This has been the “nickel and dime” winter so far for the DC and Philly metro regions with respect to snowfall as both locations have had many small snow events, but nothing on the big side, and time is growing short.There is still some hope; however, for some serious snow in both of these areas as long as the North Atlantic Oscillation (NAO) remains negative and it appears that that will be the case for the next couple of weeks.A negative NAO promotes a “blocking” pattern in the upper atmosphere in the northern latitudes (e.g. around Greenland and Hudson Bay, Canada) and this, in turn, increases the chances for cold air masses to drop into the central and eastern US and, just as important, it also tends to help generate sustained cold shots in this part of the country.
All indications are that the NAO will drop this week as blocking develops up north and this could actually produce a noticeable effect on the actual weather in the US as early as late this week.A storm will cross the country later this week after coming ashore on the west coast in the next couple of days.By late week, this storm will try to turn northward into the Great Lakes region, but because of a negative NAO and a block in the upper atmosphere, that low will come to a grinding halt over the Great Lakes.Strong surface high pressure will exist across the Hudson Bay region of Canada late this week reflective of the upper atmospheric blocking pattern setting up in the northern latitudes.After this initial low grinds to a halt, it will weaken and a second storm will likely take shape over the Carolinas, forced to the south with the evolving blocking pattern, and this type of setup will increase the chances for some frozen precipitation in the Mid-Atlantic region at the end of the week or this weekend.Without a negative NAO and the developing blocking pattern, plain rain would more than likely be the outcome in the Mid-Atlantic as milder air would more easily move into the region on the southeast flank of the initial low.This same kind of influence from a northern latitude blocking pattern is likely to occur over the next couple of weeks as suggested by the persistent negative forecast of the NAO.
The first map below shows the 00Z Monday GFS ensemble 500 mb height anomaly pattern in North America for later next week (at 252 hrs) and the positive anomalies near Greenland and Hudson Bay (orange, yellow) are typically seen with a northern latitude blocking pattern.Also, note the negative height anomalies (blue, purple) stretching from coast-to-coast across the US which is suggestive of a nationwide cold pattern that could be somewhat sustained because of what is going on farther north.The next two maps show the forecasted NAO index values based on the 00Z Monday GFS and Euro ensemble runs with negative values persisting in both cases right through the medium-term (all three maps below from the raleighwx.americanwx web site). For more on the late week event check out the latest video at thesiweather.com
First things first...we have a real threat to deal with at the end of this week in the Mid-Atlantic. However, if I were to look ahead, I would say this: there have been many times over the years when the "end of a period of negative NAO" (ie a transition period) yields a big storm. Therefore, the timing may just be right in early March for a transition period - and a big storm. We'll see.