The picture is coming into a little better focus for late this weekend and early next week, but I'm still being cautious.
It's like trying to read a poster or sign from really far away (comparing a long-range forecast). You may get 4 or 6 of the ten words on the poster, but as you get closer you get 8 or all 10. Similarly, forecast confidence generally rises the closer we get in time to the actual event. We may have it 30 or 40% correct a week out, but then 90-100% correct one day out. So keep this in mind that I'm writing this blog with the end of this forecast period still 138 hours out, aka 6 days.
So I'm feeling about a 50-70% confidence level with this one. A couple things are going through my mind. One is how dry it is out there right now:
All of New England is in some kind of drought. I've heard of the phrase, "when in drought, leave it out." referring to impending rain or snow activity following a dry phase. I've seen precipitation dwindle as it moves in, because the ground and atmosphere is so dry.
I think the start of this precip is going to be later than what models are picking up on at the moment. Bands of rain are forecast via the NAM/GFS to start near the Canadian border midday Friday. I'm having a hard time believing this given dry conditions lately. So once we saturate the layers of the atmosphere I think we'll have a better chance to get some precip in here.
That being said, I think saturation will take place by Friday night and Saturday with models increasing relative humidity levels from 20k feet down to the surface by Sat. AM. From north to south I'm expecting at least some light rain shower activity with our region being in a good spot for lift (via entrance level on the jet stream) and strong temperature gradient with a stationary front.
So far for Saturday models are picking up on anywhere from a tenth of an inch (south near Massachusetts) to close to 1.25" near the Canadian border. I'll downplay those to a tenth of an inch to a half inch possible south to north.
Sunday through Tuesday the forecast gets a little more interesting. Here's one of the latest updates from the European model:
From top left to bottom right you're looking at 500mb heights, surface projections for highs and lows (pressure systems:surface analysis), 700-850 mb relative humidity, and the 850 mb temperatures. Note a few things, top left picture is showing a deep 'u' shape which is a long wave trough. The tilt is 'negative' which means it looks like it is leaning right. That is a strong phase with 'waves' in the atmosphere. Top right shows an area of surface low pressure with a pretty strong center to our south, with lots of moisture available (bottom right) and a strong warm air advection over us (bottom right). The combination of these parameters tells me we'll have a good chance of precipitation in our area, especially getting a little additional moisture feed off the Atlantic.
Now the tricky part is: This is only a projection. It's not what will happen, but what could happen. So if it pans out differently west, east, north, south, whichever...that could mean a big change to how much we get.
Still models are spitting out some pretty large numbers for precip in our area.
This is the American GFS model updated 2 a.m. EDT Wednesday, April 16th. It's for precipitation totals Friday through Monday night. The red areas show precip at or above 2.5". The model run before this had steeper accumulations between 3 and 5 inches. Again for now, I'm downplaying these numbers a bit, going with a widespread 1 inch to 3.5 inches. That's a pretty big range but I'll narrow it down once it gets closer. Looking on the hydrology side the ground can get about 2.25"-3.0" of rain in a 6 hour period before flash flooding becomes an issue, based on this website from NOAA. We'll have more later this week/weekend.
-Meteorologist Steve Glazier