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NYC Blizzard Update #2 – Mar 13, 2017

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The track of the imminent nor’easter and blizzard has continue to trend west of earlier model runs. This has introduced increasing uncertainty as to overall snowfall totals along the immediate coastal areas along with possible wintry mix/freezing rain. On the other hand, confidence is increasing in heavy snow for areas north and west of the city, with snowfall totals 18″+ looking likely.

Impacts – heavy snow north and west of the city, snowfall accumulations in the interior 16-20″+ looking likely. Sharp gradient in the heaviest snowfall totals approaching the coast where wintry mix, freezing rain, and even plain rain could mix in during the early afternoon Tuesday. The exact location of the extent of warm air intrusion off the ocean will ultimately determine the difference between routine totals over 1 foot and areas that receive much lower snowfall totals. The difference in some cases could be a matter of mere miles. This line could fall in and around NYC itself. Still expecting a period of blizzard or near blizzard conditions for NYC during the course of this storm even if a transition to mixed precipitation does occur. Snowfall totals could still pile up close to a foot or over prior to this transition. The possibility of freezing rain and ice introduces an additional hazard on the roads and when coupled with strong winds of 30-35mph and gusts 45mph+ could result in power outages.

Timing – light snow should begin to fall early into the overnight hours, progressively growing in intensity. In the city, overnight accumulations of 3-6″ possible, with a sharp increase in totals north and west. Heavy snow with sleet and wintry mix during the morning and early afternoon in the city, all snow in points north and west. Blowing and drifting snow with blizzard conditions possible. Snow begins to wind down during the evening hours.

Caveats – any further movement of the storm track west would result in drastically lower snowfall totals for NYC and points east along the coast. Correspondingly, an eastward shift of the storm track would put NYC itself squarely in the bullseye for the heaviest snows.

NYC Weather – After the Blizzard – Jan 27, 2015

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As many of you woke up this morning, you were probably wondering, what happened here? This storm seemed like a dud. True, the snowfall totals in and around NYC were lower than initially forecast, but portions of the area did get slammed with over 20″ of snow. To put it simply – as I had laid out in the caveats from the last post, slight changes in track of this storm dramatically altered the placement of mesoscale banding features, hence leading to a sharp cutoff in snowfall totals from east to west.

Snowfall Totals

Looking at this snowfall totals map, you can easily make out the line that demarcates where the heaviest snow bands stopped their westward march. We got what appears to be around 6-8″ in the city, but just east of us in Nassau the totals jump to 12″ and then just east of that in Suffolk we see 20″+ totals.

Snowfall total map
Snowfall total map

On a larger scale, you can see the areas that got the highest snowfall totals, corresponding to where the heaviest bands of snow were able to pass over.

Screen Shot 2015-01-27 at 12.48.04 PM

Forecasters are saying that the storm tracked further east than they expected by about 50-75 miles, which literally would have meant the difference from us having 6-8″ snowfall totals here in the city to easily 12-18″+. Now before we all go blaming them for a dud forecast (that some are already saying wasted taxpayer money), there are a couple things to consider:

  1. Forecasters utilize a suite of different computer models to come up with these forecasts, in this case they favored the NAM (North American Mesoscale) and ECMWF (European Center for Medium Range Weather Forecasts) over the GFS (Global Forecast System), UKMET (United Kingdom Meteorology), and Canadian models. There were specific reasons for doing so, one of which is based on forecasters’ experience with these types of systems and which models have historically done a better job with them. ECMWF often does a very good job of modeling strong cyclonic systems like this (and is often favored in forecasting tropical cyclones). In this case, the NAM and ECMWF had the storm moving just far west enough to put us on the cusp of those high snowfall totals.
  2. Mesoscale (medium scale) banding features are notoriously difficult to forecast in these types of storms. The exact position, orientation, and movement of these banding features is often not known until the storm is already ongoing. This is due to a number of complex atmospheric interactions that are responsible for these features forming. To take another example, the Storm Prediction Center can forecast that severe thunderstorms are likely over a certain area 2-3 days out. However, it is exceedingly difficult even the day of to know which towns will get hit by individual storm cells/tornadoes, etc. Despite advances in supercomputing power, forecast skill and accuracy, and understanding of meteorology, these mesoscale events remain challenging to get right even for the most experienced forecaster. So while the general public can say, “I told you so”, I’d challenge any one of them to forecast the next Nor’easter, and the one after that, and so on, and get them right with more skill than a straight up guess.

Anyway, since we all got the day off (the political response to this storm is a whole other debate beyond the scope of this blog), forget about the forecast being a dud and go out to take advantage of a day off!

NYC Blizzard Update – Jan 26, 2015

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A major nor’easter will be bringing a high impact blizzard throughout the NYC region tonight through Tuesday midday. Beginning this evening, snowfall will increase in coverage from south to north. Periods of heavy snow are expected with snowfall rates of 2-4″ per hour, with thundersnow occurring in the heaviest bands of snow. Extended periods of high winds, with sustained winds at least 35mph and gusts over 40mph (even up to 60mph) will lead to blowing and drifting snow, blizzard conditions, and near-zero visibility. Widespread snowfall totals of between 18″-24″ possible throughout the area, with locally higher amounts.

Snowfall Totals

Since yesterday afternoon, forecast models have backed off a bit on the overall total accumulations. At this point yesterday, the quantitative precipitation forecast (liquid water equivalent total) was as high as 2.7-2.8″ and broadly over 2.5″ across much of the area. As seen below, those liquid water equivalent numbers have gone down by about half an inch.

Liquid water equivalent forecast totals
Liquid water equivalent forecast totals

This decrease in overall precipitable water has led to a corresponding downgrading of the overall snowfall totals. Using a 12:1 snow to water ratio, a drop in precipitable water by half an inch translates to a decrease in snow totals by 6″. Indeed, the National Weather Service has decreased the likely totals from 27″+ for many places to closer to 20-22″.

Snowfall accumulation total forecast
Snowfall accumulation total forecast
Highest probabilities for snowfall totals in excess of 18" during the 48 hour period ending 7AM Wednesday now rests over Southern New England, with the bullseye setting up right over Boston and immediate suburbs.
Highest probabilities for snowfall totals in excess of 18″ during the 48 hour period ending 7AM Wednesday now rests over Southern New England, with the bullseye setting up right over Boston and immediate suburbs. Here in NYC, we are in the range of 30%-50% probability of receiving over 18″ – notice how abruptly the probability goes up as you go further north and east.

Timing

Light snow has already begun falling over the area, however the heaviest snow will not be occurring until after 8PM tonight. The most intense period of this storm is expected overnight tonight, as a deformation banding feature forms and begins moving slowly inland. Based on the simulated radar loop below from the NAM (North American Model), the peak intensity of snowfall looks like it will occur between 1AM-7AM overnight into Tuesday. Snow appears to taper off over NYC by 1PM tomorrow.

It does appear that a secondary set of banding snow will linger over eastern areas of the metro region throughout the afternoon Tuesday until as late as 4PM tomorrow.

By the way, part of the reason the main area of highest snowfall totals is now shifting to Southern New England and Boston is because it is anticipated that these areas will see moderate to heavy snow for much longer than we will here.

NAM model simulated radar output for this storm
NAM model simulated radar output for this storm

Impacts

Snow, heavy at times, beginning in earnest this evening especially after 8PM. Some thundersnow possible, with the most intense snow occurring overnight. Strong sustained winds up to 35mph with gusts over 40mph and as high as 60mph will lead to blizzard conditions with blowing and drifting snow cutting visibilities down to near zero.

Caveats

Forecasters have been pointing out that there continues to be some slight disagreements between different models even though this storm is imminent. Snowfall total forecasts for storms like this one are notoriously difficult to predict with a high degree of accuracy prior to the event itself. This is due to the fact that the exact placement, movement, and intensity of the deformation banding feature on the northwest flank of the low pressure center is hard to foresee until the storm is ongoing. With snowfall rates as high as 2-4″ per hour within this banding feature, a difference of a couple hours under it could translate to 4-8″ difference in overall accumulation in any one location. Same thing with location, as one location that misses the most intense portion of the banding feature could easily miss out on 6″+ in total accumulation. Furthermore, a slight change in the thermal profiles within this type of storm could alter the snow to water ratio to be higher, and of course, a change from 12:1 to 15:1 would result in a dramatic difference in totals.