Even with the monsoon on possibly permanent pause, storm chances will continue through the week with large hail and damaging winds a concern as well. We need the rain as portions of the Metro area are now experiencing severe drought. Late in the week, winds shift and drier air will work into Colorado from the southwest leading to reduced storm chances. We discuss this and more in our weekly outlook.
This week’s highlights include:
- Despite the halt on the monsoon, daily thunderstorms have produced 1 to 2″ of rain in portions of the Front Range over the last five days
- The weather pattern early in the week will support scattered severe thunderstorms across northeast Colorado each day through Wednesday
- Drier air takes hold late in the week, but storm chances won’t go away entirely
- Temperatures throughout the period will be near seasonal normals in the upper 80’s to lower 90’s
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.
Severe threat early week
Despite the cessation of monsoonal flow into the southwestern United States, daily showers and thunderstorms have continued across north-central Colorado over the last several days. Persistent northwest flow combined with surface upslope have even produced a few severe thunderstorms at times over the weekend along with localized heavy rainfall. The areas hardest hit were near Longmont, Lyons, and Estes Park where 1-2″ of rain has fallen. Boulder and Denver have observed only 0.12″ and 0.17″ respectively in the last seven days.
7-day radar estimated rainfall totals ending Sunday night
The latest update from the United States Drought Monitor last week now has 99.4% of the state within a classification. This includes Moderate to Severe drought in portions of the Denver Metro area. The week ahead will offer daily chances for thunderstorms, but unfortunately nothing organized enough to provide any real drought relief.
The large-scale atmospheric set-up to start the week has a double-lobed ridge of high pressure across Arizona stretching offshore of Baja California. This is the weather pattern we’ve been discussing for the last two weeks which has put a halt on monsoonal moisture flowing into the Desert Southwest. The clockwise flow around the ridge is steering most of the moisture westward and away from Arizona.
Elsewhere, a deep trough is present from the Great Lakes region to the Gulf Coast. Tropical Storm Isaias remains offshore with little in the way of impacts to the shorelines of Georgia and Florida. Isaias is expected to make landfall tonight somewhere in the Carolinas as a ragged tropical storm.
GOES-East visible satellite animation from Monday morning
As has been the case seemingly for the last five days or so, with the ridge axis to our west and trough axis to our east, northwest flow aloft will stay the course today and into mid-week across Colorado (see above). At the surface, a weak frontal boundary remains parked across the Metro area, with moist southeasterly flow spreading into the Front Range (see below).
These differing wind directions with height in the atmosphere, also known as vertical shear, will set the stage for severe weather each and every day through Wednesday across eastern Colorado with a focus right along the stalled frontal boundary. The Storm Prediction Center includes a narrow sliver of Colorado at “Marginal Risk” Monday with hail and strong winds the primary concerns with developing thunderstorms.
High-resolution model simulated radar for later Monday shows the threat of hit-or-miss severe storms will exist from mid afternoon until sunset anywhere from Montana to New Mexico along the frontal boundary.
HRRR model-simulated radar for late Monday afternoon across the Front Range. Developing storms will be rolling off the Foothills to the adjacent Plains during this time.
As the week progress, the ridge axis aloft will push ever-so-slightly to the east along with the surface front. Thus, the threat for severe weather will slowly migrate eastward as the low-level moisture and notable vertical shear do as well. The most probable location of severe weather both Tuesday and Wednesday will be just east of the Metro area. The Storm Prediction Center already highlights a “Slight Risk” for Tuesday and a “Marginal Risk” for Wednesday.
The set-up early in the week will support only scattered storms and a few severe ones across the Front Range. Rain is not a certainty for all locations though, with only a 20-40% chance of storms each day. Highs through Wednesday will be seasonal in the upper 80’s to near 90 degrees. Expect morning sunshine but then thick clouds for the late-day periods.
Drying out and warming up
By Thursday, the ridge of high pressure will have weakened and shifted eastward into New Mexico or Texas. Flow aloft will back to southwesterly by Thursday opening the door for warmer and slightly drier air to enter Colorado from the southwest.
It still appears that there will be enough moisture around for isolated to widely scattered showers and storms over our area. The threat of severe weather should be non-existent. Highs will warm slightly into the lower to middle 90’s. As or writing, the upcoming weekend looks pleasantly dry for much of the state, though temperatures will be on the the warmer side.
Forecast Specifics:
Monday: Mostly sunny early, then mostly cloudy with scattered thunderstorms, some of which will be severe and capable of large hail and strong winds. Highs in the upper 80’s on the Plains and middle 70’s in the Foothills.
Tuesday: Morning sun, then mostly cloudy with scattered thunderstorms, some of which will be severe and capable of large hail and strong winds. Highs near 90 degrees on the Plains and middle 70’s in the Foothills.
Wednesday: Increasing clouds with widely scattered afternoon and evening thunderstorms, some of which could turn severe near and east of Denver. Highs in the mid to upper 80’s for the Plains and middle 70’s in the Foothills.
Thursday: Morning sun, then partly cloudy with widely scattered showers and thunderstorms developing through the day. Highs near 90 degrees on the Plains and middle 70’s in the Foothills.
Friday: Warmer and slightly drier with only isolated thunderstorms developing across the area. Highs in the low 90’s on the Plains and upper 70’s in the Foothills.
Weekend: Storm chances should lessen or disappear, along with highs in the lower to middle 90’s for both Saturday and Sunday.
High Country: Scattered showers and thunderstorms will be likely Monday through Wednesday for the central and southern mountains of Colorado. A lack of moisture to the west will mean mainly dry weather for the western and southwestern ranges. Thursday and Friday will have isolated to scattered daily storm coverage statewide with flow turning southwesterly. Check our SummitCAST page for daily updated forecasts for more than 120 mountain hiking destinations across Colorado. Here’s a look at peak-top winds speeds for all the 14ers on Monday:
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