Tuesday, December 25, 2012

Shifting Pattern Favors Central, East US For Snow

Early January appears to be favoring the Ohio Valley and Northeast for the best snowfall and cold. I'll explain why.

This is the GFS forecast for Hour 168, or the afternoon of January 1st, at the 200mb level. The 200 millibar level is commonly identified with the level best used to observe the jet stream. If we look at the GFS forecast above, we see that there is a deep trough in the Southwest/eastern Rockies. Such a disturbance causes high pressure to form on the West Coast, creating a positive PNA index. This positive PNA would then act to keep the storm system going to the south rather than north and into the Plains.

The ECMWF forecast for the same timeframe and the same level in the atmosphere shows a very similar set-up, with high pressure on the West Coast and a depression in the jet stream over the eastern Rockies. Again, we see a very amplified jet stream in the Mid-Atlantic. Considering both models also show high pressure prevalent over eastern Canada, it's likely that storm systems exiting the Rockies will be forced into a track that goes through the South US, and eventually up the East Coast.

In the long range, both models show a below normal height anomaly at the 500mb level, indicating low pressure will be present in the Gulf of Alaska. In response, high pressure attempts to form on the West Coast and thus tries to initiate a positive PNA situation. As exhibited by the amplified jet stream in the Mid-Atlantic, we would then see the storm track favor a track that goes along the Ohio Valley and possibly up the Northeast.

ECMWF (left) and GFS (right) long range 500mb height anomalies
Problems arise when both models show a positive North Atlantic Oscillation (NAO) evolving, with a stormy pattern over Greenland. When you get a positive NAO, storms are not favored to go up the coast. However, the jet stream is aligned in a more zonal (west to east) pattern that then goes into the Mid-Atlantic. This would explain why models amplify the jet stream in the somewhat-long range.  If we have a positive PNA in place with this positive NAO, I find it plausible for storms to push into the Southwest US. After doing so, they will eject east into the Southern Plains and get picked up by a powerful jet stream. When that storm gets picked up, the jet stream will be forced to buckle south, much like when you place a brick on a horizontal bendy-ruler. As the system's vorticity orientation tilts to the southeast, it attains a negative tilt, meaning the highest storm's vorticies are tilted to the southeast. As this happens, a strong severe weather situation is likely to set up in the Gulf Coast region, and this severe weather will be a multi-day event, likely starting in the western Gulf Coast before going to the Southeast over a period of a few days. On the wintry side, the storm system will slowly move northeast, spreading snow across the southern Midwest, southern Ohio Valley before getting off the East Coast and bringing a better snowstorm to the Northeast. A strong cold outbreak then follows.

If you didn't get that, here's what I think will happen:



armando said...

Hey Andrew merry christmas and thanks for the post. So I have a few questions; will this pattern stay just only the beginning of Jan? Also what would happen if the NAO crashed negative would that only be better for the east? When you have a chance could you make a post about this weekend storm it looks good for the east coast! Thanks

Anonymous said...

same damn crap that just happened. tennesse and kentucky gets screw for snow. i now see why they said 50 degrees in 2 weeks.like i said. they was right

Anonymous said...

What is your opinion as to what Iowa's winter will be like?

Anonymous said...

what is the latest on that new years day storm?
will it hit the central plains?

Martha E. said...

Andrew what happened to your gerbi adventure videos on YouTube? I see a playlist with that name, but all of the videos are deleted. What were they?

Anonymous said...

Accuweather is saying the temps will be near the seaonal average for NYC, which basically means chilly rains if true. I'm feeling blue.

Randy the Random Dude said...

Andrew what is the final stage in the life cycle of a Baroclinic low? Thanks in Advance

Andrew said...

Armando: Anytime the NAO is negative is a good time for the East.

Anonymous #1: Again, you cannot forecast a temperature for a single location 2 weeks out. That has an enormous potential for failure.

Iowa Anonymous: My final winter outlook should show the graphics. That's all I can say.

New Year's Storm Anonymous: It looks to be suppressed south.

Martha: I don't use YouTube anymore- I find it better to use Facebook.

NYC Anonymous: You're putting all your eggs in one basket. That never ends well.

Randy: I don't know at the moment, I'm busy preparing a huge post for later today. I can research it at a later date.