Lake Erie Convergence Zone Strikes Back!

The Lake Erie Convergence Zone isn't a magical place, where snow shovels itself and it's always a balmy 80 degrees.  From a forecast perspective, however... It will either make you or break you.

Allow me to explain:

Winds are driven by high and low pressure systems.  They don't just move clockwise and counter-clockwise, however but spiral inward and outward, due to the external forces acting upon them.  These forces include the Coriolis and Pressure Gradient Force (PGF).  To a lesser-extent, friction also influences wind direction at the surface.  Friction is simply the result of topography.  The more obstructions, the more friction becomes a factor.

Take the lake shore for example.  When a strong southerly wind is present, the City of Erie can be several degrees warmer than locations further inland.  This is caused by warm air advection and friction from a sloping Lake Erie shoreline.  The same process occurs on the leeward side of the Rocky Mountains.  As air travels toward sea-level, it accelerates and warms adiabatically- a phenomena known as the Chinook.

Obviously, there are no such obstructions in the mid and upper levels of the atmosphere.  Therefore, friction is non-existent here.  To a lesser-extent, this is also the case over vast, open bodies of water.

Here's where the Lake Erie Convergence Zone comes into play!

Winds are mainly synoptically driven.  That is, winds are driven by high and low pressure systems.  Winds over open water, however can vary by as much as 45-90 degrees than those over land.  One reason: There is less-friction present over water than on land.  Add a strong 500mb trough to the equation (see below), and you have increased wind speed with height over a virtually frictionless body of water.

Where these northwest and southerly winds meet, is called a convergence zone.  This typically occurs along the lake shore.  Thus, the textbook definition of "lake effect" can be reversed for places like Erie, where a mainly southerly or even calm wind is present at the surface.  All it takes is a -13 (on average) degree difference between the air and lake water temperature, and a mainly northwest flow aloft.

So, you see how Mother Nature deals us a wild card every now and then.  A textbook lake effect event in Erie is a west-to-north flow, with almost no wind shear present.  In this event, however Erie and the lake shore saw a mainly southerly flow, with some wind shear (winds backing to the northwest with height).  That, combined with moisture from a relatively warm Lake Erie provided enough instability aloft to produce a 3-5 inch swath of snow along and north of I-90, with lesser amounts south across the traditional snow belts.

I might also add that a similar process occurs during the spring and fall months.  Anytime wind direction changes with height, you have a highly sheered environment.  When storms encounter this "twisting" with height, brief tornadoes can and often do with storms moving onshore.

I plan to spend more time talking about this during the 2012 Severe Weather Workshop and Symposium, January 10th at Penn State Erie- The Behrend College.

Tim Samaras and I hope to see you there!

It is About to Get Much Colder

It is no coincidence the south has been under the gun for severe weather lately.  After all, fall is considered the second severe weather season in the U.S. (see the November 10th "Veteran's Day" tornado outbreak of 2002), spring being the first.

Why?

The location of the 300mb jet stream is key.  As the transition occurs from summer to winter, the sub-tropical jet (associated with warmer air, as the name implies) retreats south.  This "void" is filled by much colder air that has been bottled-up over the poles.  Among other factors, the location of this air mass is dependent on the polar jet.

The polar jet has remained mainly north, however.  Evidence of this lies in the historical stretch of cold temperatures that have been plaguing Alaska.  To-date, Fairbanks has experienced 5 consecutive days of temperatures at or below -35 degrees Fahrenheit.

Once again, however a weak La Nina will cause a ridge of high pressure to amplify north across the Pacific.  This, combined with a negative North Atlantic Oscillation (NAO) and a blocking ridge over Greenland will force much colder air south across the eastern third the Contiguous United States.

What does this all mean?

For starters, the extended stretch of mild air Erie has been enjoying for much of November is about to take a break, probably until mid-January.  That means an active December shaping-up, with heavy lake effect snow for the snow belt.  It also means frequent northern-tier storms and perhaps Nor'easters for the New England States.

Stay-tuned...

2011-12 Winter Outlook

Synopsis:

We are currently entering a weak La Nina period, meaning cooler than normal Equatorial waters off the Peruvian Coast.  Also, indications are that the North Atlantic Oscillation is entering a negative phase.  The NAO refers to the east-to-west movement in the upper-level high across the North Atlantic.  Typically, this works in tandem with the Arctic Oscillation.  In a negative phase, we see a blocking pattern setup over Greenland, forcing the polar jet farther south across the eastern third of the U.S.  La Nina further amplifies this trough, by setting-up a ridge of high pressure in the North Pacific.

Outlook:

Indications are we could see deep trough of low pressure setup across the eastern U.S. by December, accompanied by below-average temperatures.  A record setting Lake Erie water temperature this past summer would provide for an active lake effect season across the snowbelt, at least to start.  This is also the time in which we could see some big synoptic players (i.e. Nor'easters) for the Mid-Atlantic and New England states.

Climatologically speaking, January is the coldest of the winter months.  This year, however a large upper-level high may anchor itself over the southeast U.S.  This would keep the polar jet and thus, the bulk of the colder air farther north.  A good portion of the eastern U.S. may see above-average temperatures and below-average precipitation during this month.

Summary:

December... Negative NAO & AO... Temperatures: Below-Average/ Precipitation: Above-Average (especially for the snowbelts)

January... Positive NAO... Temperatures: Above-Average/ Precipitation: Below-Average

February... Temperatures: Average/ Precipitation: Average

**La Nina should be in a weakening state at this time.  February starts dry, but trends wetter heading into March.

JFB

Baron Tropical Weather Discussion

Issued:  August 27th, 2011          

Time:    8:00pm (CDT)

Storm:   Hurricane Irene

Storm Discussion:

As of 8pm, Hurricane Irene centered over about 100 MI SSW of Ocean City, MD and is steadily moving North-Northeast at about 16 mph.  NHC is reporting that Irene is still a Category 1 storm with maximum sustained winds of 80 mph, however, a NOAA C-Man station at the mouth of the Chesapeake Bay is reporting a maximum of 52 mph and gust of 57 mph.

Forecast track for the next 48 hours from NHC continues to waver little as does the BAMS.   The NHC track skirts the coastline with Irene to make landfall again somewhere near Jones Beach, New York on Sunday morning.  BAMS take a more land-based route over the Delmar peninsula, through mainland New Jersey, to New York City with the same timing.  Currently, NHC has Irene remaining as a Category 1 storm at 2^nd landfall, but minimally, and likely with only a tight core of hurricane force winds.  Current water vapor pretty much supports this and showing a large area of the dry air to the west and south of the storm which is being entrained into the southeast side currently.  Hurricane Force winds extend out up to 85 miles with tropical storm force winds being the larger factor as that wind field extends outward up to 290 miles.  Tropical storm force winds will likely be into the New York City area by late this evening.  With Irene’s relatively slow forward speed, heavy flooding rains will be a factor from New England into the Middle Atlantic states. 

Another factor with Irene’s slow movement is that for the next 12-24 hours areas from the Middle Atlantic into the western end of the Long Island sound will be the favored quadrant for sustained, surging water flow.  High tide is already above normal this weekend and storm surge can as much as 4 to 8 feet above ground level in the hurricane warning area North from Virginia to Cape Cod.  The mouth of Chesapeake Bay already has a storm surge of about 4 ft. 

Hurricane Irene’s winds increase drastically with height.  As Irene goes past the Delmar peninsula and approaches larger metropolitan areas, high-rise buildings will experience considerably stronger wind speeds than those at the surface.

As Irene moves into central New England early Sunday afternoon she will quickly diminish to a tropical storm then become absorbed into the westerlys by across eastern Quebec late Monday afternoon.

Baron Forecast Team

Curtis Brideau

Baron Tropical Weather Discussion

Issued:  August 27th, 2011          

Time:    3:00pm (CDT)

Storm:   Hurricane Irene

Storm Discussion:

As of 3pm, Hurricane Irene centered over the northeastern end of the Albemarle Sound of North Carolina and is steadily moving North-Northeast at about 10-12 mph.  NHC is reporting that Irene is down to a Category 1 storm with maximum sustained winds of 85 mph, however, a significant number of Buoys, C-Man stations and METARS are reporting a maximum of 65 mph.

Irene came ashore about 7am over Beaufort, NC as a Category 2 storm.  However, the highest gust report was 115 mph by the Cedar Island Ferry Terminal. Maximum report sustained winds were reported between 72 and 85 mph at stations across south-central North Carolina.

Irene should be exiting back into the Atlantic somewhere along the Virginia/North Carolina state line by 7pm (EDT). This is base on a quick extrapolation of movement and is in line with the BAMS.  NHCs track and movement also looks good but has Irene’s position by 7pm (per last forecast) was to be near the southern tip of the DelMarVa peninsula.

Forecast track for the next 48 hours from NHC continues waver little as does the BAMS.  With Irene still being guided by the western Atlantic ridge to the southeast and the trough across eastern Canada into the central Appalachians.  Irene should make landfall again somewhere near Jones Beach, New York in the late morning Sunday.  Currently NHC has Irene remaining as a Category 1 storm at 2^nd landfall but minimally and likely with only a tight core of hurricane force winds.  Current water vapor pretty much supports this and showing a large area of the dry air to the west and south of the storm which is being entrained into the southeast side currently.  Tropical storm force winds is the larger factor as that wind field extends outward up to 200 miles.  Tropical storm force winds have already shifted into the Cape May area of New Jersey and will likely be into the New York City area by late this evening.  The rain shield associated with Irene is also large with forward bands already moving across southern New England.  With Irene’s relatively slow forward speed, heavy flooding rains will be a factor from New England into the Middle Atlantic states.  Another factor with Irene’s slow movement is that for the next 12-24 hours areas from the Middle Atlantic into the western end of the Long Island sound will be the favored quadrant for sustained, surging water flow. 

As Irene moves into central New England early Sunday afternoon she will quickly diminish to a tropical storm then become absorbed into the westerlys by across eastern Quebec late Monday afternoon.

Baron Forecast Team

Brett Campbell

April Tornado Outbreak One for the Record Books

April was a record setting month for tornadoes across the U.S. Preliminary estimates by the National Weather Service suggest more than 600 tornadoes occurred last month, smashing the previous record of 542 for a single month set in May 2003.

---
Online:
http://www.reliaweather.com/articles/april_tornadoes.html

Remembering the 1974 "Super Outbreak" 37 Years Later

The worst tornado outbreak of the 20th century occurred 37 years ago to-date. Dubbed the "Super Outbreak," 148 tornadoes touch downed between 1:00 PM EST April 3rd and 4th, resulting in 315+ fatalities, over 5,000 injuries and an estimated $600+ million loss (in 1974 dollars). The combined path length of all the tornadoes was approximately 2,500 miles across 13 states in the Eastern U.S., marking the longest on record for a 24-hour period.

---
Online:
http://www.reliaweather.com/articles/super_outbreak_37.html