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1. Yesterday's warmup went south on us in eastern Massachusetts and northwestern Rhode Island last night. As a weak low pressure area drifted past the area last night, low-level winds from the north returned cold air that had just left hours earlier. Then a band of rain moved through, and the resulting ice ruined rush hour for many.
2. At 2 p.m. ET, it was 50 at Boston, 60 at NYC, 72 at D.C., and 70 at Myrtle Beach, SC. However, it was 56 at Detroit at 8 last night and is now more than 20 degrees lower. Temperatures will be approaching or passing the freezing point overnight as precipitation arrives in Philadelphia and then New York City.
3. Snow will fall heavily from northern Maryland to Massachusetts then Maine tomorrow. I want to really stress how fast conditions will deteriorate on roads and walkways. Conditions will rapidly deteriorate on roads and walkways.
4. The storm should last 12 hours or less at many places, though blowing and drifting snow may persist afterwards. These maps show the pressure pattern and weather features around 1 p.m. today:
Earlier, the 200 mb map showed a spread of about 15 degrees between the warmest readings at the bottom of the short wave that will support the storm and the downstream ridge. Since this situation was still evolving, we should expect to see a greater spread on the map tonight. Previously, I explained this rough tool, saying that the projected snow accumulation in inches might approximate the Celsius temperature difference between trough and ridge divided by two. I do not claim this is a scientific theory, so I should not meet the same fate as Galileo did when he said the Earth was round.
5. Prediction of exact snow amounts is made difficult by the fact that snow is largely air, and so an inch of water might make 5, 10, 15 or 20 inches depending on how wet or dry it is. Computer models help with the forecasting process, especially as we get closer to the event. The following charts show the spread in accumulation estimates between a number of models: (These were based on models run last night.)
First, for New York City:
...and this is for Boston:
Although there is a lot spread between individual solutions, they virtually all call for a disruptive snowstorm tomorrow at both locations.
These maps show where the storm should be at six-hour intervals tomorrow. On each map, the shaded area represents precipitation in the six-hour period leading up to map time.
Southwestern and central Pennsylvania as well as western Maryland will get heaviest snow on this map between 7 p.m. and 1 a.m.
The area from Baltimore to Hartford join the fray by 7 a.m.
New York City and then Boston get pounded tomorrow.
6. As hard as they work, weekday snow falling at 1-2 inches per hour makes it very hard for road crews to keep up. However, if you do get stranded at your destination tomorrow, keep in mind this does not look like one of the long-lasting storms that have caused ongoing challenges for more than day in this region over the years. I know that's cold comfort, but by preparing, it can be a manageable experience.
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