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:

-Andrew