This week will feature a continuation of the dangerous, unprecedented heatwave in the Pacific Northwest, but cool and rainy weather in the Front Range. Go figure! Our temperatures will remain below normal through the week with daily chances for showers and thunderstorms, helping to alleviate but not remove wildfire concerns across the state. We also discuss the status of monsoon season, the active tropical eastern Pacific, and what may lie ahead for the Fourth of July weekend.

This week’s highlights include:

  • The historic heatwave continues in the Pacific Northwest while unseasonably cool and rainy weather lingers in the Front Range
  • No, this is not the monsoon yet, but it is looming!
  • Temperatures rise from the lower 70’s Monday into the lower 80’s by Friday
  • Rain chances each and every day!
  • June 2021 will conclude with a prolonged cold stretch for us after beginning as one of the hottest

DISCLAIMER: This weekly outlook forecast is created Monday morning and covers the entire upcoming week. Accuracy will decrease as the week progresses as this post is NOT updated. To receive daily updated forecasts from our team, subscribe to BoulderCAST Premium.

Weather Swap: Denver <-> Pacific Northwest

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he extreme and dangerous heatwave across the Pacific Northwest will rage on early this week while our area continues to feel like Seattle with cool weather and rain showers lingering. Many cities in Oregon, Washington and British Columbia obliterated their existing all-time record high temperatures on Sunday. Incredibly, those freshly minted records could get rebroken on Monday as the heatwave maximizes in many locations. Each “X” in the map below denotes a city that smashed an all-time record high temperature on Sunday! Portland (112°F) and Seattle (104°F) were among them.

The historic, omega blocking ridge responsible for this epic heatwave will slowly weaken and migrate eastward over the next few days (see below). In our neck of the woods, a troughy pattern has facilitated persistent northerly flow and frequent cold fronts which have kept conditions cool and rainy since Thursday of last week. Boulder has reported wet weather each of the last four days with nearly 2″ of total rainfall, a stark contrast to the almost bone-dry rest of June beforehand.

It’s been so rainy that many of you have reached out to us asking if this recent cool/wet stretch we’ve been experiencing is the summer monsoon getting started already. Sorry to disappoint, but no this is not the monsoon pattern yet. While subtropical moisture has been flowing into the Desert Southwest and Colorado at times of late, this is only in response to the highly-amplified ridge/trough pattern embedded over North America right now. Remember that the monsoon is a seasonal shift in the winds that slowly develops over time. A record-shattering ridge in southwest Canada is not part of the monsoon equation. As further evidence, it is largely agreed upon that monsoon season’s official start is when the average daily dew point in Tucson Arizona is at least 54 degrees for three consecutive days. As you can see below, there were two days in a row above this threshold last week, but things quickly dried out as the moisture waned.

With that said, it’s likely that the monsoon could get going a little bit sooner than normal later this week as moisture resurges into the southwestern United States. As mentioned earlier, a single weather event cannot bring on the monsoon – it is a large scale persistent pattern. However, Hurricane Enrique which is currently located off the western coast of Mexico, will help things along by drawing a huge batch of moisture northward into Arizona during the middle to latter part of the week ahead. Enrique was a Category 2 hurricane recently, but is expected to weaken in the coming days as it heads over cooler waters and landfalls in Baja California.

Infrared satellite image from early Monday morning of Hurricane Enrique off the coast of Mexico in the Pacific.

Looking at the forecast for Tucson this week, it’s clear something is up with the monsoon especially Wednesday and beyond. This desert area receives almost no rain from April through the beginning of the monsoon. Things are looking good for the official beginning of monsoon season later this week in southeast Arizona. It’s unclear if this initial surge of monsoon moisture will ever reach Colorado, but it is a good sign of the seasonal change about to take shape.

Source: NWS Tucson

As the week unfolds, temperatures will slowly decline across the Pacific Northwest (but still remain incredibly hot) while they increase marginally each day for the Denver Metro area. We’ll be starting out on Monday much like the past weekend in the lower 70’s. We then rise into the upper 70’s by midweek and warm further into the lower 80’s by Friday. Talk about comfortable temperatures! Friday will mark the 9th consecutive day of below normal conditions….a stark contrast to the record warmth we saw during the first three weeks of June!

Rain showers and thunderstorms will be in the forecast each and every day this week, particularly Wednesday and Thursday. For the most part these will be late-day rain chances, but given the moisture and lingering trough/upslope across eastern Colorado, overnight and morning rainfall can’t be entirely ruled out either on some days. Even with the scattered nature of the showers, most of us should actually see rainfall two or three days this week, if not more. At this point, the severe threat is low to non-existent this week for us, which is good news!

GFS ensemble precipitation plumes forecast for this week. Daily rain chances are expected, generally late in the day.

Given the setup, the central and southern Mountains will do quite well, better than Denver even. The map below shows the GFS model forecast precipitation totals through Friday night. These rains should help with the ongoing fires and the drought to some extent. However, the most extreme drought conditions in northwest Colorado will not find any relief this week.

Total precipitation forecast through Friday night. Lots of rain expected in the central and southern Mountains.

The holiday weekend ahead will see continued but declining storm chances with temperatures generally in the 80’s across the Plains. If you’re planning to recreate in the Mountains over the Fourth, expect daily chances of scattered thunderstorms, with the activity being more isolated around Boulder and Denver. Good news for those of you sticking close to home for cookouts and firework displays!

RELATED POST:
It's Lightning Awareness Week! Find out why Colorado is one of the most deadly states for lightning

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Forecast Specifics:

Monday: Morning sun, then increasing clouds with scattered afternoon and evening showers and storms. Cool with temperatures in the lower 70’s on the Plains and in the lower 60’s in the Foothills.

Tuesday: Still cool with increasing clouds and scattered late-day showers/storms. Highs in the middle 70’s on the Plains and middle 60’s in the Foothills.

Wednesday: Mostly cloudy with rain showers possible in the morning, then numerous showers and thunderstorms in the afternoon and evening. Temperatures close to 80 on the Plains with upper 60’s in the Foothills.

Thursday: Late-day storms likely with highs near 80 degrees across the Plains and upper 60’s in the Foothills.

Friday: A mix of clouds and sun with scattered late-day storms. Highs in the lower to middle 80’s across the Plains and lower 70’s in the Foothills.

Mountains:  It will be a stormy week in the Mountains with daily chances of afternoon and evening thunderstorms. Given the unseasonably cool temperatures, these showers will mix with snow above 12,000 feet elevation with even light accumulations possible on the highest peaks, especially Monday and Tuesday. As always, plan any Mountain activities with dangerous late-day lightning in mind!

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Ben Castellani

Ben grew up in southwestern Pennsylvania and holds both a bachelor's and a master's degree in Meteorology, the latter being from CU Boulder. His hometown received nearly three feet of snow from the Storm of the Century back in March of 1993, sparking his initial interest in impactful weather. Ben currently works on remote sensing and data analysis software at L3Harris Technologies in Boulder.

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