11:45 AM | *Classic "omega" blocking pattern in the upper atmosphere to play a role in meandering coastal storm*

00Z GFS Ensemble forecast map at 500 millibars for Friday night with the classic signature of an "omega" block: deep low pressure trough along both coasts and strong high-pressure ridging in the central US; map courtesy tropicaltidbits.com, NOAA

*Classic “omega” blocking pattern in the upper atmosphere*

Overview
Weather systems generally move along at a pretty good clip in the mid-latitudes thanks to usually reliable west-to-east winds in the upper part of the atmosphere.  Every once in awhile, however, the westerlies become disrupted as a blocking pattern forms in the upper part of the atmosphere.  This in turn can lead to the stagnation of surface weather systems with several days of nice weather or several days of poor weather in a given area depending on which part of the upper-air block resides overhead. There are several different types of blocks known to meteorologists and one of these is referred to as the “omega” block since the wind flow pattern in the upper atmosphere resembles the Greek letter by the same name.  Indeed, a classic “omega” upper-air blocking pattern is now unfolding across the US and it will soon feature deep upper-level troughs of low pressure along both coasts with strong high-pressure ridging in between.  As a result, we’ll be stuck with inclement weather right through Friday and the weekend could also contain some shower activity in the Mid-Atlantic region.  

12Z high-resolution NAM surface forecast map for Thursday morning featuring strong low pressure off the Mid-Atlantic coastline; map courtesy tropicaltidbits.com, NOAA

Omega-block and a meandering coastal storm
As colder air pushes towards the east coast from the Upper Midwest over the next 24 hours or so, a deep upper-level trough will form along the east coast.  At the same time, deep upper-level trough will intensify along the west coast and strong ridging will pop up across the middle of the country.  The 00Z GFS Ensemble forecast map (above) at 500 millibars for Friday night shows the classic signature of an “omega” block in the US with the wind flow generally following the shape of the Greek letter (red line).  Air over Mexico and the Southwest US will be forced to the north into the Northern Plains and then back south again into the southeastern US and Gulf of Mexico region by the vast high pressure ridge in the central US.  

In the Mid-Atlantic region, where we are stuck under a deep upper-level trough, surface low pressure will form just off the coastline on Thursday and then meander around for a couple of days.  The latest (12Z) version of the high-resolution NAM computer model features strong low pressure off the Mid-Atlantic coastline on Thursday morning and a very similar-looking pattern on Friday morning.  The result around here will be periods of rain on Thursday and Friday and - given the long time period of onshore flow - this scenario could result in some coastal flooding. The weekend should feature some improvement with the return of some sunshine; however, given the "omega" blocking pattern, don't be surprised if there are some lingering showers in the Mid-Atlantic region. Meanwhile, the middle part of the nation is experiencing several days of fine weather and warm temperatures with high pressure ridging sitting overhead and the western US is having prolonged cool, miserable weather - just like as in the eastern US.

For an extended video discussion on the "omega" block visit the Meteorology 101 page of the Vencore weather web site.

12Z high-resolution NAM surface forecast map for Friday morning featuring strong low pressure off the Mid-Atlantic coastline; map courtesy tropicaltidbits.com, NOAA

   
Meteorologist Paul Dorian
Vencore, Inc.