Recent pattern flip has brought four noticeable changes

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Pattern Change Brings Several Changes Across Nation

You've probably noticed a change in your weather, more than just the change of the seasons.

In the West, the winter was warm and, of course, very dry. Now, cooler temperatures have developed, along with some unusual May rain and snow. On the other side of the country, those in New England, in particular, saw a very snowy and cold winter, which has now been replaced with warm and drier conditions.

(MORE: Summer Temperature Forecast)

Conditions during this past winter were influenced by the jet stream, which for most of the season kept a ridge of high pressure over the West, pushing the jet stream far to the north into Canada and a southward dip or trough in the East. This setup brought the record heat in the West, as well as the record snow to parts of New England.

Now, as we move forward through May, a change has occurred with a dip in the jet stream setting up over the West, allowing cooler conditions, as well as disturbances bringing rain and snow into the Southwest. Meanwhile, in the East, the jet stream has pushed northward, allowing warmer temperatures to surge into the Northeast and parts of the Midwest.

Let's take a look at four interesting aspects of this pattern reversal.

Cooler West

This past winter was persistently warm across the West, with above-average temperatures dominating the region. In fact, many cities experience their warmest winter on record when looking at the average temperature, factoring both the high and low temperatures for a given location. Las Vegas, Seattle, Portland, San Francisco and Salt Lake City all recorded their warmest winter.

(MORE: Warmest Winter)

However, the jet stream has shifted, with a trough now set up over the West. This has allowed cooler air to slide southward, bringing an end to the seemingly endless parade of record highs.

San Francisco, as of May 17, is about 1.5 degrees below average for their average temperature. The last time they recorded a month with a below-average temperature was October 2013, when temperatures were 1.7 degrees below average. This is also a big difference compared to this February when temperatures were more than 5 degrees above average.

Temperatures were below average last week, and they are expected to be at least slightly below average across most of the West through the week ahead. Phoenix, for example, saw a high of 76 degrees on Friday and only reached 75 degrees on Saturday.

Looking ahead, daytime highs are expected to be as much as 5 to 15 degrees below average through the first half of the week. Las Vegas will have highs in the mid-70s to low 80s to start the week, and the marine layer will keep Los Angeles in the mid-60s throughout the week.

Warm Temperatures in the East

After a winter that didn't seem to want to end, warm temperatures have finally returned to the East in May.

Boston finally reached 80 degrees on May 4, and this is after setting a new record for the most snow in a season this winter with 110.6 inches of snow falling. It was even chilly through April, with Houlton and Presque Isle, Maine, setting all-time monthly April record low temperatures with lows of 7 below zero and 4 below zero respectively.

(MORE: Worst and Best Cities of Winter 2014-2015)

As the calendar turned to May, however, the pattern has changed, with the jet stream sliding farther north.

Mother's Day weekend was very warm across much of the East, with highs well into the 80s. This past week, cooler temperatures returned, with low temperatures dropping into the 30s and 40s in portions of the Midwest and Northeast.

Except for coastal New England, the warm temperatures are making a comeback, and it will feel more like summer, especially in the mid-Atlantic. The warmth will last until Tuesday due to southwesterly flow ahead of a slow-moving cold front. Although the interior Northeast will cool down into the 50s and 60s on Wednesday, low 70s will continue to hug the East Coast from Boston to D.C. through Friday.

(MORE: Cooldown Incoming to the Midwest, Northeast)

Rain and Snow for Southern California and Arizona

The winter of 2014-2015 saw a lack of rain and snow in the drought-stricken West. The period from November-April is considered the "wet season", and it was unusually dry.

It was the driest January-April period on record for San Francisco, with just 2.89 inches falling. Los Angeles in the January-April time period received just under 3 inches of rain, which is more than 7 inches below average.

According to the latest Drought Monitor, 46.8 percent of California is experiencing exceptional drought conditions, which is the highest category of drought.

(MORE: Record Low Sierra Snowpack | Missing More Than 2 Years of Rain)

The good news is the recent change in the jet stream has brought rain and mountain snow to parts of southern California and the Southwest, as an unusually strong upper-level low was able to push farther south, whereas during the winter, the jet stream focused low pressure systems into the Pacific Northwest and Canada.

On Thursday, record rainfall was seen in southern California, including in San Diego, where it broke the record for the wettest day for the month of May with 1.63 inches falling. The previous record was 1.49 inches on May 8, 1977.

(FORECAST: Record California Rain)

Arizona is also benefiting from rain and snow this week. Phoenix measured 0.93 inches of rain on Friday, setting an all-time daily rainfall record for the month of May. More than 7 inches of snow was even reported in the Flagstaff, Arizona, area on Friday. Flagstaff also received more than 5 inches of snow the Friday and Saturday prior to Mother's Day, and on average, they see one day of snow in May.

Some additional rain is possible for northern California as well as much of Washington this week. Additional snow will also be likely in the Sierra through Friday, providing welcome precipitation to areas that are still suffering from extreme drought.

Drought Reduction in Texas

The May 12, 2015 drought monitor reported no exceptional drought in Texas for the first time since July 17, 2012. There has been a huge improvement in drought conditions in the southern Plains since last year. The drought monitor report released a year ago noted 20.7 percent of Texas in the exceptional drought category.

Most of eastern and southern Texas are no longer experiencing drought conditions at all, which is great news.

Recently, there has been plentiful rainfall and even flooding in the region, as the subtropical jet stream has brought moisture into the area. This is quite the change from the past several years.

(MORE: Texas Drought Improvement)

However, there is still severe and extreme drought conditions in portions of central Texas and the Texas Panhandle, and some reservoirs are still running at less than 10 percent of capacity.

More rain is expected in the southern Plains in the week ahead. Locally heavy rain and flash flooding is possible.

(FORECAST: Severe and Flooding Threat)

The pattern is not expected to change anytime soon, and the chance of rain and thunderstorms is expected to persist in the southern Plains through much of next week.

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