It’s that time of year when the Sun retreats to the Southern Hemisphere taking the warm air with it. The cold air deepens near the North Pole and slowly slides southward, ushering in a new, but temporary era of cold and snow. We defend ourselves with jackets, boots, mittens, and hats. We celebrate a slew of holidays in an effort to forget the misery that is intertwined with this weather. We even take a holiday, extend it into a season, and celebrate the cold and snow as part of its’ appeal. In short, we respond to adversity with a smile as best we can. We are human.
How’s that for waxing poetic?
It appears this week will bring the first Winter-like weather to the U.S. Granted, there has already been snow at high elevations in the western mountains and even in the Appalachians, where snow fell the earliest it ever has on record in North Carolina. This time, it’s expected to fall at lower elevations and actually accumulate beyond a half-inch.
Winter storm warnings are in effect for southern Wyoming and much of Colorado beginning this afternoon and continuing through Wednesday evening. In total, 4-8 inches of snow are expected in the high plains east of the front range. The foothills and mountains can expect high amounts ranging from 6-12+ inches. This includes the cities of Cheyenne, WY and Denver, CO.
This is a great example of how valuable forecasting is when compared to 50 years or so ago. Yesterday, Denver set a record when they hit 80 degrees in the afternoon. It’s currently about 50 degrees and will likely fall below 30 tonight as the snow begins. This would be hard to do without a network of observations and computer programs to simulate the progression of the atmosphere ahead of time.
(By the way, it’s not uncommon to have strong southerly winds bring in very warm air ahead of a strong cold front. In fact, it’s that sharp contrast in air temperature that can help to create a strong low pressure center and subsequent storm event. I once spent a Fall day chasing severe thunderstorms/tornadoes in 70-80 degree weather. The next day it snowed.)
The forecast for New England is a little more tricky. There are a lot of factors working against a snow event here:
- Forecast timing – This event is still a few days away and we always have to be cautious about predicting precipitation totals and type this far in advance… especially when it’s a little abnormal. This brings us to…
- Climatology – While it’s not unheard of to get snow in New England before Halloween, it’s not “normal” either. That is, climatology would not expect a snow event this early in the year. Also, average temperatures would not support a snow event with highs averaging in the mid-50s and lows in the mid-30s.
- Temperatures aloft – The models suggest temperatures aloft will not be cold enough to support snow at the outset of the event, meaning temperatures will have to cool during the event to change the precipitation from rain to snow. And while that may happen, a snow event isn’t a snow event if the snow isn’t accumulating, which brings us to our last point…
- Temperatures at the surface – Again, temperatures should be above freezing at the start of this event and rain will likely be falling to begin the day on Thursday. Now, the evaporation of water (thermodynamics) and the advection of cold air into the area (dynamics) will work to cool surface temperatures. The question is how much cooling occurs. Does it cool enough for snow to accumulate on grassy surfaces? Does it cool enough for snow to accumulate on roadways? Does it cool enough for any snow to accumulate at all?
To give you a visual aid for how tough this forecast is, consider the following model forecast:
To convert liquid water into solid snow, we need to figure out a conversion factor. In other words, one inch of water is equal to “some amount” of snow. That amount will vary depending on the environment in which it forms and falls. You may have heard 10:1 in the past for a ratio (one inch of rain is equal ten inches of snow). This is a decent rule in the dead of Winter, but it’s not applicable here where temperatures will struggle to support snow. I think a ratio close to 6:1 is more accurate than 10:1. So, if this precipitation fell entirely as snow, you may see 3-5 inches of the white stuff pile up on the surfaces most receptive to accumulating snow.
Ahh, but will it fall entirely as snow in that 6 hour window? That’s why meteorologists get paid a little bit of money. I don’t want to go into extensive detail here, so let’s just take a look at 850 mb (about 1 mile or so above the surface) temperatures during this 6 hour window:
If you juxtapose the 850 mb temperatures with the area of heaviest precipitation from above, you can clearly see why this is a difficult forecast. It appears southern Vermont and New Hampshire along with eastern upstate New York and western Massachusetts have the best shot for snow. Still, it’s a very difficult forecast. If the precipitation moves out before the air can cool, they won’t see much snow. Likewise, if the cold air doesn’t move in as quickly as forecast, the rain won’t change over to snow.
This is truly a difficult forecast. One that will test these men and women forecasters; perhaps like never before. The dreariness of the weather will only be met by the dreariness they feel in their hearts as they make a less-than-confident forecast. A correct forecast will not result in praise or new-found fortune. They’ll simply be told they performed their task as expected. An incorrect forecast will lead to vitriol, spite, and a continued lack of trust from the citizens they walk amongst on a daily basis. Their children will be taunted in schools, their relatives…
OK, I can’t do this anymore.