September 11, 2023 – Boulder Cast

It dries quickly at the beginning of the week

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The seemingly endless summer days we endured at the beginning of September are now behind us with cooler weather waiting for us for next week. This shift in pattern began with a cold front moving in last Saturday evening. Since then, a sloping runoff has been identified across eastern Colorado bringing low clouds, fog, cooler temperatures, poor air quality, and, at times, actual raindrops falling from the sky – a welcome change indeed!

After a cold front at night, low clouds and Canadian smoke hang in there #boulder. Seeing some fog sometimes! 🌫️ #cow pic.twitter.com/tqUSak5MFG

— Boulder CAST Weather (@BoulderCAST) September 10, 2023

Rainfall totals fell overnight into Monday morning between 0.25 and 1.00″ in the Boulder-Denver metro area, with the highest totals coming from around Longmont, Lyons and Niwot, which were hit by a weak thunderstorm Sunday evening.

24-hour precipitation totals end ~7 a.m. MT Monday 11/9/23

The highest elevations saw snow accumulation overnight, the first white coat since early June! This is the view from Rocky Mountain National Park this morning (about 12,000 feet):

Likewise, the view is also wintry from the 14,200-foot summit of Pikes Peak, which received more significant accumulation from this departing storm:

As of early Monday morning, the bulk of the wet weather has shifted south and east of the Denver metro area. Widespread light to moderate precipitation continues across the southeastern part of the state while drier air creeps in from the northwest fairly quickly – something that will contribute to a rapid drying trend for us early in the week.

The pattern that appeared Monday across the west is complex with several short wave disturbances and even a weak ridge embedded in the westerly-northwest flow (see below). The system that brought rain Sunday night is departing to the southeast with a ridge now moving in from the northwest. With clouds remaining for much of Monday and stable skies, the arrival of the ridge will only help keep things under control over the Front Range. A weak secondary short wave will come ashore in Northern California on Monday and reach Colorado by Tuesday. Current model trends take this small-scale system south of our area with more rain falling in areas south of Denver, but little to no rain is expected in our immediate area.

The best chance of rain on Monday actually comes in the pre-dawn hours (but these rains have already cleared the area as of this writing) with a slight chance of redeveloping some popcorn showers in the afternoon and early evening. Additional rain falling late Monday is expected to remain confined to the hillsides where there is some instability, especially southwest of Denver. Monday’s highs will be well below seasonal averages in the mid-60s – we should be near 80 degrees this time of year!

A weak system passing to the south on Tuesday could bring some rain or isolated storms along and south of the Palmer area. For the most part, Tuesday will be a pleasant day in the Boulder-Denver area with temperatures back into the mid-70s. In fact, as you’ll soon see, Tuesday will be the most beautiful day of the week!

Wet weather returns mid to late week

As we move into the middle part into the latter part of the week, a wide, slow-moving area of ​​low pressure is drilling across the central and northern Rocky Mountains. It’s not a huge bottom by any means, but it will keep things cool and choppy across the front range from Wednesday through Thursday and potentially Friday as well. Below is a view of the forecast for the EC Rising Anomaly of 500MB from Wednesday to Friday. Behind our soft bottom, a more significant ridge is developing across the Pacific Northwest – something that will be a player for us this coming weekend.

Although we still have several days to go, the models are coming to better agreement with how this system will develop late in the week, keeping it mostly as an open wave across the region. Previous model runs were showing potentially lower cutting pressures which would have had more wetter effects. This seems increasingly unlikely now. However, we expect a significant increase in precipitation chances starting Wednesday in the form of scattered thunderstorms late in the day with additional Pacific moisture from the west before bottoming out.

From Thursday to Friday, a slow moving low will be placed directly over Colorado making these the most turbulent days of the week. GFS in particular, as seen below, stops and lengthens the roughly west-to-east bottom through the frontal range on Friday. The EUR is moving things a bit faster but both models have cold and rainy weather on both days.

However, Thursday appears to have the best chance of rain this week with scattered to widespread thunderstorms expected late in the day with temperatures dropping to close to 70°C. Friday would also be a very good opportunity for a shower besides ending up being the coldest day of the week. Both the GFS and Euro models dovetail well with highs struggling to reach 60°F on Friday with the arrival of a minor cold front ahead of the deep northerly flow.

GFS precipitation averages about 0.75″ in Boulder in the back half of the week (Wednesday through Saturday), with a clear focus on late Thursday for the heaviest precipitation and best overall chances for wet weather.

Plan for Tuesday to be the nicest day of the week, although Monday won’t be bad either. After a short warm-up midweek, things got ugly again. Yes, the late week will be cold and wet, which is typical for us this time of year. Fortunately, unlike the record-breaking September 2020 storm, it won’t be cold enough for snow just yet! Enjoy the weather!

The Great Colorado Flood: 10 years ago this week

Ten years ago this week, torrential rains fell across the Front Range, resulting in one of the worst floods in Colorado’s short recorded history, accompanied by a staggering repair bill of more than $3 billion. Boulder County was at the center of the disaster, receiving more than a year’s rainfall in just one week. The road to recovery has been long and arduous, and surprisingly, it continues today. We take a look at this historic flood event, explain how it happened, and provide an update on relevant analysis that has been done since. Read the full post here.

Forecast details:

Monday: There is a slight chance of showers in the morning as the low clouds clear to reveal some sunshine in the afternoon. Scattered thundershowers may develop late in the day, especially across higher terrain southwest of Denver. Most of us stay completely dry after the morning blues lift. Highs reach the mid 60s on the plains with mid 50s in the hills.

Tuesday: It’s partly sunny on the most pleasant day of the week. One or two storms may develop south of Denver along the Palmer Barrier. Temperatures remain cool in the mid 70s for highs on the plains with lows in the 60s in the hills.

Wednesday: The sun will rise early, then clouds will increase with scattered thunderstorms late in the day. Highs are in the middle to upper 70s across the Plains with mid 60s in the Hills.

Thursday: It will be cold with multiple to widespread showers and some thunderstorms expected. Highs will be around 70 degrees on the plains with upper 50s in the hills.

Friday: Staying gloomy with sporadic showers and frigid temperatures only hovering in the high 60s at best. Some places may stay in the 50s. Highs in the ridge will be in the lower 50s.

Disclaimer: This weekly forecast forecast is generated on 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, among many other perks, subscribe to BoulderCAST beloved.

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Ben Castellani

Ben grew up in southwestern Pennsylvania and earned his bachelor’s and master’s degrees in meteorology, the latter from the University of California, Boulder. His city received nearly three feet of snow from the storm of the century in March of 1993, sparking his initial interest in impactful weather. Ben is currently working on the remote sensing and data analysis program at L3Harris Geospatial Solutions in Boulder.

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