Weekend Nor'Easter Update # 3: Pressure Gradient Means Business!Last Edited:
It's really intriguing to see a storm signal stick around (and be rather consistent on placement of the low) after 10 days. I often make fun of the GFS model, but even I have to admit, it DOES do a good job at times picking out general patterns in the medium range. Sure there have been deviations in track over the past 10 days but it's remained consistent with the idea of having a coastal storm pull north along the Mid-Atlantic. The one thing the model is likely going to miss the mark on? Intensity. The low itself won't be as strong as it modeled last week (at one point it was down to sub 970mb) but it will still bring strong winds due to a very tight pressure gradient. I will talk more about what that means in just a second. First I want to focus on WHY this storm is pretty much unavoidable.
Negative North Atlantic Oscillation (NAO)
Our weather is directed by areas of high pressure and low pressure and their position relative to our location. There are several "atmospheric conveyor belts", one of which is "close by" called the Greenland Block. When the NAO goes negative, high pressure south of Greenland allows the storm track to buckle towards the coast and slows down systems trying to creep north. This isn't going to be one of those situations where we will wake up tomorrow morning and the storm changes course. We find ourselves preparing for either rain and wind or A LOT of rain and wind. There's no real in-between. We aren't going to see it escape out to sea with the amount of blocking modeled.
Alright let's take a look at some of the model guidance real quick. Here's the storm moving slowly north by midnight Friday (Saturday morning) showers begin to break out and winds pick up. It's just getting cranked up.
By mid-morning we're seeing moderate rain spreading across the region extending into the pocono mountains. Winds are whipping up at about 15-20mph with gusts over 45mph at the coast.
By Saturday night and early Sunday morning the storm is still continuing to pull SLOWLY north. Winds shift direction and heavy rain turns into scattered showers.
Forecast Rainfall (EURO MODEL):
A widespread 1" for most, perhaps more at the coast with 1-2"+ across portions of PA. I talked about this potential track earlier. Most could escape the highest rainfall totals if the storm traveled due north over our area. Why? Dry slotting. Most of the precip looks to be on the west side. I think an inch is reasonable as of right now.
Wind Gust Forecast (EURO MODEL):
Here's where that pressure gradient comes into play. Remember, I said the low ITSELF won't be too strong, but the high pressure system doing the blocking to the northeast IS strong. Advance a slow moving low, even a weak one, towards a high of that magnitude and the isobars (lines of equal pressure) are going to constrict and come together. The closer they are together, the windier it gets. Unfortunately it appears as though South Jersey could take the brunt of the worst winds. EURO model suggests gusts over 50mph at the coast. I've circled the key things you need to know. Don't be disillusioned by the "weak" low.
This is the biggy for those of you who live at the coast. Growing up in Brigantine, I understand the concern. I've made it clear that I expect moderate tidal flooding out of this system. No way to sugar coat it. Why? We will be 2 days removed from the full moon, tides are already astronomically high. Add an on-shore flow for 24 hours and a slow moving system? We could EASILY see 1.5-2 feet of additional water propagate towards the coast. Saturday morning would be the worst, Saturday evening could see a second round due to something we call "stacking" of the back bays. This happens when a strong east or northeast wind prevents the bays from completely draining at low tide. With the wind whipping all day Saturday, I think stacking will be an issue for Saturday evening's high tide cycle.
Here's the chart showing potential tide heights for Atlantic City... Cape May county will be a little higher.
Here's something else that concerns me. Bring that center of the low close enough and we could be talking 10-12' waves battering the beaches. Obviously beach erosion is going to be an issue. Large waves also help pump in extra water...
The storm should begin to wrap up late Saturday night but it will remain breezy on Sunday with perhaps some showers lingering behind the main event. Another weaker storm is possible on Monday. Stay tuned and feel free to ask questions! Overall impact to our region? 3 out of 5 for the mainland, 4 out of 5 for the coast.