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Requirements for a Major Snowstorm That Incorporates Heavy Snow Event (HSE) Criteria in the Philadelphia Metropolitan Area. Widespread Snowstorm in the Delaware Valley (southeast PA, southern NJ away from the coastal regions, northern DE, and northeast MD) of 8 inches or more in a 24 hour period.

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  1. Requirements for a Major Snowstorm That Incorporates Heavy Snow Event (HSE) Criteria in the Philadelphia Metropolitan Area Widespread Snowstorm in the Delaware Valley (southeast PA, southern NJ away from the coastal regions, northern DE, and northeast MD) of 8 inches or more in a 24 hour period

  2. Thank You • Very special thanks to two prominent figures in the meteorological business—Glenn “Hurricane” Schwartz and David Tolleris. Without them, this work would NOT be possible. I hold these gentlemen in high esteem and admire their work and expertise immensely. NBC-10 Chief Meteorologist Glenn “Hurricane” Schwartz initiated the idea of a major snowstorm decision tree for the eastern seaboard (checklist) in the 1980’s. Meteorologist David Tolleris expanded on the original prototype and has applied such checklists to his forecasts. The professional feedback I received from them during this research endeavor was greatly appreciated. Their continued support and collaboration in reference to ongoing meteorological issues is priceless and I am extremely grateful for their contributions. Additionally, thanks also to Larry Cosgrove, Paul Kocin, Louis Uccellini, Henry Margusity, Rob Guarino, Norman Wes Junker, HM, Bill Conlin, Joe Bastardi, and Kathy Orr for their peer review and feedback.

  3. Storm Checklist • State of the NAO • 50-50 Low • Polar Vortex • Character of 500 hPa Trough • Character of Surface Low Pressure • Character of Surface High Pressure • 850 hPa Low Strength • 850 hPa Low Track • Low Level Cold Air • Other Factors: (700 hPa Low Track, 700 hPa Low Strength, PNA Pattern, MJO Phase, EPO Index, ENSO, Southeast Ridge, Ocean Temperatures, Moon Phases, Weekend Rule, Phases of a Snowstorm, and Local Climatology)

  4. State of the NAO Positive phase of the NAO is not conducive for HSE’s in the Delaware Valley. Snowstorms can happen during a positive phase, but it is rare for PHL to receive a HSE during a + NAO (January 2000 was an exception to the rule). A positive NAO usually exhibits a fast west to east zonal flow and/or ridging east of the Mississippi River.

  5. State of the NAO A negative NAO in conducive for East coast snowstorms because it usually allows for cold air to spill into the eastern United States. It also allows for a blocking to take place in Eastern Canada, a subsequent trough to develop over the eastern United States, and coastal cyclogenesis to sometimes occur. A negative NAO is by far the most important factor in East coast snowstorm development. Negative NAO’s and their associated blocks usually last 10 to 14 days. Models sometimes break them down too early.

  6. Major Snowstorms and the NAO Index Most big snowstorms for Philadelphia occur during periods of negative NAO when it is trending sharply towards neutral or positive. Sometimes they occur when the NAO is positive or neutral trending sharply negative. Click here to see current NAO values: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/new.nao_index_ensm.html

  7. Major Snowstorms and the NAO Index The NAO index at times can be deceiving. For example, the NAO can be negative (good for snowstorms) yet there could be no classic Greenland block (High Pressure over Greenland). The block is instead situated elsewhere or not in the most favorable position for northern Mid-Atlantic snowstorms. Also, the NAO index can be extreme negative which can prompt a suppression of storms. In this case, the lower Mid-Atlantic and/or southeastern states could receive the heaviest snow. Therefore, when it comes to the NAO, the following must be taken into consideration. 1. Negative or positive? 2. Trend? 3. How negative or positive? 4. What type of NAO? Where is the block?

  8. NAO Index (Blizzard of 2006) • NAO was weak negative prior to the storm, but trending sharply positive (this is called “relaxing”). • This NAO trend of negative to neutral of positive is often key in the development and propagation of East coast snowstorms.

  9. Other Factors: NAO and EPO Index • The NAO, basically, is the dominant upper wind flow pattern over the North Atlantic influenced by the ocean. While in a negative phase, the NAO sometimes tends to act as a block (or dam) to the upper wind flow over the eastern half of North America. This blocking effect, in turn, tends to deliver the polar/arctic air into the eastern half of the country and Great Lakes more readily. • Click here for current EPO index and forecast

  10. Other Factors: NAO and EPO Index • The Eastern Pacific Oscillation (EPO) is the upper wind flow over the Eastern Pacific influenced by the ocean. When in a positive phase, the EPO generally is reflected by dominant stronger zonal flow and/or “troughing” along the West Coast of the U.S. This combination, in turn, tends to funnel milder Pacific air well inland into the country and thus, limits arctic outbreaks by holding them at bay up in Canada. When the EPO is dominated by a negative phase (as with the NAO), more ridging develops along the West Coast as higher pressure extends from the Gulf of Alaska south along the West Coast of Canada (opposite of the positive phase). This, in turn, encourages a northwesterly flow from Canada into the middle and eastern sections of the US and thus, the delivery of polar or arctic air. • Click here for current EPO index and forecast

  11. Blizzard of 2006 (EPO Index)

  12. 50-50 Low The 50-50 low is put in place by a negative NAO. This low is called such due to its location about 50 longitude and 50 latitude. This low is important because it blocks coastal storms along the East coast against a wedge of arctic high pressure to the storms’ north This high pressure is usually located south and/or west of the 50-50 low.

  13. 50-50 Low According to Dave Tolleris, the “50-50 Low” affects the overall pattern across eastern North America in several ways. Enhances the intensity or amplitude of the negative NAO in general and the Greenland Block negative NAO in particular. Keeps cold air source (high pressure) in place. Trap and lock example. Thus, more precipitation falls as snow due to the prevailing NW, N, or NE wind. 3. Prevents systems in Plains/Midwest from passing west of the Appalachian Mountains.

  14. High (Greenland Block), Low (50-50 Low), High (Arctic High) Configuration

  15. February 2003 Presidents Day Storm (50-50 Low, PV, and High Pressure)

  16. Blizzard of 1996 (January 6-8, 1996): 50-50 Low, PV, and High Pressure

  17. Polar Vortex • The polar vortex is analyzed at 500 millibars. The polar vortex occurs above the core of the coldest polar air. Since frigid air is dense, heights are lower aloft because cold air has a lower thickness than warmer air. • At the surface of the polar air mass will be high pressure, but low heights will occur aloft at 500 mb since the air is compacted due to high density air near the surface.

  18. Polar Vortex cont. • The polar vortex can often be located over Canada since the coldest surface air is often found over high latitude icy/land locations. The polar vortex aloft propagates toward where the polar air mass moves. The vortex often moves very slowly or is stationary, and its position determines what part of the USA the Arctic air will invade. • When the vortex is centered over the Hudson Bay, as shown above, arctic air usually plunges south over the Dakotas and the northern Plains. If the vortex center shifts to the east, the core of the Arctic air invasion usually shifts east with it.

  19. Polar Vortex cont. • There are many regions where the polar vortex can be located for it to be marginally conducive for East coast snowstorms. Most important is that the PV is on our side of the northern hemisphere and away from the pole (displaced). • However, in my opinion, the polar vortex is best situated in southeastern Canada just prior to an East Coast snowstorm. However, for a storm to come up the coast the PV needs to weaken and/or move out of the way (northward,northwestward, or backwards diving south and east of the pumping PNA ridge). Otherwise the storm will be suppressed to the south. North Hudson Bay, Canada is also a favorable position just prior to a snowstorm.

  20. Polar Vortex cont. • Just keep it away from the Great Lakes region or just above it. In this position, short waves have trouble slowing down and digging (thoughts courtesy of Dave Tolleris).

  21. Polar Vortex cont.

  22. Polar Vortex and 50-50 Low (Blizzard of 2006)

  23. Character of 500 hPA (mb) Trough Cutoff Low. Low remains closed at 500 mb. This setup gave Philadelphia 30 inches of snow.

  24. Character of 500 hPA (mb) Trough Open wave that evolves into a closed wave at 500 mb. This setup gave Philadelphia over 12 inches and areas to the north and west over 30 inches.

  25. Character of 500 hPA (mb) Trough Open wave that remains open at 500 mb. This setup gave Philadelphia over 20 inches of snow.

  26. Character of 500 hPA (mb) Trough In addition to there being a trough in the first place, the trough must exhibit specific properties to be conducive for heavy snow fall in the Philadelphia region. The 500mb vorticity maxima needs to be south of Philadelphia and propagate in a northeastward direction near to the East coast. If the vorticity maxima propagates northeastward too far west and away from the coast, it is likely that the precipitation associated with the storm will NOT remain all snow or even mostly snow. Click here for current 500mb chart (Chart 1) Click here for current 500mb chart (Chart 2)

  27. Character of 500 hPA (mb) Trough

  28. Blizzard of 2006 (Vorticity Maximum) Note the energy transfer. Vorticity maximum (violet) just SE of Philadelphia. Note the light blue shading over Harrisburg, PA and NNE—the best snows are in between the two.

  29. Other Factors: Upper Level Signatures Prior to Heavy Snow Events

  30. Character of Surface Low Pressure (Miller A or Gulf Atlantic Low)

  31. Character of Surface Low Pressure (Miller A or Gulf Atlantic Low) Miller A: Surface low tracks in a general northeast direction from the southeastern United States or the Gulf of Mexico. For us to have a good chance at receiving heavy snow, the low needs to stay to our south and east and preferably stay off the Delmarva Peninsula. It is also beneficial for the low to be just off the Virginia Capes while deepening and moving slowly in a NE or ENE direction.

  32. Storm Track (Blizzard of 2006)

  33. Character of Surface Low Pressure (Miller B or Atlantic Redevelopment Low)

  34. Character of Surface Low Pressure (Miller B or Atlantic Redevelopment Low) Miller B: Surface low tracks from Northern Plains or Canada and dives southeast. This low weakens while a secondary or coastal low redevelops off the East coast. For Philadelphia to receive a heavy snow fall from a Miller B low requires a unique set of characteristics. The secondary (coastal) low must form to the south and east of Philadelphia. This low must also intensify, preferably in a rapid, bomb like, fashion yet be moving slowly. This low unlike the Miller A, is best for heavy snow when it moves in a more northerly direction rather than a NE or ENE direction.

  35. Character of Surface High Pressure An arctic high pressure is key for heavy snow to fall in Philadelphia. An arctic high pressure, if positioned correctly, and with enough strength, will aide in keeping the storm mostly snow or all snow. In addition, an arctic high pressure with good position and strength will also slow down the movement of the coastal low allowing for precipitation to fall for a longer period of time. For heavy snow events to occur in PHL, the best position for a high pressure is to the west of Maine and to the north of the PA/NY border. The high can also be located in the Northern Plains/Midwest. As far as strength goes, pressures above 1020mb are adequate enough to provide decent low level cold air.

  36. For current surface data, click here

  37. 850 hPa Low Strength and Track The strength of the upper air low, the 850hPa (mb) low, is one of the factors that aides in the rapid development of a coastal low. Thus, an intensifying 850 mb low, or an already strong 850 mb low, greatly aides in the development of heavy snowfall near the Philadelphia region. This is especially true in Miller B systems. For a Miller B storm to be deemed a heavy snow event for Philadelphia it is crucial that the 850 low goes through a rapid intensification cycle. However, even more important than strength is track. The 850 hPa is best to the south of Philadelphia, yet north the NC/VA border moving in an easterly direction.

  38. 850 hPa Low Track Click here for current 850 mb data

  39. 850 hPa Low (Blizzard of 2006) • Heaviest snow band over the I-95 corridor at this time. Rates of 2 to 5 inches per hour. Phase 3 of the storm (wrap around).

  40. Low Level Cold Air One factor often overlooked during a heavy snow event, even when the event is going full throttle, is the amount of low level cold air. Philadelphia is a major boundary area due to its proximity to the Atlantic Ocean, Gulf Stream Current, and Appalachian Mountains. Thus, even if an arctic high pressure is in perfect position, the precipitation can still change to sleet, freezing rain, snow pellets, or rain. Thus, temperatures throughout all layers of the troposphere need to be analyzed in real time and by using numerical model guidance. Click here for local data from upper air soundings

  41. Other Factors: Local Climatology Note how the cities along the I-95 corridor are often along the boundary between frozen and liquid precipitation. This makes winter weather forecasting one of the toughest forecasting aspects in ALL of meteorology.

  42. Other Factors: Interior Snowstorm Characteristics: A Miss for Philadelphia The cyclone track is further west, usually crossing the Delmarva Peninsula. The cyclone also tracks in a more northerly direction going west of Cape Cod, Mass. Also, the high pressure is either too far north or too far east.

  43. Other Factors: Characteristics of All Snow Cases Versus Changeover Cases All snow cases. High pressure west of the state of Maine and north of PA/NY border. Changeover cases, high pressure is on top of the state of Maine or eastward.

  44. Other Factors: Characteristics of All Snow Cases Versus Changeover Cases To remain all snow, it is best if the low tracks NE or ENE staying away from coastline of the Northeast U.S. Why? If the low rides right up the coast, it pulls in too much low level warm air from the Atlantic Ocean. Therefore, the precipitation has a difficult time staying frozen throughout the entire atmosphere. Philadelphia can receive heavy snow from a changeover event, an interior event, and/or a moderate event. However, the chances of a widespread 8 inches or more across the entire Delaware Valley is very unlikely.

  45. Other Factors: Characteristics of Moderate Snowstorms High pressure systems are in a wide variety of locations. Many cyclonic redevelopment cases from the west and from the Gulf of Mexico. Wide variety of surface low tracks as well. However, all low tracks are south of Philadelphia. Moderate snowstorm events are the most capable events, outside the heavy snow events, to give Philadelphia an 8 inch or more snowfall.

  46. Other Factors: PNA Pattern A negative PNA pattern favors a trough along the West coast of the United States and ridging along the East coast of the United States. Thus, when the PNA is negative, the pattern is not extremely conducive for East coast snowstorms. Can snowstorms still occur along the East Coast? Yes, but the storm track still has to be south and east of Philadelphia. Best option here for snow is a negative PNA pattern with a split flow jet stream and other conducive factors in place like the 50-50 low, negative NAO, and position of surface high pressure.

  47. Other Factors: PNA Pattern

  48. Other Factors: Positive PNA Pattern A positive PNA pattern favors a trough along the East coast of the United States and ridging along the West coast of the United States. Thus, when the PNA is positive, the pattern is conducive for East coast snowstorms. However, many other factors must be in place as well. This is the case with every single one of these points. Just one will not make a snowstorm. Having all of them in place does not mean a snowstorm will happen. However, the more you have, the better chance that exists for a heavy snow producing storm over the Delaware Valley. Click here for current PNA index values

  49. Other Factors: Positive PNA Pattern

  50. Other Factors: PNA Pattern Major snowstorms have occurred in the Delaware Valley during times of a negative PNA. However, it is more of a rare occurrence when compared to times of a positive PNA. If a snowstorm does develop along the East coast during a time of negative PNA, there has to be some evidence of ridging to the south and/or west to promote the digging of a trough near the East Coast. This ridging may be displaced just east of the West coast (Rockies/Plains) or sometimes as far east as the Deep South, thus the PNA remains in a negative state. Click here for current PNA index values

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