September 18, 2023 – Boulder Cast

Some disorders require low chances of shower initiation

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The first few days of the week will see some shortwave disturbances tracking west to east across the northern Rockies. The hills from the weekend will slide to the south and allow somewhat wetter areas to flow through. Integral to this flow will be some shortwave energy passing through Colorado on Monday (shown below) and Tuesday.

This week’s GEFS precipitation forecast shows us the chances of rain or storms on Monday and Tuesday that are directly related to normal shortwaves in zonal flow. We also see continued but lower rain chances for the rest of the week – more on that in a moment!

The figure below shows energy tracking eastward on Tuesday afternoon and evening. You’ll see very little change starting Monday, although a trough over Montana and Idaho will help provide more lift across the Front Range. As such, rain chances are expected to be higher on Tuesday compared to Monday.

The NAM rainfall forecast for Tuesday shows shower activity developing first across the high terrain and then reaching the plains later in the day into the evening. In general, rain chances at the beginning of the week will be at the low end of the spectrum, only about 10 to 30%. Any potential storms are not expected to be severe given weak shear and low instability across the plains.

Temperatures on Monday and Tuesday, and actually most of the week, will hover around 80 degrees. There will largely be a lee bottom most of the week, favoring an overall downhill flow pattern east of the mountains. This is one reason why our rain chances will be low at the beginning of the week because that will promote sinking and drying of the air.

Most active late in the week – but rain chances uncertain

As we head into the middle and latter part of the week, the hills will turn north on Wednesday. When that happens, the disturbances will move slightly to the north and should keep us largely dry until midweek. However, late in the week, the ECMWF, GFS and other modeling trends showed a deep trough that will follow eastward Thursday into Saturday. As usual, the ECMWF and GFS have not agreed on its location, timing and strength at this point. The ECMWF shows a deep trough over northern California on Thursday and reaching western Wyoming on Friday. The GFS shows a more elongated low on Thursday, centered in eastern Oregon, and tracking east into eastern Wyoming later Friday.

The only similarity between the models so far is that a strong cold front will likely persist sometime through Friday or Friday night (see below). Before that happens, models show a deeper wave of moisture rising from Mexico on Thursday. By Friday, most of the guidance dries out as the cold front slides east. This frontal corridor may combine with a tighter elevation gradient to enhance dry and gusty winds Friday afternoon or night, potentially increasing fire danger to the state. Some signs also indicate the development of a mountain wave with strong downward winds behind this front.

As for rain chances, confidence is not very high about when and if we will see measurable rain. First, the wide flow slope on the roof may limit shower coverage with this system. Second, the current path will favor areas east and north of Colorado. Right now, it looks like Thursday will be our best chance for rain late in the week, albeit still on the low end at around 20%.

While we are still hopeful that most of our area will see at least one spell of rain this week, we are not overly optimistic. Even the wettest days won’t affect everyone. With that in mind, enjoy the continued warm weather in the days ahead – stretches in the 80s like this are sure to be on borrowed time! The first day of fall is Saturday…

Next weekend should be somewhat cooler than normal behind the aforementioned cold front. Look for highs to “drop” back into the lower 70s this weekend as sunshine becomes increasingly widespread. Enjoy!

Forecast details:

Monday: Partly sunny skies give way to increasing clouds and a 10% chance of rain or light showers. Highs are in the low to mid 80s on the plains and lower 70s in the hills.

Tuesday: Partly cloudy with a 20-30% chance of rain and storms with high temperatures in the upper 70s on the Plains and upper 60s in the Hills.

Wednesday: The weather is mostly sunny and largely dry, with high temperatures in the 80s on the plains and 70s in the foothills. There may be a stray shower or two late in the day with little consequence.

Thursday: Partly cloudy with a 20% chance of rain or storms. Highs are in the low to mid 80s on the plains and lower 70s in the hills.

Friday: The weather is mostly sunny, windy and mild with a 10% chance of rain. Highs near 80 on the Plains and upper 60s in the Hills. The downslope wind event may unfold Friday evening or night.

weekend: Dry behind a departing storm system, but cooler. Look for highs in the lower 70s on Saturday and Sunday with mostly sunny skies.

Disclaimer: This weekly 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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dew

Andrew was born and raised in St. Louis, and earned his Ph.D. He received his PhD in Atmospheric Sciences from the University of Colorado in 2015. From 2015 to 2020, he worked remotely in Boulder as an atmospheric scientist with the National Oceanic and Atmospheric Administration’s (NOAA) Atlantic and Meteorological Laboratory in Miami. Andy is now a full-time meteorologist.

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