The seemingly endless wave train of low pressure systems emanating from the Northern Pacific will continue chugging along throughout the upcoming week. The train will make stops along the West Coast of the US and Canada this week, resulting in a progressive weather pattern largely identical to that which spawned this weekend’s historic snowstorm in parts of the Southeast. Another storm is expected to intensify in the Gulf of Mexico in response to the wave’s eastward propagation late this week into the weekend. Though widespread snow is not expected at this time, the overall pattern and large spread in guidance suggest it cannot be discounted everywhere entirely.
Last month we warned in our winter forecast that most of the storms impacting the eastern US during the first half of winter would originate in the Gulf of Mexico or the lee side of the Rocky Mountains, partially as a consequence of the brewing El Niño. Generally the upper-level components of these systems are expected to originate in the northern Pacific Ocean, trekking southeastward toward southwestern North America. After generating mudslide-inducing rains in California, these upper-level waves of low pressure generally straddle the southern half of the United States, eventually interacting with the Gulf of Mexico. This is where the surface components of the storms are expected to originate as a response to the temperature gradient between the warm Gulf and the cold continental US. Where these storms proceed after the Gulf of Mexico largely depends on the properties of the northern jet stream. The ongoing cross-country storm that brought mudslides to California, heavy rain to the Gulf Coast, and historic snowfall to parts of the Southeast followed such an evolution. A shortwave trough embedded in the northern jet stream in southern Canada suppressed this weekend’s storm southward enough for the Southeast to indulge in wintry weather while the Northeast missed out on precipitation entirely. This week’s storm will exhibit a similar evolution until leaving the Gulf of Mexico. Current model guidance suggests the inverse northern jet stream pattern to impact next weekend, which would draw the system northward toward the Northeast.
That the evolution of storms lately and in the foreseeable future all follow a similar pattern until reaching the Gulf of Mexico emphasizes the importance of northern stream features. The 500 mb height plot (roughly 3-4 mi bove the surface) from the 18z RAP model courtesy of the Storm Prediction Center’s (SPC) mesoanalysis is presented below for 2:30 pm December 9. Both the ongoing snowstorm and the northern stream trough suppressing it to the south are labeled. The same plot from the 12z December 9 GFS for 12z December 15 (7am Saturday EST) is shown beneath it, courtesy of Pivotal Weather.
These plots illustrate the critical differences in the northern jet stream between the current system and next week’s system. Note that in the first plot, for this week’s image, the storm’s upper-level feature is a shortwave completely embedded in the southern jet’s flow. It is completely at the will of the overall flow, so the trough in the northern jet completely suppresses it to the south. Conversely, next week’s system will be supported by a much more robust upper-level feature that nearly cuts it off from the jet stream. The strong jet streak rounding its base will help propagate the future storm to the northeast. A ridge downstream will allow the storm to travel further northeast as there is no southward component directing the storm out to sea.
This scenario is not ideal for snow lovers. Some blocking is needed to keep cold air in place ahead of the storm’s arrival. Without this, precipitation will likely fall as rain almost everywhere except far northern New England, where influence from the weak surface high associated with the upper-level ridging will maintain marginally cold air for snow.
Snow is doubtful for most but not impossible. Run-to-run variability in global deterministic models suggests there is great utility in ensembles for this upcoming system. A peak at them suggests that the upper-level pattern next weekend will not be so clear-cut. The GFS Ensemble, for example, demonstrates large spread in the track of the surface low-pressure centers for next Saturday morning, especially in the Ohio Valley. The ensemble has solutions for the low ranging anywhere from southern Illinois to the North Carolina Coast. The former would be an all rain event everywhere east of the Mississippi. The latter could result in snow for some inland portions of the Northeast and sleet, ice, and possibly snow for mountainous areas of the Southeast. The ensemble plot below is courtesy of Tropical Tidbits.
Regardless of the track, heavy rain is expected next weekend. The further east the track, the closer to the coast the heaviest rain will likely fall. The 12 UTC GFS suggests 1-3″ of rainfall across most of the East Coast through next Sunday.
This heavy rainfall especially needs to be monitored for the Southeast. Much of the Southeast recorded 2-4″ of rainfall or liquid equivalent over the past few days. Where that precipitation fell as snow, heavy rain late this upcoming week will force nearly instantaneous snow melt between Friday and Saturday, doubling down on the flooding threat. Locations like Asheville and Roanoke in the Blue Ridge Mountains are especially susceptible to mudslides and flooding as some areas here will record over a foot of snow from the current system. Gravity will pull the rainwater and snowmelt down mountainsides, causing streams, rivers, and valleys to overflow with water much more rapidly than elsewhere in the Southeast.
There is a lot to work out with the upcoming system. Those recently impacted by the ongoing storm in the Southeast ought to pay extra close attention to this heavy rain rain event. Be sure to check back with WeatherOptics as the uncertainty clears throughout the week.