It's Just a Phase...but Maybe We Should Worry

First, a few words about jet streams and then we'll talk about Tuesday - which could be yet another significant snowfall for the DC/Baltimore region...

Perhaps you have heard of the jet stream.  It is a relatively narrow, fast moving belt of air high in the atmosphere.    What you might not know, is that there are several jet streams.  The two that matter for us are called the Polar Jet Stream and the Subtropical Jet Stream.

The jet streams have a lot to do with the type of weather we experience.  Areas of the country north of the Polar Jet experience cold air outbreaks.  Sometimes meteorologists (especially on TV) will refer to the Polar Jet "diving" southward but what really happens is that the cold air is forced southward by other atmospheric interactions which, in turn, causes this ribbon of fast moving air to move along with it.

In any case, low pressure areas form as ripples in this wave of air.  These are the storms we experience.  The Polar Jet, also referred to as the "Northern Branch" sometime brings snow to our area - but it's usually relatively light.  Have you heard of Alberta Clippers?  These are fast-moving storms that come down on the Polar Jet from Canada.  Since they are continental in origin, they don't have a lot of moisture to work with and we typically get a few inches at the most from these storms.

Low pressure systems also develop in the Subtropical Jet.  These are very wet storms as they usually pass over the Gulf of Mexico and draw moisture from there and the Atlantic Ocean.  In the Winter, subtropical storms bring a lot of precipitation - but without any cold air, this can be rain.

Sometimes, these branches of the jet stream will join.  This is referred to as phasing and when it happens, we can get some pretty big storms.  To give you a visual sense of what this looks like on a map, the diagram below shows the northern and southern (Polar and Subtropical) jet streams phasing over Kansas back on December 6, 2009:

Essentially, phasing combines the energies of both jet streams and produces a strong area of low pressure.

On Tuesday, a low pressure system is forecast to move in from the Midwest.  Sometime on Tuesday, the northern and southern branches of the jet stream are forecast to phase.  If this happens where the models currently suggest it will, a strong low pressure will form off the coast of North Carolina or southeast Virginia and will move up the coast and give us some snow.

Right now, that is the most likely scenario but there are others.

The jet streams could phase later, which could cause a weaker low pressure to form which would, in turn, result in much less precipitation for us.

Forecasting the timing of jet stream phasing is difficult - and meteorologists rely on models and satellite images to try and make the call.

Timing of phasing will be everything with this system.  For now it looks like this could be yet another significant snow event for the DC/Baltimore region.  We may have more issues with possible rain/sleet/snow mixing during the event, but we could still see accumulating snow.

How much?

The GFS shows 1.00 - 1.25 QPF forecast.  (QPF is a shortcut way to say "liquid equivalent precipitation")  Even with mixing issues, that at least 6 and maybe as much as 10 inches of snow.  But again - it all depends on the phase timing.  This is the type of forecast that could change radically 24 hours away from the storm if the atmospheric conditions don't come together just right.

Stay tuned!

On to the Next!

This storm did not disappoint, even if it didn't break the all-time record of the Knickerbocker storm of 1922 (28 inches).  National will end up reporting about 17-18 which is about 2-4 inches lower than the surrounding areas. For them, that's pretty "accurate".

Dulles, on the other hand, checks in around 30 inches as do other places to our west and north.  Here in Arlington, I recorded just over 20 inches as did Loran and Joan in Alexandria.  Reports are still coming in.

We did, however, break a 110-year-old record for snow in a single day - even with National Airport's useless measuring system (I picture an NWS guy holding his hands in the snow, one at the bottom, one at the top, and then walking 200 yards back to the tower with his hands held apart to have a colleague measure the space between them).

Now for Tuesday.  Models have a pretty good storm and this could be the one that New England has been waiting for all Winter.  While we here in the Mid-Atlantic region have been unusually snowy, places from New York City northward haven't seen much - both relative to us and relative to their seasonal norms.

We're relatively close to this storm, temporally, so the models are already coming into line.  Storm track, as always, is a factor.  Below you can see a snapshot from this evening's GFS model at 84 hours (overnight Tuesday - morning Wednesday) showing a pretty good low pressure system on the coast, the DC area north of the blue freezing line (indicating that our precipitation would be of the frozen variety - probably snow) and showing about a half inch of liquid equivalent precipitation being tossed back over the area.

Since this is a snapshot, you can expect that there was a little precipitation on the model for us in the time frame before and after this snapshot.

At the moment, that argues for at least a 5-inch storm.  But this time, the models don't agree all that well and some have a stronger storm, some have it closer to the coast.

It bears watching - especially since there are a couple of scenarios that could give us issues with precipitation type.  We don't need any rain on top of this crazy amount of snow.

More to come on this next storm in an incredibly busy season.

Hope everyone enjoyed this storm and is safe and warm indoors!

OBS: WDX and NAX

Observations from two of our reporting stations:

First my parents (WDX) report snow rates of an inch an hour throughout this morning and total snowfall approaching 15 inches.  All that remains of the company name on the official measuring stick is "Reamer Company".

And from reporting station NAX, in north Arlington, Bryna and Russ report 17 inches and these scenes.  Cleverly, the furniture rental place included their contact information twice on their official measuring device:

Mother of God. Part II

Shoveling that snow takes some effort.  Wow. 

Almost 19 inches here in Arlington at 1:00 PM.  Snow still falling at a moderate to occasionally heavy rate.

Awesome!

The Dry Slot

Well, as the transfer of energy between the low out west and the low off the coast happened overnight, some dry air began to intrude into northern Virginia and the DC metro area.  Very early this morning, that caused snow to temporarily decrease in intensity.  Meteorologists refer to this as a "dry slot".

The dry air never really made it much further north and as a result, the snow totals in the DC area this morning range from 14-19 inches whereas in Baltimore they have already seen 2 feet and in Columbia, MD they are reporting 28 inches.

Snow continues to fall and will continue for most of the daylight hours into the evening tonight as the main surface low pulls away from the coast and the upper level lows rotate through the region.  Today's snow would be a normal "big" snow for DC - probably 4-8 inches of accumulation on top of whatever you have already seen.

For us, that's 17 inches and counting!  Measurement is becoming difficult as the winds have picked up and snow is blowing everywhere.

Thundersnow!

While watching a particularly heavy snow burst at 1:28 AM (that had to be at least a 3-inch-per-hour burst), I saw lightning in the southern sky twice and heard thunder.  Incredible.  I've only seen thundersnow one other time - during the Veteran's Day storm of 1987.

We're over a foot now here at Potential Vorticity central.  The ruler has disappeared.  Going to get the yardstick involved next.

And, of anecdotal interest: while watching the Weather Channel earlier this evening, both the current conditions and the forecast for Saturday simply had one word:

Blizzard.

Radar at 12:14 AM

Tonight I believe I have seen some of the heaviest snowfall I've ever observed.  Rates are 1-2.5 inches an hour and more very heavy bands are headed this way.  In the radar below, you can see two things.  The first is that as the low in the Appalachians transfers its energy to the coastal low off of North Carolina, it has brought a surge of warmer air with it.  Radars tend to be poor at judging rain/snow lines, which is why it looks in the radar below that rain has edged into DC and points south when I can see outside that it has not.

But there is some mixing going on in central Virginia and further south of the DC metro.

As the low off the coast rapidly intensifies (mets refer to this effect as "bombing") more cold air will be drawn in and will change the precipitation that is mixing back over to snow, possibly as far south as Norfolk.

The other thing you can see is the dramatically heavy precipitation that's going on.  Those dark blues are the bands I spoke of and they are where the snow is falling at tremendous rates.  See that yellow line above Richmond?  That's another locally heavy band of what is likely sleet.  As it moves due north and hits the colder air, it will be yet another burst of heavy snow for our region.

11.5 inches here in Arlington.  More to come...