Couple of things on the agenda tonight, starting off with a quick explanation of what changed on the models in the last couple of days to make the weekend snow threat all but vanish. Then, we’ll move to the extended day by day forecast, and finally, talk about a potential storm for Christmas eve and how it will help set up a much more wintry pattern in it’s wake.
First, the weekend storm(Or what was the weekend storm). At the time of our last post, models had fairly strong consensus on a coastal storm gathering in the gulf and coming up the coastline, bringing a round of precipitation and possible wintry weather to the region. However, over the last 36 hours or so, we’ve seen a strong trend in the models towards deamplifying this system, and as a result, a storm forecast to be sub 990 milibars will now struggle to break below the 1,000 barrier. As a result, the storm won’t have the energy to come up the coastline and pull in cold air, and as a result, we get nothing but some light rain and warmer temps.
Why is this happening? There are a few reasons, but by far the largest has been the trend for the next system, currently modeled to be Christmas eve, to move forward in time on modeling. This causes the pacific jet to become stronger, and when it smashes into the ridge in the west, it knocks it down as it enters the CONUS, and as a result the mechanism for amplification is not there. Previously, it was modeled to stay offshore long enough to allow the first storm to amplify, but now, it has sped up in time, and as a result, when the first storm is trying to come up the coast and amplify, it cannot do so.
Some weak waves of moisture may still reach us, but given the warm antecedent airmass, would expect mainly rain showers on the coastline with rain and/or snow showers inland. Minor accumulations may be possible inland, but nothing of any significance is expected with this event. There is a low(~10%) probability that this trends back towards the original depiction, but for all intents and purposes, this event is done for anything more than a minor impact.
An aside for a second. This is exactly the reason why one shouldn’t be speaking in specifics or certainties when you’re 5+ days out, because all it takes is a small change on modeling to make a completely different scenario. It’s not that we don’t want to answer your questions on amounts and impacts from such a long lead-time, but that we can’t do so with any sort of reliable accuracy. Sure, sometimes a storm remains consistent on modeling for the entire period leading up to it, but more often than not, you see these sorts of changes and that’s why we don’t like to speak in specifics when the leadtime is more than a few days.
We’ll see generally calm and quiet weather for the first half of the period, before things get more active towards the end of the forecast. Let’s break it down day by day, interjecting with some commentary as needed.
Tomorrow: A chance of morning snow/rain showers, otherwise, partly sunny, with highs in the lower 40s, except in the NW hills, where highs will be in the mid 30s. Chance of precipitation is 40%
Modeling swings an upper level disturbance over the region tonight, but it’s lacking any moisture for more than a few snow/rain showers. An isolated dusting to an inch is possible in the higher terrain, but the most likely result for any area is nothing.
Friday: Sunny, with highs in the mid 30s.
Saturday: Sunny, clouds build later, highs in the mid to upper 30s.
Sunday: A chance of rain and/or snow showers, with primarily rain showers on the coastline and a higher chance of snow showers inland. Otherwise, cloudy, with highs in the mid to upper 30s. Chance of precipitation is 50%.
Monday: Cloudy, with highs in the low to mid 40s.
Now, this is where the simple forecast gets a little more exciting. Models are in pretty strong agreement that we’ll see a very strong system coming out of the south on Tuesday, but where it goes is still up for debate. The ECMWF and it’s ensembles track the system almost due north, and by Christmas morning, have tracked it basically on a straight line through the Midwest and into the Great Lakes, where it’s gotten down to an impressive 962 milibars on the operational run!
Here’s images of the 12z EURO at hours 168 and 192, showing the positioning of the storm Wednesday and Thursday mornings.
All three of these solutions would have the same sensible impacts for most of the storm, that is, heavy rains and strong winds, but the GFS solution would likely lend it’s self to backend snows/snow showers as the low passes over us and lingering precip is left behind as the area moves into the cold sector. As a result, will paint a 50% chance of rain in the forecast for Tuesday, a 70% chance of rain in the forecast for Wednesday, and a 50% chance of rain or snow for Christmas day. Temps will be above normal for Tuesday and Wednesday before crashing to below normal on Christmas day.
While this storm is a Grinch storm for anyone looking for a white Christmas, it serves a fundamental role in reshuffling the pattern to bring us a much better window for wintry threats in it’s wake. The strong storm slamming into the PV to the north causes the PV to weaken, and as a result, leads to above normal heights at high latitudes and flips the NAO, EPO, and AO(The three regions of high level blocking) negative, meaning that above normal high latitude blocking is present. This weakening of the PV allows for a rush of cold air to funnel into the country, setting the stage for a return to a much colder pattern sometime between just after Christmas and the beginning of 2015 depending on which model projection you believe. Furthermore, the development of high latitude blocking promotes a positive storm track for coastal storms, and given that the STJ looks to continue to be active, there are plenty of reasons to be optimistic for the future if you’re a snow lover. It can find ways to rain/not snow in a good pattern, so don’t lock anything yet, but I’d think we’re in a good position for a flip to more wintry weather in the period between Christmas and New Years, with that regime lasting into at least early January and possibly beyond. This doesn’t guarantee a storm by any means, but it’s certainly a good look for one if you’re looking for snow…
Here’s an image of 500mb at hour 240 from the 18z GFS. Notice the strong positive height anomalies in the northern tier, suggesting a weak PV and negative values of the teleconnectors mentioned earlier. In future discussions, we’ll go into more detail about how to interpret these 500mb images, but for now, know that it’s a really nice look for a wintry pattern to develop.