Thursday, February 12, 2015

Long Range Outlook (Made February 12, 2015)

This is the Long Range Outlook, valid for the middle and end of February into the start of March.

Ensemble guidance close to 9 days out from today shows a pattern highly conducive to cold weather across North America. We see a strong ridge positioned across the west coast of North America, pushing into Alaska. The alignment of this ridge closer into the Gulf of Alaska as opposed to along the shoreline indicates a negative East Pacific Oscillation (EPO) pattern. This sort of pattern allows for sustained northwest flow (winds out of the northwest) in the Central US, and this is well shown by the strong upper level low buckling south into the Great Lakes. Short range projections show very cold temperatures slamming much of the nation, hitting the northern Ohio Valley and Northeast very hard in particular. This cold pattern looks to be enhanced by some ridging, albeit suppressed, south of Greenland. I am concerned that the ridge not being positioned closer to Greenland could make this cold weather more progressive and not as long-lasting as current ensembles are making it out to be, though those details can be sorted out further down the road. For now, the opening days of this 7-31 day forecast period are looking very cold.

By Hour 348, which is about 14 days out from today, we see a shifting of the members mentioned above to the west. Our negative EPO ridge has now been shunted west, and now occupies the Bering Sea into waters to the southwest. The strong upper level low still remains on our side of the hemisphere, but the core of this low has been retracted into the North Atlantic, with a leg of negative height anomalies stretching out to the Rockies. As a result, the general idea is that more progressive flow can be expected as we round out February, which should be able to moderate any cold blasts near the end of the month. We also see ridging coming up offshore of the East Coast, which could put an end to what looks to be an incredibly cold and snowy February for millions in the East.

We now turn to tropical forcing to help us identify the pattern for the beginning days of March. Looking at the bottom panel, we see a significant swath of enhanced tropical convection blossoming just south and east of the subcontinent of India, depicted by the deep blue shadings. If we match the positioning of these anomalies to the eight phases of the Madden Julian Oscillation, we can expect the emergence of a Phase 3 or Phase 4 event, as the composite chart below shows.

Generally, Phase 3 events support cooler than normal temperatures in the United States, while Phase 4 composites indicate warmth prevails across the country. Purely due to how February looks to shape up, I would favor a chillier outlook to kick off March. This method of persistence forecasting tends to work well, particularly when the pattern seems to 'lock in' to a certain temperature alignment; in this case, a cold Central and East US, with warmth in the West.

Looking even further out, just for kicks, let's go over sea surface temperature anomalies, under the surface. Anomalies show the 'warm part', the upwelling phase of a Kelvin wave moving eastward at a depth of about 100 to 200 meters. We're currently seeing the 'cold part' of that Kelvin wave, the downwelling phase, hitting the surface from about the 100 west longitude line on eastward. The warmth from the upwelling is already at the surface in the Central Pacific, and this could be setting up a more El Nino-like pattern. I personally find this to be plausible, as El Nino winters are notorious for a slow start and furious end, as we're seeing in real time right now in the East US.
In springs with an El Nino, temperatures in the North US will generally be warmer than normal, while the opposite scenario plays out in the South, with colder than normal temperatures. It remains to be seen if this will play out in the March-April-May period, but it'll be something to watch.

To summarize:

- A very cold pattern will unfold over the next 7-14 days.
- There are hints of a warmer pattern coming around for the period beyond 14 days, but this needs to be watched for potential failure.
- It is expected that the East will remain overall below normal throughout the entire period in temperatures. A similar story, though not as cold, may be expected in the Central US.
- Snowfall should favor the East US in this entire pattern, though a shift to a more inland track could occur in late February if model guidance ends up being correct.


1 comment:

Anonymous said...

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