Labor Day Weekend forecast is decent, but uncertain

We provide the forecast for the upcoming holiday weekend, one which is not a simple as you might expect. Right now, model agreement is rather poor in regards to the finer details this weekend. Unfortunately, this will prevent us from issuing confident specifics about the weekend forecast this far out.

Let’s take a step back. Here is what the models do concur on…

As has been the case much of this week already, Colorado will continue to sit between a trough in the Pacific Northwest and high pressure to our south through the holiday weekend. As you can see in the 500 mb height map below for Friday, this set-up will result in a broad-scale west-southwest flow pattern across Colorado. Based on this general pattern, we expect temperatures through the weekend will be near normal and somewhat calm across the state. Moisture will ever-so-slightly be increasing with each passing day, too.

GFS 500 mb vorticity map for Friday

And now, the cause of the uncertainty…

While the main jet stream is hugging the United States and Canada border, a weaker secondary jet streak is wrapped around the northern end of the high pressure off the California coast. The left-exit region of this jet streak is predicted to spawn numerous small-scale disturbances from Thursday all the way into the middle of next week. The 250 mb diagram below shows this quite well. The time-stamp of this forecast map is Sunday evening.

GFS 250 mb wind map for Sunday

Model guidance has been really struggling to resolve the frequency and timing of these small disturbances. Seemingly every model and new run comes in with differing results, leading to a tricky forecast. Each small wave will slightly enhance atmospheric lift across Colorado and if the timing lines up with peak daytime heating, some areas could see decent chances for showers and thunderstorms, especially across the Mountains.

With this uncertainty in mind, we’re thinking that there really won’t be a “bad” day anywhere across Colorado this weekend. If we had to place a bet on the wettest day, that would probably be Saturday when a potentially stronger disturbance is looking somewhat likely….

Here are the forecast details through Monday….

Friday: We’re confident that the first small disturbance will traverse across Colorado during the day Friday with a weak cold front moving into northeast Colorado during the afternoon or evening. Moisture is lacking, but the timing of the wave is fairly good. A couple of isolated high-base thunderstorms may form across the Mountains and lower elevations. Temperatures will probably remain just shy of 90 degrees around Denver.

GFS 800 mb temperature map for Friday evening.

Saturday: There is decent agreement in the models on another, slightly more potent wave for Saturday afternoon/evening. This should produce scattered afternoon and evening storms for much of Colorado. Temperatures will be cooler east of the Continental Divide following the cold front…highs in the upper 70’s to lower 80’s.

Sunday: A chance of a new, but weaker, disturbance means another round of late-day isolated storms for the Plains, with possibly scattered coverage across the Mountains. The reduction in upper level support suggests Sunday will be somewhat drier for our region. Temperatures warm back into the low to middle 80’s.

Monday: This is the day with the greatest uncertainty in our opinion. Some models show another disturbance moving across Colorado. Others keep the area mainly dry. Furthermore, the latest model trends favor southern Colorado this day. When the operational forecast models can’t decide on an solution, we often turn to the ensembles! The GFS ensemble temperature forecast has a range of 68 to 90 degrees for Monday, also trending with that larger uncertainty.

What’s up with Ensemble Member #12 predicting 68 degrees on Monday for a high? You can see what a major outlier that member is in the forecast panels below. We’ll also mention this particular member dumps 1 to 2″ of rain in Boulder on Monday. Let’s hope that doesn’t happen! For now, just note that the ensemble average forecast daytime high temperature for Monday is between 75 and 80 degrees in Boulder (bottom right map). This is the best forecast at this point.

GEFS forecast high temperature panels for each member for Monday, and the ensemble average (bottom right)

Overall, the weekend will be fairly nice in the immediate Denver area. While we can’t totally rule out late-day rain at all unfortunately, most of the time we will be dry. And of course, many areas may not see any raindrops at all given the lower percentages.

If you’re heading into the Mountains, expect at least isolated to scattered storm coverage each day. Saturday looks particularly brutal for central and southwestern Colorado. Consider the threat of heavy rain and lightning when making your High Country plans. SummitCAST has your covered, though!

Much of our team will be off-the-grid this weekend, so this is the last you’ll likely hear from us. Have a great and safe week’s end and Labor Day!

DISCLAIMER: This forecast was posted Thursday morning and covers the entire upcoming weekend. Accuracy will decrease as time progresses as this post is NOT updated. To receive daily updated forecasts, subscribe to BoulderCAST Premium.