March 17, 2016: Will Spring Start With a Snowstorm?

Forecast Highlights:

e_apcpsfc_f84In 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.

Storm Synopsis:

 na_absvprs_f1212z 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.

 

 

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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.

Forecast Uncertainty:

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.

 

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12z GFS hour 84, valid at 0000 UTC Monday, depicting MSLP, 6-hour precipitation and 850 hPa temperatures. At the same valid time, the ECMWF depicts a much stronger low pressure with significantly heavier precipitation over the area.

The deterministic model guidance highlights this uncertainty with run-to-run variability, with solutions ranging anywhere from a weak low pressure only rapidly deepening once well offshore on the GFS, to a rapidly deepening low closer to the coast producing heavy snow over much of the I-95 corridor on the ECMWF. Differences between the two models appear to emerge with their handling of the eastern US trough and the upper level low over eastern Canada; the GFS is more progressive with the trough, resulting in weaker surface cyclogenesis farther to the east, and is slower with the Canadian upper level low, resulting in a more confluent flow over the northeast US, with the net effect a much faster and weaker surface low pressure than the ECM.

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18z GFS ensemble members; this figure depicts 12 out of the 20 ensemble members, which show roughly an equal split between hits and misses. Image from PSU e-Wall.

The ECMWF has been fairly consistent in its depiction of a major snowstorm across the region, although it is always important to keep in mind that consistency does not necessarily imply accuracy; this has been exemplified numerous times over the last few years, most notably with the 26 January 2015 blizzard, in which the ECMWF was consistent with over 24 inches of snow in NYC for multiple runs, yet the outcome was barely 9-10 inches. Cases like these illustrate the usefulness of utilizing model ensembles, in order to gain a better perspective into the extent of the uncertainty. The ensembles are obtained by running the same model multiple times but with slightly different initial conditions and model physics, to attempt to account for uncertainty with the initial conditions. There are 20 GFS ensemble members and 50 ECMWF ensemble members; the latter have often been assessed to be more skillful than the GFS ensembles. The latest GFS ensemble members are split approximately equally between a major impact event in the region and a near miss offshore, while the ECMWF ensemble members are more heavily weighted towards significant impact, with at least 27 out of 50 members depicting 6 or more inches of snow in Newark, NJ.

Potential Impacts:

The deterministic and ensemble guidance spread indicates that the potential for a significant nor’easter to affect the region exists, but with variability in the handling of the eastern US trough and the eastern Canadian upper-level low resulting in uncertainty regarding the evolution of the coastal low pressure, specifically its intensity, track and the timing and location at which it begins to rapidly intensify once the eastern US trough interacts with the approaching central Canadian trough. This affects the potential impacts in the region; while given the spread in the model guidance, it is too premature to make a deterministic forecast regarding impact in the Northeast US, several potential scenarios can be identified based on their probability of occurrence given the latest data.

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12z GFS hour 54, valid at 1800 UTC Saturday, depicting the antecedent cold air mass at 850 hPa over the region ahead of the developing low pressure.

As previously noted, an antedecent cold air mass will be in place over the northeast US ahead of the low pressure, with 850 hPa temperatures below -5C to -10C forecast over the region. Given widespread cloud cover and sustained precipitation, this would support high temperatures at least in the low to mid 30s, indicating that unless the low pressure tracks close enough to the coast for significant warm air advection to extend into the area, the primary precipitation type would be snow. Precipitation totals, and accordingly snow totals, depend on the evolution of the low pressure; not only on the track, but also on the timing of the rapid deepening of the low pressure. For instance, a 1004 hPa low pressure off the coast of New Jersey (e.g. GFS) would likely produce widespread light to moderate snow, but with the lighter snowfall rates not accumulating as quickly owing to warm ground temperatures and the high sun angle in late March, whereas a rapidly deepening 990 hPa low pressure in the same location (e.g. ECM) would be associated with heavy frontogenetic snow bands in the northwestern quadrant of the low pressure associated with deformation; such snowfall rates would easily overcome any warm ground and high sun angle issues (e.g. October 2011 nor’easter) and would result in significantly higher snow accumulations, perhaps over a foot of snow.

In terms of snow accumulation, the best forecast that can be provided at this time is a probabilistic one, owing to the wide range of snow accumulations on the deterministic and ensemble guidance, ranging from no snow to over 16 inches of snow. The majority of the deterministic model guidance, with the exception of the GFS and to some extent the NAM, is currently weighted towards significant impact over the region. The CMC and ECMWF ensemble members are heavily weighted towards significant impact as well, with the CMC currently the westernmost outlier, depicting the low pressure track close enough to allow for rain for parts of the area. Despite the deterministic GFS remaining near the eastern end of the envelope, nearly half of its ensemble members depict moderate to significant precipitation over the tri-state area. As it stands now, the current model guidance would favor higher odds of a major snow event, although this does not account for potential future trends with the handling of the aforementioned troughs that may alter the development of the system and accordingly shift the model consensus either east or west. As such, the current probability forecast is weighted slightly lower than most models would support to account for the uncertainty in the forecast.

Forecast Summary:

Based on the latest available data, snow is likely to begin developing on Sunday afternoon, towards the 12-4 PM time frame; precipitation type may initially start as rain especially south and east of NYC, but assuming that at least a sustained period of light to moderate precipitation occurs, precipitation type would likely change over to snow for most of, if not all of the area with temperatures hovering close to freezing. The peak of the storm is likely to occur around Sunday evening, with snow likely to begin tapering off past 12-4 AM overnight.

Regarding snow accumulations, as the analysis section above discussed, there is too much uncertainty at this time to issue a deterministic snow forecast. Taking the latest model guidance and ensembles, as well as past and potential trends, into consideration, a probability outlook for snow ranges was made to accompany this forecast post. These probabilities are focused on New York City, and would increase towards less snow to the west and increase towards more snow to the east.

0-1″: 5%
1-3″: 20%
3-6″: 35%
6-9″: 25%
9-12″: 10%
12″+: 5%

Stay tuned for additional updates on this storm over the next several days.

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