Hey everyone! It's been quite a while since I posted on this blog, but what better time to post than right before a major winter storm is set to impact the region? Below, I will try and outline what you can expect from this developing winter storm, and why there remains some uncertainties, despite us being less than 12 hours away from the first flakes. With that said, let us begin with the active weather alerts issues by the National Weather Service:
-- BLIZZARD WARNING: Suffolk County, NY...Coastal NJ (Monmouth/Ocean Counties)
-- WINTER STORM WARNING: All Counties except Warren County, NJ...Sussex County, NJ...Orange County, NY...Dutchess County, NY
-- WINTER WEATHER ADVISORY: Counties listed above not in the Winter Storm Warning
**note: there are also Coastal Flood Advisories posted for coastal communities
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First off, I want to begin with the latest water vapor satellite imagery. In the imagery below, we can make out several features that are key into our developing winter storm. We can see a deep upper-level trough diving SE'ward from the Upper Midwest and Canada. Notice how to the east-southeast (downstream) of the base of the trough, there is a plume of green extending NE'ward across the Carolinas and along the Southeast coast. That green plume represents upper-level water vapor, or moisture and is the early stages of a baroclinic leaf. To put it bluntly, that is the beginnings of our coastal low. Mid and upper-level dynamics are allowing for rising air near the coast there and subsequent pressure falls.
Over the course of this afternoon, the trough/upper-level disturbance(s) to our west will propagate towards the coast while continuing to interact with the S'rn-stream disturbance. This will only aid in allowing the coastal low to develop further and take shape. This can be seen quite nicely below in the 500 mb forecast height field/vorticity (courtesy of tropicaltidbits.com).
In terms of surface development, and relative impacts (what you'll see outside), we are now at the point where we can now-cast and monitor short-range higher-resolution model guidance. So, let's discuss timing first. Many of us saw light snow this morning. That was not associated with our coastal storm, but rather a precursor disturbance dropping through. The snow from our highly-anticipated coastal storm arrives this evening, with snow breaking out along the coast and across S'rn NJ/S'rn PA around 6-8 pm EST. The snow will gradually move N'ward and E'ward to encompass N'rn NJ, NYC, Long Island, Lower Hudson Valley, and CT by mid to late-evening. As the hours progress, you will notice the snow shield to intensify with banding features becoming apparent. This can be seen in the HRRR evolution below (take with grain of salt on exact evolution - use as an idea!)
You can sort of get a sense of the interaction and phasing of the disturbances in the above forecast gif as well. Anyway, the storm really intensifies as it moves NNE'ward and will meteorologically "bomb out" off the Southern New England coast Saturday afternoon. This the region that will likely experience the most intense conditions; with very heavy snowfall and strong winds expected. For the NYC Metro, we can expect the snow to begin winding down from west to east early to mid-afternoon as the snow is raging across Southern New England. Banding features in our area will become dominant. In other words, where you have strong banding, you will likely also have nearby subsidence. It's a saying as old as time: "what goes up, must come down." So, if you have strong upward motion, caused by frontogenesis, slant-wise instabilities, etc., you will also have nearby sinking air. That's often why we see such localized snowfall maxima after major winter storms. In other words, strong upward motion results in snowfall (given the right conditions) and often healthy snowfall rates/ratios, while downward/sinking motion results in lighter precipitation/no precip altogether.
As always, the question becomes this: where do these snowfall bands establish themselves? It's difficult to say with much certainty even just hours before the snow begins to fall. This is usually an aspect of a storm that is now-casted (i.e. monitored in real-time). Hence also why meteorologists and forecasters give snowfall ranges.
The above plot, courtesy of PolarWx/Tomer Burg (@burgwx) shows 10m forecast winds from the NAM model. Winds will be strongest along the coast; hence the hoisting of Blizzard Warnings for Suffolk County, Coastal NJ, and SE'rn CT.
Lastly, you can find my latest snowfall map below. As always, this is just my thinking, and for official decision-making, I advise you to consult your local National Weather Service (NWS) office for official products.
Now is the time to now-cast. If you don't have to travel later this evening and tomorrow, stay off the roads. It'll be dangerous and risky. In many cases, travel will be near-impossible, or impossible altogether. Snow moves out later Saturday. Be safe, and thank you for reading!
-Zack J
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