1:00 PM | ***Some accumulating snow likely tomorrow night…major weekend storm likely to bring rain, ice, snow to the I-95 corridor…potential “flash freeze” late Sunday as an Arctic blast arrives***

A noticeable trend in recent computer forecast models such as the Euro has been for a a bit farther south “digging” of the southern wave of energy and a bit more separation from the northern stream system. This loop shows the changes from the Monday night Euro model run to the Tuesday night version. Maps courtesy WSI, Inc., ECMWF

Overview

A cold and stormy weather pattern is getting locked in for the eastern US and there may be three different systems to deal with over the next week or so.  On Thursday, low pressure will head in this direction from the Ohio Valley and likely produce some snow around here on Thursday night.  A much more significant storm is going to impact the I-95 corridor this weekend and everything is on the table for this event including rain, ice and/or snow.  It is still too early to determine the magnitude of each precipitation type for the DC-to-Philly-to-NYC corridor, but accumulations of ice and snow are on the table.  In addition, as the weekend storm pulls to the northeast later Sunday, Arctic air will flood the Mid-Atlantic region and any precipitation that lingers can change to ice and then snow before ending.  Also, as temperatures plunge late Sunday, a “flash freeze” is possible in some areas with a quick ice up on roads, etc.  The coldest air mass of the season so far will likely result in single digit lows in parts of the region by early Monday morning and highs may be confined to the teens to start the new work week.

12Z GFS forecast map at 500 mb for Thursday evening with the wave of energy (circled) that will play a role in the Mid-Atlantic snow, courtesy NOAA/EMC, tropicaltidbits.com

Thursday night

A weak cold front will slide through the Mid-Atlantic region later night and it’ll actually play somewhat of an important role in the next precipitation event as we’ll turn slightly colder and drier on Thursday just ahead of the next low pressure system.  Snow should break out late in the afternoon or early evening hours on Thursday in the DC-to-Philly-to-NYC corridor and there can be 1-3 inches by early Friday in the Philly and NYC metro regions and a coating to an inch or two in DC – higher amounts in those ranges to the north and west and lower amounts to the south and east.  This storm will be supported by a relatively weak and fast-moving wave of energy in the upper atmosphere (see forecast map). The snow may end briefly as a period of freezing rain or sleet early Friday morning which could result in some slippery spots for the AM commute.  Temperatures may then actually reach into the 40’s on Friday afternoon as slightly milder air moves into the area.

12Z GFS forecast map at the surface for Saturday evening with snow (in blue) north of the PA/MD border; courtesy NOAA/EMC, tropicaltidbits.com

General discussion of the weekend storm

By the early part of the weekend, low pressure will be intensifying over the Mississippi Valley and it’ll feature lots of snow to its north and west and rain to its south and east.  At the same time, high pressure will be building over southeastern Canada and this will push a reinforcing cold air mass into the Northeast US.  Precipitation could arrive in the I-95 corridor by later in the afternoon – perhaps in the form of snow north of the PA/MD border and rain to the south of there.  As low pressure moves near or over the I-95 corridor later Saturday night, enough milder air may push northward up along the eastern seaboard for any snow that is falling to change to rain.  By Sunday, once the low pulls off to the northeast of here, much colder air will move in behind the system and likely produce a changeover from rain to ice and then to snow in the I-95 corridor before the precipitation ends. Interior sections in the Northeast US from northern PA northward through much of New England could end up with 1-2 feet of snow from this system by late in the weekend. 

12Z GFS forecast map at the surface for Sunday morning with rain over the I-95 corridor (in green) and snow across interior sections of the Mid-Atlantic; courtesy NOAA/EMC, tropicaltidbits.com

Temperatures are likely to drop sharply late Sunday and this Arctic air invasion could cause a rapid freeze-up or “flash freeze”; especially, in those areas that end up receiving heavy rainfall.  By early Monday, the coldest air of the season will likely result in single digit temperatures and highs may do no better than the teens in much of the I-95 corridor as we being the new work week.  In addition, the winds will be quite strong out of the NW on Sunday night and Monday causing wind chill values to be considerably lower than the actual air temperatures.

12Z GFS forecast map at the surface for Sunday evening with Arctic air pouring into the Mid-Atlantic region on the back side of the departing low pressure system; courtesy NOAA/EMC, tropicaltidbits.com

Factors to monitor in coming days

One factor to monitor over the next few days will be the cold air intrusion into the Northeast US on Friday night and early Saturday as high pressure builds into southeastern Canada following the passage of an upper-level trough.  If this trough is a bit stronger than currently projected then the reinforcing shot of cold air early this weekend may also be a bit stronger. This kind of a change would likely result in more of a quick thumping of snow and ice in parts of the Mid-Atlantic region on the storm’s front end (i.e., later Saturday).  One other note to consider, computer forecast models tend to underplay the magnitude of low-level cold air intrusions this time of year.

In addition, there will be two waves of energy in the upper atmosphere that will need to be closely monitored in coming days and the one that will be the main driver of the weekend storm is still way out over the Pacific Ocean.  One wave of upper-level energy will be dropping southeastward through the northern branch of the jet stream and the Pacific Ocean system will eventually move across the southern states.  The ultimate interaction of these two upper-level features will dictate what kind of storm track the weekend system will end up taking.

Recent model trends (e.g., Euro) suggest the phasing of the two upper-level waves of energy may be slower than earlier projected.  A quick phasing would tend to strengthen the storm and pull it farther to the north and west resulting in an overall warmer solution for the Mid-Atlantic region.  A slower phasing would result in an overall colder solution taking the surface low farther to the south and east.  In fact, one reason the phasing may now appear to be slowing is that the southern wave is digging farther south and this too favors a colder outcome this weekend… stay tuned, and another system may impact the region by the middle of next week.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

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