2:00 PM | *Chaotic pattern setting up…touch of spring tomorrow, sneaky system to watch for later Sunday…two storms threaten Tuesday-to-Thursday with odds favoring rain…longer term signals for cold*

Overview

A chaotic weather pattern is shaping up for next week and it will be a very challenging forecast period.  The weekend will begin on a very mild note with temperatures flirting with the 60 degree mark in some spots along the I-95 corridor and there will no doubt be a feeling of spring in the air. Colder air will then filter back into the region from the northwest late Sunday night and early Monday and it looks like a sneaky low pressure system may accompany that cool-down.  After that, it appears we may have to deal with two separate low pressure systems later next week in the eastern US with odds favoring rain in the I-95 corridor.

Sneaky system late Sunday/early Monday; multiple storms threaten Tuesday-to-Thursday

It’ll stay relatively mild on Sunday, but not quite as warm as tomorrow promises to be, and a fast-moving low pressure system will approach the Mid-Atlantic region from the Midwest.  This sneaky system could very well spread some rain - perhaps even a soaker -  into the region from later Sunday into late Sunday night and there is a shot that the rain changes over to snow before ending - primarily to the north of the PA/MD border - as colder air filters into the area.  Two separate low pressure systems are likely to then affect the I-95 corridor with the first system from Tuesday into early Wednesday and the second - and stronger system - impacting the area later Wednesday into early Thursday.  Odds favor rain from both systems in the immediate I-95 corridor and points eastward to the coastline, but it is still a close call and mixed precipitation cannot be ruled out. 

12Z Euro forecast map for Wednesday morning showing two separate systems; map courtesy WSI

Longer term signals for cold

Looking beyond the active mid-week time period, signs point to a prolonged period of colder-than-normal weather in the eastern US from later next week into mid-March and this period could be quite stormy as well.  By no means does this mean each and every day in this time period will be below-normal, but the period as a whole is likely to be colder-than-normal for much of the eastern US when we look back on it later in March. 

12Z GFS 500 millibar height anomaly forecast map for Thursday morning; map courtesy tropicaltidbits, NOAA

The 12Z GFS height anomaly forecast map at 500 millibars (above) for next Thursday has some important features that point to a cold weather pattern in the eastern US.  First, there is an eastern trough of low pressure (blues) associated with low pressure in this part of the country.  Second, there is extensive high pressure ridging near the west coasts of the US and Canada and this combination generally allows for the penetration of cold air masses from northern Canada into the northern US.  In addition, this forecast map suggests high-latitude blocking will unfold “over the top” in Greenland, northern Canada (highlighted orange region) and this type of pattern generally allows for cold air to become sustained in the Northeast US. 

Arctic Oscillation index for current, past (black) and near future (red)

High-latitude blocking can be tracked by meteorologists through an index value called the Arctic Oscillation (AO).  The AO refers to opposing atmospheric pressure patterns in middle and high latitudes. When the AO is positive, for example, surface pressure is low in the polar region and this helps the mid-latitude jet stream to blow strongly and consistently from west-to-east keeping Arctic air locked up in the polar region. When the AO index is negative, there tends to be high pressure in the polar regions (i.e., high-latitude blocking), weaker zonal winds, and greater movement of polar air into the middle latitudes. The latest AO index plot (above) shows past and current index values (black) and forecasted levels (red) into early March.  The general tendency for the AO index in the near term is for a drop into negative territory which supports the idea depicted by the 12Z GFS of high-latitude blocking by later next week – and a potential prolonged colder-than-normal period for the eastern US.

Meteorologist Paul Dorian

Vencore, Inc.