Modeling is now in relative agreement on a storm passing on top of or just west of the 40/70 benchmark, starting most of the state as rain except for far interior areas, and then, as the low bombs out and brings in cold air, flipping all areas to snow from NW to SE. The warm ocean will have an influence here, but given that modeling usually overestimates coastal impacts, I don’t think it’ll have much impact on totals outside from right on the beaches to maybe a mile inland or so. Otherwise, a general NW to SE gradient looks to be in effect here, relating to when the event changes over to snow. Models all in good agreement on this gradient.
What models are not in agreement over is the exact track and temperatures, as well as to a lesser extent, the amount of QPF that we have to work with. Let’s look at the latest guidance one by one and break down what solution it would have for our area, and then we’ll talk about what we’re weighting and why.
To start us off, here’s a gallery of the NAM, GGEM, GFS, and UKMET’s MSLP positions, all for 7 PM Wednesday night. We can’t include the EURO here due to copyright restrictions, but it’s similar to the GFS, perhaps a hair west of it.
Good evening to you from Southern Connecticut Weather!
As we’ve been talking about for almost a week now, a coastal storm is expected to develop in the gulf tomorrow and come up the coastline, passing near the 40/70 benchmark on Wednesday and bringing significant QPF to the state, with quite a bit of that being in the form of snow due to strong dynamics and a fresh source of cold air from a front coming in from the west. This event will produce accumulating snow for the entire state, and in the process, thoroughly mess up thanksgiving travel.
This discussion is broken into two parts: The first breaks down the models and provides technical discussion; the second is the actual forecast and impacts. If you’re interested in the science, read on, otherwise, skip down towards the end of the discussion for the map, forecast, timing, and impacts. We’ve bolded where you should join us again, look for it right below the snowmap.
In terms of models for this event, we’ve had an interesting progression over the last several days. Modeling was previously in relatively good agreement in having this event pass well to our east, bringing no impact to the region. However, a few days ago, we began to see the EURO model bringing the event west, introducing some interaction between the northern and southern streams and as a result bringing the storm further west as well as making it stronger. The EURO continued as the western outlier for several runs before the rest of the guidance began to catch on Thursday and Friday, bringing the event closer to the coast and introducing precipitation into the forecast. Here’s the GFS for 0z Friday and 0z Saturday for this event, showing the shift west(Notice it's not even on the map on Friday it's so far east!)
Finally, we have the 12z UKMET. The UKMET is an outlier in having a strong tucked in low close to the coast, flooding the midlevel’s with warm air and producing a ton of rain/sleet across the region with little to no snow except for far NW areas. As of right now, it’s a significant outlier, and considering the general east trend, we are tossing it for the purpose of the forecast, although we’ll mention it here just to cover all the bases. It’s the biggest way that this storm could melt down and result in mostly rain for us, but as of right now, it’s a low probability scenario. Here’s a map of 850 mb temps from the UKMET, showing the vast intrusion of mid level warmth and rain as a result.
So, what guidance are we weighting in this forecast? The EURO and GFS are the two middle ground solutions here, and as a result, we will give both fairly high weight, with a 50% weight to the EURO/EUROens and 30% to the GFS/GEFS. We will assign 10% weight to the GGEM and NAM as outliers, and will discount the UKMET entirely. When we put it all together, we get this map.
We’re forecasting one to three inches of snowfall in the SE corner of the state, two to five inches for the majority of the coastline, four to eight inches for most of the interior, and six to ten inches in the NW hills. In general, we went towards the more conservative side of the forecast, considering we’re still 2 days out and there’s several ways this can trend into a lesser event, either too far to the east and limiting QPF or too far to the west and bringing in warmth. If modeling holds or becomes more optimistic on amounts tomorrow, we will likely issue an updated map tomorrow evening raising totals, and likewise, if it becomes less aggressive, we will lower totals or hold with this map for a final map. This map is slightly lower than the model consensus, so if the models remain exactly the same, we’ll probably nudge up slightly tomorrow afternoon. As always, exact totals will end up being determined by mesoscale banding and timing differences, and some areas will bust high and some low, but for the most part, this is a good starting point.
Timing: Most modeling is in relatively good agreement as far as timing goes; although there are some minor differences, which could influence start and end times. For the most part, the mornings commute looks to be OK, with skies likely dry or perhaps some light rain or snow falling. Do not make the mistake though that it is OK to head to work or school, because precipitation arrives with a vengeance from SW to NE throughout the course of the morning on Wednesday, and by noon or so, most areas are seeing moderate to heavy precipitation. Northern interior areas should start as snow from the start, while southern areas likely start as a mix or rain, before changeover takes place from northwest to southeast over the course of the early afternoon. By the time the afternoon commute rolls around; pretty much everywhere should be seeing snow, with heavy snow quite possible in many areas. The afternoon commute will be nothing less than a disaster, with heavy snow and exceptionally heavy traffic due to the thanksgiving travel rush. Conditions will not begin to improve until after midnight, and it may be until Thursday morning before all of the roads are back to semi-normal conditions.
We highly recommend travelling tomorrow to your thanksgiving plans if at all possible, but if you must travel on Wednesday, try to leave as early in the day as possible and expect major delays, poor conditions, and dangerous driving conditions. Have an emergency kit in your car with warm clothes, food and water, and a fully charged cell phone, and be prepared to deal with accidents and spinouts if they come up. Stick to major roads, which will be better plowed if possible, and don’t be above pulling over if conditions get too dangerous for you to handle. Better to be late for thanksgiving than not make it at all because of an accident that could have been avoided by waiting.
Air travel is also likely to be impacted. In general, the earlier in the day your flight is on Wednesday, the better chance you’ve got to get out OK, but it’s possible that airlines will make pre-emptive cancellations and flights earlier in the day will be impacted as well. Thursday should be OK for the most part, although residual delays and cancellations can be expected.
Many schools are closed for Wednesday, but those that are open certainly could be impacted either in the terms of a closure or early dismissal, depending on timing shifts over the next few runs and the whims of each individual district.
Bust potential: For the interior, bust potential is low with this event, as all guidance is in good agreement. For the coastline, bust potential is moderate, and it comes down to how models are estimating temperatures and track vs. reality. A little warmer than forecast, and we see mainly rain at the coastline, a little cooler, an all snow event. The current forecast takes a middle ground. A solution like the UKMET, bringing all rain for all, or like the JMA, which fringes NW areas and lowers accumulations for all due to less QPF, is possible, and would result in a more spectacular bust of the forecast to the low side, but the odds of that are low, likely less than 10%. Similarly, we could see something like the ARW and SREF members have which is a full out snow bomb with a western track and cold temps and be looking at widespread 12”+ amounts, but this is also comparatively low, likely less than 10%. Overall, forecasters confidence is moderate with this event.
To summarize: Precipitation moves in Wednesday morning, mainly as snow north and rain south, before changing over to all snow throughout the afternoon. Major travel impacts for the Thanksgiving period, with possibly treacherous conditions on roads, especially later in the evening. Storm moves out Wednesday night, with total accumulations of 4-8” interior, 3-6” coastline, and 2-5” SECT.
We are in storm mode here at SCW, with a forecaster on duty around the clock watching the models and updating as needed. We will have a small update tomorrow morning, with our final call package and map tomorrow at around the same time as tonight’s update. We will be updating rapid fire during the event on Wednesday on Facebook and Twitter, and will post significant updates to the site as well. Don’t forget to follow us @SouthernCTWX if you haven’t already and like us on Facebook at www.facebook.com/southernconnecticutweather for the most up to the minute info.
In closing, enjoy the snow, and remember, when deciding to travel or postpone, that your safety is more important than whatever plans you may have. If you’ve got specific questions, comments, or want advice on travel etc. leave us a comment here or on Facebook (quicker response likely), and we’ll get back to you as soon as we can, although if the response is load is overwhelming, we can’t promise a response, although we’ll do our absolute best.
As always, thank you for reading Southern Connecticut Weather! We’ll have another update tomorrow morning.
-SA (Map by TW).