Sunday, April 22, 2012

"Say it isn't so"

Photo from the Slusser family outside State College, PA
as rain changed to snow around 9:30 AM this morning
Winter Storm Warnings: Western PA to southern NY 
Extensive power outages expected in Appalachians
Flood Watches in eastern New England


11:20 AM EDT 4/23/12 Millions of residents along the Appalachians may have spoken this lyric from Hall & Oates when they awoke to snow this morning.  This bizarrely historic Spring storm that has already turned rain to snow in the Laurel Highlands of Pennsylvania, according to several reader reports from earlier this morning. 

It's supposed to be Spring! How is this happening? Scientifically it's very interesting to forecasters that an almost August-like sun angle this time of the year can even permit accumulating snow in places that were 75 F on Friday! As our meteorologists say, the reason why this can occur in a "rain changing to snow" situation like is seen in the Appalachians is due to  "dynamic cooling." This 1998 research paper by a National Weather Service meteorologist notes an example of when this process has produced snowfall following rain, in the case of a February 1998 storm in Tennessee:
"dynamic cooling was the primary mechanism that resulted in cooling the lower and middle levels of the atmosphere, allowing the rain to change to snow much earlier than anyone expected." (The article goes on to provide a working definition: "Dynamic cooling is therefore the cooling that results from decreasing pressure."                          - Henry Steigerwaldt, Science and Operations Officer - NWSO Nashville, Tennessee 
How much snow then? Lower elevations of areas appearing to receive snow in the radar may only eek out an inch or two, (that's still pretty epic for April!) higher elevations above 1500 feet are likely to see 10 or more inches. This may produce crippling effects in those areas which could equal the power outage problems following the October 2011 storm. Please scroll down to our earlier statements for an overview of impacts expected in this event.

April 22-29, in addition to being recognized as Earth Week, is also NOAA's first ever nation-wide Severe Weather Awareness Week. This wild but timely Spring Storm gives all of us a fresh reminder of the value of being prepared for all weather hazards, regardless of the time of year.  Visit the NOAA site for more details on how you can keep your family or business well-prepared for Severe Weather in the months ahead.

Storm Impact Statement  
Forecasters Andrew Barney, Jason Mitchell, Greg Jackson

6:30 PM EDT 4/22/12 Winter Storm Warnings are in effect until tomorrow night for much of western Maryland and western Pennsylvania including the Laurel Highlands. This storm is likely to produce 12 or more inches overnight Sunday into Monday in these areas, as noted by the latest snowfall projections of the State College PA National Weather Service. Impact in western Pennsylvania and New York may be equal or greater than the October 2011 storm, including:
  • Low visibility from high snowfall rates in short periods of time
  • Numerous road closures and delays due to downed trees
  • Thousands of power outages and interrupted public services
  • Disrupted ground and air transportation in local airports
  • Delays or cancellations for some schools and colleges
Our Three Rivers Team in metro Pittsburgh is tracking the event closely on their local forecast page in Facebook. Please visit their page for the latest updates on this significant event. Regional updates on impacts in the Mid-Atlantic are also posted in our Winter Stormcast page.

4:00 PM EDT 4/22/12 A strong low pressure system is rapidly moving towards the area, and there is significant snowfall potential in western Maryland, western Pennsylvania, and the mountains of West Virginia. Here is an overview of the timing of the storm and locations most affected:

TIMING: Rain and snow has begun to overspread these areas from south to north. 
  • Sunday PM: Precipitation rates will become moderate to heavy by the predawn hours Monday, and a changeover of rain to snow is expected likely by sunrise. 
  • Monday PM: Precipitation intensity will wane by late afternoon, although there will be a few lingering snow showers through the evening hours. All precipitation should end Tuesday morning.

AMOUNTS: Despite temperatures have reached the 70s on Friday, a

 heavy, wet snow is expected across the region tonight into Monday. In higher elevations above 1500 feet, 6-12” of snow is expected with less than 6 inches in lower elevations and primarily on grassy surfaces. A total snowfall graphic by the State College NWS is shown below:

IMPACTS: The main concern for most areas will be reduced visibilities and a potential for some slushy road conditions. The areas most affected by this storm will be along the higher elevations of mountains and ridge tops. 

If snowfall becomes this heavy in the regions highlighted in bright white on the map above, downed trees and power lines will become a concern because trees are now almost in full leaf at this point. These areas could see up to a foot of snow, with upward potential. 

(Forecaster Andrew B., Contributor: Forecaster Jason M.)

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