Nearly one full week of January has already passed yet parts of the Carolinas have recorded more snow this winter season than most of the big cities in the Northeast. Instead of snow, most of the East has been inundated with a series of heavy rain events, with the Southeast taking the brunt. Last week we addressed some of the reasons for this pattern and when to expect the arrival of more persistent winter weather. This week we elaborate further on some of those details.
A Powerful Pacific Jet Stream:
The “Pacific” subtropical jet stream has dominated the weather pattern since early December due to a developing El Niño. The warmer than normal waters of the eastern equatorial Pacific have intensified the temperature gradient between the equator and the arctic closer to the mainland US than normal. As the Pacific has warmed, so too has the air above it. Warming air expands, which has resulted in higher pressure in the upper levels of the troposphere than normal. Cold air is more compressed than warm air, resulting in lower pressure in the arctic compared to the equator. Air, like all fluids, flows from high pressure to low pressure. Consequentially, the larger than normal difference in pressure between the equator and the arctic in the eastern Pacific has resulted in a stronger poleward acceleration of air. Before the warm tropical air can reach the arctic, however, Earth’s rotation deflects the air eastward, intensifying the west-to-east subtropical jet stream in the pacific.
The strong Pacific jet stream has limited the southward intrusion of cold air by inhibiting the eastward propagation of atmospheric waves (troughs and ridges) known as Rossby waves. A zonal (straight) west-to-east jet stream acts like a wall preventing air masses from protruding north or south. Rossby waves are needed to “bend” the jet stream and draw air from the tropics northward and air from the arctic southward.
The jet stream weakens upon reaching the West Coast of North America due to cooler sea surface temperatures which weaken the temperature gradient between the tropics and the poles. The deceleration is maintained by a thermally indirect circulation in this region of wind deceleration, whereby air rises on the cold (northern) side and sinks on the warm side. The rising air on the north side results in low pressure at the surface, which explains why so many storms that have impacted the US recently have originated in the eastern Pacific Ocean. These low pressure systems have amplified the jet stream over the western US through the development of troughs. These troughs have mostly proceeded across the southern tier of the US, straddling the border with Mexico rather than Canada due to momentum from the subtropical jet.
The troughs have let cold air spill southward over the western US, which is why parts of the Desert Southwest recently observed a heavy winter snowfall just before the New Year. The troughs have simultaneously drawn warm air from the Gulf of Mexico northward to the east. This is why New York City observed temperatures near 60 degrees to ring in the New Year following the same storm system that resulted in a rare blizzard warning for Albuquerque, NM.
Overwhelming Gulf of Mexico Moisture:
Momentum from the subtropical jet stream has caused many of these troughs to proceed eastward across the southern tier of the US rather than the northern tier. Cutoff from the polar jet stream and therefore the cold air required to sustain them, these systems have tended to degrade in their march eastward. That they tend to take a southern route across North America such that these upper-air systems pass within close proximity to the Gulf of Mexico, where they become rejuvenated with their interaction with the Gulf’s starkly warmer and moister airmass.
Still too weak to overcome a ridge of high pressure over the Caribbean, the invigorated storms have been directed north up the Mississippi River toward Canada. Since the Pacific jet stream has inhibited waves from penetrating the arctic, cold air has generally been locked in Canada. Without cold air in place over the Midwest and Northeast, the reinvigorated Gulf of Mexico systems have been mostly rain-producers up and down the East Coast. That this pattern has been stagnant has resulted in weekly or semi-weekly heavy rain storms in the East. Collectively, these systems have produced impressive rainfall, especially over the South, where some areas have received over 20″ of rain in the last 30 days. That’s impressive for the summer, let alone December!
Intrusion of Cold Air On the Horizon:
Nothing lasts forever. As we discussed in our Winter Outlook and in last week’s Sunday Storm, this active warm, wet pattern was expected to persist through mid-winter. We still expect a thorough change to a pattern more susceptible to cold outbreaks by the third week of January. The Pacific jet stream is expected to remain active, but several signs suggest cold air will finally be available over the Eastern US throughout much of the second half of winter.
The Southern Oscillation Index (SOI) has been steadily decreasing since late December, per values listed in the below table provided by Queensland, Australia’s Long Paddock initiative. The SOI is based on the normalized pressure difference between Tahiti and Darwin, Australia and is a key metric used to evaluate El Niño and La Niña. In its negative phase, the pressure is greater further west in Darwin Australia than in Tahiti further east in the south-central Pacific Ocean.
The warmer waters of the central and eastern Pacific Ocean than the western Pacific Ocean have contributed to the reversal of the pressure difference. The reversal of the pressure difference suggest Rossby waves are likely to propagate across the Pacific Ocean as a consequence of an intensifying El Niño. These waves will disrupt the zonal (straight west-east) Pacific jet stream. Storms that develop along these waves may permeate into the arctic and displace cold air southeastward toward the Eastern US in roughly two weeks.
The Polar Vortex Breakdown:
Changes to the Polar Vortex (PV) are likely to be the most significant factors in bringing about a pattern change. When the polar vortex is confined to the arctic it typically intensifies, as without heat input from the south during the long polar night, the arctic continuously cools. The vigorous and zonal Pacific jet has contributed significantly to the intensification of the PV by isolating it from disturbances from the south. A stronger than normal polar vortex (or weaker than normal high pressure near Greenland) is characterized by the positive phase of a teleconnection known as the Arctic Oscillation. As discussed last week, anomalously low pressure in the arctic during the positive phase of the Arctic Oscillation (AO) helps keep the jet stream blowing from west to east, preventing arctic air from intruding into the mid-latitudes.
The graphs above from NOAA’s Climate Prediction Center (CPC) display observed values of the AO over the last 120 days plus a two week forecast from 11 members of the GFS ensemble. A complete description of the plots can be found here. They demonstrate that we are in a declining AO, which is expected to turn negative over the course of the week. A negative AO suggests cold air is more likely to infiltrate the eastern US and western Europe.
A possible contribution to the reversal of the AO and the upcoming pattern change is the recent conclusion of the sudden stratospheric warming (SSW) event discussed last week. The effects of SSW on the troposphere, where most sensible weather is observed, were discussed in last week’s Sunday Storm. It will take a few weeks before the response of the SSW appears in the troposphere. It now appears that the corresponding tropospheric response will manifest as a “displacement” of the Polar Vortex (PV), the region of low pressure situated near the North Pole, into three pieces. When projected onto the northern hemisphere, a continuous Rossby wave pattern is visible. Recall that Rossby waves are necessary for arctic air to intrude southward and for rapid intensification of coastal storms. A peak at recent runs of the 12z January 6 GEFS courtesy of Tropical Tidbits illustrates this pattern initially emerging around January 20. Ridge axes are drawn in red, trough axes are outlined in blue, and the wavelike pattern is outlined in blacked.
Winter is Coming:
What the described pattern change means is that winter is coming. The PV breakdown will provide long-term cold for the Eastern US, cold which could last from the end of January through most of February, possibly through early March. The continuous activity of the Pacific jet will mean that unsettled weather will likely continue on the West Coast and the Eastern US, but matched with perpetual cold air, there will be greater chances for snow all the way to the coastal Northeast. The cold will occasionally reach the Gulf Coast, resulting in some snow developing over portions of the Southeast too. Many storms are still expected to develop and rejuvenate in the Gulf of Mexico, but there will be greater chances for clipper-type systems emerging from Canada. These will bring occasional bouts of moderate snow to the Midwest, and could become powerful snowstorms if they survive the arduous journey to the Atlantic Ocean.
Two consecutive low-pressure systems are expected to cross the Midwest and intensify over the Northeast between Monday night and Wednesday. Both systems are expected to bring moderate snow to parts of Upstate New York and Northern New England, while bringing light rain to the Ohio Valley and the northern Mid-Atlantic. Southern New England, including Boston, may receive a quick coating to an inch of snow Monday night into Tuesday morning, but any snow accumulation is expected to be washed away by light rain Tuesday night. Heavier snow is expected further north and west, with high elevations of Northern New England and New York’s Adirondack Mountains picking up 6-10″ with lower elevations accumulating 3-6″. Significant lake effect snow is expected on the backside of theses systems Wednesday and Thursday for the typical lake effect Snow regions downwind Lakes Erie and Ontario. Besides these two systems though, no big storms are on the horizon until late next weekend, when another storm comes up from the Gulf of Mexico. For now, it looks to stay suppressed in the Southeast thanks to the arrival of cold air from the newly negative Arctic Oscillation. But at one week’s lead time, anything can change. Be sure to check back throughout the week to learn more about these upcoming storm systems.