Last Wednesday, a strong Arctic front swept across much of the Northeastern US, impacting many areas with a line of heavy snow showers, then ushering in record-breaking cold. The line of heavy snow immediately preceding the frontal boundary set off Snow Squall Warnings, which many readers would have seen on their mobile devices. The Snow Squall Warning is a new type of warning that went live nationwide on November 1, 2018. I believe that last Wednesday’s event was the first time National Weather Service forecast offices issued this new warning type for a widespread frontal snow squall. In this post, I’ll share my thoughts on the new warning type, and some observations about the event itself.

The New Snow Squall Warning

The relatively new snow squall warning product is, like other existing warnings, an effort by the National Weather Service to inform the public about imminent hazardous weather impacts. The main motivation behind this new warning type is to try and reduce the number of potentially fatal multi-vehicle accidents that can occur in snow squalls. Snow squalls can cause these kinds of accidents because the intense snow and wind in them can rapidly reduce visibility to near whiteout conditions with little advance warning. The heavy snow can also result in quick accumulations that make driving even more dangerous. Snow squalls can occur along frontal boundaries, like what we saw last week, but they can also be isolated or form in conjunction with lake effect snow. Although I haven’t as yet seen a clear-cut definition of what triggers this new warning, the criteria I have seen tie in directly with the hazards mentioned above: visibility less than 1/4 mile (whiteout conditions), strong wind gusts (above 35 mph, it appears), heavy snow, and surface temperatures below freezing.

Snow squall warnings are functionally similar to severe thunderstorm warnings, which makes sense because snow squalls and severe thunderstorms share some sensible weather impacts and meteorological properties. In this case, with a frontal snow squall, my professor and seasoned forecaster Steve Corfidi observed that “Essentially, a winter cold-frontal snow squall band is simply a summer cold frontal squall line with its bottom two-thirds or so chopped off. For all practical purposes today you simply experienced the passage of a narrow, fast-moving band of convective cirrus!”

Snow Squall Event in NYC

The snow squall that hit NYC last week was associated with a strong Arctic front. As I’ll discuss below, this frontal boundary provided the necessary lift to generate a narrow band (along the east-west dimension) of heavy snow along much of its length. Light snow began falling around 3:30PM by my estimate. The intensity of the snow picked up moderately over the next 15 minutes. However, it wasn’t until close to the end of the event that snowfall rates truly kicked into high gear, along with the winds. During a span of about 5-10 minutes, as the snow and wind rapidly picked up, visibility dramatically decreased, with scenes like the one below typical.

Once the worst of the snow squall cleared, conditions rapidly improved, with visibility recovering quickly and precipitation ending rather abruptly. Following the passage of the Arctic front, forced subsidence with the much colder and denser air behind the front sinking to the surface helped mix down some very strong wind gusts, and helped usher in some of the coldest air of the season.

Why the Squall Seemed to Peak at the End

The snow squall started off as a few flurries, and for most of the duration of the event, it seemed like that was all we’d get. Then, within a very brief span, the squall peaked in intensity, and as quickly as it had peaked, it was over. So, why did this event appear to unfold this way to us as observers on the ground? It all has to do with the profile of the winds above surface relative to the Arctic front and the squall line.

In the sounding above, we can observe that the wind barbs on the right side of the sounding are generally increasing in speed up to 600 mb – triangles represent 50 knots, each full tick represents 10 knots, and half a tick is 5 knots. At the surface, winds were west-northwest at 10 knots, but at 600 mb, winds were at 90 knots! Quite a difference. The second thing to note is above 850 mb, the wind barbs are oriented roughly at the same angle, indicating winds from the same direction at these levels (west-southwest) . This is what forecasters refer to as a “unidrectional” wind profile. The result here is that we had a set up where there was significant vertical speed shear. This has tangible effects on the structure of the clouds/convective activity within the snow squall, as shown below.

In the diagram above, as the leading edge of the Arctic front progresses, the air ahead of it is mechanically lifted above the dome of cold air behind the frontal boundary. Once the air reaches the LCL (lifting condensation level), it’s saturated and clouds begin to form. In this case, there’s enough lift and available moisture that precipitation begins to fall. Temperatures at the time supported all snow. Lift provided by the front would continue allowing the clouds to grow until they hit a stable layer – I won’t go into specifics about this but suffice to say that at this point, the cloud can’t keep growing vertically. This results in the cloud spreading out horizontally, creating an “anvil”. Because the wind speed is so much faster at this level, the anvil is sheared away from the direction of oncoming wind producing a “leaning” effect.

This radar image from the Newark TDWR (Terminal Doppler Weather Radar) at 2:58PM last Wednesday has a good depiction of the light snow falling ahead of the main squall line that’s coming from the sheared anvils of the main convective line. Note the scattered lobe of light snow ahead of the solid band of darker blue hues indicating heavy snow.

Because the winds were coming from the west-southwest, the anvils leaned in the opposite direction to the east-northeast. For us on the ground, that meant the light snow preceded the heart of the action, “the worst of the storm” that was closer to the leading edge of the Arctic front itself. Had winds aloft been weaker, or from a different direction, suppose more parallel to the frontal boundary itself, the contrast between the light and intense snow wouldn’t have been as dramatic.

Thoughts on Improving the Snow Squall Warning

This was the first widespread use of this new warning product, and it’s not surprising that this led to some confusion. I had several people ask me when the warning was issued and the snow started “How long is this going to last?”. Some people even did the exact opposite of what the warning is intended to prevent: they rushed out to “beat the snow” since it started off light and they didn’t realize it would be over in a short span of time.

I think that these warnings could be improved if a specific duration of the event were mentioned in the warning text, something along the lines of “Expect snow squall conditions to last between 30-45 minutes”. Some other weather forecast offices issue warnings with such text. As discussed above, frontal snow squalls are similar in nature to their warm season relatives. While people are used warm season convective activity ending pretty quickly, intense snow squalls here are often caused by the mesoscale bands accompanying Nor’easters. These can last several hours. In general, many winter weather warnings are long duration, which I believe contributed to some of the confusion that people had about this new type of warning.