The first week of October will be busy as a series of systems trek through the state from the southwest. The first will be the remnants of Hurricane Rosa. The second will be a quick-moving cut-off system. These will lead to the chance of showers and a few thundershowers through the week. We’re also tracking unsettled weather for the weekend with both rain and snow for the state of Colorado.
Monday and Tuesday “influenced” by Rosa
Tropical Storm Rosa is expected to make landfall in Baja California Monday evening and then will move into Arizona on Tuesday. Despite Rosa being so far away, moisture and lift are already streaming northward into Colorado today. This will increase clouds across the area, as well as bring slight rain chances region-wide.
GFS 500 mb map for Monday morning
Yesterday’s frontal boundary has moved eastward and we’ll thus see temperatures return back into the mid to upper 70’s. Here’s the satellite imagery from GOES-East yesterday showing the persistent low clouds dissipating just before sunset as the upslope weakened across eastern Colorado.
Although Rosa will bring in more moisture into the region, the best chance of rain these next few days will reside across the High Country of western Colorado where moist southwest flow will interact with the terrain. The Metro area will be under a general downslope flow today and Tuesday, which will “dry out” some of the precipitation as it treks eastward from the Mountains. This is evident below from the GFS model, which depicts much of the rain and thundershowers over the western part of the state. Nevertheless, we do think an isolated thundershower this afternoon and evening is possible, and potentially again tomorrow. However, most of us in the Metro area won’t see much if any rainfall at all from Rosa, unfortunately. Her eventual track will be some 100 to 200 miles further west than we discussed last Friday,
GFS precipitation type today (left) and tomorrow (right)
Minimal rainfall is expected for our area, but closer to the landfall of Rosa, some desert areas could see 2 to 5″ of rain….nearly a year’s worth! Some parts of the San Juans could see 1″ of rain or more. Flash Flood Watches are in place for this region. Rosa’s airmass is very warm, so generally all precipitation will be rain, even on the highest peaks.
GFS total accumulated precipitation through Wednesday afternoon.
Even though precipitation chances are somewhat meager for our region, clouds today and tomorrow will be quite prevalent. Shown below is the 700 mb relative humidity for the next few days. Thanks to Rosa’s deep tropical moisture, lots of mid and high-level clouds will be overhead. This will inhibit temperatures from getting too warm despite the strong southwest flow, likely 70’s to near 80 through Tuesday. This is still some 5 to 10 degrees above average, by the way.
GFS 700 mb relative humidity today (left) and tomorrow (right)
Warm and dry (most of) Wednesday
On Wednesday, downslope flow will maximize as a trough axis separates the state with westerly flow fairly strong during the day. As a result, warm anomalies show up in our forecast (shown below). Wednesday will be warm, sunny, and dry with highs in the low to middle 80’s. This warm weather will be short lived – so take advantage while it lasts!
GFS 700 mb temperature anomaly on Wednesday
Rain and cooler to end the week
The better chance of rain for the week, at least at this point, appears to begin Wednesday night after dark. During this time, many of the weather models are showing a stronger cold front sliding through the area once again. Below shows that it will be parked across eastern Colorado, with east-southeasterly winds Thursday morning. As a result, highs should tumble – albeit given we’re a few days days out, temperatures are somewhat uncertain. Look for middle 50’s to lower 60’s.
GFS 800 mb temperature on Thursday morning
Thursday should be quite cloudy as another trough moves ashore and into Colorado during the day (below). This trough is more favorably oriented compared to Monday and Tuesday. With the trough and upslope flow at the surface, rain chances will be elevated late Wednesday night and early Thursday, at about 50%. This rain could fall in the form of drizzle or light showers.
GFS 500 mb absolute vorticity for Thursday
Another trough approaches Friday, albeit weaker looking than the one on Thursday. This coupled with the slowly retreating cold front from Thursday will likely end the week with partial sunshine, a slight chance of showers and temperatures remaining on the cooler side in the 60’s.
Weekend trending unsettled…
On the upcoming weekend, things are looking rather interesting to say the least. Both the global GFS and ECMWF models are showing a deep trough across the West. Below shows the 500 mb height anomalies for Saturday from the GFS and Sunday from the ECMWF models. Both are surprisingly similar, and there has been some consistency in the weekend forecast over the past day or two. Both models show a trough digging into the Four Corners region and persisting over the area for several days. As a result of this, our weather looks to turn unsettled, especially Sunday onward.
500 mb height anomalies Saturday (GFS) and Sunday (ECMWF)
If this pattern holds true, snow levels (below) over the weekend will dip down to about 8,000 feet for the Foothills and Plains in eastern Colorado. As a result, snow appears likely under this scenario over the Mountains, with rain and possibly a mix with snow in the Foothills.
GFS snow level for Saturday
As for the Plains, the chance of snow will be dependent on the exact track of the trough and how much cold air it can bring down from Wyoming. Several GFS ensemble members produce snow in Boulder, with many others showing rain.
GEFS model outlook for precipitation type this weekend and early next week
Nonetheless, this will be something to closely watch as the week goes on, given the slow moving nature of this potential trough, its position, and the cold air moving in. Overall, expect a colder and more unsettled weekend for the entire state.
GFS ensemble 500 mb height spaghetti plot for Sunday Oct 7
Forecast Specifics:
Monday: Lots of mid and high-level clouds during the day, with a slight chance of isolated thundershowers in the afternoon and evening. High temperatures in the mid to upper 70’s on the Plains and middle 60’s in the Foothills.
Tuesday: Overcast skies with isolated showers through the day, mainly across the higher elevations. Highs in the upper 70’s to near 80 for the Plains and upper 60’s in the Foothills.
Wednesday: Mostly sunny, dry, and warmer with highs in the low to middle 80’s on the Plains and low to middle 70’s in the Foothills. Scattered rain showers will be around after sunset into early Thursday morning with low clouds as well.
Thursday: Overcast in the morning with a chance of drizzle, then partly cloudy in the afternoon. Highs in the middle 60’s for the Plains and upper 50’s in the Foothills.
Friday: Partly cloudy with a slight chance of rain showers. Highs in the mid to upper 60’s on the Plains and upper 50’s in the Foothills.
Weekend: The upcoming weekend will trend colder and more unsettled, likely dipping into the 40’s with rain on the Plains and possibly a rain/snow mix in the Foothills.
High Country: Rain showers will be across the mountains today and tomorrow. Heavy downpours associated with the remnants of Hurricane Rosa could produce isolated flash flooding on the Western Slope. Dry and more mild weather is expected on Wednesday. Thursday and Friday turn unsettled again with clouds and rain showers likely. The upcoming weekend looks to showcase rain and snow chances as well, especially during the latter part of the weekend. Check out our SummitCAST page for 6-day forecasts for more than 120 Colorado mountain destinations!
DISCLAIMER: This weekly outlook forecast was created Tuesday 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, subscribe to BoulderCAST Premium.
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