National high temperatures for Saturday. Note how much colder the northern tier of the United States will be compared to today. Click to enlarge.
Current visible satellite imagery for the East Coast. Click to enlarge.
Current enhanced infared satellite imagery for the East Coast, showing low clouds in light blue and higher clouds in yellow. Click to enlarge.
Average high temperatures this time of year are right around 45 degrees. So far today in Lowell, we have managed to make it just up to the 40 degree mark, with some high cloudiness helping to keep things cooler by filtering out some of the sunlight. These high clouds are associated with a weak area of low pressure near the Great Lakes which will be visiting us for tomorrow.
By tomorrow, the weak system passing well to our north will give us the chance of some scattered morning flurries, and as the day warms up, temperatures would become too warm to support any snow, so we run the risk of a quick shower or two through Thursday afternoon. This little storm does have an accompanying cold front with it, which means things will start to cool down quite a bit for the weekend. Most of the weekend looks to remain on the dry side, but by Sunday, a potent storm system looks to get going over the middle of the country and track to our NW over the Great Lakes.
Cold air + strong storm.... sounds like things could get interesting, huh? And they could. However, with a storm passing to our west, this means warm air will flow up from the South into our region as the storm passes by. This happens because storms spin counterclockwise. If the storm were forecast to pass to our east, then we would be in for a big snowstorm as the cold air would drain down from the north, keeping temperatures at or below freezing for most of the event.
As it looks now, we could be dealing with a mixed bag of precipitation, starting out as snow Sunday evening and changing to sleet, freezing rain, and then to plain old rain by midday Monday.
I am often asked how does sleet and freezing rain form. Check out the diagram below - it nicely illustrates the concept. Snow occurs when a thin layer of warm, moist air rides over a deep cold layer of air. However, the thicker the layer of warm air, the greater the chances that the snow will melt on its way down. Why then, don't we just get plain old rain?
If there is a layer of cold air at the surface, the rain will re-freeze on the way down. This looks like it could be the case for the weekend storm, as we will have cold dense air at the surface, with warmer air aloft sliding over it. As more warm air moves in, the precipitation will transition from snow, to sleet, and then to freezing rain, and finally to plain rain. Tired of reading my explanation? Check it out graphically below....
Graphical representation of vertical temperatures and expected precipitation type, which could be a factor by late Sunday and Monday. Click to enlarge.
Computer model showing the forecast location of the upcoming storm as of 7 PM Sunday night. The storm is forecast to be strong, but to pass well to our west and north.
To the north of the blue line, temperatures will be below freezing at the surface, which is important in forecasting precipitation type. Note that this blue line will be very close to Lowell, making for a tricky forecast. Click to enlarge.
So by Sunday evening, things could be getting pretty tricky around here. A few things should be noted:
1. This storm is several days away, and computer models aren't always accurate from several days out.
2. Any change in the storm track will change the forecast.
3. If the storm re-develops near the coast, we will have a colder, snowier scenario.
4. How much cold air will be entrenched in the region by Sunday?
5. Timing. Does most of the precipitation fall during the colder night-time hours, or during the day on Monday?
There are more subtle variables at play such as "bagginess" in the low, and if the storm moves closer to us but remains west, stronger SW flow may scour out the colder air more quickly. I won't go into everything because my job is to attempt to forecast and educate - not to bore!
We will be tracking the upcoming weekend storm and will have an update tomorrow. Again, this does not appear to be a big snow maker, but with frozen precipitation possible, conditions could become tricky late Sunday and Sunday night. We should have a much better picture of the event by Friday and especially Saturday.
Whew! Well, if you made it this far, you deserve the detailed forecast. And here it is.....
Rest of Today: Sunshine filtered through high clouds. High near 40.
Tonight: Clouds lowering and thickening. Chance of a flurry late. Low 27.
Thursday: Cloudy with a morning snow shower possible, followed by a chance for rain showers in the afternoon. Warmer, with a high near 45.
Thursday Night: Decreasing cloudiness. Low 27.
Friday: Partly cloudy and cooler. High 41.
Saturday: Mostly sunny and seasonably cool. High 35.
Sunday: Becoming mostly cloudy. Chance of light snow and sleet by afternoon. High 33.
Monday: Cloudy with a wintry mix or rain possible early, becoming all rain by afternoon. High 41.
For WUML Lowell 91.5 FM this is WUML weather director and UMass Lowell student meteorologist John Webster.
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