I am monitoring the potential for a winter storm from the fluid timeframe of January 31 - February 2.
It does appear that a storm system will be delivered upon the Southwestern US in the closing days of January, and will subsequently move east. This storm was discussed in an earlier post, but again, a strong storm system could eject from the Rockies and center itself over Gary, Indiana, as shown in the above forecast for January 30th. This forecast displays mean sea level pressure anomalies, with cool colors displaying low pressure areas and warm colors indicating high pressure areas. However, look closely at that second low pressure system that has formed right over Brownsville, Texas. This storm system appears to be on the tail of the dominant storm system of Gary Indiana, and that's where our story begins.
The 500 millibar pattern for this timeframe suggests a strong high pressure system will form over the northeast Pacific and West Coast in a classic pattern called the 'positive Pacific-North American pattern', or Positive PNA for short. In a positive PNA, high pressure forms in the aforementioned areas, and, as a response to the jet stream shooting north to get around the high pressure, it must collapse down into the southern Rockies as a response, creating a very favorable set-up for storm systems impacting the Central and East US. This idea is displayed well in the 500mb pattern shown above, which is valid for the morning of January 30th.
Now, the storm system is clearly shown by the dip in the colors on the map above (depressions in lines and colors show low pressure systems, arching patterns indicate high pressure), and is placed in western Texas. Let's revert back to our positive PNA pattern. The first part of the pattern (where high pressure was placed out west) was already explained. But, there is another, more important factor that concerns storm tracks of systems emerging from the Southwest. In a positive PNA, high pressure will tend to form in the Southeast US. This persistant high pressure system is also called the 'Southeast Ridge', a name that lives in infamy for those in the Northeast, as it diverts storms away from that region. However, the phrase lives in glory for its winter storm capabilities in the Central and parts of Eastern US. I drew three arrows in the map above; the single arrow represents a rough sketch of what the system would do if it abides by the positive PNA, while the set of arrows that crawl up the East Coast demarcate what the system would do if it managed to bypass the Southeast Ridge.
There is one more thing that is helping the Midwesterners and Ohio Valley folk in this situation- a negative North Atlantic Oscillation (NAO). The negative NAO involves high pressure being present over Greenland, but its the placement of that high pressure that matters. You see, if the high pressure system is west of Greenland, it is appropriately named a 'West-Based negative NAO'. while high pressure centered east of Greenland results in the 'East-Based negative NAO'. If you look closely at the same 500mb map, you see an intrusion of blues in eastern Greenland. These are higher height anomalies, indicating high pressure, then indicating an east-based negative NAO. Why do we care about this? Because an east-based negative NAO enhances the potential for storms over the Midwest and Ohio Valley, while a West-Based negative NAO encourages the storm formation over the Northeast, which are known as Nor'easters.
This is going to be a very potent time period, and the dates of this are very fluid. We will need more model runs to either confirm or deny the potential of such an event occurring, but right now, things are looking up for the snow-starved Midwest if this model forecast is to come true.
Andrew
