In a season marked with numerous extremes ranging from 70s in December to NYC’s second biggest snowstorm on record, then back to summer-like warmth in early March, the latest extreme appears to be the potential for a snowstorm to mark the beginning of astronomical spring. Much of the deterministic model guidance depicts a major nor’easter affecting the East Coast; while variability among the models precludes higher confidence in the forecast details, the model guidance nonetheless offers some clues regarding the potential impacts in the region.
Overview: From 80s to Snow in March
Much of March has been highlighted by an increasingly active pattern over the eastern US, beginning with colder than average temperature and a major snowstorm just east of the tri-state area, which quickly transitioned into summer-like temperatures with highs in the 70s and 80s last week. Temperatures trended slightly cooler but still warmer than average this week with an upper-level cutoff low over the northern US, along with frequent convection both yesterday evening with the passage of an occluded front and this afternoon associated with very steep low-mid level lapse rates and weak instability.
The relatively mild weather will be brought to a temporary end tomorrow as a cold front moves through the tri-state area; synoptic-scale forcing for ascent is generally weak, although as with today, somewhat steep lapse rates and weak instability along with the surface-based convergence along the cold front could result in another round of scattered showers tomorrow afternoon. Temperatures are projected to peak in the low to mid 50s tomorrow; cold air advection in the wake of the frontal passage will bring overnight lows into the low 30s in NYC and mid-upper 20s elsewhere, while setting up the stage for the next incoming low pressure system.
Sunday – Monday: Snowstorm Potential Exists, But Uncertainty Lingers
The main story over the last several days has been the somewhat consistent signal among the deterministic model guidance, especially the typically reliable ECMWF, for a major snowstorm to affect the Northeast US. This would be yet another unusual event added to the list of anomalies this winter; for parts of the northeast US, the modeled snow totals on the ECMWF would exceed their entire seasonal snowfall to date, which also implies that their largest snowstorm during the 2015-16 winter season would have actually occurred during the spring. Despite the consistency with multiple ECMWF runs, large variability remains between different models as well as with the model ensembles, with an increasing likelihood of measurable precipitation but with considerable uncertainty regarding the evolution of the low pressure system and the resulting impacts over the northeast US.
This post focuses on the forecast synoptic-scale evolution of the storm, areas of uncertainty in the forecast, and any clues for potential impacts that can be identified from the latest model guidance. A brief summary is included at the end of the post.
12z GFS hour 12, valid at 0000 UTC Friday, 18 March, depicting 500 hPa gepotential heights and absolute vorticity.
As the previous section noted, a cutoff upper-tropospheric low has been centered over the northern US over the last several days, following the trough gaining a rapid negative tilt and resulting in cyclonic wavebreaking around 15-16 March. The end result is a lingering transient block-like feature over southern Canada, with a broad field of cyclonic vorticity to its south and a deep upper-tropospheric low associated with a cold air mass situated over eastern Canada. The northerly flow to the west of the upper-level low and behind the surface cold front expected to pass through the region tomorrow will result in cold air advection into the Northeast US, establishing an antecedent cold air mass over the region, often a crucial ingredient for a snowstorm potential.
12z GFS hour 66, valid at 0600 UTC Sunday, 20 March, depicting 500 hPa geopotential heights and absolute vorticity.
In the short term, ridge amplification is expected over the western US associated with warm and moist air advection downstream of a deepening cyclone in the Northeast Pacific Ocean. With a digging shear vorticity maximum over the Rockies, as well as part of the northern US cutoff low splitting and quickly progressing eastward, a closed 500 hPa low is expected to organize over the Dakotas before progressing east towards the Ohio Valley by Saturday night. By this point, the aforementioned upper-level low over eastern Canada will move out, resulting in decreased confluence east of the region, while interaction with an approaching trough over central Canada will result in the eastern US trough becoming neutrally tilted, with increasing cyclonic vorticity advection facilitating rapid surface cyclogenesis off the Northeast US coast.
Even though the nor’easter is at least 3 days away from developing, the model guidance remains somewhat inconsistent with its depiction of the storm. Numerous factors are likely contributing to this uncertainty, especially the forecast evolution of the vorticity maximum over the Rockies, an area which models often struggle to handle correctly in the medium range, which is partly dependent on the evolution of the upstream ridge building over the western US. This is complicated by the fact that the low pressure responsible for building the ridge is still over the Northeast Pacific Ocean, a region with sparse data sampling. Additional areas of uncertainty are farther downstream, including the emergence of the upper-level low from the current broad vorticity field over the northern US, as well as the amplitude of the upper-level low over eastern Canada and the timing of its departure. These factors will influence the amplitude of the eastern US trough and accordingly the position and intensity of the surface low pressure.