More Clarity Coming To The Weekend Setup
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There are pros and cons when it comes to improving numerical weather prediction (NWP) - The good part is that forecasts (even if you don't believe me, which I'm sure many of you don't) are getting better and better. Remarkably better in the case of long-term outlooks. The bad part is that we now end up talking about weather events a week or more out, because we know about them farther in advance than ever before. So there's more time to discuss the details, the scenarios, and changes along the way. Unfortunately in the end, that's what most people remember! The forecast they heard 6 days before a potential storm. But the process is what it is now, whereas if you DON'T talk about it you just look like a dunce who doesn't know what's going on. As it stands, we have some more details coming into focus regarding this storm as the weekend approaches.
In general, the trend has locked in on a solution that produces storminess along the East Coast, consolidating into a powerful low pressure center, but just a little too far east for significant southern New England impact. Any good New Englander knows that a track difference of 50-100 miles can have massive implications on what happens in the end, and since we're still a few days out I'd say that's the current room for error. Here's the breakdown of what's expected.
Live in western or central New England? The potential for significant issues due to this weekend's weather look low. Will it be cold? You bet. We're talking highs in the 30s and 40s on Saturday, and almost exclusively in the 30s on Sunday for towns north of Route 2. Will it be windy? Definitely, especially Saturday night into Sunday night. This is where gusts could be in the 25-40mph range. When it's in the 30s and you have wind, guess what you get? Wind chill! Yep it's time to dust off those maps. It will feel like the 20s on Sunday (especially after the sun goes down, which will be around 4:30 since DST ends Saturday night. But will you see much rain/snow? Not likely. The storm should be too far out to sea to wrap any meaningful precipitation your way. Some areas of rain/snow showers are probable, but as of right now that's it.
Live in eastern New England? You need to keep tabs on this forecast over the next 48 hours. While it's not likely that you'll see major impacts, the storm should track close enough to be wary. A little wobble west, a change in the jet stream energy, a trough digging in a slightly different direction, and we'll be talking about a different story. As of this writing, I'm expecting areas of rain and drizzle to develop on Saturday, making for a raw and chilly day. Highs will stay in the 40s for most, and a breeze will really start to pick up (especially near the coast).
Saturday night into early Sunday is where we have a *chance* at bigger issues. Temps may fall enough to change any rain over to wet snowflakes, and there could even be some icy surfaces to the NW of Boston. But mostly what I'm paying attention to is the wind potential. Even with an offshore storm, we could be talking about widespread 25-40mph gusts, but locally up to 60mph on Cape Cod/Nantucket. Most of southeast Mass still has enough leaf coverage for this to lead to downed trees/limbs and power lines.
12z ECMWF featuring 50-55 knot wind gusts over Cape Cod and the Islands Sunday. This is one of the bigger factors to watch for this weekend. Source: Weatherbell
If you have any interest/plans in downeast Maine - this may be the place to be for snow. It stands the highest risk for accumulating wet snowfall and strong winds. As you can expect the models will shift over the next couple days, but if anyone gets the goods, this should be the place along with eastern Canada.
The JMA - one of a few models that's still close enough to lead to some discomfort for forecasters. Source: Weatherbell
If you're interested, here's a look at some of the more 'behind the scenes' technical stuff. While the overall trend is certainly offshore, we'll be dealing with a beast of a storm. This should undergo 'bombogenesis' like some of our winter nor'easters, meaning it will drop at least 24mb in pressure in 24 hours or less. There's going to be a bowling ball of short-wave energy plowing into the southeast helping to energize this coastal low. In fact, it's that reason that may help save us. It dives so furiously and deep into the southeast that it turns the corner up the coast a little too late/too far east to draw the low toward the coastline. These powerful storms can sometimes have surprising lives, and so that's why I still want to stay vigilant until we're assured it's a miss.
18z NAM depicts this 'bowling ball' as shown by 500mb heights. Source: Weatherbell
You may have seen me write about 'ECMWF EPS' on the blog here. When we're talking about that, we're talking about the European model's ensemble prediction system. There are 51 total ensemble members which make up the group. Each member has its initial conditions 'tweaked' a little bit, which gives a selection of different results. The more of them that are in agreement, the better your odds at the outcome. But when they're all over the place, uncertainty is high.
A look at half of the members of the EPS from 12z Wednesday. We don't look at the totals per se, so much as the 'hits' and 'misses.' You also have to take these into account when looking at the ensemble mean. While a good tool, a couple huge hits can skew the mean in the end...making it misleading. Source: Weatherbell
Looking at the 12z run on Wednesday, there are still 14/51 (or 27%) of the members forecasting significant accumulating snowfall in New England. That's enough to give pause and create some uncertainty about where exactly the low will track. We've also seen a trend (in a couple other models too) of a lead low spinning up the coast on Saturday with the deeper northern stream energy producing the stronger low heading into Sunday. It's a pretty complex setup with some strong dynamics at work, so the bottom line is that the case isn't closed on this forecast yet.
