The Good News… Friday looks fantastic!
Sunshine and blue sky look to be the dominant weather story on Friday. Our northwest flow out of Canada will give us low humidity, and afternoon highs are right where they should be for this time of year (Avg. High = 83°)…
The Bad News… Our 4th of July weather, has just done a complete 180° turn.
When we looked at everything Wednesday night, Saturday was looking like a winner. A weak disturbance was going to pass well to our south. High pressure was going to control everything, and sunshine would be the lead weather story.
Fast Forward 24 hours… and the picture looks completely different as of Thursday night. A weak low pressure should develop, and spread rain into Pennsylvania and southern NY early on Saturday, and that will progress east during the afternoon hours. This would translate into a rather cool (if not chilly) Saturday in the Hudson Valley… with a good chance of rain starting in the early morning, and that threat should extend into the mid afternoon.
It still appears the Fireworks will be “Green lighted” for Saturday night, as the rain should exit by sunset… giving us drying conditions and clearing skies for Fireworks around 9pm. But the change to Saturday’s forecast is a substantial one… and with it being a holiday, A LOT of plans will be impacted.
How Does This Happen?
In weather forecasting… a dramatic forecast adjustment inside of 36 hours is a headache. On a holiday weekend… it’s a migraine. The Independence Day forecast appeared to be a ‘case closed’ scenario, with not a whole lot of room for a major surprise. Then suddenly… SURPRISE!
So what is the culprit? How does the data change so drastically from one day to the next? Well… it’s really not a major change at all, it’s actually quite subtle. The problem occurs in the upper air pattern, at the jet stream level. Take a look at the GFS Model from Wednesday night…
Focus on the circle over the northeast. The blue area represents a very weak trough, and you’ll notice the thin black lines (height lines / jet stream) dip over the northeast ever-so-slightly. You’ll also notice that they don’t really begin to rise up again, until they are off the east coast… and over the Atlantic Ocean. …Now look at the Thursday Night GFS model…
You’ll notice immediately, that while rather subtle… the Thursday night GFS digs the trough deeper over the Northeast. You will also notice how the heights (thin numbered black lines) begin to rise again before reaching the ocean. The slightly deeper trough allows for a weak storm to develop along the base of the trough… and since the trough lifts before reaching the ocean, the storm lifts up the east coast, and spreads rain into the Hudson Valley.
I know that’s a bit technical… and perhaps somewhat boring. But when you have a forecast shift like this one… sometimes it makes the pill a bit easier to swallow, when you have the “why” behind the forecast. It’s such a subtle change in the positioning of the weather features, but it’s just enough to make a complete mess of our weather… and the forecast.
Have a great Friday, Hudson Valley!