Finding Nemo – Blizzard Forecast to Impact New England

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For some reason I think it’s just hilarious that The Weather Channel has named our Friday blizzard Nemo! While naming storms is just a silly enterprise it looks like Nemo may really be a beast!

Earlier today we were all having fun with the 12z NAM that was showing 40-60″ of snow in parts of New England. No big deal, right? The NAM is just not designed to be used for rip and read snowfall forecasts. It’s a curiosity and is of no use. Toss it. You can toss other QPF forecasts from non-hydrostatic models out the window at this juncture as well.

The global models are in exceptional agreement that this storm is going to be huge. The GFS/Euro combination along with their respective ensemble members have honed in on an impressive solution. Here’s my latest thinking in a probabilistic way.

chart_1

While a chart like this would give TV news consultants heart palpitations it’s really the best way to express forecast uncertainty! I know my blog readers have a lot of weather geek in them… so enjoy! If you notice here the odds of more than 18″ of snow are pretty low… there’s a reason for that! The greater Hartford area has only recorded 5 snowstorms (officially) of 18″+ in the last 108 years! The Bridgeport coop observer has never recorded an 18″ snowstorm since the 50s! They’re just not all that common. We’re also still 36 hours before the storm’s onset and a lot can change.

Before I get accused of being a debbie downer. Let me now talk about how amazing this storm looks meteorologically. Here’s the 18z GFS valid at 6z Friday.

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Here’s the GFS which is in fair agreement with the Euro (though the GFS is a bit more impressive with an earlier phase/capture). Synoptically, a digging northern stream disturbance mananges to be timed and located perfectly to dig and capture a moisture laden southern stream disturbance. Beauty! Without blocking downstream there’s not much wiggle room. The timing has to be PERFECT for this to work out for Connecticut. 6 hours in either direction will make a huge difference (keep in mind the northern stream disturbance is over Montana and the southern stream is over Texas).

All of our models show the perfect phase though the Euro is a bit late and therefore a bit less impressive in Connecticut compared to places around Boston. So assuming that actually occurs and it’s not late (a late phase would still clobber the Cape and eastern Massachusetts but give us a more pedestrian storm) let’s watch the beauty unfold at 700mb from 18z Friday to 6z Saturday in 6 hour increments on the 18z GFS.

700mbEvolution

I mean if that’s not breathtaking I don’t know what is. We even manage a little loop-de-loop there as the 700mb low tightens and matures. Exceptionally powerful frontogenesis on the northwest flank of that mid level low would result in a super band the likes of which you rarely experience. All of this is taking place under an area of strong divergence thanks to a coupled jet streak (classic KU setup).

The result of this “perfect scenario” is a large swath of 1.5″ to 3.0″ of liquid and likely a snowfall on the order of 1 1/2 to 3 feet.  Wowzers.

The perfect scenario is only one such possibility, of course, and small changes in the next 24 hours with the 2 disturbances we’re watching can make a large difference down the line. In order to start picking up over 15″ of snow in this part of the country you need small-scale (mesoscale) features on your side. These are notoriously challenging to forecast even 6 hours ahead of time!

That said, as of right now this has all the makings of a classic. Odds are better than 50/50 that many inland areas see a foot of snow. Along the shoreline some sneaky mid level warmth may  bring a period of sleet and a bit of mid level drying may promote some dry slotting. Big “IFs” here though with plenty of potential for a crippling snowstorm if the shoreline is able to hold the sleet and dry slotting at bay.

Will this turn into an historic storm? It’s possible. Too early to say for sure. The amount of liquid being generated by the normally reliable models (like the GFS and Euro – ignore the NAM) are staggering. I’m excited for this one 🙂

The Great Southeaster – November 25, 1950

Storm surge in Southport / Courtesy: Pequot Library Association

Connecticut meteorologists love talking about nor’easters but do you know about the state’s biggest southeaster? The “Great Appalachian Storm” of November 25, 1950 was one of Connecticut’s most violent wind storms on record. In some towns the wind speeds in 1950 were only exceeded by the great hurricane of 1938!

The storm was only of modest strength in terms of central pressure – 980ish mb. But what made the winds vicious was the 1050ish mb high near Maine. The freakishly strong pressure gradient produced violent southeasterly and easterly winds across New England. Here are some of the wind gusts recorded in Connecticut on November 25, 1950.

  • Bridgeport – 88 m.p.h.
  • New Haven – 77 m.p.h.
  • Hartford – 100 m.p.h.

The 70 m.p.h. 1-minute sustained wind in Hartford remains the strongest wind recorded for the official Hartford records since observations began in 1904. The second highest value is 64 m.p.h. recorded during the October 3, 1979 tornado. In Bridgeport the sustained wind of 62 m.p.h. is one of the highest on record (since 1948) with the highest occurring during Gloria in 1985 at 74 m.p.h. sustained and two other higher wind speeds during the winters of 1964 and 1969. Note on record: 2-minute sustained winds replaced 1-minute sustained winds in 1995.

Courtesy: PSU/Richard Grumm

For a non-tropical storm there’s no question in my mind that the 1950 southeaster was the most violent windstorm we’ve seen. The standardized anomalies from Richard Grumm at the NWS in State College shows a wide area of +4 standard deviation 850mb winds. That’s quite a low level jet! The winds reached 160 m.p.h. on Mount Washington in the core of that LLJ.

Looking at the reports from that day here in Connecticut temperatures in the warm sector came close to 60º with highs in the upper 50s in Hartford, Bridgeport, and New Haven. The unseasonably warm weather, when coupled with a a ripping low level jet, lead to enough turbulent mixing to mix down destructive winds – in some cases to 100 m.p.h.!

Here are some of the comments from the official weather bureau reports.

  • Hartford – “Of paramount interest in this month’s weather is the occurrence of “The Great Wind Storm of November 25, 1950″. Considering its great extent, extreme weather of various types, and its unusual meteorological character, this storm will be long remembered. At Hartford, E’ly winds averaged the amazing speed of 38 m.p.h. for the entire day of the 25th, and attained gust speeds of at least 100 m.p.h. on at least 3 occasions between 4:30 and 7:30 p.m..”
  • Bridgeport -” Storm of Nov. 25th worst since 1938. Station inundated with 4 to 5 feet of water.”
  • New Haven – “Severe southeast storm on 25th. Extensive wind and water damage to shore fronts. Many trees, wires, antennae down, roofs damaged. Max for 5 minutes: 50 SE at 1:55 PM (17 Hrs.), probably exceeded 8:00-9:00 PM. Fastest single mile: 57 SE at 1:56 PM (17 Hrs.), possible exceeded  8:00-9:00 PM. Gusts: 55 MPH at 1:35 PM; 66 MPH at 4:20 PM; 66 MPH at 7:40 PM; 77 MPH 5 second gust at 4:45 PM… 5 min. max record; fastest mile exceded in Sept. 1903.”
12 UTC Surface Analysis / Courtesy: MWR/NOAA

The strong winds produced widespread tree and power line damage across the state. The winds tore a roof off a dormitory at UConn and ripped shingles off roofs across the state. Several shoreline homes lost their roofs according to a Hartford Courant article from shortly after the storm.

The storm surge flooding was extensive on the coast. In New London the tide reached an impressive 7.58ft MLLW. The only storms higher in the last 100 years are the 1938 hurricane, hurricane Carol, and hurricane Sandy. In Stamford at the Hurricane Barrier the tide reached 9.5ft NGVD which was similar to Irene’s tide level.

Laguardia Airport on 11/25/1950       Courtesy: NYC OEM

Much like Sandy, the storm surge flooded large portions of New York City including the lower east side and Laguardia Airport. Sandy’s surge, however, was much more powerful in the parts of the City, like Staten Island and the Rockaways, with Atlantic exposure.

On the Connecticut shoreline houses, cottages, railroad tracks, and beaches were swept away. Newspaper accounts indicate that the sand was several feet deep on coastal roads and was removed by snow plows. Many people had to be rescued from their homes after refusing to heed evacuation orders.

The storm resulted from an exceptionally deep dip in the jet stream and monstrous closed low that formed over the Appalachians in a trough the went negatively tilted. The easiest way to visualize this is to look at the 500mb height anomalies associated with the low. Much like Sandy, there was a large “block” downstream. In the 11/25/1950 case there was a 350+ meter positive height anomaly over eastern Quebec. The upper level low itself over the central Appalachians was an exceptionally impressive 450+ meter negative height anomaly.

This type of unusually deep system lead to unusually cold weather and extreme snowfall in the Appalachians and the Ohio River valley. The storm is one of the worst blizzards in parts of the country. Steubenville, Ohio recorded 44 inches of snow while the synoptic desert of Pittsburgh dug out from 30.5 inches of snow!

In the southeast U.S. the backside of the storm delivered a bitterly cold air mass. The mercury dropped to -3º in Atlanta, GA. Many observing sites saw their coldest November temperatures on record. 850mb temperatures reached an INCREDIBLE -20ºC over northern Georgia at 12z 11/25/1950.

Courtesy: PSU/Richard Grumm

Not very often do you see a -6 sigma 850mb temperature! The exceptional baroclinicity and phasing resulted in what amounted to one of the most impressive east coast storms of the 20th century.

PSD 20th Century Reanalysis

While we frequently refer to the March 1993 storm as the “storm of the century” the November 1950 storm gives ’93 a run for its money. For east coast storms we really had 2 storms of the 20th century.

Epic Upslope

Courtesy: Stowe

I got an email this morning saying:

“I have a condo up north in the mountains and I was very concerned after reading your tweets about up to 2 feet of snow this weekend… you need to be careful about your alarmist tweets”

It turns out that some areas in Vermont will wind up with 3 1/2 feet of snow from an absolutely exception upslope snow event along the spine of the Green Mountains from Killington on north!! Killington has picked up a foot, Stowe and Jay over 30″ as of 7 p.m. Saturday and it’s still dumping.

Snow like this is localized to along the spine of the mountains that run the length of the state. In this case the snow was focused in central and northern Vermont with relatively little in the south.

With westerly and northwesterly wind and plenty of low level moisture the rapid rise in elevation from the Champlain Valley to the Greens forces the air to rise rapidly. The lift  is comparable to a storm that just parks over the same spot and doesn’t move (you can’t move mountains like you can move a storm). Instead of lift being cause by processes in the atmosphere in “upslope” we get lift cause by the wind blowing up over terrain which yields upward vertical motion and if strong enough clouds and precipitation.

Here’s the radar from Burlington that shows the impressive dump of snow that continues in the mountains.

Courtesy: WeatherTap (click on image to animate)

It looks like many areas in central Vermont will see 1-2 feet total from this storm. The mountains in northern Vermont north of I-89 should see around 40″ of powder. Absolutely epic!

Light Saturday Snow

In this winter even a light snow event is enough to garner some extra attention! The setup that we have is actually quite close to developing a fairly sizable snowstorm but the pieces will not come together just right this time.

Here’s the GFS 500mb height/vorticity forcast for 1 p.m. Saturday.

I circled a “vort max” or “shortwave” off the east coast. Think of this as a piece of energy in the mid levels of the atmosphere. To the west there is a large and relatively impressive trough digging through the Great Lakes. These 2 features will remain separate. They will not phase into one. That means we’re left with a relatively large but diffuse area of lift in the atmosphere offshore as opposed to a concentrated area of strong lift – which is how we get our biggest storms.

You can see the impact of that by looking at the sea level pressure prog for the same time.

Notice how diffuse the low is (i.e. not tightly packed/wound up)? This shows that this storm is going to have trouble getting organized until it reaches or passes our latitude. Not good for a big storm.

That said there will be enough lift and convergence to produce a widespread swath of snow across southern New England. Many locations will see 1″-3″ of snow. It’s possible that if the storm winds up organizing faster and closer to the coast we could see more impressive snow as a comma head develops.

There is one other thing to watch out for but at this point seems unlikely. The 00z and 06z NAM is developing an inverted trough from the low offshore back to New York City and southern Connecticut. This essentially maximizes low level convergence and

produces a narrow, but intense, band of heavy snow. At this point the NAM is the only model showing this so it’s hard to get too excited about it but it’s something to watch.  The 2 panels on the left show the 06z NAM at 1 p.m. Saturday with a relative QPF max over eastern Long Island and southern Connecticut with the lower panel showing why – a band of convergence stretching north from the Atlantic toward Connecticut.

For what it’s worth the 03Z SREF guidance shows relatively robust probabilities for >4″ of snow across parts of southern New England. The blue shading is >25% chance while the green is >50% chance. At this point I think these numbers are a bit high but there is a chance that this storm trends a bit more impressive for some of the reasons discussed above.

Sneaky Snow?

For people who love winter this weather pattern just plain sucks. Surprisingly, I haven’t been complaining about it. After October a break from winter has been nice – and it’s been nice for our heating bills too!

It appears that this may be one of the rare times that we are able to get a couple inches of snow to fall in a hostile early season weather pattern.

12z NAM / 48 Hours 500mb Vorticity & Height Forecast

Our computer models are showing a very powerful area of upper level energy ejecting from the southern Plains toward southern New England by Thursday. This vorticity maximum gets deamplified as it is crushed by a growing area of confluence over the Canadian Maritimes and the North Atlantic.

Occasionally this kind of setup can deliver what I like to call a drive-by snowstorm. A 6 hour window of snow in a somewhat localized swath. The storm will never become a powerful or mature cyclone as its upper level energy is zipping along and weakening. In addition in the mid levels of the atmosphere a low pressure system never really closes off and you’re left with an “open wave” effectively limiting the amount of precipitation that can fall.  See the 700mb NAM forecast to the left.

So what’s the bottom line? In a storm like this there’s little room for error. The storm has to “thread the needle” so to speak with a perfect track of the 500mb shortwave. At this point our models are in fairly good agreement with a track just to the south of Connecticut. With warm waters and a period of upper level divergence we should see a deepening surface low and a period of warm advection produce precipitation along and north of the shortwave’s path.

At this point things look cold enough for a rain/snow mix or snow as long as the storm maintains its current forecast track. Any adjustment north would back winds near the surface enough to warm the boundary layer and result in more rain especially for the valleys and shoreline.

The maximum amount of snow we can reasonably expect in a setup like this is around 6″ given an “ideal” track. Something less than ideal will result in less snow whether it’s due to mixing or due to the storm tracking too far south. Odds favor less than ideal but it’s worth watching.

By midday tomorrow we should have a better idea how this “drive-by” snow event will play out. It would be pretty incredible if such a mild meteorological autumn and beginning of winter was able to produce 2 plowable snowfalls before December 10. If you like snow don’t get too excited yet and if you don’t like snow don’t worry nothing is set in stone at this early juncture.