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Don Paul: This storm could be 'most serious ice storm hazard' we've seen in a while

The National Weather Service has elevated  a winter weather advisory for all but the three Southern Tier counties to an ice storm warning. A winter weather advisory remains in effect for the Southern Tier for somewhat lesser ice accumulations.

Quick answer: Not yet imminent, but likely.

Cold weather precipitation type is largely determined by the depth or thickness of the cold air. If abundant moisture is going to precipitate entirely through freezing or subfreezing air, it will generally fall as snow. Even if the temperature near the ground is a few degrees above freezing, there usually isn’t enough time for snowflakes to melt in shallow, somewhat milder, air, so you can get wet snow that doesn’t stick or slushy, water-laden snow.

For precipitation type, or p-type, deep, cold air is an easier call. But when the moisture is precipitating through warm air into shallower cold air, p-type becomes more problematic. If the subfreezing air is shallow but at least 2,000 feet deep, that gives time for raindrops, even when falling from much milder air above, to freeze into ice pellets before they hit the ground. Sleet is slippery and can accumulate on surfaces, but it does not produce the treacherous glaze that freezing rain does.

Freezing rain occurs if the subfreezing air is only near the surface. In that situation, the rain will fall as liquid rain and freeze on contact when it hits the ground, trees and power lines.

(National Weather Service)

The simple diagram at right illustrates the cold air depth dilemma.

I call it a dilemma, because some models show enough slightly deeper cold air Saturday afternoon into Sunday morning to allow freezing rain to transition over to sleet pellets, especially over the Niagara Frontier.

Believe me, we’d be lots better off if we got sleet instead of freezing rain, in terms of ice accretion on tree limbs and power lines, as well as for driving and walking. Sleet bounces and accumulates, but it forms a coating with some texture and doesn’t glaze over.

This high-resolution National Weather Service model has surface temps at 7 p.m. Saturday below freezing over virtually all of Western New York. In its precipitation depiction, you’ll note a lot more purple rather than red over the Niagara Frontier. The model, one of the coldest of many models, is suggesting more sleet and less freezing rain where the purplish shade is seen.

The coarser, lower-resolution National Weather Service GFS model is not as cold. It depicts liquid rain over much of the Southern Tier and mainly freezing rain on the Niagara Frontier.

Most meteorologists will gravitate toward the high-resolution models as we get closer to an event because they contain so much more data/information than the lower-resolution models. So, at this point, I would lean more heavily toward the first model.

However, as I described, the difference between less-treacherous sleet and more-treacherous freezing rain is tenuous and a very difficult call. For public safety, I would assume the worst if I were forecasting for the power companies.

Either way, plenty of salting will be needed on area roads as we move further into Saturday afternoon through at least early Sunday. Initially, the precipitation may even fall briefly as snow, because the first falling drops will be evaporating and cooling the lower atmosphere. But then warmer air will deepen aloft and change any snow to rain aloft, where it will eventually transition to freezing rain and sleet as it reaches the ground, especially in the afternoon.

The amount of precipitation will be heavy, so this presents potentially the most serious ice storm hazard we have faced in a long while, especially if there is mainly freezing rain and not sleet. As much as a half-inch ice could accumulate before liquid rain returns sometime Sunday morning, as warmer air reaches the surface.

The increasing weight of this accumulating ice will be complicated by the strengthening northeast wind coming up to 15 to 30 mph by Saturday afternoon, with gusts up to and over 40 near Lake Ontario. Those tree limbs and power lines will be especially stressed by the wind.

At left is a probability forecast from National Weather Service headquarters for ice accumulating more than 0.25 inches.

I guess you’ll have to take it from me: We don’t see that large an area under 40 percent probability all that often. Even if the accumulation were quarter inch of ice instead of half an inch, with those winds, this will present a major hazard for scattered power outages and downed tree limbs over a large portion of Western New York, especially the Niagara Frontier.

I’ve little doubt utilities and road departments are prepping for this storm. You all know the song “Purple Rain.” In this case on the models, purple means sleet and red means freezing rain. So let’s hope for “Purple Sleet.”

 

 

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