chance of verifying in the DC metro.  A greater chance as you move north and west of the city.

I'm concerned about how much cold air there will be and how much precipitation will be out on the front-end of this storm.  But I expect we'll see some frozen precipitation on Saturday night into Sunday.  How much and for how long remains to be seen.

I'll update when new information comes in.

Welcome back!