Look at all that warmth! Not so fast, shore dwellers….
I can’t say that I’ve ever lived on Cape Cod. I know that it’s absolutely glorious in the summer, between the phenomenal beaches and pleasant sea breezes drifting in off the Atlantic. I love spending time there in the fall, when the tourists leave but the days and water are both still warm. It’s actually pretty great in the winter too, as there are few places as serene and beautiful during the cold months of the year (snow on the sand is a personal favorite). But in spring, I can only imagine that can be a very frustrating place to live. When you tune into the local news and see “60s and 70s ahead!” and you know that it’s just not meant to be outside your door. And anyone who’s lived around here for a while knows why – that ocean is C-O-L-D this time of year!
Source: NOAA NODC
Here’s the good news – at least it’s warming up. Since water has a very high heat capacity, it responds to the changing seasons much more slowly than air does. So even though the days have become markedly warmer, the water is just barely creeping up from its winter lows. Most of the surface temperatures around here are now in the low 40s, up from the 30s just a few weeks back. Progress, but still awfully chilly. So when the wind direction comes from anywhere over that water, you feel it.
The way to get truly mild or warm days on Cape Cod/Islands this time of year is to have a brisk WNW wind. A persistent flow of dry air, just shooting the gap to travel mainly over land and not blow across the coastal waters, is the way to deliver more pleasant air to the region. For the rest of eastern Massachusetts, all you need is any westerly component to the wind. And it needs to be at least 5-10mph, if not more like 8-16mph, to keep the sea breeze at bay. If that’s the case, you’re on your way to a mild afternoon.
When high pressure is overhead and there isn’t a strong pressure gradient to drive that wind across the state and out to sea, that’s when a sea breeze can develop. A sea breeze kicks in when the sun heats up land at a faster rate than it heats up the water. The warm air over land creates a ‘thermal low’, and starts to rise. Since air always flows from high pressure to low pressure, the air over the water starts to move in toward the land. It replaces the air that just rose, heats up, and heads back out over our heads toward the water where it sinks again toward the surface. The process continues until the sun starts to go down and the heating stops.
I put this together to track the influence of the wind over the water for the next few days. Here’s how it breaks down:
Friday: a cold front stalls near the south coast. Most of the area will be under the influence of westerly winds, but a WSW component just south/along of the front will lead to just a sliver of water influence over Cape Cod and the Islands. Inland highs 60-66.
Saturday: A very weak pressure gradient is expected, so a sea breeze should develop by midday and last into the afternoon. So instead of just the Cape feeling the cooling breeze, it’ll be all coastal communities. Inland highs 61-67.
Sunday: The gradient picks up again and our winds turn more southerly. The most significant cooling influence will be felt along the South Coast, Cape and Islands. Inland highs 64-70.
Monday: A gusty SSW wind will keep the South Coast, Cape and Islands cooler again, while areas outside of the ocean’s influence skyrocket into the 70s, perhaps even a couple 80s!
Current SST departures from average. Source: WeatherBell
Interestingly, we’ve got some unusual water temps off our coast at the moment. The water immediately along the coastline features SSTs that are well below average for this time of year (thank our cold winter and persistent NW winds for that). But the Gulf Stream is downright balmy! This was part of the reason there were so many monstrous North Atlantic storms over the past several months. Parts of the G.S. are 6-7º above average, the biggest departure from average of any ocean area on Earth right now! And almost all of the tropical Atlantic continues to feature above average surface temperatures. You may be wondering if that will factor into hurricane season. Well the water was plenty warm last year but a continuous flow of Saharan dust and significant wind shear helped to keep the season at a minimum regardless of water temps. In all honesty, the tropical Atlantic is always warm enough to support plenty of hurricanes. However, it’s just one of many variables involved.