I'm watching a very interesting system that may be developing for the Wednesday/Thursday time frame here in the DC area.

Most models show a phased system moving out of the Southeast, off the coast of the Carolinas and then up the coast.

The GFS run for Wednesday night shows a low pressure system rapidly intensifying off the coast

and throwing moisture back into our region.  The temperatures here are noteworthy.  The blue line is sub-freezing temperatures and we are north of that line.  This model suggests that the precipitation that falls will fall in the form of snow.

Interesting.

The total precipitation forecast from the same model through the duration of the storm for us looks like this:

It's a little tough to see but our area falls within the 0.1 - 0.5 range with a very sharp gradient towards the coast.  Using the old 10-to-1 ratio for liquid precipitation to frozen, this implies 1-5 inches of snow for our area.

More interesting.

As always, this is dependent on a lot of things but especially on storm track.  A track closer to the coast would mean more precipitation but also warmer temperatures as air is pulled from the relatively-mild Atlantic.  A track further from the coast would mean less precipitation - and a few solutions have a total miss with the storm moving off the coast too far to affect our weather.

Here's the GFS estimate for snow cover after the storm:

In this scenario, Delaware gets the bulk of the snow but you can see how close the areas of heavier snow are to this region.

The bad news in all of this is that the storm is likely to slow down and pound areas affected by Sandy (the Jersey shore especially) with winds, rain and surf.  Just what they don't need.

Let's see what the evening models hold but for now, there's a chance that some rain will start Wednesday, possibly changing to snow as nightfall approaches with better chances of snow north and west of DC but less overall precipitation the further west you go.

That's the weather forecaster in me being careful with the forecast.  The GFS above has no such hedging.  If I made a forecast entirely based on that, I would say "plan for snow Wednesday night".

We wait for the evening model run.

Possible Nor'easter MIdweek

Hope everyone did well during and after Sandy.

Models (the GFS and Euro) have been trending towards a solution that would bring a phased low pressure system out of the Gulf of Mexico northward along the coast as a Nor'easter for the Tuesday-Thursday time frame.

I'll post some model data later but the last few runs have turned away from an "off the coast" solution and now bring precipitation chances to our area.

Temperatures will be cool but not cold so precipitation type for the DC area looks to be rain at this point but stay tuned.

Here's Something You Don't See Everyday

The warning map from the Sterling NWS:

The pink color over the water is a hurricane warning.  The bright red in West Virginia?

Blizzard warning.  Wow.

6-12 inches of heavy snow possible with very strong winds.  A lot of that depends on how much cold air wraps around the system.  The atmosphere is marginal for snow but the dynamics of this storm are impressive and all of the models point to snow. 

Sandy Still Moving NE

The 11 PM update - Sandy is still tracking as expected.  Officially, the storm is still moving northeastward but has come about 15 degrees westward in the past 6 hours.  A previous heading of 050 is now 035.

This left turn should continue to a northwest heading by midday tomorrow.  Landfall still looks like Monday evening in south-central New Jersey.

No practical changes to our forecast.  Winds are picking up and rain is falling throughout the DC area.  This will continue and get more intense throughout the day tomorrow.  Gusts to 65 mph in localized areas will be possible tomorrow evening.

Stay safe!

Sandy Located 250 Miles off Cape Hatteras, Moving NE

Here is the latest advisory from the Hurricane Center:

Right now the storm is moving to the Northeast so the "capture" and turn in towards land has yet to happen.  Until it does, the location of landfall will be a bit of a guessing game with everywhere from Long Island  to Delaware still in play.

The most likely scenario is for a central-New Jersey landfall which would likely produce a very serious situation for New York City and areas along the Long Island Sound.  Because winds flow counterclockwise around a hurricane, the winds north and northeast of the center will be "piling" water up on shore.  Combine that with a full moon and high tide and you get a life-threatening storm surge situation.

Here in the DC area things are still forecast as described in the last post.  The weather for the balance of today should be tolerable.  Rain will start later this afternoon / evening and things will get breezy.  Overnight, though, the rain and wind really get going and by morning we should be seeing winds consistently around 30 mph and heavy rain.

Tomorrow starts bad and just goes downhill from there.  Winds increase throughout the day, rain continues.  By nightfall gusts to 60 mph are possible.

On Tuesday the rain hangs around but the winds start to decrease somewhat although I think breezy conditions persist until later in the week.

Expect 5-8 inches of rain, stream & creek flooding, maybe some flooding issues in the Old Town, Alexandria and Georgetown, DC areas that are prone to it.

Still watching the models to see how they track the storm.  Until that left turn happens, there's still some uncertainty.

Storm on Track for Landfall

Here's the latest from the National Hurricane Center:

Based on the models I've seen in the last few hours, I think their track might be a touch too far south.  The location of landfall isn't significant when you talk about whether or not we'll see any effects (we will) but it does have an impact on how extreme the effects could be.

For us, the size of the storm pretty much guarantees we will experience the expected effects:  rain and wind.  I think the weather starts deteriorating tomorrow afternoon with rain spreading in from the southeast.  The winds will be picking up too.  Expect sustained winds tomorrow night around 20-25 mph.

Monday is the most unpleasant day of the next three - especially as the day wears on.  Expect the wind to continue increasing through Monday afternoon and evening.  By Monday night we could see sustained winds around 40 mph with gusts over 50.  On the present track. the rain would fall heavily and fairly steadily throughout Monday.

By Tuesday things should start winding down but I think it stays windy until later in the week.  Rain is still possible on Tuesday - especially for the first half of the day. 

The big concerns for our area are:

1. Flooding.  We could see 5 or more inches of rain over the next three days.  Especially if the strike is south of the projections.  

2. Wind damage.  There are two factors to consider.  The first is that the winds will be quite strong for an extended period of time - maybe as much as 12 hours of tropical storm-force winds (40 mph+).  It is unusual - especially for this area - to experience a wind event like this one.  The second factor is that the ground will be extremely saturated during the period of strongest winds.  There will be tree-related damage.  Power outages are almost certain.

GFS-forecast wind gusts for Tuesday morning.  Our area is just south of the 70 mph zone indicating possible gusts in the   60-65 mph range for Monday night / Tuesday morning.

Much like winter storms - this will soon be down to small-scale forecasting.  Predicting where exactly the heaviest rain will fall and where the strongest winds will blow is very difficult.  So it makes sense to be prepared for the worst and hope we get something better.


Make no mistake, these models show a virtually unprecedented situation for this extended region over the next three days.  In a central-New Jersey strike scenario, New York City would see at least 5 feet of storm surge as winds and high-tide combine. 

Keep an eye on the conditions, use good sense, stay home on Monday if you can and treat the conditions with respect.  Be safe.

I'll post an update later today.

Track Shifts Southward

Here's the 11 AM update from the National Hurricane Center:

showing that the anticipated landfall has shifted south into southern New Jersey / Delaware  However, and I can't stress this enough:  don't focus on where the center makes landfall.  Yes, a southward shift to the track means that we in the DC area will likely experience more effects from the hurricane.  High winds and rain will definitely be a problem.  But this storm is large.  And if the extratropical transition I mentioned a few posts ago happens, the effects of the storm will be felt over a very large area.

On the coast, the big concern is from the center northward.  It is there that the winds will be strongest, and the problem with tides and surf will be the greatest.

Right now, as you can see from the graphic above, Sandy has weakened considerably due to dry air intrusion.  This dry air has disrupted the circulation and caused the cyclone to be a minimal hurricane.  Forecast models suggest that the storm will reintensify once it moves north and passes over the warm waters of the Gulf Stream.

After that, the models suggest a lot of cold air will enter the cyclone's circulation which will start that process of tropical-to-extratropical transition.

Hurricane Sandy.  You can see the exposed center of circulation and the thickest clouds to the northwest.  This is caused by dry air surging into the hurricane's circulation.

A hurricane is a complex engine requiring precise circulatory patterns of air to sustain intensity.  Storms like this become so large, they create their own environments but larger-pattern flow can affect them.

 The graphic above shows the typical Northern Hemisphere hurricane circulation pattern.  The spiral rain bands bring warm surface air into the cyclone where it spins up into thunderstorms near the center of circulation and especially in the area right around the center called the "eyewall".

Air then descends within the eye back to the surface.

As you might remember from previous posts:  ascending air creates clouds and descending air dries out the atmosphere.  This is why the eye is clear.

Above the storm is the outflow sector.  Air that ascends in the storm must flow outward to complete the cycle.  If the outflow or the inflow is blocked, the storm cannot intensify.  If strong upper-level winds cut across this circulation and disturb the outflow, the hurricane can fall apart.  Check out this dramatic example of windshear tearing a hurricane apart:

 See how the low level circulation is exposed as the thickest clouds are blown away by the shear?  Some of this is happening to Sandy right now.   Because of the shear, it wouldn't be entirely surprising to see Sandy being downgraded to a Tropical Storm later tonight.

 Unfortunately, there's plenty of time left for the storm to strengthen again before landfall.

So...where are we?

For now, expect a southern New Jersey landfall sometime on Tuesday.  We should start seeing some initial effects here as early as Saturday night with some initial rain bands.  The winds will increase throughout the period and the rain/wind maximum should happen by Monday night into Tuesday.

What makes this storm so unusual and the reason for the hype surrounding it is the unusual path it is expected to take.  I can't remember nor can I find an example of a storm that "hooked" to the west and made a direct-hit landfall on this part of the country.  There have been plenty of "drive by" storms (Irene comes to mind) but this left turn to a due west heading may well be unprecedented.

It's not unusual to have this kind of flow pattern in the fall and winter.  Sometimes big winter storms get caught up in this type of flow and hover in an area for a couple of days.  In fact, with winter storms, we snow lovers hope for that type of situation.

But for a hurricane to be in the right place at the right time to be caught in that kind of flow?  Unprecedented.

And for a hurricane to be absorbed into another system instead of torn apart by it?  Well, I can only think of the Perfect Storm example.

While the entire region will be affected in some way, the coastal areas north of the center could experience conditions that haven't really been seen there before.

And in DC, we could see winds on Monday night and Tuesday sustained in the 40-60 mph range with higher gusts.  If the storm track is further south, look for that to shift to Sunday night / Monday.