12:10 PM | *Unfolding high-latitude blocking pattern ensures winter won’t go down without a fight…powerful ocean storm to form late this week during the transition period*

High-latitude blocking pattern develops later this week with anomalous high heights (purple region) over Greenland and northern Canada and a powerful ocean storm will form; map courtesy NOAA/EMC, tropicaltidbits.com

Overview
High-latitude blocking refers to an atmospheric pattern in which higher heights (and pressure) compared to normal sets up in high latitude regions such as Greenland or northern Canada and it can remain in place for an extended period of time leading to a large-scale obstruction of surface weather systems.  High-latitude blocking tends to be more likely during periods of low solar activity and that is certainly the case now as (weak) solar cycle 24 heads towards the next solar minimum.  During the latter stages of winter, this type of setup can result in a persistent colder-than-normal weather pattern for the central and eastern US and perhaps stormy conditions as well.  All signs point to a strong high-latitude blocking pattern to develop later this week across the North America side of the North Pole and this virtually ensures winter will not go down without a fight in March in the central and eastern US. This change in the overall pattern will also likely result in the generation of a powerful ocean storm late this week just off the Mid-Atlantic coastline and it will likely be a slow mover. 

Low solar activity periods have an increased chance for episodes of high-latitude blocking as depicted by the composite map of 500 mb height anomalies (left) shown for solar minimum years.  The current solar image (right) continues a recent stretch of spotless days (9 consecutive days) as we approach the next solar minimum.  High-latitude blocking was a focus of the 2017-2018 Winter Outlook issued last fall by Vencore Weather. Maps courtesy NOAA, spaceweather.com

Tracking the prospects of high-latitude blocking
In addition to the monitoring of solar activity for insight into the potential for high-latitude blocking, meteorologists track an index known as the North Atlantic Oscillation (NAO) in order to get some information on temperature and pressure patterns in the northern latitudes.  When this index drops sharply into negative territory for an extended period of time this time of year, it generally signals an increased potential for high-latitude blocking.  The NAO index is about to tank to very low negative levels and computer forecast models are supporting the notion for high-latitude blocking to form later this week.  The latest 500 mb height anomaly forecast map by the 06Z GEFS features very strong high-latitude blocking by Friday morning with abnormally high heights over Greenland and northern Canada.  

North Atlantic Oscillation (NAO) index is forecasted to drop sharply in coming days to values well below zero; map courtesy NOAA

Its impact on the weather
During this transition period in the atmosphere later this week to one in which high-latitude blocking forms on the North America side of the North Pole, it appears that a powerful ocean storm will form likely to be sitting just off the Mid-Atlantic coastline by Friday morning.  This storm is likely to result in another soaking rainfall for the DC, Philly, NYC corridor later Thursday into Friday and winds will become quite strong as well; especially, down at the NJ and Delmarva coasts. In fact, this storm may result in near hurricane-force winds later this week in places like Long Island and southern New England.  Finally, in a somewhat unusual fashion, this powerful ocean storm will be actually be forced to eventually move southeastward the end of the week and not take the more typical track up the coastline.  In fact, there can even be a "loop" of the surface low pressure system before it pulls away to the southeast – all a result of the high-latitude blocking pattern.  The slow movement of this system ensures there will be an extended period with strong winds and high waves and coastal flooding may become a serious concern at times of high tide.

12Z GFS surface forecast maps from Thursday morning-to-Saturday morning in 6-hour increments with the powerful ocean storm taking a "loop" as a result of the strong blocking pattern that sets up to the north across Greenland and northern Canada; maps courtesy NOAA/EMC, tropicaltidbits.com

In terms of the potential for snow, this storm is lacking cold air and there is no strong, cold high pressure system to the north (usually an important factor for snow this time of year). However, there are two ways in which this late week system can still result in some accumulating snow for parts of the Northeast US.  First, elevation can play a key role in the chances for snow at interior locations during this upcoming storm.  For example, in higher-elevation locations such as the Poconos and Catskills, there may very well be enough cold air for some heavy, wet snow.  In addition, in areas experiencing strong upward motion (i.e., dynamics), there is the potential that enough cold air gets pulled into the lower atmosphere for a changeover to snow for at least a brief period of time late this week.  This may occur in areas right near the track of the upper-level low in the cold conveyor belt (CCB) region of the storm where upward motions are strong. It is entirely possible that a changeover to accumulating snow takes place as far south as around the PA/MD border – still too early to call. 

06Z GEFS is forecasting progressively colder weather "relative-to-normal" for the Mid-Atlantic region as we begin the month of March; maps courtesy NOAA/EMC, tropicaltidbits.com

In addition to playing a role in the generation of a powerful ocean storm later this week, this unfolding high-latitude blocking pattern will result in a colder pattern "relative-to-normal" for much of the central and eastern US as we begin March.  The 06Z GEFS 850 mb temperature anomaly forecasts averaged over 5-day periods show colder and colder levels as we progress from the current five day period into mid-March.  

Warmer-than-normal water temperatures exist in the Gulf of Mexico and western Atlantic Ocean and will likely contribute to a stormy period; map courtesy NOAA, tropicaltidbits.com

The overall weather pattern is also likely to remain somewhat stormy going forward in the month of March. One contributing factor to this possibility will be the abnormally warm sea surface temperatures in the western Atlantic Ocean and Gulf of Mexico.  There will be this strong western Atlantic ocean storm late this week – aided in part by these warm sea surface temperatures – and then perhaps another storm to deal with near the east coast during next week.  In fact, the system next week is likely to have more cold air to work with in the Mid-Atlantic region so it’ll have to be very closely monitored over coming days (i.e., even more potential for snow).

Bottom line…winter will not go down without a fight.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com 

Extended video discussion on the upcoming pattern change: