High latitude blocking increasingly likely through November

Good afternoon! Autumn like weather is in place as we speak across much of the Northeastern United States, with relatively seasonable temperatures. Enjoy it while it lasts. It will feel much more like winter very soon, as an arctic blast is set to arrive from Friday into Saturday. Very quickly, temperatures will fall 15 to 25 degrees below normal for this time of year, with record low temperatures possible over reporting climatology sites in the Northeast states. While this intensity of cold air will be short lived, colder than normal air will remain persistent.

A vigorous polar disturbance inside a shortwave trough will be swinging through the Great Lakes and Northeast tonight and Friday. This feature will amplify into a small closed low over Quebec later on Friday Night. The result will be a frigid airmass from Northern Canada swinging southward with a vengeance and reaching the Northeast USA from Friday into Friday Night. This type of southward movement of polar/arctic air is unusual this early in the season.

High temperatures will occur during the early morning hours on Friday. Despite plenty of sunshine, temperatures will likely fall through the 30s and then into the mid-upper 20s during the day, as colder air continues to filter into the Northeast. It will also be quite blustery as area of low pressure associated with the polar shortwave trough deepens over Southeast Canada. Northwest winds sustained 15-25mph could gust to between 30-35mph on Friday. This will support wind chills in the teens and lower 20s over parts of the Northeast. Winds will gradually diminish Friday night and Saturday morning. But the first hard freeze of the season is likely for the major cities and urban areas along the I-95 corridor, as temperatures drop into the lower to middle 20s. Over the Interior sections, temperatures will drop well down into the teens. Windchill values could still run several degrees lower.

The NAM model showing the vigorous shortwave energy moving into Queebec on Friday

The NAM model showing the vigorous shortwave energy moving into Quebec on Friday

The polar shortwave trough will lift out of Southeast Canada over the weekend, due to a lack of high-latitude blocking over the Atlantic early in the period. This will allow for a zonal (west to east) jet stream to return and allow some moderation of the airmass as the weekend goes on. Temperatures will remain several degrees below normal despite the moderation, which is important for heating demand as the weekend goes on.

Changes in the atmospheric pattern will likely occur in the high-latitudes as we move into next week, favoring an increased propensity for cold air and opportunities for storms across the entire Northern 1/3 of the USA through Thanksgiving. Model and ensemble guidance continues to show an anomalous -WPO/EPO ridge over Aleutian/Bering Sea building towards the North Pole and then another anomalous -NAO ridge building over Greenland/Davis Strait. This pattern evolution of high latitude blocking almost always results in cold air being displaced further south from the arctic into Canada and the USA’s Northern 1/3.

Here’s how we expect it to play out: First, a piece of the polar vortex over the Hudson Bay will break off into Alaska and Western Canada. This will dump arctic air into the Gulf of Alaska and British Columbia. Thereafter, a disturbance is expected to move through the Northern Plains and then amplify as it reaches the Northeast USA next week. This could lead to a developing coastal storm system with the potential for some significant precipitation. This could be especially true for parts of Upstate New York and New England, where wintry precipitation is a possibility during this time frame.

eps_z500_anom_noram_312disco

This system will enhance ridging out ahead of it, pumping atmospheric heights up in Greenland and towards the Davis Strait – otherwise known as a Negative NAO. This causes the polar vortex to buckle and become more established over the Northeast Pacific Ocean. In response, a stronger ridge will be develop over the Western US and Canada and carve an even deeper trough over much of the Eastern US. The Negative NAO blocking ridge over the Davis Strait will keep the pattern persistent, with colder than normal air a distinct possibility in the Northeast states through Thanksgiving.

To wrap things up: This developing atmospheric pattern over the Western Hemisphere will result in the possibility of sustained cold air over the Northeast and much of the Northern 1/3 of the USA. Disturbances emanating from Canada will have the potential to amplify and develop off the USA East Coast. Colder air will remain in place through Thanksgiving and the end of November, suggesting the potential for wintry weather from the Great Lakes into Interior New England through that time frame.

GWHDD will very likely reach near or above the 10 and 30 year moving averages – but some uncertainty still exists in regards to how much ridging can build in the Central United States to balance out the demand. At the present time our forecast through November carries slightly above normal GWHDD numbers.

Stay tuned for more updates over the next couple of days as we continue to fine tune the forecast.