The calendar will roll over into a new year Tuesday, but the wet weather pattern that dominated the last six weeks of 2018 will continue. Two additional storms will emerge from the Gulf of Mexico and soak the Eastern half of the country this week. This relentless pattern featuring Gulf of Mexico moisture swamping the East has many wondering when winter will finally show its face.
This is your Sunday Storm.
The first rain storm this week will be drawn northward into the Mississippi River Valley Sunday night by an upper-level low pressure system crossing the Rocky Mountains and the Southwest. A strong low-level jet will develop between the temperature contrast of the warm Gulf and cold continent ahead of the upper-level low. This jet stream will pull the developing system to Northern New England by the time the ball drops over Time Square Monday night. The heaviest rain will follow the jet over the Mississippi and Ohio River Valleys, where 1-2″ of rain can be expected, and with lighter amounts of 0.5-1″ in New England and the Mid-Atlantic. The storm’s swiftness means the rain will not last long, but its poor timing will nonetheless make for a wet start to 2019 for most in the East.
The rain New Year’s Eve should clear the East by the first sunrise of 2019 but by then all eyes will have turned once again toward the Gulf of Mexico as more rain is on the way for the South and the lower Mid-Atlantic. A second system is brewing in the Gulf, which will be picked up Wednesday morning by yet another shortwave trough of low pressure from the Pacific. Another 1-3″ of rain can be expected from this second system between Wednesday morning and Friday night, which could provoke more flooding for the already saturated region.
The tendency for storms to be dispatched from the Gulf of Mexico ahead of the arrival of a western trough in this fashion has become a broken record by now. But as evidenced by the record snowfall over North Carolina and Virginia earlier this month, these Gulf-based storms can quickly become efficient snow producers given sufficient cold. Cold air has rarely been readily available so far this winter outside of this isolated snowstorm.
The upper-level components of the East’s recent storms have tended to split apart from the polar jet stream before the birth of the surface components. The polar jet stream can be viewed as holding a reservoir of cold air to its north. With no source of cold air, the default mode for these storms has been rain even after reaching the Atlantic Ocean and quickly intensifying. But even reaching the coast has been a struggle for these systems. A ridge over the Caribbean has forced the majority of them to cross the Mississippi River Valley toward Ontario or New England without reaching the coast, allowing subtropical air to spill over the entire Eastern Seaboard. Brief warmth and unseasonably torrential downpours have been the result.
Snow lovers shouldn’t fret. This Gulf of Mexico-dominated pattern was anticipated in our Winter Outlook through the first half of winter. We still anticipate a colder, snowier pattern to arrive during winter’s second half.
There are a few reasons that this warm and wet pattern has been so prominent in the East thus far into meteorological winter. The most advertised reason earlier this year was the brewing El Nino. Warmer than normal sea-surface temperatures in the Pacific tend to deflect storm systems in the North Pacific toward California rather than Canada or the Pacific Northwest. A southerly track toward California favors separation from the polar jet stream and eventual interaction with the Gulf of Mexico. This is why so many of the East’s rainstorms have technically originated south of Alaska. But the brewing El Nino does not explain everything. One reason in particular is soon expected to significantly change the weather pattern.
Cold air has been sorely missed in the East. Strong high pressure anomalies near Alaska, owing partially to warm sea surface temperatures, have helped to confine the polar vortex to the arctic. The polar vortex is the region of upper-level low pressure that typically resides at the poles and grows stronger with colder temperatures. 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. A stronger than normal polar vortex is characterized by the positive phase of a teleconnection known as the Arctic Oscillation. During the positive phase of the Arctic Oscillation (AO) the anomalously low pressure in the arctic helps keep the jet stream blowing from west to east, preventing arctic air from intruding into the mid-latitudes.
The graphs below 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.
Notice that the AO has been in its positive phase throughout most of December, but that it is forecast to dip back into negative territory by the second week in January. The negative phase of the AO characterizes a weaker than average polar vortex. The lighter pressure gradient between the arctic and the mid-latitudes will permit the intrusion of storms and airmasses from the sub-arctic or even the mid-latitudes towards the North Pole. The jet stream will then retreat from the arctic and buckle in a north-south direction in response, causing cold air to spill southwards over eastern North America under a persistent trough.
Just because the AO is expected to dip into negative territory by mid-January does not mean it will stay there for an extended period. Previously this season, negative phases of the AO would only last a few days before returning to positive territory. But several signs suggest that the availability of arctic cold will last.
The Climate Forecast System (CFS) and other climate models are forecasting the breakdown of the ridging of high pressure near Alaska. The breakdown of the ridge can be partially attributed to an eastward propagating complex of thunderstorms over the tropics known as the Madden-Julian Oscillation (MJO). It is forecast to propagate across the Pacific Ocean over the next few weeks. As it does so it will generate atmospheric waves known as Rossby Waves that act much like ripples observed when dropping a stone in water. Some of these waves are likely to reach the arctic and weaken the polar vortex, thereafter weakening the jet stream and allowing cold air to flow into the mid-latitudes.
Perhaps the most notable factor to consider in the transition to a colder weather pattern is an ongoing Sudden Stratospheric Warming event. The graphic below from the CPC plots 45 day forecasts of the mean temperature in the stratosphere in the arctic. The stratosphere is the layer of the atmosphere just above the troposphere, where most weather occurs. Forecasts follow forward slanted lines whereas observations are represented by following the horizontal axis extending from Lead Time of 0 days. Purple and blue are cold and red and grey are warm. Notice that near December 28-30, the temperatures suddenly warmed in the stratosphere.
Stratospheric warming can have a slew of impacts on the global weather pattern. Since the stratosphere and troposphere are largely independent and decoupled, warming of the stratosphere can occur without warming in the troposphere. What happens instead is analogous to a s’more. Since warming air expands, the stratosphere squishes the troposphere beneath it. But since the troposphere is approximately incompressible, air spreads horizontally across Earth’s surface, like pressing on a toasted marshmallow with a graham cracker. When this occurs over the arctic the polar vortex, a fortress of cold air, is forced to split southward into several pieces. Typically the process of the polar vortex splitting takes place over the course of several weeks. As stratospheric warming is ongoing as of December 30, the arrival of cold air associated with the polar vortex can be expected some time around mid-January, which is also the mid-point of meteorological winter.
The presence of cold air in eastern North America for the described reasons will have a variety of consequences on the weather pattern. First and foremost, surges of Gulf of Mexico warmth will become less frequent ahead of storms. Surface components to storm systems may still originate over the Gulf of Mexico, but such systems will be less frequent after mid-January. The cold will be contained beneath troughs that develop and perpetuate over most of the eastern US. The troughs will draw most of the storms that do originate in the Gulf of Mexico towards the Atlantic Ocean–rather than the Mississippi River or Appalachian Mountains–before trekking northeast. This will keep cold air in place as the coastal storms race toward the Canadian Maritimes, even near the coast. These storms will consequentially be stronger and snowier, as there will be more cold air readily available in conjunction with Gulf moisture.
Clipper-type systems will represent a larger proportion of storms that impact the Midwest and the Northeast. Troughs over the East imply ridging over the West. The break down of the Alaskan ridge will permit more Pacific storm systems to make landfall over British Columbia. The clockwise flow downstream the western ridge and the counterclockwise flow upstream the eastern trough will likely draw these systems southeastward from western Canada into the Midwest and/or Northeast. These systems generally produce only small quantities of snow, but they tend to become significant East Coast snowstorms if they survive the journey to the Atlantic Ocean.
The transition to a colder, snowier pattern in the East is still at least two weeks away. Until then, check back with WeatherOptics to stay up-to-date with the barrage of rainstorms and to be alerted when snow is finally on the horizon.