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Cold April weather pattern means more snow, fewer tornadoes over Lower 48

Six- to 10-day temperature outlook from the National Weather Service. (NOAA)
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From the Rockies to the East Coast, the normal spring progression toward warmer weather is set to suffer a serious setback, as chilly weather settles in over the next six to 10 days.

In some places, including the Rockies and the interior Northeast, wet snow is expected over the next several days, while most areas in between endure chilly temperatures that more resemble March than April.

On the plus side, warm, humid and unstable air over the Plains and South, which acts as fuel for severe thunderstorms and tornadoes, will be notably absent.

The cool April pattern could last through the end of the month, suppressing severe weather but prolonging wintry weather, especially in northern and mountainous areas over the eastern two-thirds of the nation.

Some of the cold entering the Lower 48 can be traced back to a record-setting blast of Arctic air that gripped Alaska last week, linked to the springtime disintegration of the polar vortex.

The West Coast, however, will be mostly left out of the cool regime. Very warm and dry conditions are anticipated west of the Rockies, worsening drought conditions.

Snow in the Rockies

Starting Tuesday and continuing through Friday, as cold air pours south through Montana, Wyoming and Colorado while a disturbance drops into the region from the Pacific Northwest, snow — heavy at times — is expected to fall in waves.

The National Weather Service has already issued winter weather advisories and winter storm warnings in parts of southern Wyoming. In the South Laramie Range, the Weather Service is calling for up to 10 to 14 inches of snow and winds gusting as high as 35 mph. Snow is expected to begin Tuesday but probably will not pile up until late Wednesday, when colder air and heavier precipitation becomes established. The Weather Service anticipates extending winter weather advisories into southeast Wyoming and western Nebraska Thursday and Friday, including for the area around Cheyenne and Laramie.

Into Colorado, including Denver, Boulder and Fort Collins, rain and snow showers are forecast intermittently through Thursday, but generally light amounts are expected. Weather5280.com, a website for Denver forecast information, calls for a trace to two inches through Wednesday morning in the Mile High City. After the chance of more intermittent rain and snow showers into the day Thursday, the Weather Service writes “there is a good chance much of our forecast area receives a period of moderate to heavy snow Thursday night,” with one to five inches in lower elevations and higher amounts in the mountains.

Denver averages about six inches of snow in April, so this latest bout is not terribly unusual. That said, it comes after an exceptionally snowy March when 34 inches fell, second most on record.

Temperatures over much of the Rockies are forecast to average about 10 to 20 degrees below normal for the next week.

Snow in the Northeast

At the same time snow blankets the Rockies, a disturbance dropping into the Northeast containing very chilly air will deliver wintry weather in interior portions of the Northeast and New England. A wintry mix of precipitation is anticipated Thursday and Friday in western New York as well as Vermont and western Massachusetts.

The Weather Service office in Buffalo expects rain to change to wet snow in the Adirondacks and Tug Hill region in western New York Thursday night “with a light slushy accumulation not out of the question by early Friday morning.”

In Vermont, the Weather Service in Burlington writes that there is “the potential for some decent higher elevation snow across the southern Greens [mountains],” while the office in Taunton, Mass., notes that some heavy, wet snow could also fall in the Berkshires depending on where the disturbance tracks.

While snow is not expected near the coast, Boston, Providence, R.I., and Portland, Maine, can expect a cold, wind-driven rain late Thursday into Friday, with high temperatures in the 40s and winds gusting to around 30 mph.

Dearth of severe thunderstorms, tornadoes in the Plains and South

The overarching pattern is also one that is unfavorable for severe weather on the Plains, at least into the forecastable future. The jet stream pattern, which features ridging, or a northward jaunt, in the West and a prominent dip in the East, will induce persistent northwesterly flow over the central United States. That will keep dry air and cooler than average temperatures over the eastern two-thirds of the Lower 48, reinforced by scouring cold fronts that will surge all the way to the Gulf Coast.

Instability, or the warm, moisture-laden fuel needed to spark thunderstorms, will be hard to come by. That should cut down markedly on precipitation chances across the central and northern Plains. Any disturbances embedded in the jet stream will dive south and east with comparatively little fanfare before encountering moisture and producing downpours along the Gulf Coast. That is where the best chance for above-average rainfall will be in the next week to 10 days.

Dry conditions will continue to prevail over the Plains late in the month.

April is usually the second-most-active month for tornadoes on the Plains, with Texas, Oklahoma, Kansas and Nebraska averaging more than five dozen total. This April will inevitably come up dramatically short, much to the relief of residents but to the chagrin of storm chasers.

That is not to say that localized batches of severe weather are impossible — especially from East Texas through southern Louisiana, Mississippi, Alabama and the Florida Panhandle over the next week. It just appears that the broad-scale pattern will not be conducive to more regional- or national-scale outbreaks that are more typically seen this time of year.

There are signs that, by mid- to late next week, the weather pattern may allow periodic influxes of mild, more humid air over the southern Plains and Mississippi Valley. That could reawaken the severe weather threat toward the end of April.

Some very coarse long-range models hint at a pattern reversal into early May that could bring more typical conditions and seasonal severe weather chances to much of the central Lower 48.

Warm and dry in the West

The configuration of weather patterns, shaped by the jet stream, means that while the eastern two-thirds of the nation is unusually chilly, the West will be rather mild.

From the Pacific Northwest to Southern California, temperatures are predicted to be about five to 10 degrees warmer than normal for the next week.

More significant than warmth, however, will be the lack of rain. The Weather Service Climate Prediction Center’s 6- to 10-day and 8- to 14-day outlooks both call for below-normal precipitation throughout the West.

This means drought conditions will persist and, in some areas, worsen, further increasing the wildfire risk as the even hotter, drier summer months approach.

Drought-plagued California and western U.S. may see another devastating fire season

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