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Monthly archives for December, 2011

A Year (2011) in Review

Dec31
2011 Leave a Comment Written by Meteorologist Cory Mottice

Below are some of the stats for this site during the year 2011:

  • 58,478 page views (37,874 visits)
  • Visitors from all 50 states and 6/7 continents (not Antarctica)
  • Top 3 most viewed posts:
  1. Deadly Joplin, MO Tornado (2,547 views)
  2. April 16, 2011 Tornado Outbreak (2,170 views)
  3. How the AO, NAO, and PNA Affect Winter Weather Patterns (2,168 views)
  • Most views in 1 day: 4,299 on April 26, 2011
  • 3,365 referrals from facebook, 659 from twitter
  • Most searched term that led to this site: Joplin, MO tornado (searched over 1,650 times)

Thanks for reading everyone! Happy New Year!

Posted in General Weather - Tagged ,

January 1-3, 2012 Preliminary Snowfall Map for Ohio

Dec31
2011 Leave a Comment Written by Meteorologist Cory Mottice

Here is a preliminary snowfall map for Ohio for the impending lake-effect snow. It is important to note that the snow will not be spread evenly throughout the areas highlighted. For instance, wherever the heaviest band sets up in far northeast Ohio, there will likely be 12-18″ of snow, but outside of this band, there may be even less than a foot of snow. It is still too far out to pinpoint where the snow bands will set up, but as the event draws nearer, I will post an updated snowfall map.

Preliminary lake-effect snowfall forecast for Ohio.

Posted in Winter Weather - Tagged , ,

January 1-3, 2012 Lake Effect Snow

Dec30
2011 4 Comments Written by Meteorologist Cory Mottice

There has been a lot of talk lately about a significant lake-effect snow event for the Great Lakes, and for good reason. We haven’t really had any significant lake-effect snow events so far this season, but it looks like the first good lake-effect snows will occur from January 1-3. In my blog yesterday I talked about the necessary ingredients for lake-effect snow, so I will be referring to that frequently.

I am going to break this down by lakes. I will talk about the potential for lake-effect snow off of Lake Michigan and Lake Erie. If you would like a detailed break down for any other lake, just let me know and I will see what I can do.

Lake Michigan

A strong cold front will move across Lake Michigan Sunday night, bringing an end to the relatively mild weather. Due to the lack of cold so far this winter, the surface water temperature of Lake Michigan is a relatively warm 3-4 degrees Celsius. The temperatures at 850 mb will be -12 to -14 Celsius, so the temperature difference of 15-18C is plenty for lake-effect snow.

The winds behind the front will start off westerly on Sunday and will then turn more northwesterly Sunday night into Monday. The directional shear from the surface to 700 mb will tend to be less than 30 degrees throughout the duration of this event, which as I mentioned yesterday, is ideal for heavy lake-effect snow. One factor that may limit the overall intensity of the snow bands will be the excessive wind speed throughout the boundary layer. The wind speed from 925 mb up to 700 mb will range anywhere from 45-60 knots (~52-70 mph). Considering the ideal average wind speed throughout the boundary layer is less than 40 mph for heavy lake-effect snow, this will likely minimize the potential for snow bands producing snowfall at rates of greater than 1.5″/hr. One exception may be up across far northwestern Michigan where the lake fetch is essentially from Lake Superior and northern Lake Michigan.

All in all it looks like areas east of Lake Michigan should still receive a good amount of lake-effect snow just due to the duration of the event. It will last from Sunday through Monday night before ending. The winds will be shifting quite a bit from the west to the northwest and eventually almost to the north. This will help to spread the snow out over a greater area, which will also limit the total amount of snow. It is too hard to forecast snowfall amounts at this time, however the heaviest snow will fall across portions of far northern and northwestern Michigan and across far southwestern Michigan (from Allegan county on southward). The area in northwestern Michigan (near Otsego and the surrounding counties) has the best *potential* to receive a foot or more of snow. Something else to keep in mind is that with such strong wind speeds, the heaviest snow will not fall along the lake shore, but rather 10-20 miles inland. The strong winds will also make for blizzard-like conditions at times. Below is a county map of Michigan so you can reference the counties I was talking about.

Michigan counties.

 

Lake Erie

The cold front will move across lake Erie during the day Sunday. The water temperatures and 850 mb temperatures will be nearly the same as for Lake Michigan, so we know that there will be potential for lake-effect snow. The lake-effect event off of Erie will run from Sunday night through the first part of the day on Tuesday.

The winds behind the front will initially be westerly and this will continue through Sunday night. The winds from the surface to 700 mb will tend to have less than 30 degrees of directional shear, which is ideal for heavy lake-effect snow. As we head into Monday, the winds will turn more out of the northwest and will remain this way through much of Monday night. Early Tuesday morning into the early afternoon hours, the winds will try to become more north northwesterly.

The winds throughout the boundary layer will average between 30 and 40 knots (~35-45 mph), quite a bit weaker than the winds over Lake Michigan. This will greatly enhance the chance of significant lake-effect snow. Much like Lake Michigan, the wind direction does shift quite a bit throughout the entire event. This will be the one limiting factor as far as snowfall totals go off of Lake Erie. Sunday night into Monday will be when the heaviest snow falls in this area, since drier air will move in after this timeframe.

The heaviest snow off of Lake Erie will fall across far western New York (south of Buffalo), far northwestern Pennsylvania (just inland from Erie), and across far northeastern Ohio (east of Cleveland). In Ohio the hardest hit counties will likely be Lake, Geauga, Ashtabula, and Trumbull. As the winds turn more out of the north Monday and Monday night, counties farther to the west and south will have the potential to pick up some accumulating snow as well. This includes Portage, Cuyahoga, and Summit counties. It would not surprise me if some places in Ashtabula and far northwest Pennsylvania pick up 1-2 feet of snow. The strong winds at the surface may also make for blizzard-like conditions at times. Below is a county map of Ohio for reference.

Ohio counties.

 

Here is a great map from accuweather.com to help you visualize what areas downwind of the lakes have the best chance at lake-effect snow.

The map represents the areas that will likely see lake-effect snow from Sunday through Tuesday morning. Courtesy accuweather.com.

Posted in Winter Weather - Tagged , , , ,

Ideal Setup for Lake Effect Snow

Dec29
2011 1 Comment Written by Meteorologist Cory Mottice

Lake-effect snow events occur most commonly in the Great Lakes region and can produce extremely heavy snow. Snowfall rates can reach several inches per hour and there are many reports of 2 or more feet of snow falling in a 24-hour period from lake-effect snows.

Lake-effect snow generally occurs from November to February, and tends to be most intense in the late Fall and early Winter because the temperature gradient between the lake and the cold air is relatively high. So what factors need to come together to produce a lake-effect snow event?

  • You want the temperature difference between the lake surface and 850 mb to be at least 13 degrees Celsius. (i.e. you need a wave of cold air to be moving over the lakes) This wave of cold air should be deep, at least to 850 mb for lake-effect snow, but deeper than 700 mb for heavy lake-effect snow. This is because shallow arctic air limits the amount of convection that can take place.
  • You want very little directional shear from the surface to 700 mb. Less than 30 degrees is ideal, but you can still get weaker lake-effect bands with up to 60 degrees of directional shear. Any more than 60 degrees and you will tend not to get anything more than some flurries. This is because directional shear decreases the moisture convergence out over the lake and causes the snow to spread over a larger area.
  • You want a lake fetch of at least 60 km for lake-effect snow showers, and of at least 100 km for heavy lake-effect snow. The “lake fetch” is the distance the air must travel over the water before reaching land.
  • You don’t want the winds that blow across the lake (in the boundary layer) to be too strong. They need to be light enough for moisture convergence to occur. If the winds are too strong, say 50 mph, enough moisture may not be able to evaporate to produce heavy lake-effect snow. The ideal wind speed is between 10 and 40 mph.

Below is a sketch from theweatherprediction.com of a typical lake-effect snow event.

Sketch of a typical lake-effect snow event.

Those are the key ingredients needed for lake-effect snow. Of course, depending on just where you are compared to the lake will make a huge difference. Lets take Lake Erie for example. If you have a northwesterly wind and all the above criteria is met, where would you expect the heaviest snow off of Lake Erie to occur?

If you guessed across far northeastern OH, especially in Geauga and Ashtabula counties, you would be correct. We can use December 14, 2010 as an example. During this day, the winds were generally out of the northwest and up to 2 feet of snow fell across portions of northeast Ohio. Below is a map of some of the snowfall totals that I posted last year. You can see just how sharp some of the gradients were. Central Trumbull County received nearly 2 feet of snow, while the southern portion of the county only received 3 inches!

Lake-effect snow totals from December 14, 2010.

The terrain also plays an important part in lake-effect snow. Once the air traveling over the water hits land, frictional convergence plays a big role. Essentially, the air gets “piled up” just inland from the lake shore and this helps to intensify snow bands. You will rarely see the heaviest snow occur right along shore, but rather a few miles inland. The winds will blow the developing snow downwind from the lake shore.

Posted in Winter Weather - Tagged , ,

100 mph Winds Batter Scotland

Dec28
2011 Leave a Comment Written by Meteorologist Cory Mottice

A strong synoptic wind event battered Scotland earlier today, Wednesday, December 28, 2011. More than 50,000 people were left without power due to these high winds. Most winds were 70-80 mph with a few gusts near 100. An even more impressive wind event occurred just earlier this month on December 8th.

  • A wind gust was of 165 mph was recorded on the 8th at Cairngorm Summit, just 5 mph shy of the all-time British record high wind speed set back on March 20, 1986.

High winds create massive waves off the coasts of Scotland. Image courtesy dailymail.co.uk.

Waves batter drivers along a road off of Troon harbour wall in Scotland. Image courtesy dailymail.co.uk.

 

A large part of the UK was also affected by December 8th storm system, not just Scotland.

  • A car was swept into a river in Swaledale, North Yorkshire, where three people had to be rescued by helicopter.
  • The winds were so fierce off the East Yorkshire coast, that a ferry with more than 300 passengers was stranded due to fears that high winds could damage the ship as it entered port.

Wind gusts across most populated areas ranged from 75-85 mph. Several bridges and roads were closed to all traffic for a time and ScotRail was forced to completely shut down operations. Schools across much of Scotland were closed and commuters were advised to head home from work early.

Posted in Severe Weather - Tagged , ,

Don’t Get Your Hopes up on Major East Coast Snowstorm

Dec27
2011 Leave a Comment Written by Meteorologist Cory Mottice

There has been a lot of talk the last couple of days about a major East Coast snowstorm during the first week of January. Two of the major models (the GFS and ECMWF) show a storm sometime during either the middle or latter part of the first week of January. While I do believe there will be a trough that swings through the eastern part of the country, the GFS and ECMWF are extremely over amplifying this feature.

NOTE: I will be writing this post assuming you already know what the AO, NAO, and PNA are, if you do not, you can refer to yesterday’s post that explains these indices.

As I mentioned yesterday, I do believe there will be periods of cold air and snow that will affect the eastern portion of the country starting during the first week of January. However, I do not believe that there will be a major east coast snowstorm such as the models are showing. Here is why:

  • The AO is still extremely positive, and although it is forecast to become less positive over the next two weeks, it will not become negative. On top of that, the AO is more positive than any model has forecast, so I would expect the actual AO index to remain higher than the average of the forecasts. Below is the two week AO forecast, where the yellow shading shows the ensemble mean, while the upper and lower red lines show the range of the forecasted indices, respectively.

Two week AO forecast. Yellow shading = ensemble mean; redlines = upper and lower forecasted index.

  • The NAO is positive, and just like the AO, it too is at the high (positive) end of every model forecast. While the NAO will likely approach neutral, or perhaps even turn slightly negative, it won’t be enough.

Two week NAO forecast. Yellow shading = ensemble mean; red lines - upper and lower forecasted index.


  • To get a massive east coast snowstorm, you want both the AO and NAO to be negative, and the more negative the better. Also, you want the PNA to be positive, and it is forecast to be right around neutral, or perhaps even slightly negative.

Now this is not to say there will not be cold air and snow somewhere in the northeast U.S., or perhaps even the mid-Atlantic, during the mid or latter part of the first week of January. As I mentioned yesterday, with the AO and NAO forecast to become less positive, this will allow for cold (arctic) air to sink southward into the eastern U.S., especially the northeast U.S. The ingredients just do not appear that they will come together for a major east coast snowstorm during the first week of January.

Posted in Winter Weather - Tagged , , ,

How the AO, NAO, and PNA Affect Winter Weather Patterns

Dec26
2011 5 Comments Written by Meteorologist Cory Mottice

Now that winter has arrived, you will likely be hearing these terms used more frequently by meteorologists. Here, I will discuss each of the following terms and explain the affects they have on winter weather patterns. The terms are: AO (Arctic Oscillation) index, NAO (North Atlantic Oscillation) index, and PNA (Pacific-North America) index.

The first term I want to talk about is the AO index. This is defined by surface atmospheric pressure patterns. When the AO index is positive, surface pressure is lower than normal in the polar region and higher than normal in the mid-latitudes. This causes westerlies to increase over Canada and keeps the cold air farther north.

The warm (positive) phase of the Arctic Oscillation (AO). Courtesy NWS Little Rock.

When the AO is negative, surface pressure is higher than normal in the polar region and lower than normal in the mid-latitudes. This means the westerlies are not as strong since the pressure gradient force is weaker, and the cold air is able to move southward into the mid-latitudes.

Currently the AO is extremely positive, thus the frigid air is being bottled up in the polar regions. The AO is forecast to become less positive over the next two weeks, and this should allow for periods of cold air to sink southward into eastern parts of the country, especially as it approaches zero during the first week of January.

Observed AO (black line) and the forecast AO (red lines).

The NAO is closely related to the AO. Essentially, the NAO index is based off the difference of the atmospheric pressure at sea level between the Icelandic low and the Azores high. The strengths and location of the the Icelandic low and the Azores high vary from year to year and this variation is known as the NAO.

Comparison of a positive and negative NAO.

Much like the AO, if the pressure gradient between the Icelandic low and Azores high is strong, then the corresponding westerlies are strong. This is known as a positive NAO. Since the Azores high is stronger (and typically farther west), more of southwesterly flow is present across the northeastern United States, meaning warmer than normal temperatures. When the high is weaker (and farther east), the Icelandic low tends to influence the northeastern US more than the high. This means more of a northerly component to the wind, thus allowing the colder air to intrude farther south.

Currently the NAO is positive (as shown below), but is forecast to become less positive over the next two weeks. Again, this will allow for periods of colder air to makes its way into eastern sections of the country, especially towards the latter part of the first week of January.

Observed NAO (black line) and forecast NAO (red lines).

Lastly there is the PNA. When the PNA is positive the surface pressures are higher than normal over Hawaii and the inner-mountain west of the United States. The PNA pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. When the PNA is positive, this jet stream is enhanced and shifted eastward to where the exit region of the jet is near the western United States.

When the PNA is negative, this jet stream is weaker and shifts farther to the west toward eastern Asia. This effectively blocks activity over the high latitudes of the North Pacific and causes a strong split-flow to develop over the central North Pacific.

The positive phase of the PNA is associated with above average temperatures across the far western US and western Canada, and below average temperatures across the south-central and southeastern U.S. This also tends to lead to above average precipitation across the Gulf of Alaska and the Pacific Northwest and below average precipitation across the Upper Midwest. The negative phase of the PNA is associated with exactly the opposite.

Temperature anomalies associated with a positive PNA. Courtesy Climate Prediction Center.

Precipitation anomalies associated with a positive PNA. Courtesy Climate Prediction Center.

Currently, the PNA is slightly positive and is not forecast to change to much over the next week. In fact, it will likely rise a little if anything. Perhaps as we head into the first week of January it will try to become neutral or negative, but we will have to wait and see.

Observed PNA (black lines) and the forecast PNA (red lines).

Much of the Pacific Northwest has been unusually dry as of late, but with the PNA forecast to rise a little over the next week, we can expect to see this dry spell come to an end. Much of the Pacific Northwest will see a good amount of rain over the next week, and the PNA supports this.

Hopefully you now better understand how the AO, NAO, and PNA affect the weather patterns across the US. If you have any questions feel free to leave a comment or send me a message.

Posted in General Weather, Winter Weather - Tagged , , , ,

Max 24 Hour Snowfall Totals this Winter

Dec23
2011 Leave a Comment Written by Meteorologist Cory Mottice

Here is a list of the top 24 hour snowfall totals per state so far this winter (October 20th – December 23, 2011). The 24 hour time frame runs from 6Z – 6Z. NOTE: Only states with totals over 12″ are included.

  1. Alaska – 37″
  2. Washington – 34″
  3. Colorado – 30″
  4. Wyoming – 25″
  5. Arizona – 25″
  6. New Mexico – 24″
  7. New Hampshire – 19″
  8. Montana – 16″
  9. Texas – 15.5″
  10. Utah – 15″
  11. South Dakota – 15″
  12. Minnesota – 15″
  13. Maine – 15″
  14. Kansas – 15″
  15. New York – 14″
  16. Vermont – 13.7″
  17. Michigan – 13.1″
  18. California – 12″

**All data is unofficial**


Science Blogs
Science links

Posted in Winter Weather - Tagged

Aftermath of December 19-20, 2011 Plains Blizzard

Dec21
2011 2 Comments Written by Meteorologist Cory Mottice

I was out in Garden City, Kansas during this blizzard, and I can honestly say, I have never experienced anything quite like this before. I was born and raised near Akron, OH, so getting a foot of snow was not too unusual, but getting a foot of snow with winds gusting to 50+ mph, now that was crazy. At times, it was hard to even keep your balance with the combination of 50 mph wind gusts and the ice and snow under your feet.

Courtesy NOAA: A map of the snow depth after the blizzard. As you can see, I was in one of the heaviest bands of snow in Kansas. (Click image for full NOAA resolution)

I arrived in Garden City around 3 PM, just about the time the rain changed to freezing rain and was mixing with a little sleet. I checked in to my hotel room and began to prepare for the long night ahead of me.

For nearly two hours the precipitation was a mix of a little bit of everything. For brief periods of time it would change to all snow, but then it would mix right back with sleet again. By 5 PM the snow/sleet was beginning to accumulate, but very slowly.

Snow/sleet beginning to accumulate.

By 9 PM there was only 2 inches of snow/sleet on the ground. The sleet was really cutting down on accumulations, and I began to worry I hadn’t driven far enough west/north.

About 2 inches of snow/sleet had fallen by 9 PM.

Finally, by 10 PM or so, the precipitation had changed to all snow. Around 11 PM the snow really began to pick up in intensity and the winds continued to consistently gust above 45 mph. The snow began to accumulate quickly (in the somewhat sheltered areas) and the drifts began to pile up. For a time, mainly between 11 PM and 2 AM, the combination of the high winds and heavy snow was so intense that the Comfort Inn sign in the above pictures was barely visible.

Below is some footage I took during the blizzard. This was taken around 11 PM.

 

By the morning, all the snow had come to an end. The winds were still gusting to 35 mph and blowing some snow around, but nothing too severe. This is what I woke up to.

What I woke up to the morning after the blizzard.

I took multiple measurements around the hotel that morning, and the average snow depth was 13 inches (in sheltered locations), with drifts of 2 – 3 feet. I began my trek home around 10 AM, and below is a little of what I saw along the way. Roads were completely snow packed with a layer of ice underneath. While driving east on US 400 that morning, I did not see one other vehicle for nearly an entire hour, a moving vehicle that is. There were probably 30-40 vehicles stranded along the highway between Garden City and Dodge City, some were even on their sides. The average snow drifts outside of town were 4 – 5 feet, while a few isolated snow drifts reached 7 FEET in remote locations just east of Garden City. When I arrived in Pratt, Kansas, about 1 hour west of Wichita, trees and power lines were coated in about 1/4 inch of ice.

A truck was flipped on its side off of US 400.

The average snow drifts outside of Garden City, KS were 4 - 5 feet.

A few isolated snow drifts reached 7 FEET!

Trees and power lines were coated in approx. 1/4 inch of ice near Pratt, Kansas.

This was definitely an experience I will never forget, and hopefully I will get to go blizzard chasing again sometime soon!

Posted in Winter Weather - Tagged , ,

Updated Forecast for December 19-20, 2011 Winter Storm

Dec17
2011 3 Comments Written by Meteorologist Cory Mottice

A major winter storm will affect portions of the southern Plains this coming Monday and Tuesday. The heaviest snow will fall over southwestern Kansas and the Oklahoma and Texas panhandles. In these areas, isolated amounts of up to 18 inches are possible. Winds will be gusting up to 45 mph at times in these same areas and will cause near blizzard conditions.

There will be a very sharp gradient with this storm and several locations on the southern edge of the snow area will see a transition from rain to freezing rain to sleet and finally to snow. Up to an inch or two of sleet will be possible where this occurs.

Forecast map for the upcoming winter storm.

Posted in Winter Weather - Tagged , ,
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