A powerful Arctic air mass is surging southeast, leaving the Plains and Intermountain West squarely in the throes of the icy tongue of air. Temperatures dipped to nearly minus-35 in Utah, with a number of other locales set to plunge below zero toward the mid- and late week. Meanwhile, a strong storm will develop along the chilly air’s leading edge, bringing a soggy Halloween to the East.

The setup

The uncooperative weather — which is all tricks and no treats — is thanks to a serpentine jet stream, a river of strong winds in the upper atmosphere. Nudged north by a bubble of warmth over Alaska, the jet then collapses south, allowing a pool of cold air to spill across the Canadian border and into the Lower 48. Long-standing record cold temperatures could fall in a number of spots, with snow a likelihood in some areas, as well.

The mercury bottomed out at a bone-chilling minus-34.7 degrees in Peter Sinks, Utah, on Monday morning before rebounding more than 30 degrees in 30 minutes as the winds kicked up and mixed out the cold. Peter Sinks is a notorious spot for achieving some obscenely low temperatures on calm, clear nights. It’s quite high, at an elevation of around 8,000 feet.

“It’s essentially a high-elevation valley,” explained Mike Seaman, lead forecaster at the National Weather Service in Salt Lake City. “It’s basically a big hole that the cold drains into. On cold, dry cloudless nights, [any remaining heat] radiates out.” A nearby thermometer located just 250 feet higher failed to drop below zero.

This appears to be the lowest-ever October reading across the Lower 48, but it will not go into the books as breaking any records. “It’s not a long-running official climate station, so it’s not used for records,” said Seaman.

The cold

That bitterly cold reading is just the tip of the iceberg. As the cold sloshes down, it will expand east — covering the Plains on Wednesday, the Mississippi and Ohio valleys on Thursday, and the East Coast late Friday and into Saturday. Temperatures will bottom out near record lows over a wide swath of real estate.

After up to a foot of snow through Wednesday, Denver’s temperature may plummet to near zero Thursday morning. That could threaten Denver’s coldest October temperature on record, a minus-2 reading from 1917. If the current forecast holds, however, it would still be Denver’s coldest October temperature in over 100 years.

Temperatures in the Mile High City dropped below freezing at 1 a.m. Sunday morning, and they probably won’t climb back above the freezing mark again until Halloween afternoon at the earliest. It will likely wind up as Denver’s coldest four-day stretch on record for the month of October.

Denver’s had a rough go of it lately. A widespread four to eight inches of snow fell across the metro area on Sunday and Monday, shuttering businesses and delaying the start time of the city’s public schools. A snowfall of 3.8 inches was measured at Denver International Airport. Depending on what happens with Wednesday’s storm, 2019 is likely to be just the third year in the past 50 in which Denver has seen more than 10 inches of snow in a single October.

It’s not just Colorado, though. Casper, Wyo., set an October record low of minus-4 Sunday evening.

The cold is on the move.

Oklahoma City won’t get out of the lower 40s until Friday; its average high for late October is 68 degrees. It’s a similar story in Kansas City, with rain and snow in the offing Wednesday/Thursday when typical late October highs peak around 60.

Temperatures in Chicago will run 15 degrees below normal, with a few flurries possibly mixing into the rain showers overnight Tuesday into Wednesday.

The cold even reaches the Gulf and East coasts on Friday; New Orleans will see highs around 62 or 63 degrees instead of 75, with overnight lows in the 40s.

The storm

Ahead of the impending cold, the Eastern Seaboard has been enjoying a rather balmy period of highs in the 70s, with anomalous warmth banked east of the Appalachians. Washington, D.C., saw its most humid October reading on record Sunday, when the dew point hit a sultry 74 degrees. But the approaching dome of cold is kicking that air mass away.

Along the interface, the clash of seasons will stir up a sprawling storm. Heavy rain will fall on the east side, starting over the Ozarks and the Tennessee Valley on Wednesday before spreading east into the mountains Thursday. Timing is a question for the Atlantic coastline, which will see increasing clouds during the day Thursday, with rain arriving from the west late in the day. There is a shot that, if the system slows down a tad, trick-or-treating could go off without a hitch. However, there is the distinct possibility of a waterlogged late evening and night.