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Eastern U.S. Winter Storm Threat Dec 27th

OKpowdah

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Let's begin with the state of winter right now. Where is it? Recent reports say Russia has found it

(http://rt.com/news/r...emperature-379/).

In other news, the planetary reconfiguration that is occurring right now. Major ridging burgeoning from the Aleutians to the Bering Strait and right up over the North Pole by Saturday. The effect? Well at 22/00z that's 125kts at the DT crossing the NP toward the western hemisphere.

blogentry-128-0-68120600-1356042720_thum

Clear at this stage the magnitude of nonlinear processes leading to anticyclonic breaking of the ridge over the NP toward Greenland this weekend. Quite a signal for developing negative AO and negative NAO regime.

At the same time, we have this blizzard ongoing over the Plains associated with an impressive trough, continuing to amplify tomorrow (around -2.5 to -3SD anomaly at 500mb). The trough breaks over the Northeast into Quebec this weekend, amplifying downstream ridging from the northern Atlantic into the Davis Strait. By Christmas Eve, this ridge is starting to bridge with the NP ridge. By Christmas Day, we're actually on our way to achieving a quasi-stable anticyclone from the Hudson Bay to Baffin Island ... i.e. a west-based negative NAO.

blogentry-128-0-14386000-1356042742_thum

This pattern is clearly characterized by a tumble in both the AO and NAO domains as seen on the forecast maps I'm presenting here.

Notice also in the above map the disturbance (lowered DT / PV anomaly) over the Davis Strait. Tracking it back in time reveals that it originates over eastern Russia currently and is brought around the Pacific ridge as it is bursting poleward. From the Davis Strait, this disturbance continues westward (south of the anticyclone) into eastern Quebec and Newfoundland and Labrador by the 27th.

In the end this disturbance in fact plays a crucial role in the 50N/50W slot, with its miriad relationships in Northeast snowstorms.

So I see you perked up at the mention of the word "snowstorm." Yes, I suppose I should get to the actual storm itself.

The storm of interest originates as a shortwave from the GOA vortex. The shortwave is ejected into the Pacific Northwest on the 24th. At this time, attention turns back to the North Pacific where a strong shortwave is plowing under the recently broken-off ridging from the Aleutians to the NP. The shortwave continues eastward, amplifies and breaks just off the west coast. This results in the Pacific jet briefly splitting as ridging amplifies downstream over the western U.S. ...can also note a relate spike in the PNA at this time ... so certainly convergence of teleconnections supporting this event.

blogentry-128-0-12035800-1356052593_thum

Coinciding with this PNA spike, the shortwave associated with our storm amplifies into the southern Plains by late on the 25th-26th.

The general progression of events that I outlined above is supported in the model guidance. But some minor discrepancies leads to large disagreements in the end result for the eastern third of the nation.

Some of these disagreements I think can be related to the span of time between the ejection of the shortwave and the PNA spike ... the greater separation there is between these two events, the further east the shortwave will amplify.

...Can be thought of simply as timing of the group energy catching up to the shortwave.

By the evening of the 26th, we're watching the development of beautiful dual-jet structure, with excellent upper level mass evacuation over an intensifying low.

The ECMWF has tended to want to cut this low inland ... actually track it along the spine of the Appalachians, meanwhile attempting but failing to produce a secondary low along the coastal baroclinic zone. The GFS and CMC both achieve this secondary development with the coastal low taking over.

Below I've included some maps from the 20/12z run of the GFS that presents the features I discussed above. It's worth noting on the DT theta map signs of convection wrapping into the system from the central Atlantic -->

Meteorology ... helps amplify downstream ridging and the overall system.

Modelology... can introduce potential convective feedback errors in the model forecast.

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blogentry-128-0-62447000-1356040310_thum

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Some things to think about for the coming week.



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Do you teach? That was an excellent presentation which was easy to follow and understand. I've written quite a few technical documents in my day and that was a textbook example of taking a complicated topic and making it simple to comprehend. Well done and very informative. Thanks!

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Thanks very much! I've been trying to get a little better at writing discussions that can be followed easily while still managing information from all different directions. It's a fantastic challenge in meteorology where there are so many different aspects to be considered in any one given situation!

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I also enjoyed that.
I hope we can look back in a week, using this post, to understand how and why it ultimately plays out.

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