Author Archives: Meteorologist Cory Mottice

Severe Weather Outbreak Later Today/Tonight

Posted by on January 22, 2012 No comments

There is a significant threat for severe thunderstorms later today and tonight from the Ohio Valley southward to the central Gulf Coast. A strong mid-level trough will become negatively tilted this evening over the Ozark Plateau and will then eject northeastward over night across the Tennessee and Ohio Vallies.

Severe weather outlook from the Storm Prediction Center (SPC).

 

An associated surface cyclone will move across eastern Kansas this evening and should be near the Chicago area by early Monday morning. The attendant cold front will trail to the southwest and will move eastward through the Mississippi Valley, while a warm front will move northward across the mid-Mississippi Valley and eventually into the lower Ohio Valley by late tonight/early Monday morning. Dewpoints in the 50s will surge as far north as southern Illinois and southern Indiana, while dewpoints in the 60s will be widespread across the Mississippi and lower Tennessee Vallies.

There will also be a very strong low level jet of 50-70 knots, increasingly strengthening as the night wears on. Jet streaks of 100+ knots at 500 mb and 250 mb will provide essential upper-level divergence across the warm sector, and instability at the surface will approach 500-1000 J/Kg as far north as the lower Ohio Valley and as high as 1500 J/Kg across the lower Mississippi Valley.

Damaging wind outlook from the SPC.

The Total Totals Index (TTI) will be around 54-56 for much of the warm sector. The TTI is a combination of the Vertical Totals (VT) and Cross Totals (CT). The VT is the temperature difference between 850 and 500 mb while the CT is 850 mb dewpoint minus the 500 mb temperature. A value of 54-56 means widely scattered severe storms are possible.

The SWEAT (Severe Weather Threat Index) values will range from 300-500. Any value over 400 suggests tornadic supercells are possible.

SRH (Storm Relative Helicity) values will range from 250-450 m**2/s**2. SRH “is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. Larger values of 0-3-km SRH (greater than 250 m**2/s**2) and 0-1-km SRH (greater than 100 m**2/s**2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear ‘boundaries’ between non-tornadic and significant tornadic supercells (2012, SPC).”

Tornado outlook from the SPC. Large and long tracked tornadoes are possible in the 15% hatched area!

Everything appears to be coming together for a significant severe weather outbreak over the areas mentioned later today and into tonight. Below is a list of the threats you can expect…

  • Damaging winds in excess of 80 mph will be widespread
  • Tornadoes, some large and long-track tornadoes possible, especially across western Tennessee and upper Mississippi Vallies 
  • Hail to 2″ in diameter

If you live anywhere within the outlooked areas shown above, please pay attention to every warning issued by your local NWS office and take them seriously. If you are under a tornado warning take shelter immediately as you will not be able to see the tornado coming after dark. If you know of anyone living in the above areas, give them a call and let them know what to expect.

Historic Snow in Store for Pacific Northwest

Posted by on January 16, 2012 No comments

Multiple waves of low pressure will move ashore over the next several days across the Pacific Northwest. An arctic front will move through western Washington late tonight into Tuesday morning, dropping freezing levels to around 500 feet Tuesday afternoon and eventually to the valley floors Tuesday night.

The Washington and Oregon Cascades will get absolutely pounded with snow throughout much of this week. In general, the Cascades will receive a wide spread 6-10 FEET of snow by Friday, but some of the highest peaks could receive more than 16 FEET! With the pattern the Pacific Northwest will be locked into the remainder of the month, it is not out of the question that Mt. Rainier in Washington sets the new record for the most snowfall in a month. The current record of 304 inches, or 25′ 4″, is held by Mt. Baker in Washington.

While the Cascades will definitely receive feet upon feet of snow, what about the lowlands, including Seattle and Portland? As the freezing levels drop to the valley floors Tuesday night into early Wednesday, both Seattle and Portland will likely see accumulating snow. Prior to Tuesday night, both cities will see some light snow showers, accumulating an inch or two. The storm really gets going Tuesday night into early Wednesday before the snow changes back to rain. Below is a graphic from the University of Washington depicting 24-hour snowfall accumulations for western Washington ending Wednesday afternoon.

University of Washington WRF 4-km forecast of 24-hour snowfall accumulations.

As you can see, the UW 4-km WRF brings significant snow across just about all of western Washington. During this period Seattle is forecast to receive around 6 inches or so. Just north of Seattle is expected to get quite a bit more snow than downtown. Do you know why this is? This is due to the Puget Sound Convergence Zone. My friend Luke Madaus wrote a great blog about this a while back.

While I do think Portland will see some accumulating snow, they will not see near as much as Seattle. Portland will be straddling the rain/snow line for a while Tuesday night, but I still expect downtown to pick up an event-total of 2-4 inches. Below is a list of a few cities in the Pacific Northwest along with my snowfall forecast for that city ending Wednesday afternoon.

  • Olympia, WA: 16-24 Inches
  • Everett, WA: 8-14 Inches
  • Seattle, WA: 5-10 Inches
  • Longview, WA: 4-8 Inches
  • Gresham, OR: 4-8 Inches
  • Portland, OR: 2-4 Inches

**Worth noting: Average annual snowfall, as measured at Sea-Tac Airport, is 8.1 inches.**

Tornado Tears Through Rutherford and Burke Counties in NC

Posted by on January 11, 2012 No comments

**UPDATE: NWS confirms the storm damage near Ellenboro, NC was from an EF-2 tornado.**

An apparent tornado tore through Rutherford and Burke counties in North Carolina Wednesday (January 11, 2012) evening. The likely tornado toppled trees, knocked down power lines, and severely damaged a mobile home park, according to hamptonroads.com.

A house was severely damaged from an apparent tornado in North Carolina on January 11, 2012.

Three people were airlifted from the Ellenboro area in eastern Rutherford county. The hardest hit area was eastern Burke county where a mobile home park was nearly wiped out. Several people were trapped under debris in this area.

WLOS ABC is reporting that at least 10 people were injured near Ellenboro, one seriously. An unknown number of people were injured in Burke county. Whether this was one or more tornadoes is not yet know, but the NWS will survey the damage tomorrow (Thursday, Jan. 12).

Jan. 1-3, 2012 Lake-Effect Snow Totals for Ohio and Michigan

Posted by on January 3, 2012 No comments

Here are the snowfall totals from the NWS for Michigan. The totals are overlaid with my forecast for this event (the legend is on the left).

Jan. 1-3, 2012 lake-effect snowfall totals for Michigan. (click to enlarge image)

 

Here are the snowfall totals from the NWS for Ohio. The totals are overlaid with my forecast for this event (the legend is on the left).

Jan. 1-3, 2012 lake-effect snowfall totals for Ohio. (click to enlarge image)

How the MJO Affects Weather Across the Western US

Posted by on January 1, 2012 No comments

The Madden-Julian Oscillation (MJO) is a coupling between atmospheric circulation and deep tropical convection. In other words, it is an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Oceans. Variations in wind and temperature produced by the MJO can have a significant influence on global atmospheric and oceanic circulations. The MJO is also known as the 30-60 Day Tropical Wave since it typically circles the globe along the equator in about 30-60 days, or 45 days on average.

A graphical depiction of the MJO from the NWS Boulder Office.

MJO activity in the pacific is greatest during weak La Ninas (such as we are currently in) and ENSO-neutral conditions. During moderate to strong El Ninos, MJO activity is minimal or even absent. The MJO plays an important role on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics, but for the purposes of this blog, I am only going to tell you how it affects the weather across the western US.

The greatest affects from the MJO in the United States are felt during the winter months along the west coast. As the MJO propagates eastward across the Pacific Ocean, subduction in advance of this oceanic tropical wave (Kelvin wave) anomalously warms the sea waters.

As this Kelvin wave, and its associated deep convection, nears the central Pacific Ocean, it encounters the sub-tropical jet. This sub-tropical jet will then transport this moisture northeastward into the mid-latitudes. This is also known as the Pineapple Express. A perfect example of this (that I happened to document) occurred last winter during the week of December 19, 2010. Below is the picture I posted in the blog “The Pineapple Express“.

A clear depiction of the MJO and the Pineapple Express from December 19, 2010.

The large cluster of thunderstorms (the reds and yellows) over the central Pacific Ocean is actually the MJO. The sub-tropical jet is then tapping into this moisture and transporting it northeastward, as is depicted by the arrow. This particular event brought extremely heavy rain to the vallies of California, and over 10 feet of snow to the Sierras.

When the MJO is affecting the western United States, effects typically seen are:

  • Unusually heavy precipitation
  • Greater cloud cover
  • Warmer than normal nighttime temperatures

The MJO recently peaked in late November and early December of 2011. Perhaps you can recall, during this time, multiple upper level mid-latitude troughs (Rossby waves) formed or passed over the western US, and often remained over this area for several days at a time. This resulted in periods of moderate to heavy rain/snow over portions of the southwest during this time.

Time series of daily MJO Index. Data supplied by the CPC and graph supplied by the Boulder NWS.

The pattern in early December of 2011 was more like what we see during an El Nino as opposed to what we typically see during a La Nina. This was due to the MJO peaking during that time. After the MJO passed, the southwestern US saw dry conditions, which is what we expect to see during La Nina winters.

Currently the MJO is becoming better organized once again across the Indian Ocean, however, it does not yet appear to be propagating eastward. This will be something to keep an eye on, because if it does strengthen and begin to move eastward, this will have a significant impact on the weather across the western US in the next month or so.

The MJO also influences the frequency and intensity of cold air outbreaks across the eastern US, along with tropical cyclone activity in both the eastern Pacific and Atlantic basins during the Northern Hemisphere summer. I will save this for another time.

January 1-3 Forecast Lake-Effect Snowfall Totals for OH and MI

Posted by on January 1, 2012 1 comment

Click here to see the final snowfall totals from this event

Here are my forecasted snowfall totals for Michigan for the entire lake-effect snow event. The higher amounts will be found wherever the lake-effect bands persist the longest. If you are not under one of these bands, then the total snowfall in that area will be closer to the lower end of the amounts shown.

Forecast lake-effect snow totals for Michigan for areas that will receive at least 3" of snow.

Here are my forecasted snowfall totals for Ohio for the entire lake-effect snow event. The higher amounts will be found wherever the lake-effect bands persist the longest. If you are not under one of these bands, then the total snowfall in that area will be closer to the lower end of the amounts shown.

Forecast lake-effect snow totals for Ohio for areas that will receive at least 3" of snow.

A Year (2011) in Review

Posted by on December 31, 2011 No comments

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!

January 1-3, 2012 Preliminary Snowfall Map for Ohio

Posted by on December 31, 2011 No comments

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.

January 1-3, 2012 Lake Effect Snow

Posted by on December 30, 2011 4 comments

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.

Ideal Setup for Lake Effect Snow

Posted by on December 29, 2011 1 comment

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.