Meteorological winter is officially here. Just two days into the season, a cross-country winter storm is already on the horizon for late this week. It has the potential to bring flooding rains, ice, and heavy snow from California to Maine. This is your Sunday Storm.
The precursor to the potential winter storm is a powerful 983 mb extra-tropical cyclone swirling over the northeastern Pacific Ocean between Alaska and British Columbia. Its moisture feed is flowing along a vast and vigorous atmospheric river stretching nearly 2000 miles from the tropical eastern Pacific to the seas just south of the Gulf of Alaska. The storm and its moisture feed will travel southeastward toward California alongside a broadening planetary wave of low pressure and weaken as it approaches land. The surface low and its parent trough will bring heavy rain to parts of California Tuesday through Thursday and the Desert Southwest Thursday through Friday as it straddles the coastline and makes landfall on Mexico’s Baja Peninsula. By this point, the once mighty oceanic storm will have been reduced to a meager cluster of downpours with only an innocuous parent shortwave for dynamic support. Further degradation will ensue as the dying system crosses the rugged terrain of northern Mexico. If this system were really to menace the Eastern US, it would need to enlist support from external sources. The Gulf of Mexico will provide the perfect environment for this support.
The factors contributing to the Pacific low’s re-intensification are continental in scale. A cold core high pressure system will slide southeastward from western Canada into the northern Plains Wednesday night on the tail of a northern-stream trough propagating eastward across the northern tier of the US and Canada. The high will be parked in the upper Midwest just as the Pacific low emerges into the Gulf of Mexico on its last legs Friday evening. Arctic air will be pumped southward all the way to the lower Mississippi Valley ahead of the Pacific low’s arrival. This arctic air will be starkly contrasted by the subtropical Gulf of Mexico, from which warm, moist air will flow northward and collide with the arctic airmass. The resulting baroclinic (temperature contrasting) environment will serve as the perfect mechanism for low-pressure intensification.
The baroclinicity near the Gulf Coast will have already resulted in the formation of a wave of low pressure along the boundary between the two airmasses ahead of the Pacific system’s arrival. As the warm Gulf of Mexico airmass slides above the cold arctic airmass, moisture will cool and condense, resulting in a wide swath of heavy rain in central and eastern Texas and a corridor of frozen precipitation from the southern Plains to the middle Mississippi River. The lack of upper-level dynamic support will limit intensification and eastward progression of the wave of low pressure. The idling of the surface wave will compound the flooding impacts of heavy rain, especially in Texas where the transport of warm air will be strongest.
The arrival of the Pacific shortwave and the once-potent Pacific storm into the Gulf’s baroclinic environment will dramatically change the fate of both the Pacific and Gulf surface low pressure systems. The shortwave, albeit a shell of its former glory, will still retain from its arctic origins potential vorticity, a conserved quantity that describes the potential to generate local spinning motion in the atmosphere. Increased quantities of local spinning correspond to more intense low pressure systems. Its arrival will thus serve to intensify the Gulf low by enhancing upward motion (and therefore precipitation) and by drawing more cold air toward the low. Both of these processes increase the local spinning of the atmosphere by strengthening low pressure systems. The shortwave will also serve to push the low pressure system eastward along the Gulf Coast, where the difference between the two airmasses will be greatest.
The dying Pacific low will eventually be absorbed by the invigorated Gulf low due to the aid of its own backstabbing parent shortwave. Torrential rain will flood parts of eastern Texas and the Gulf Coast late this week as the Pacific shortwave and the Gulf of Mexico low pressure system engage in positive feedback. Some areas of eastern Texas eastward through the Florida panhandle could be submerged by 6-8″ of rain in less than 36 hours before. This flooding rain will come on the heels of the wettest fall on record for many cities between Texas and the Carolinas. Dallas, for example, measured 29.21″ of rain between September and November, shattering the 2015 record by over 7″ and almost three times the climatologist normal, according to the Dallas/Fort Worth National Weather Service office.
While downpours flood the Gulf Coast, snow, sleet and freezing rain will bring wintry conditions to northern Texas, Oklahoma and southern Kansas eastward to the middle Mississippi River Valley of southern Missouri and northern Arkansas. The strongest forcing will be south of here but a heavy snowfall on the order of 6-10″ is possible just north of the corridor of heavy rain. Significant icing could along a corridor of the southern Plains and Mississippi River Valley where the Gulf’s warm subtropical air gradually and just barely overwhelms the cold arctic air.
Upon reaching Florida late Friday night the evolution of the storm becomes highly unclear. The storm may intensify and travel northeastward along the East Coast of the US or it may continue east-northeastward harmlessly out to sea. The former scenario would likely deliver a widespread, significant accumulating snow from Georgia to the Mid-Atlantic or New England. The latter would confine the snow to inland parts of the Southeast.
The northern extent of the low pressure system will largely depend on the ultimate intensity of the re-intensifying Pacific shortwave, of a future northern stream Canadian shortwave, and of the cold high pressure system. If the intensification of the cyclone is too slow over the Gulf of Mexico, perhaps due to a weak temperature gradient, the parent shortwave will remain broad and the jet stream will be suppressed to the south. Likewise, if the future shortwave is too quick to trek southeastward or if the cold high pressure system situates itself too far south over the Midwest, the trough will be suppressed south. In the latter case, the storm would quickly intensify, but it would go out to sea before impacting the Northeast. Parts of the inland South would pick up heavy snow between late Friday night into early Sunday if this scenario were to pan out.
With more than one week’s lead time model guidance is still too sporadic to be of much use. Deterministic guidance is unhelpful, as each model cycle presents vastly different upper air patterns. Global ensembles are not of much use either so far in advance, as both timing and intensity are vastly spread across the members. Just a peak at the cyclone centers of the 12 UTC December 2 EPS for next Sunday night, courtesy of weathermodels.com, highlights the confusion. The members depict the coastal storm centered anywhere between the Carolinas, New Brunswick, Canada and the open waters of the North Atlantic.
Deciphering the probability of one scenario over the other is far from trivial given the complex dynamics involved. However, forecasts from global models, especially from the GFS, tend to trend westward with time. Additionally, given the northerly origins of the system over the North Pacific (as opposed to the Rocky Mountains or exclusively the Gulf of Mexico), the storm already has the potential to intensify. Therefore, a storm track that at least brings precipitation to the lower Mid-Atlantic is favored at this time. Regardless, residents from California to New England ought to pay close attention to this system over the coming days, as significant impacts of this storm are likely from coast to coast.