Objectives:

Types of blocking ridges

Storms that typically accompany the block.

Numerical models and ensembles forecast these ridges as much as 10 days in advance of occurrence.

Blocking Patterns

Definitions: Eastern Pacific Oscillation Arctic Oscillation Pacific North America or Positive/Negative Anomaly North Atlantic Oscillation

Omega Blocks The wind and height contour fields are

distributed in a manner mimicking the Greek letter Omega - .

The ridge is abutted by two storms on either side.

Alaska Omega block

Omega Blocks

Because of the closed nature of the anticyclonic circulation, Omega blocking in winter months can result in extremely high surface pressures.

The ridging may last a very long time, as the warmer values aloft only gradually cool when shortwaves (or other cyclonic systems) pass south of the core of the ridge center.

Arctic Canada Omega block

Rex Blocks

Consists of a higher latitude ridge that is positioned directly north of a closed cyclonic circulation.

Rex block anticyclone consists into four recognizable patterns.

Classic Rex Block Signature

Rex Blocks

Classic:

Produces a "figure 8" arrangement in the height contour field.

Staring with warm advection aloft toward the northeast.

A shortwave to the right of the building ridge tunnels in a southwestward manner.

Classic Rex Block Signature

Rex Blocks

Classic:

This pattern brings unseasonable cold and storminess to the western U.S.

May result in shortwaves ejected out of the resultant closed low.

Classic Rex Block Signature

Rex Blocks

Lateral:

The blocking signature extends in an west-to-east manner (often through merger with another positive height anomaly).

The resulting stretched appearance aids in building surface pressures over a wide west to east area.

Lateral Rex Block Signature

Rex Blocks

Split Flow:

Jet stream splits into two parts.

The southern branch is as strong, or stronger, than its higher latitude counterpart.

Prevalent during El Nino years, a split signature is a true block.

Ridge centers often appear above 45° N latitude, and act to keep storms in the subtropical or lower polar flow from phasing with the traditional polar westerlies.

Split Flow Signature

Rex Blocks

Split Flow:

Jet stream splits into two parts.

The southern branch is as strong, or stronger, than its higher latitude counterpart.

cPk and cA regimes do NOT migrate southward from Canada.

Snow only occurs in the strongest storms through dynamic cooling processes, cold sector convection, or in nocturnal patterns.

Split Flow Signature

Rex Blocks

In Situ:

In cases where a strong closed low separates from a high-velocity Arctic or Polar jet stream, two impressive baroclinic zone may set up in the corridor between the storm and the fast upper flow.

The presence of the ridging slows the forward motion of the disturbance while at the same time acting to hold colder air in place. In Situ Split Flow Signature

baroclinic zone - A region in which a temperature gradient exists on a constant pressure surface.

Blocking Patterns

NAME THE BLOCKING PATTERN

Blocking Patterns

NAME THE BLOCKING PATTERN

Blocking Patterns

NAME THE BLOCKING PATTERN