The GFS has been trending further and further northwest with each run. This was an "ok" trend at first because it brought the heaviest core of precipitation northward towards us. However, it's starting to become a touch uncomfortable because a northern track is also a warmer track which we don't want.
Temperatures for this event were going to be marginal anyway.
The official NWS call for the DC area is 2-4 inches east of I-95 and then a fairly tight gradient of about 6-8 inches just west of DC to over a foot for the western suburbs.
We need to keep a southern component to the track or we'll definitely have precipitation type issues.
Most March storms are like this but the snowfall forecasts are too close for comfort:
I should note that the GFS has the furthest northward track of any model and has been leading the northward charge for the last few cycles. In fact, the Euro - which is considered a better model - insists on a more southerly track. The disparity between the models - especially after the storm gets off the coast - is quite remarkable at this timeframe and highlights the complexity of the atmosphere and the interactions between pressure systems.
All I can say is "stay tuned". Snow still seems likely but we're getting close - too close - to having rain mixing problems, especially near the city itself.
You can see we're in the "bullseye" for the NAM which shows 1.5-2+ inches of liquid precipitation from this storm. Even losing some of that to rain/slop and assuming low snow-liquid ratios, that suggests at least 6 inches of snowfall.
It's down to a question of "how much?" which will be strongly driven by the ultimate storm track and the intensity of the storm as it passes through. We could "lose" some precipitation to rain at the beginning (Tuesday afternoon/evening) but the changeover to snow should start from west to east overnight Tuesday.
The closest analog to this storm and the pattern seems to be February 5-6, 2010 which dumped 18-32 inches of snow in the DC area. This will not perform as well for two reasons:
1 - Lack of abundant cold air at the surface. This storm will depend on dynamic processes to cool the air and change initial precipitation to snow. The forecast high for DCA on Tuesday is 47 so there will be some distance to go - even at night - before the atmosphere is fully conducive to supporting snowfall.
2 - Lower liquid-snow ratios. The rule-of-thumb is 1 inch of liquid = 10 inches of snow. Because of the marginal temperatures, the ratios are likely to be more like 8:1.
Still, most of the models produce significant QPF (precipitation forecast amounts) for our area - well over an inch in many cases. Those of you along and east of I-95 stand the best chance of having precipitation problems with rain mixing in even on Wednesday which may keep totals down. To the west, the precipitation is more likely to remain snow.
The timing for this is:
Tuesday afternoon/evening: rain starts and changes to snow overnight.
Tuesday night: rain/snow changing to all snow by morning
Wednesday: snow (possibly mixed with rain to the east)
Wednesday night: snow ending from west to east
Here is the NAM clown map from this morning's run:
Looks like 6-12 inches from DC westward, over a foot in the mountains. Less as you go east. The gradient is pretty tight along I-95 so this will be a tough call. I can see scenarios where only rain falls in the I-95 corridor and I can see others where all snow falls.
It's a tough forecast as almost all March snowstorms tend to be.
I'll say a general 6-inch snowstorm is likely now for the DC metro with greater amounts to the north and west and lesser amounts to the east.
We'll see what the models have to say this afternoon.
As you've probably heard by now, the potential for significant snow exists for Tuesday night through Wednesday night in our area. This is a potentially serious situation because the precipitation, if it indeed falls as all snow, would be wet and heavy and according to some models I'm looking at now - there could be a lot of it.
Here's the GFS "clown map" for example:
showing our area "bullseyed" with 12-16 inch snows. The GFS does some strange things in this run so we'll have to watch to see if it's repeated in tomorrow's run and if the other models agree.
There's a ton of data to look over and most importantly, the track is going to be a big big deal. In March, the sun angle is higher and it's going to be hard to get temperatures down to the level where everything sticks (or even falls as snow). But if the track is just right, this could happen.
I'm still checking things but let's just say that the chance of significant accumulating snowfall starting Tuesday night and continuing through Wednesday night is better than 50%.
The rain/snow line is more of a question now rather than the question of whether or not anything will fall.
I hadn't posted because, really, there's not much to get excited about when Snow Chance #1 pretty much slopped for 3 hours and went away.
Friday night's chance looks like that's still a possibility but a few models have a different, more significant idea for Friday night into Saturday.
It's a mess of temperature issues and whether or not the storm can get going off the coast before it moves too far away. If it can do this - and it's still a big "if" - we could have a better chance at accumulating snow here in the DC area.
Hopefully all of our friends and family in the northeast - especially Connecticut and Massachusetts - are doing well after the historic storm last week. Truly amazing amounts of snow fell in parts of northern New England.
If you haven't seen this video yet, give it a look. This is a time-lapse video of the snowfall near Hartford, CT.
When you consider NYC even got in on the action at 8-12 inches, why didn't we see any snow here? It sleeted/snowed for about an hour on Thursday night and then rained a bit into Friday morning.
Boring!
Let's have a look at what happened because this has bearing on the upcoming possibilities. To save you from sensationalistic plugs that make you read an entire post before you get what you're looking for, I present a digest:
3 chances in the next 7-10 days:
- Wednesday 2/13
- This coming weekend
- Next Monday-Tuesday or so
Those of you with short attention spans can now go merrily on your - LOOK! SOMETHING SHINY!
Onward...
It's all about storm type and phasing. There were two systems moving eastward in the two main branches of the jet stream. The Northern Stream storm would head on a path across the Upper Midwest and to the north of our area.
Since winds move counterclockwise around a low pressure system, this created a southerly flow around our area that quickly changed frozen precipitation to liquid overnight on Thursday.
This was a Miller B type storm (as they all have been this year) and as the Northern Stream system moved eastward it transferred its energy and phased with the Southern Stream system which had moved out of the southeastern US and up the Atlantic Coast.
Now, storms moving along the coast from the south in the winter is usually a good thing for us. However, with that Northern Branch system moving north of us and the phase happening just to our northeast, we were caught in the middle - in an area between two phasing low pressure systems. This is almost always a zone of minimum precipitation and it's a big problem with Miller B storms that don't phase to our south.
The Southern Stream storm would move from the southeast and then up the coast, phasing with the Northern Stream late in the afternoon on Friday - well to our northeast. The precipitation gradient was extreme with parts of the Eastern Shore seeing heavy rain all day Friday while we in DC saw some sunshine in the afternoon.
Here are the two storms last Thursday morning as they began to phase.
Watch the following animation and see the phase as the southern system absorbs the northern system.
Notice a couple of things here. First, the northern storm starts to weaken (the 1008 mb contour disappears from the center of that storm towards the end) and the storm on the coast significantly intensifies as it moves up the coast (the last few frames show a center contour of 996 mb). Where are we?
Right in the middle.
Ok - congratulations to our friends up north. What about our snow? Let's talk about the first chance.
WEDNESDAY 2/13 - THURSDAY 2/14
The models have been showing a "Valentine's Day Storm" for a while now and it's encouraging that this close to the event, they still have it.
What's discouraging is that they seem to have little consensus about what path its going to take and whether temperatures in our area will be suitable for frozen precipitation.
At the moment, I would classify the event as marginal as we're in one of those situations where a more northern track would produce heavier precipitation but also warmer temperatures (rain) and a southern track would produce lighter precipitation but colder temperatures (snow). In this case (ironically), the best place for us to be would be in the middle of those track solutions.
Here's the Euro at hour 72 (Wednesday night) showing the storm center in the southeastern US:
And then off the coast by hour 96 (Thursday morning)
The GFS is in favor of a more northward track:
which, as you can see, would move the higher axis of snow accumulations northward.
This bears watching and until there's a track consensus, anything is possible from warning-criteria snow to a rain event here.
We'll see what the evening model runs hold.
I'll post later about snow chances on the weekend and then a bigger chance early next week.
