A Block in the Weather Pattern

 

An omega block is being indicated by the models for the latter part of the upcoming work week. An omega block pattern is one that contains a high pressure system sandwiched in between two low pressure systems so that it looks like the Greek symbol omega. It’s called a block pattern because the pattern is blocked up as a result of the upper level jet stream, which allows systems to steadily traverse through the United States, becomes stopped up. There is too much north to south or south to north flow to keep systems moving west to east through region. In the classic example, the main jet stream moves west to east north of the block and at the bottom of the trough where the stopped-up upper low pressure systems reside. This wouldn’t allow a strong jet stream of wind to flow in from the northwest and act as a pivot point for any of the troughs to easily kick them out to sea and so they remain nearly stationary and cause flooding issues. Indications are that the upper level low pressure system on the east side of the omega block will have this pivot point so that it will still be able rotate through the region quicker than is normal with these blocking situations.

The idealized depiction of an omega block with pressure center positions indicated with letters and jet stream streamlines shown in blue.
The idealized depiction of an omega block with pressure center positions indicated with letters and jet stream streamlines shown in blue. Image courtesy of COMET.
Jet stream level winds generally follow the idealized situation asside from the northwest winds coming in from Southern Canada, which act as a pivot point for the low pressure to the east.
Jet stream level winds generally follow the idealized situation aside from the northwest winds coming in from Southern Canada, which act as a pivot point for the low pressure to the east. Image courtesy of tropicaltidbits.com
Forecast depiction of the general location of high (red colorations) and low pressure (blue colorations) centers
Forecast depiction of the general location of high (red coloration) and low pressure (blue coloration) centers. Notice the low pressure system in Northeast Canada serving as a pivot point even while the low pressure system on the East Coast is cut off. Image courtesy of tropicaltidbits.com

 

Generally, since the low pressure systems coming together to create these blocking patterns come in from the north, they create plenty of cool air in addition to precipitation. Forecasts currently have below normal temperatures at the surface, underneath where the low pressure systems are forecast to set up and above normal temperatures underneath where the high pressure system is forecast to set up. Temperatures along the West and East Coasts are forecast to be 5 to 15 degrees below normal with temperatures expected to 5 to 15 degrees above normal in the Central states. Furthermore, precipitation is forecast to fall along the East Coast and the West Coast. Precipitation anomalies are above normal along the West and East Coasts with below normal precipitation in the Central states. In some situations, the upper level low pressure system on the West Coast is able to draw in enough warm and moist air from the south that it is able to cause severe weather in the Plains. This blocking pattern doesn’t appear to have enough warm and moist flow over the Plains to be able to create an outbreak, but that does bear watching.

Temperature anomalies during the blocking pattern. Above normal temperatures are forecast over the East and West Coasts with below normal temperatures in the Central US.
Temperature anomalies during the blocking pattern. Above normal temperatures are forecast over the East and West Coasts with below normal temperatures in the Central US. Actual forecast temperatures at various locations are in black for Thursday May 5th. Images courtesy of NOAA.
Forecast precipitation for day 4 and 5 via the WPC. Rain in the east and west with a lack of rain in the Central US.
Forecast precipitation for Wednesday and Thursday. 
7-day precipitation anomalies during the blocking pattern. Above normal precipitation is forecast over the East and West Coasts with below normal precipitation in the Central US.
7-day precipitation anomalies during the blocking pattern. Above normal precipitation is forecast over the East and West Coasts with below normal precipitation in the Central US. Courtesy of tropicaltidbits.com

 

In many cases for the Northeast, enough clouds remain around the region for a prolonged period of little to no sunshine. Also, in addition to some precipitation, there are also periods of dry weather and sometimes even sunny peaks, but forecasting exactly where these sensible weather highlights might occur is tricky. Tracking individual pieces of energy and areas of upward motion or weather-causing forces is often the best way of figuring out the exact timing of precipitation and lulls, but these often change on a day to day or even model to model basis. Running forecasts at slightly different time periods and with slightly different conditions (an ensemble forecast) allows the forecaster some probability of precipitation and some possibility of being correct during the period with a lack of predictability.

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