[Updates follow original entry; final at Noon EST Thu Feb 11]

I'll never forget when last Thursday, at the end of the briefing I gave during TWC's morning news meeting, Renee Willet, weather.com Content Manager, looked at me and said, "You're scaring me, Stu." It was at that moment that I realized how hyped up I must have sounded in my description of the weather situation, as if I had consumed three cups of coffee but actually I hadn't had any caffeine -- it was all adrenaline from the upcoming storm which came to be known as "Snowmageddon."

On Monday, I started my briefing by saying to Renee and the other folks in the room, all non-meteorologists, "What if I told you that 'Snowmageddon' wasn't a 'bomb,' but this storm is going to be one?"

I quickly qualified that by noting that weather bombs aren't inherently associated with precipitation amounts (though they can produce a lot of it), so it's not like we expect accumulations of snow to equal much less exceed the maximum amounts of nearly 40" with Snowmageddon! This storm doesn't have as much moisture and the duration of the snow won't be as long. Rather, it's the sudden drop in the cyclone's central pressure which is going to be memorable this time, and that's what's going to make this one a "bomb."

Yes, that's a formally accepted meteorological term!

Its definition in the American Meteorological Society's Glossary of Meteorology:

"An extratropical surface cyclone with a central pressure that falls on the average at least 1 mb h−1 for 24 hours."

Translation: a non-tropical low pressure system whose barometric pressure drops at least 24 millibars (.71") in 24 hours.

In turn, although I couldn't find "bombogenesis" in the AMS Glossary, that's a term commonly used by meteorologists to describe the process, following from "cyclogenesis," which is the development of a cyclone.

This model analysis shows the pressure Tuesday evening at 7 p.m. EST:

[Source of images: wright-weather.com]

The "secondary" low developing in eastern North Carolina ("primary" low at the time was over Ohio) had a pressure of ~1006 millibars (29.71"). Models have consistently predicted that by this evening the central pressure will have plummeted into the 970s and possibly the upper 960s tonight.

What does all this mean, other than if you were on a boat (not recommended today!) on the ocean offshore of the northeast U.S. with a barometer in the middle of this storm you'd be able to measure the drop in pressure?

It means that winds are going to rapidly increase today!

These were the wind measurements in mph at 5 a.m. EST:

Boston 5 (with no gusts, ditto the others below)
Plymouth 0
Providence 3
Montauk 0
JFK 10
Philadelphia 12
BWI 6
Dulles 7

Per warnings which have been issued by the National Weather Service for blowing snow, power outages, etc., people in the path of this storm should not be misled by the early morning conditions. (And in addition to the light winds, in some places such as Philadelphia there has been a lull in the precip and/or a change to rain.)

You don't need to be a meteorologist to be able to see that the map below shows something really intense!

It's a model forecast for this morning of an exceptionally potent disturbance aloft which is crashing overhead of the developing surface cyclone, whose pressure has already dropped below 990 mb: a buoy just east of the Delaware coast is measuring 988.5 mb at 8 a.m. EST. So that's a drop of the system's pressure of ~17 1/2 mb in 13 hours, on track to exceed the "bomb" criterion!

And the wind is already starting to respond, with Dulles Airport now reporting gusts to 44 mph.

As always, at any time you can get the latest updated TWC forecast for your location by typing in your city/town or zip code in the box at the top of this page. If you live in the path of the storm, batten down the hatches, and stay safe!

UPDATE 2:30 PM WED

Strongest winds so far (sustained/gusts in mph):

Boston 24 G33
Plymouth 14 G25
Providence 20 G27
Montauk 17 G33
JFK 31 G43
Philadelphia 21 G32
BWI 27 G40

The pressure at a buoy south of Islip, LI has dropped to ~979 millibars (28.92"), and it's not exactly where the center of the low pressure system is so the pressure of this system is lower than that and solidly in the 970s. (That buoy has also had sustained winds of gale / tropical storm force.)

Thus this cyclone has -- unlike the one associated with "Snomageddon" -- met (and easily exceeded) the criterion for being a "bomb," dropping at least 26 millibars in 19 hours.

That also means that the current cyclone already has a central pressure lower than the Blizzard of 1978 in February of that year in New England. Its pressure only got as low as 984 mb. However, there are a few important differences between the two storms.

Ultimately what matters for wind is the pressure *gradient*, not just the actual pressure, and the Blizzard of '78 was butting up against a much stronger high pressure system than today's low is pressing against, so winds today and tonight are not expected to be as strong along the Northeast coast as in '78, when a gust to 92 mph was measured in southern New England. Also that high built across New England and was accompanied by very cold Arctic air, whereas today the air there is not particularly cold (in fact, temps in much of southeastern New England are currently above freezing with the precip changing to rain in some places, while temps in Pittsburgh are in the teens, in the west/southwest part of the system).

And the intense winds and high snowfall rates lasted a relatively long time in '78 (severe coastal flooding in New England and up to 38" of snow in Rhode Island), whereas this time the low will eject out to sea quickly tonight.

UPDATE 6PM EST WED

Blizzard or near blizzard conditions have been reported today in many places on the I-95 corridor from DC to NYC. The definition of "blizzard" takes into account wind speed and visibility, but not the number of inches accumulation. In that department, the highest totals so far have been in excess of 20" and approaching 2' in west-central Maryland and just across the border in south-central Pennsylvania. The snow better diminish soon or I'll have to revise what I said about no amounts near 40"!

At the moment a band of enhanced snowfall rates is in that region from south and west of Baltimore to near Chambersburg PA. It's at the very edge of the storm; that's a feature you often see in these systems but usually on the northwest not the southwest fringe! There is also an area of particularly heavy snow near Philadelphia in PA and extending across parts of New Jersey to Long Island. Ironically, farther north, in places such as Providence, rain has been mixing in.

Speaking of strong winds ... at about six miles above Nashville this morning, the velocity was measured at 246 mph! That's strong even by jet stream standards, and is an indication of the energy feeding into this storm.

Although that wind maximum is in mid latitudes, it is hooked up with a subtropical jet stream coming across Baja, with the beginnings of a new system evident on satellite imagery via clouds streaking across the south-central U.S.; it'll bring snow to parts of the Deep South during the next couple of days!

And if you look closely at the image below, there is a thin streak of high cirrus clouds connecting that bright band of clouds coming from the Pacific with the storm over the Atlantic!

FINAL UPDATE NOON THU

Yesterday's storm itself wasn't of the magnitude of the worst blizzards of the past in the northeast United States, but what made it historic is the way it came back-to-back in the northern Mid-Atlantic with the one just a few days before it, and the cumulative impact the two have had. In particular, the amount of snow that has fallen since late last week from the Baltimore-Washington metro area across north-central Maryland and into parts of Pennsylvania is amazing.

Here are maps from the National Weather Service (the basemap on the second is courtesy of Google Maps) showing the accumulations from the two storms in a portion of that region.

FEBRUARY 5-6, 2010

FEBRUARY 9-10, 2010

I'll leave you with a few more images.

The first is how yesterday's bomb now looks over the Atlantic Ocean. The impact would have been even worse if the cyclone hadn't moved out to sea last night!

[Image source: NASA Earth Science Office]

Snow piled up on Capitol Hill in Washington yesterday:

[AP Photo / J. Scott Applewhite]

And, the southern snowstorm begins in downtown Dallas this morning (and according to the AP, the dog's name is Dallas too!).

[AP Photo / LM Otero]