Ready for winter? Got your snow blower working? Got enough firewood and milk to last you a few days without power? If you answered no to any of these questions, I hope you don't live from Virginia to New England. Winter is coming early to portions of the mid-Atlantic and Northeast this weekend and it is going to be a brutal awakening for many. My latest thoughts on the storm along with snowfall map are below.
The Set-Up
There is no better place to start this discussion than at the beginning so let's begin with the current atmospheric set-up as of 3 AM (7 UTC) on Friday, October 28th 2011. Potentially the eve of an historic early season winter event.
The image below is a water vapor satellite image and I have highlighted the important areas using the extremely outdated and archaic "Paint" program. In basic terms, this image shows how much water vapor (or moisture) is in the mid to upper troposphere (part of the atmosphere where weather happens). Moist areas are whiter while dry areas are darker.
Courtesy of PSU E-Wall: GOES-13 Water Vapor 0615z 10/28
A definition of each area...
1) Northern branch shortwave (piece of energy)
2) Trailing southern branch shortwave
3: Leading southern branch shortwave
Rina: Tropical Storm Rina
Green arrrows: moisture plumes from subtropical jet and Rina
Over the next 36 hours, the three pieces of energy (1,2,3) will phase (or join together) over the Ohio Valley. All of this combined energy will allow an area of low pressure to form off the Eastern Seaboard Friday night. Let's take a look at a forecast model to get an idea of how the storm will evolve.
How the Storm Takes Shape
Below are 3 frames of the 0z/28 GFS (06 hours, 18 hours, and 30 hours). You can clearly see the 3 shortwaves fully phase together by 30 hours.
00z/28 GFS; 20 hrs; phased
The first two images (6 and 18 hours) are more for illustration purposes of how the shortwaves phase. The last image (30 hours--Friday night) is what I want to focus on to explain the set-up ahead of the storm.
I want to point out several key factors here...first is the trough across the lower Ohio Valley that is responsible for the precipitation circled on the bottom right chart. Second is the area of confluence (2 arrows) east of New England that is responsible for high pressure (upper right chart circle). Third is the shortwave trough just north of Washington. Fourth is the moisture plume being drawn northward from Rina (purple arrow on bottom left chart). All of these factors will play a role in storm evolution.
First, as the Ohio Valley trough deepens, air rises ahead of the trough in the upper levels of the atmosphere, lowering the surface air pressure, hence the low pressure off the Carolina coast. In addition, the area of high pressure over New England (1024 mb) is providing fresh cold air for the region. This is one of the major players in a winter storm in the Northeast. You almost always have an area of confluence over New England or just off the coast with surface high pressure providing the cold air.
This dome of cold air also helps to increase the precipitation. As the warm air from the south approaches the cold dome, it rides, up and over the dome because cold air is more dense. As the air rises, it saturates and it precipitates. This is a process called overrunning and is common ahead of warm fronts.
What does it mean in this situation? It means that snow is already breaking out across the central Appalachians and southern PA Friday after midnight. Then the fun begins.
The storm will rapidly intensify off the Delmarva Saturday morning as the upper air trough tilts negative. Moisture will be plentiful and thrown back into PA and NY. The cold air in these areas will keep precip all snow. Closer to the coast is where the battle ground will be and where the forecast is trickiest. How much snow will fall?
Snow Map
Snow map issued 5 AM 10/28
I apologize for the crudeness but it gets the point across. A breakdown of the zones:
Zone 1: Flurries only. Storm too far east for precip sheild to push this far north and east.
Zone 2: Coating-2" Cold but not too much precip
Zone 3: 2-4" Cold but not too much precip
Zone 4: 4-8"
Zone 5: 8-11"
Zone 6: 9-14" JACKPOT!
Zone 7: 4-8" Will begin as rain, then change to snow in much of this area.
Zone 8: 2-4" Majority of the storm is a cold rain, snow as storm pulls away, best accumulation on grassy areas.
Zone 9: Coating-2" as storm pulls away, otherwise rain.
Dashed line = rain/snow line. I expect little to no mixing north of this line.
My Reasoning
I am clearly favoring the snowier solution of the GFS and Euro and basically discounting the NAM based on the snow totals pictured above. There is startling consensus between both the GFS and Euro along with the GFS ensembles. In fact the GEFS are even farther west and wetter than the operational. The Euro has been so ridiculously consistent (since 5 days away) that it would be almost inconceivable that it ends up being wrong and the storm goes out to sea. The SREFs are also similar to the Euro and GFS although maybe slightly farther east. This is partially due to the NAM being east and the SREFs use some NAM solutions.
Of course a model consensus doesn't mean that the proposed solution will occur but other meteorological factors also point to a nasty storm, rather than a farther east solution (ala the NAM). The water vapor imagery this morning is very promising with the pieces coming together nicely. The moisture plume from Rina is working northward. As Rina moves northward you would expect this snowstorm to be farther west and north as well. Also, while an area of high pressure over the Northeast could help keep the storm south and east, I do not believe the dome of cold air is strong enough to suppress the system south.
Another tried and true method is the "Boise Ridge." If there is a ridge over Boise, ID 24 hours prior to the storm, snow is likely in the east (winter of course and temp dependent). The ridge axis looks to be just slightly east of Boise but with the rapidly amplifying trough in the east, the low should be able to pull itself close to the coast. In addition, the baroclinic boundary (warm water, cool land) is close to the coast, not off shore. The low will want to ride as close to this boundary as possible.
The model trend as also been to the west over the past 2 or 3 runs as all the pieces to the puzzle enter better data gathering areas of the US. In the end, this scenario that is unfolding is very rare and the all the pieces must fall into place at the exact right time or the storm will just slip out to sea. At this point though I do not see it happening and feel very confident about the snow map.
Final Thoughts
- Tight gradient of snow totals on northwest side of system. Slight shift west or east can drastically change area of heaviest snow.
- Tropical influence will allow for increased moisture and thus higher snowfall totals
- Rapidly deepening low will lead to convective banding. Thus, some places may see periods of light snow while areas just several miles away are getting dumped on. Convective banding is nearly impossible to pinpoint unfortunately.
- Snow will be extremely heavy due to relatively warm temperatures. Trees and powerlines will go down and thousands of people will be without power.
- It will be windy along the coast Saturday afternoon 25-35mph
- The "Big Cities" shouldn't see much accumulating snow.
Thanks for reading!
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