Understanding The Role of Shortwaves in this upcoming Nor'easter

[phil882]

Hi Everyone,

I've been largely absent from the most recent discussion on this weeks pending nor'easter, but I wanted to provide a perspective I've only seen sparingly used for this upcoming event. I'd like to document the recent changes in this upcoming event and provide a reason for why the models are adjusting their solutions accordingly.

So first things first, we saw a pretty large shift eastward in the last 2 days of numerical guidance. The models went from a fully phased powerful solution with a sub-980 hPa low pressure near the New Jersey coastline, to a a weaker, further east solution that won't cause nearly the headaches that were earlier expected with the more amplified solution. Why? A quick diagnosis looking at the last suite of model solutions verifying at the 00z analysis time last night offer some clues on the culprit.

http://www.atmos.alb.../shortwave.html

The key I want to harp on (and this may not be the only culprit, just the most obvious one that sticks out to me) is the incoming shortwave that is currently rounding the ridge over Western Canada. Models seem to have an especially hard time with shortwaves that traverse long-wave ridge lines, as they tend to speed up and de-amplify. Animated above is a d(prog)/dt animation of such a shortwave that will be a main player in the developing nor'easter over the next 24 hours. Plotted above are 500 hPa geopotential heights (black contours), relative vorticity (color shading), 500 hPa vertical velocity (blue contours, only negative omega shaded) and finally wind vectors (wind barbs) at 500 hPa. All of the images are verifying at 00z 6 Nov 2012 (7pm last night) starting with a forecast made 48 hours before the analysis and going forward in time to the analysis of the 00z model run for the same date.

One thing that should jump out within the red circle is the adjustment of this shortwave moving toward the analysis time. The shortwave seems to become less consolidated (in that the geopotential height lines are not as tightly packed). A weaker geopotential height gradient means weaker winds over the shortwave disturbance, and perhaps a slower motion of the shortwave as a whole.

So what does this mean if we have a slower moving disturbance than initially forecasted in the model runs coming from the northern stream? Well a lot of the discussion focusing on this upcoming system is the degree of phasing that is expected. Phasing is a broad term that is applied to the interaction and eventual merger of two cyclonic vorticity fields from different jet streams. For example, vorticity elements from the subtropical and polar jet can merge and create a highly amplified jet stream.

In this case the observed shortwave is riding along a weaker than initially forecasted geopotential gradient (at 500 hPa). It is difficult to get s/w interaction unless two features get close enough to be influenced by each other's flow. The weaker geopotential height gradient means the flow advecting the s/w over Canada is slower than progged, and thats why instead of a complete phasing, we see the upstream shortwave lagging behind, leading to a lagged phasing response that doesn't take place until the leading shortwave is over the Atlantic ocean, resulting in cyclogenesis further offshore.

I hope this makes a little bit of sense in explaining why we likely won't see some of the more amplified solutions that were forecasted earlier this week from the model guidance.

[BlizzardWx]

Thanks, interesting info here. I'll remember this for future reference.

[Amped]

H5 closed low also seems odd. Broad and week. Seems like it could be a lot deeper. I guess thats what you get from a PAC disturbance in November, not that much Potential Vorticity (Since it didn't originate over an arctic airmass. )