This update will focus on our first call forecast and summary of impacts for a coastal storm that will impact the state over the course of the day and night on Saturday. We’ll start with a brief overview of the models and trends since yesterdays update, and then will dive into the map, timing, and impacts. If you're just looking for the forecast, scroll towards the bottom of the discussion, otherwise, read on!
Models and Trends
In general, as we usually see, we saw the models shift towards a compromise on the track. The GFS and UKMET, which were well out to sea on yesterdays runs, have come west of that to a track near or on the Benchmark, while the EURO, NAVGEM and Canadian have come east a little bit and now track just inside the benchmark. The outlier is the NAM, which has gone from being the western outlier to the eastern one, and tracks the low further to the east, but considering the wide operational and ensemble support for a low on or just inside the 40/70 benchmark, we are inclined to discount the NAM for this forecast.
While there is relatively good agreement on the positioning of the low, there still remains much disagreement on the intensity of the precipitation shield, the total precipitation amounts, and the extent to which low and mid level warmth intrudes upon the state. All models except the GFS are in agreement on a period of front end snow for the entire state early Saturday morning, and given the GFS is notoriously warm in coastal systems, we are tossing it for this part of the forecast and believe that the entire state will start off as accumulating snow by daybreak for the coast and soon after inland.
What happens next is unclear. All guidance agrees that there will be a surge of warm air northward in the mid levels, but to what extent is still very much up in the air. On the warmest side of the guidance is the GFS, which eventually gets warm enough to turn the entire state to liquid precipitation (Notice we say liquid precipitation, not rain. We’ll explain why in a minute). On the coldest side is the EURO, which is cool enough aloft to keep the entire state except for potentially far SE areas below freezing in the upper levels. The other guidance is in-between the two, with the NAM being closer to the EURO, the UKMET being closer to the GFS, and the GGEM in between right in the middle. Eventually, the upper levels begin to cool rapidly as the storm pulls away and colder air floods in, but it’s a race between the exit of the precipitation and the arrival of the cold air to see if areas flip back to snow. The models vary widely on the timing of this race, with anywhere from no additional snow to several inches on the backside being pictured. The best chance for additional backside snows would be in eastern areas. I’m leaning towards a middle ground solution between the various models, perhaps slightly biased towards the warmer side as the trend has been slightly warmer. No real way to know for sure which is right though and given that it is a matter of miles for many, it’s going to come down to nowcasting to figure out exactly where that line sets up. For that reason have gone with broad ranges in the totals map.
Here’s images of the 850 MB temps at 1 PM Saturday from the various models. Notice how much disagreement there is as to the positioning of the 0 degree line(White/purple boundary of shading on most models, yellow/green border on the NAM).
Further complicating things is the fact that low level cold is modeled to hang strong, especially in the sheltered valleys and minimums away from the shoreline. As a result, could see potentially significant ice accretions(>0.10”) in those spots that don’t remain all snow. We’ll know more about that tomorrow as the hi-res guidance comes into range.
In general, thinking a slightly drier and warmer system then last night which will cut down on totals somewhat, but still a widespread accumulating snowfall.
Here’s our first call map for this storm. Keep in mind this includes both front end and back end snows, with a changeover to ice or rain for many in the middle.
Although temps aloft will warm, temps at the surface will struggle to warm above freezing away from the coastline, creating a potentially nasty icing situation for interior areas, especially the lower elevations. A tenth or two of an inch of accretion is possible, which combined with heavy wet snow has the potential to produce isolated to scattered power outages.
Precipitation should move out by late Saturday night, with all areas dry by daybreak Sunday morning. In total, we are expecting three to six inches of snow across the majority of the western 2/3rds or so of the state, with one to three inches in the far NW hills where total precipitation will be less. This large zone works because towards the SE of it where precip will be higher, warm air will linger longer, and the end result will be equal snow to the NW areas of the zone where there is less precip but colder temps. In the eastern areas, where the total precipitation will be highest, we’ve split the state into two zones. The interior NE and the hills are expected to receive four to seven inches of snow as a result of being in a better position for backend snows, and the southeast areas of the state are expected to receive two to five inches of snow (less than 2” is possible right on the coastline) due to being the first areas of the state to warm above freezing, but could see a bit of snow on the backend.
Overall, a low confidence forecast due to the variables of total precipitation and upper air temperatures, but in general an accumulating snowfall is expected for the entire state. Travel will be tricky on Saturday, especially during the morning hours when the heaviest of the snow will be falling. If you must travel, bring food, water, warm clothes, and a way to contact someone in the event of an emergency.
Keep in mind that this will be a heavy wet snow and will be difficult to shovel, especially when it has been saturated further with rain. Shoveling every couple inches will make it much easier for you in the long run!
We’ll have a final call map tomorrow evening around 6 PM, and will be updating throughout the storm on Saturday. We look forward to tracking this storm with you!
-Spencer, Greg and Tim