CNN  — 

Halloween weekend looks mostly warm across the US, but for the areas with rain in the forecast, trick-or-treaters will be saying “boo-hoo” instead of “boo.”

It all begins this weekend with a storm system along the Gulf Coast.

“A low pressure system will bring a quick round of heavy rain and severe thunderstorms across the lower Mississippi Valley,” said the Weather Prediction Center. “The low pressure system will then spread unsettled weather into the eastern US Sunday and into Monday.”

We are also watching a new atmospheric river event that will push into the Pacific Northwest spreading rain and high-elevation snow on Sunday.

If you have some early Halloween events to attend, or if you are going to some fall festivals this weekend, you may want to have an umbrella close by.

Stormy Saturday for Southeast

Saturday, some much-needed rain moves across the Lower Mississippi River Valley. These drought-stricken states have been dealing with historically low water levels along the river for weeks now, and could use the beneficial moisture.

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Memphis, Little Rock, and Jackson, Mississippi, are all currently in various levels of drought and are forecast to receive a total of 1 to 3 inches of rain through the weekend.

The downfall is that there is also a chance of severe storms Saturday afternoon and evening for cities such as New Orleans, Biloxi, Mississippi, and Mobile, Alabama.

The main threats will be damaging winds, tornadoes, and flash flooding.

“Rainfall rates with the strongest storms could exceed 2 inches/hour,” the National Weather Service office in New Orleans said. “A couple of hours of this over one location, especially an urban one, could cause problems, even though we’ve had next to no rain for the last 6 weeks in most of the area.”

New atmospheric river arrives Sunday

On Sunday, the system along the Gulf Coast pushes northward into the Ohio and Tennessee River Valleys.

Cincinnati, Indianapolis, Nashville, and Knoxville all have high-end rainfall chances where up to 2 inches of rain is expected the latter half of the weekend.

On the other side of the country, weather conditions are expected to deteriorate quickly for the Pacific Northwest as a new system pushes onshore.

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“The Weather Prediction Center is forecasting 3-6 inches of precipitation over the Coast Ranges of Washington and Oregon and the northern Cascades over the next 7 days,” according to the Center for Western Weather and Water Extremes. “Precipitation associated with these atmospheric rivers will help to improve severe drought conditions in the northern Coast Ranges and Cascades.”

Another benefit to the atmospheric river is that it may bring precipitation to the Willamette National Forest in Oregon and help with firefighting efforts at the Cedar Creek Fire.

Soggy trick-or-treat forecast

If you live near the central and southern Appalachians, or in the Pacific Northwest, you may need to have your umbrella handy on Monday.

“On Halloween Day, the wettest areas will be over portions of the Pacific Northwest, as well as across the Ohio Valley, Southeast, & Mid-Atlantic,” the Weather Prediction Center said.

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While rain is in the forecast on Halloween for Atlanta and Asheville, North Carolina, the good news is most of that rain clears out by the start of trick-or-treating.

Unfortunately, the opposite is true for cities like Philadelphia and Baltimore, where rain chances only increase as the day goes on.

It may not be a bad idea to have an umbrella handy if you live in Pittsburgh, Cleveland, or Roanoke, Virginia.

Snow will also be making a “spooky” appearance in the Cascade and Olympic Mountains for Halloween.

While snow levels start off relatively high on Sunday morning around 8,000 feet, by the time we get to Monday afternoon those snow levels drop to less than 4,000 feet. By Monday night, snowfall totals could reach 4 to 6 inches.

Another component of this system will be the increasing winds which will peak on Monday as the system shifts to the south. Wind gusts up to 40 mph across the Northwest will be common.

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“The majority of the US mainland should enjoy tranquil weather conditions for this weekend into Monday,” the Weather Prediction Center said. “Above-average high temperatures are forecast to span the Northern Tier from the Plains to New England, with highs into the 50s and 60s as we head into Halloween.”

CNN Meteorologist Haley Brink contributed to this story