Saturday, January 23, 2021

Winter May Make a Comeback

With the exception of a few flurries and the snow/mix event back in December, winter (other than some seasonably cold days and nights) has been largely absent this year. There have been multiple factors (and some bad luck) that have contributed to this. 

Technical Discussion

During the fall, the configuration of the upper level weather patterns over the Pacific Ocean caused warm air to essentially flood portions of North America. Our cold air "source" regions in Canada became essentially non-supportive to winter weather events. Even with storm systems tracking favorably for our region, there was simply no cold air to tap or drain into the area.

If you're a regular reader of weather blogs or discussions, you'll likely know that our best snowstorms tend to occur when certain atmospheric teleconnections are in the right configuration. Primarily, we look at the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), and the Pacific North American oscillation (PNA). In an ideal world for snow, we'd have a negative NAO, negative AO and a positive PNA. It's also important to remember that we are in a La Nina pattern this winter, which is generally not as favorable for snow as a weak to moderate El Nino. 

Interestingly enough, we've had those oscillations in our favored spots for large sections of the winter. The problem is, the cold air source regions are just not that cold. This is improving, and this is part of the reason why we now have some winter weather threats to track.

End of Technical Discussion

The first threat for winter weather will be from a storm system that will impact the area from Monday into Tuesday. As usual, the models will shift around until we are closer to the event start time. This particular system was "slipping away" as it appeared that it could be a very light event or a rain event (or both!). However, there has been some tendency in the past few model run cycles to bring the event back to a point where we could see some wintry precipitation. 

The last 12 runs of the GFS model for the Mon/Tue winter weather potential. (Source: TropicalTidbits)

You can see in the above animation that the winter precipitation threat has bounced around a fair amount. Areas that are typically favored (north and west of the metro area and into the I-81 corridor) could see the best shot at winter weather. The unfortunate thing is that those areas may also see a significant icing event caused by freezing rain. Freezing rain is a tricky precipitation type (ptype) to forecast, so this won't be clear until closer to the event. 

The Saturday, January 23 06z GFS model run showing potential freezing rain accumulation. (Source: College of Dupage)

The image above shows the 06z GFS run (the early Saturday morning model cycle) freezing rain accumulation. It shows very damaging amounts of icing for parts of the higher elevations to the west. Even some icing shows up closer in to the DC/Baltimore areas. 

The Saturday, January 23 06z GFS snowfall forecast. (Source: College of Dupage)

Meanwhile, you can see snow accumulations look A LOT less impressive based on this run of the GFS. Of course, this is all still a fluid forecast and a lot can still change. For now, if you are WELL to the west of the metro area, it wouldn't hurt to prepare for some icing. Any forecast for destructive levels of icing should hold off for now until certainty is higher with the storm system. 

Perhaps the more interesting aspect of the forecast is later in the week. Some model solutions suggest a another storm system will pass by to the west before transferring to the coast. This would be on or around Thursday. While the latest runs of the GFS model are showing a significant snowstorm for the region, other models are not yet biting on the event. Thus, we'll leave it as a note here - let's get through the Mon/Tue storm before we look too far into the next one. 

Stay tuned!