Good evening to you from Southern Connecticut Weather!
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.
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.
Now, let’s look at the GEFS ensembles. Notice that the MSLP is much more well defined than on the operational, and it is also positioned further NW. You’ll notice that to the north and west of the low, there are areas of red on the map, indicating higher pressure than normal. This is the block that we mentioned earlier, and it’s what’s keeping this storm from running further north along the coastline or inland and forcing it to move under us, keeping us in the colder sector of the event. Here’s images of the MSLP and QPF on the GEFS mean. Notice that most of the state has a couple tenths of an inch of QPF, which taken at a 10:1 ratio, would be a couple inches of snow verbatim. This is a significant difference from the operational GFS, and as a result, we are inclined to discount the op GFS.
The next camp is best represented by the Canadian GGEM model. The GGEM has a strong coastal storm off the eastern seaboard, but because the block is so strong, it is booted east before it can get to our latitude and as a result, we have nothing but cloudy skies, light snow, and cold temps. This is certainly a possible scenario, and considering that we’ve seen the block trend stronger over the past few runs, if we have a weaker and less defined trough to start with, it’s quite likely that this is what we’ll end up with. Overall, we’ll give this solution a 40% chance of verifying.
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 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.
Finally, we come to the third and most interesting scenario, that supported by the EURO ensembles and the Parallel GFS. This scenario results in a low that is strong enough to come up the coast to pass over or near the 40/70 benchmark and avoid being suppressed by the block. The block then acts to slow down the storm, allowing for it to “bomb out”, deepen, and intensify, creating it’s own cold air in the process and allowing for most if not all of the state to be cold enough for snow. In a solution like this one, we’d likely see heavy snow in the interior with rain to moderate snow on the coastline, with several inches of accumulating snow possible across the state. While this scenario has relatively little support, we are giving it fairly high weight for a number of reasons. First, the clustering of the individual euro ensembles is very high for such a long lead-time, and the agreement is excellent.
Secondly, the overall upper air pattern supports such a setup. The block in place is just the right strength to promote deepening while not being so strong as to suppress the system to our south. There is a 50/50 low in position, which is essentially a low NE of the block keeping it from receding as the storm tries to push north into it. The strong nature of the low suggests dynamic cooling would take place, and the general evolution of the event and timing in the upper levels is favorable. While it is not a sure thing for a snow event by any means, I believe that the upper air pattern is most conductive of something like the EURO ensembles happening, and as a result, I’ll give it a 40% chance of happening.
Here’s a look at the placement of the low on the parallel GFS. The EURO ensembles are similar.
Here’s a look at the placement of the low on the parallel GFS. The EURO ensembles are similar.
As you might have noticed, I’ve only given a 90% chance of one of the three above scenarios happening. The remaining 10% is to account for the possibility that all the models are wrong and that this becomes a great lakes cutter or a partly cloudy day with no storm for 1000 miles, because when you’re 5 days out, you can’t be sure of anything.
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.
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 PNA is forecast to remain positive, like it has for the last few weeks. A positive PNA means that the ridge is stronger out in the pacific, limiting the intrusion of warm pacific air into the CONUS and promoting a series of storms to ride over the top of the ridge and down into the CONUS. Generally, a +PNA can be associated with cooler than normal temps and increased storminess in the east assuming neutral values on the other indices, and given that the AO and NAO have been positive, it is the only thing that has been saving us from a very warm period.
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.
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.
The EURO ensembles are even more aggressive with the cold, and if the AO/NAO forecasts are correct as we head into January, they will continue to drop. Combine this with the fact that the storm track looks to continue to remain active, and you get a recipe for a much more exciting period in terms of cold and snow beginning somewhere between Christmas and early January. While nothing is ever a sure thing in weather, if you’re a lover of winter, it’s hard to not have a grin on your face when you’re looking at the long range. We’ll keep you posted as the pattern change unfolds, and as always, when there’s storms on the table, you’ll find no better coverage and analysis than right here at Southern Connecticut Weather!
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!
-SA
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!
-SA
