DETROIT - The forecast remains stable, and everything appears on track for our ice event. After a dry evening, freezing rain will move in between midnight and 3:00 a.m., and continue through Wednesday morning, changing to rain from south to north from mid-morning through early afternoon.
The rain could become moderate overnight, so it may be really coming down when you wake up, and don't be surprised if you hear a rumble of thunder overnight.
Since temperatures tonight will drop into the mid to upper 20s (-3 to -2 degrees Celsius) and not rise above freezing until late morning, icing will be a problem well into the day, especially to the north, where your temperature rise will be slowest.
Here is a series of maps from our high-resolution RPM model showing you the precipitation's timing:
Let's now explain what's happening:
If the atmosphere is below freezing from the ground all the way up to the clouds, then our precipitation obviously falls as snow.
Now let's bring in a wedge of above freezing air aloft. In this diagram, south is to the left, and north is to the right...so the warm air is flowing in from left to right (south to north).
Precipitation falling from the clouds begins as snow, but then some degree of melting occurs as the snowflakes fall through that warm wedge. If the wedge is very thin (to the right in this diagram), then the snowflakes don't melt completely, and just fall as wet snow. If the wedge is a little thicker, the snowflakes melt completely into rain drops, but those drops then freeze into little balls of ice (sleet) as they fall through the below freezing air beneath that warm wedge.
If the wedge is even thicker, the snowflakes melt completely, and don't have time to refreeze before hitting the ground. However, the air near the ground is still below freezing, so that water that hits trees and the ground freezes into ice. That's called freezing rain, and is the situation we'll be in tonight and Wednesday (with the possibility for some sleet -- better chance farther north).
These diagrams are nice, but would you like to actually see that warm wedge? Take a look at this:
This is a computer model forecast showing the low-level temperature profile for 5 a.m. Wednesday. The red line (temperature) and yellowish-green line (dewpoint) are basically on top of each other, thus indicating 100 percent saturated conditions here. The diagonal blue dashed line in the center of the image is the freezing line: 32 degrees (0 degrees Celsius).
Notice how the red temperature line is below freezing at the surface and then, as you go aloft, it takes a sharp turn to the right. Believe it or not, a little over 3000 feet aloft, the temperature rises to 42 degrees (5.7 degrees Celsius). So in this cross-section of the lower atmosphere, as forecasted by a computer model, you can actually see that warm wedge aloft -- the cause of our freezing rain.
Surface temperatures will slowly warm on Wednesday from south to north, and we all should rise above freezing by early to mid-afternoon. In fact, highs will reach the mid to upper 30s (2 to 3 degrees Celsius). Areas such as our South Zone, who warm above freezing earliest, will obviously see the least ice accretion.
Areas farther north, however, will see perhaps nearly a quarter-inch of ice. This is a very important threshold: ice accretion above a quarter-inch starts bringing "major ice storm" into play, with significant power line and tree limb damage. However, there are many important factors that play into how much ice accumulates on trees and power lines, such as the temperature of the rain drops when they hit the tree or power line, how hard the rain is falling, and the air temperature here near the surface.
Some computer models advocate a quicker rise above freezing, which would mean less ice for all of us. That would be great if it panned out.
The best news of all is that wind is expected to be light on Wednesday. Stronger wind puts a bigger load on ice-laden power lines and tree limbs, and causes more of them to fail, but we don't expect this Wednesday.
Always remember: If you see a downed power line, call 911, alert DTE, and stay away from it.
Needless to say, untreated roads will be downright treacherous overnight tonight through Wednesday morning. For this reason, there may be a lot of school closings, as neighborhood streets are unlikely to be pre-treated with salt. Many superintendents make the determination to close school due to snow or ice based upon the condition of the neighborhood streets. As most of you already know, the area's best school closing information is right here on ClickOnDetroit -- check with us tonight and in the morning for the very latest.
The Local4Casters will monitor this weather situation carefully, and will keep you updated on-air, online, on the Local 4 Facebook page, and here on ClickOnDetroit.com.
Wednesday's sunrise is at 7:41 a.m., and Wednesday's sunset is at 5:54 p.m.
A strong low pressure system crosses the region on Thursday. Ahead of this next storm, some light freezing rain is possible Wednesday night (north) but, this time, there will be a much quicker change to rain, so we don't expect many problems from this ice.Rain on Thursday could be substantial, possibly more than three-quarters of an inch. Since the ground is frozen, it can't soak up this rain, so it all has to run off. That's bad news for low areas that flood quicker (such as Hines Drive). Be aware of substantial puddling and ponding of water when out driving on Thursday. At least it'll be a mild day, with highs in the upper 40s (9 degrees Celsius). But don't get used to it.
A strong cold front crosses the area Thursday night, and temperatures will fall through the day on Thursday. During the daylight hours, we may not get out of the 20s (-2 degrees Celsius)...watch out for ice Friday, as all of that standing water will quickly freeze. It'll also be a windy day...welcome back, Winter.
Expect mostly sunny skies on Saturday...a nice reprieve and great day to head down to Winter Blast! Highs in the mid to upper 20s (-3 to -2 degrees Celsius) should be pretty easy to take with all of that sunshine and light wind.
Sunday may start with some sunshine, but clouds will increase and snow is possible before the end of the afternoon in to Sunday night. Right now, accumulations don't look significant...perhaps 1-2 or 1-3 inches.
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