This Is Time Of Year When Your Weather Forecast Is....Vague

Roses soak up some morning sun today in my St. Albans,
Vermont gardens.  A vague forecast over the next week
gives us chances of both sun and showers, take your pick.

 I got a good question aimed at me on Twitter yesterday:

"Can you elaborate on what the heck is going on with the VT forecast this week? My temp outlooks have changed dramatically every day. Not the actual swings, but the forecast."

The short answer is, it's just that time of year.  

But let's get into the details. 

Back when it was winter - remember that? - you get these big weather systems.  An Arctic high pressure system comes crashing down from Canada. You see it coming, so the forecast is pretty consistent: It's going to get horribly cold out.

Or, a big winter storm system approaches. There might be a lot of questions in the forecast regarding what proportions of snow, ice or rain  you'll get, but you do know in advance the weather is going to be inclement. 

This time of year, in the summer, we don't get those big huge weather systems. Usually it's just these piddling little disturbances that come through. 

The sun this time of year is at peak intensity, too. Which means a clear, sunny afternoon will most of the time tend to be much warmer than an overcast one. Clear nights will also be cooler than overcast ones. 

Part of the trouble with forecasts comes in with the timing of these little systems.  Judging the timing and relative intensity of them is pretty hard to do more than a couple days in advance.  The data just isn't as good on hard-to-pick-up little cold fronts and cloud patches than larger storms. So the forecasts are iffy. 

Broad brush, the forecasts are great. For example:  Starting yesterday and continuing through Thursday and probably beyond, we know there will be sunny periods and cloudy periods, and most days carry a risk of some showers. That will almost certainly come true. 

Getting into details, though, is tricky. For instance, if meteorologists initially think one of these quick disturbances will come through, say during the day Sunday, they'll keep forecasted high temperatures relatively cool, because the anticipation is it will be cloudy. The clouds will block the sun's heat.

Then, the next day, it appears that disturbance will actually hit Sunday night and the day will be sunnier than was anticipated earlier. So, forecasters will adjust projected high temperatures to get warmer than initially expected, and Sunday night's lows, under the increasing clouds, to dip down less than had skies actually cleared. 

This explains a lot of those changes in those weather forecasts people have been seeing. 

Summer time forecasts are often tricky this way, and the current weather pattern makes things even dicier for meteorologists.

That big, hot "death ridge" is taking shape in the western United States. Where that happens, the forecast is pretty easy: Sunny and hot. 

When a summertime "death ridge" forms out west, there's a corresponding dip in the jet stream over the eastern United States.  That puts us in position to get many rapid fire, but weak cold fronts and mini storms coming in from the northwest. 

Rainfall with these things is hit and miss, and small scale.  It usually isn't statewide, and comes in the form of scattered showers. The rain might be focused in southern Vermont, or eastern Vermont, or the north. Determining more than a couple days in advance -  or sometimes even the day before - where the rain will focus is always a challenge.

Some of these little cold fronts or storms might interact with each other, some won't. So again, your broad brush forecast is great, but picky details like timing, the amount of precipitation and the location of it is in doubt.

You'll often see a forecast in this type of pattern with something like a 50 percent chance of showers. People often blame meteorologists for a "wrong" forecast based on their perception and where they are.  If it doesn't rain in a particular spot  where there was a 50 percent chance of showers, a person might say the forecast was wrong because it didn't rain. 

That, even though the forecast made it pretty clear things would be hit and miss, hence the 50 percent chance hedge.

In the weather pattern we're in now, we don't usually get many temperature extremes, even if pinning down the precise forecast is difficult and vague. While the forecast for a few days from now might be off, at least we know we won't have late season frosts or another big torrid, awful heat wave in the near future. 

The bottom line is this: Take the gory details of what will happen a few days down the road with a grain of salt. You'll need to keep both your sunglasses and your umbrella handy.