Omega Block Pattern Shift Brings 4 Changes in the Week Ahead

Linda Lam
Published: May 1,2016
Changes are coming this week ahead due to a shift in the weather pattern that includes the redevelopment of the omega block weather pattern across North America. The changes will be particularly welcome to the severe-weather weary central U.S., but may not be cheered in the East.
(MORE: Deadly Flooding Strikes Texas)

Two areas of low pressure develop, one on the East Coast and another on the West Coast. In between, the jet stream bulges northward.

This is the second time in the last several weeks that we will have seen an omega block weather pattern develop. This time around the configuration of the omega block will be in a different geographic location and less extreme than what we saw in mid-April.
Here's how it works: an area of low pressure aloft will anchor itself in the East by midweek while another separate area of low pressure develops on the West Coast. In between will be a ridge of high pressure, or a northward bulge in the jet stream near the Plains and Rockies. Now, notice how the jet stream pattern – that ribbon of fast-flowing air some 30,000 feet above the ground - described above and depicted in the image to the right resembles the greek letter omega shown here: Ω.
The term omega block is not new — it's often first taught in basic meteorology courses. Since the jet stream is in such an exaggerated north to south alignment, this will result in the weather pattern moving very slowly for a time, which is where the term block comes into play. Essentially, the weather pattern is all clogged up which prevents weather systems from progressing at a steady pace from west to east like they normally do.
Here's a closer look at four changes we are watching due to this large-scale pattern change this week.

1.) Wetter, Cool Northeast

Jet-stream pattern in place late in the week, featuring a deep, closed upper-level low in the East.

It's been a dry spring so far in most of the East, but the pattern change will flip that around the first full week of May.
First up, throughTuesday, waves of low pressure and upper-air disturbances will bring periods of rain to the Northeast.
(MAPS: 7-Day Weekly Planner)
Then, a sharp plunge of the jet stream will nosedive out of central Canada and form a slow-moving gyre of low pressure over the Northeast late in the week into the Mother's Day holiday weekend.
It's not yet clear whether this late-week system will produce heavy rainfall in any locations, but persistent clouds and periods of rain look to be in the cards much of the week.
(FORECASTS: NYC | Boston | Philly | Wash. D.C. | Syracuse)
If any area of low pressure at the surface is able to strengthen off the East Coast, that would enhance the heavy rain threat. Again, that is not clear at this point.
Temperatures will also stay cooler than average through much of this week in the East. Highs will only be in the 50s and 60s from the Great Lakes into the Northeast.

2.) Drier Days Ahead For South-Central States

This Week's Forecast

The threat of severe thunderstorms and flash flooding will continue into Monday in parts of the South. This next round comes after storms have already brought severe weather to the region last week.
Thunderstorms are likely, especially toward the Gulf Coast, but the chance for severe thunderstorms will decrease.
By midweek, the risk of showers and thunderstorms will be confined to the Southeast. High pressure will move into the Plains, bringing a welcomed stretch of dry weather to the south-central U.S. starting Tuesday.
This is good news for the water-logged areas of the South, as this weather pattern will give areas the opportunity to dry out a bit and clean up any storm damage.
(MORE: Deadly Flooding Strikes Texas)

3.) Spring Returns to the South

Forecast Highs Early Next Week

Portions of the South have enjoyed a taste of summer this past week due to high and low temperatures that have been 10 to 15 degrees above average.
Highs have soared into the 80s and even a few lower 90s this week and low temperatures have only dipped down into the 60s and 70s, serving as a reminder that summer is right around the corner. These warm temperatures have been courtesy of high pressure, which has allowed a southerly flow to bring the warmer conditions into the South.
The warm temperatures have begun to come to an end, however, as the ridge of high pressure is sliding eastward and an area of low pressure is approaching from the west. The result will be slightly cooler temperatures, along with the chance of showers and thunderstorms.
(FORECASTS: Atlanta | Charleston, South Carolina | Jackson, Mississippi)
Temperatures will be closer to average for early May, but for many areas the temperature difference will be 10 to 15 degrees compared to the end of this past week. Highs this week will typically be in the 70s by Tuesday, with 80s toward the Gulf Coast. Lows will also be slightly cooler with temperatures dropping into the 50s for much of the South, with 60s toward the Gulf Coast and Florida.

4.) Warmer Days Ahead in the Rockies

Forecast Highs Compared to Average

Another area of the country that will see some of the big changes this week are the Rockies.
An area of low pressure brought snow to Rockies and parts of the High Plains to end April. Parts of western Nebraska and the Colorado Rockies saw near a foot of snow.
Snow showers may linger through Sunday night in parts of Colorado and northern New Mexico. Accompanying the snow will be chilly temperatures. High temperatures will be up to 30 degrees below average into early week in the Rockies and portions of the adjacent High Plains.
Mid-to-late week, the upper-level trough will be replaced by an upper-level ridge of high pressure. This will translate into warmer and drier conditions for the region.
(MAPS: 10-Day Forecast)
High temperatures will be up to 20 degrees above average by late this week in the Rockies and northern Plains. Temperatures are expected to climb into the 80s as far north as Montana. Denver will go from seeing highs in the 40s on Sunday to highs near 80 by Thursday.
MORE: Severe Weather April 26-27, 2016 (PHOTOS)