CIC Learning Points: Arctic Air is Difficult to Remove and Why Does East Encounter Persistent Coastal Flooding?

Two Trader/Analyst/Risk Manager Learning Points.

Arctic Air Is Difficult to Remove

Fig. 1-2: The last 10 days have featured an extremely cold arctic air mass in Western Canada while U.S. snow cover is now expanding.

Discussion: Climate Impact Company projects December 2021 as the second warmest start to meteorological winter in the 2000-2021 climatology trailing only 2015. The projection is based on gas population weight HDD. Official announcement by NOAA is due by the end of this week. While the U.S. was warm, arctic air was gathering in Western Canada. The last 10 days feature temperature anomalies of 15F to 30F below normal in Western Canada (Fig. 1). Any temperature anomaly of one day (or more) at or colder than 15F below normal is considered arctic air. The super cold West Canada/warm U.S. December pattern is very similar in character to modern-day La Nina climatology. Recently, pulses of the arctic air are migrating into the East meeting with the previous warm regime and causing snow events. Snow cover in the U.S. is definitely expanding (Fig. 2).

In the modern-day climate snow cover enhances the presence of cold air, especially arctic air. Arctic air masses are difficult to dislodge as they are very shallow and adhere the ground while any warming aloft tends to create a temperature inversion (over snow cover) trapping the low-level cold air while warming aloft is unable to reach the ground.

The key is wind. Strong wind will scour away arctic air. Recent warm medium-range forecasts are due to warming of the atmosphere aloft which is more difficult to reflect at ground level given presence of snow cover. Consequently, the previous warm forecasts started to ease last night and at midday reverse cold according to the 12Z GFS (Fig. 3-4).

Fig. 3-4: The 12Z GFS is much colder across the Central/East U.S. in the medium-range compared to previous forecasts.

Table 1: Indicated is the 12Z GFS projected gas population weight HDD for the U.S. compared to 12 and 24 hours ago through mid-January.

Northeast/Mid-Atlantic Coastal Flooding is Persistent. Why?

Fig. 5: The midday 12Z GFS medium-range temperature anomaly forecasts.

Discussion: Currently, Coastal Flood Advisories are in effect for northeast North Carolina to Maine. The issue is strong onshore flow following a departing storm. However, the wind pushing water toward and into the Northeast Corridor Coast is made more effective by the persistent warmer-than-normal ocean surface off the U.S. East Coast and particularly east of New England (Fig. 5). The cold air flow over warm water makes the low-level atmosphere more turbulent and wind speeds faster. Additionally, the much warmer than normal water indicates expansion of the ocean column and sea level rise. The surface wind pushes this higher sea level toward the coast causing the coastal flooding to be more extreme.