We have an absolute mess of a storm on the way tomorrow night through Tuesday evening. A powerful storm system will move through the Mid Atlantic states tonight and redevelop offshore tomorrow before heading our direction tomorrow night. I discussed each threat as well as some of the dynamics behind the storm in yesterday’s update. Because I’m short on time, I want to spend this update on explaining why we’re likely to see more freezing rain/sleet than snow.
The short reason that we won’t have all/mostly snow is that the mid levels will be too warm. The GFS 700mb plot shows this perfectly with a deep stream of S/SE winds coming all the way from the tropics. This warmth will easily displace the weak cold airmass in place now. Remember from last night’s discussion that the cold front in the mid levels did not make nearly as much southerly progress as the surface front.
The type of precipitation forecast to fall at the ground is often most easily forecast through analysis of a vertical profile like the one shown to the right. This is what the atmosphere directly over Gray is forecast to look like on Tuesday morning at 9 AM. Notice the deep warm layer between 5 and 10 thousand feet (850 and 700mb). This will allow for any snowflakes to melt into rain. Now notice the deep (5000 foot) layer of cold air near the surface. This will allow for all those raindrops (and there will be plenty) to refreeze into delightful (not really) little pellets of ice. These ice pellets are NOT hail! What they are, though, is annoying, slippery, heavy, and unfortunately for the Tuesday morning commute, abundantly plentiful.
I also highlighted locations of latent heat transfer on the sounding. At the top of the warm layer, melting will occur which requires outside energy to change from a solid to a liquid, cooling the environment. The opposite will happen at the bottom of the warm layer where rain refreezes into sleet. This process is how the warm layer will gradually descend to a point low enough to the ground to allow for freezing rain. Finally, as temps approach freezing at the surface, some melting may try to occur but as we discussed above, melting requires outside environmental energy which will cool the environment overall thus favoring a continuation of the cold air through most of the event, even along the coast.
Here’s the forecast for all the various things that will fall from the sky. We’ll start with a burst of snow that will quickly change to rain in the light pink areas along the immediate coastline of York County, SE NH, and the midcoast. The snow will stick around and pile up a little longer inland and most of the event will feature snow for places in the far NW mountains. The timeframe for this segment of the event will be from midnight Tuesday morning through around 6AM Tuesday morning.
After a little snow, the warm air aloft will begin to invade changing everyone over to sleet (abbreviated commonly as IP for Ice Pellets). The sleet will fall on top of the snow, compacting it and making it extra heavy/icy. The rain snow line will move a little inland during this time but won’t make a lot of meaningful headway. This phase of the storm will last from around 6AM to around noon.
Then, warm air will sink towards the ground due to latent heat transfer in the refreezing zone and as a result, some areas are likely to see a change to freezing rain. This will be most impactful in the foothills where up to a quarter-inch of ice is possible. For the mountains, snow will begin to mix with sleet and for the coast, the rain line will continue to inch its way inland. This phase of the storm will last from around noon to around 4/5 PM.
The final phase of the storm will see a change to sleet across even the most NW parts of the mountains and a change to freezing rain/freezing drizzle for the foothills. The coastal plain will change to rain as the cooling influences of upward motion, falling precip, and partial melting at the surface will ease. This phase of the storm will last from around 4 or 5 PM through the end of precip in the predawn hours Wednesday.
For more info about winds, coastal issues, and some exploration of isentropic dynamics driving the event, check out last night’s update. The stuff there is still relevant.
More info tomorrow.