We already posted a 7 day forecast last night, and considering most of the week is a pretty straightforward forecast with really no major disagreements on the models, we’re going to focus tonight’s update on two main themes, the first being the development of a coastal storm in the latter half of this weekend, and the second being a trend towards a much colder regime to cold out December and bring us into 2015.
Over the last few days, we’ve been keeping an eye on a coastal low that would impact the area late this weekend. Model guidance has been in excellent agreement on the idea that a storm will develop somewhere on the eastern seaboard in this time period, but is split into several distinct camps related to the track of the low and hence the impacts that the storm will have on the area. The three camps primarily differ in regards to the strength of the block to our north and as a result how far north the storm can come up the coastline. Let us examine them one by one.
The GFS model portrays the first camp. The GFS is a disorganized mess in the southwest where the energy originally forms, and as a result, when it finally ejects and begins heading east, the strength is not there to provide amplification. Furthermore, the block to our north for this event is so strong on the GFS that it prevents the low from coming up the coast, and as a result, it moves well out to sea before it can have any major impact on us, and the end result is nothing but some sprinkles/flurries as the low passes well to our SE. Considering the bias of the GFS to be too progressive, it’s overall disorganized look at the upper levels compared to the rest of the guidance, the disagreement between it and it’s ensembles, and it’s poor handling of southern stream systems so far this winter, we are discounting it as an outlier for the purposes of this discussion. We weight the odds of a GFS scenario at 10%.
Here is an image of the MSLP and precipitation type on the GFS.
Here’s a map of the GGEM MSLP and precipitation, showing the cutoff to our south. This is also supported by the 12z EURO operational and the UKMET which gets there via a different evolution, but has the same end result of a close miss.
Here’s a look at the placement of the low on the parallel GFS. The EURO ensembles are similar.
In summary, there is the potential for a winter storm towards the end of next weekend, with accumulating snow possible for the area. We’ll continue to keep an eye on it, and will update as we move through the week with more details.
Next, let’s turn to the long range. Here are graphs for the forecasted states of the PNA, NAO, and AO for the rest of the month.
The more interesting story, however, is in the AO and NAO graphs. The AO and NAO have both been predominantly positive the last several weeks, and looking at the graphs, it becomes clear that that is about to change. The AO is forecast to go negative by around Christmas, with the NAO following a few days later towards New Years. As you may recall from our winter outlook, getting these negative, especially the AO, was the key behind our forecast for a colder winter than average, and so far, it appears as though we are right on schedule with that transition. A –NAO signifies the development of high latitude blocking in the Atlantic, allowing for storms to track on the coast instead of further inland, and also allowing for more cold air to be brought in. A –AO allows for colder arctic air to flow down into Canada and the CONUS, and as a result, is very strongly correlated with below normal temperatures for our area. Given that a -EPO is also forecast, which signals for a funnel of colder Alaskan air down through central Canada into the Plains and then further east, the signal is quite strong for a colder period to develop.
Unsurprisingly, now that the long-range models are beginning to pick these changes up towards the end of their runs, they are starting to show a much colder look in the long range. Here’s the long range GEFS anomaly for hour 312, a little under two weeks from now. Note the widespread below normal temps showing up across the country.
We’ll have a more general forecast update tomorrow evening for the remainder of the week as well as an update on the weekend storm, but until then, thanks for reading and have a great night!