Though the middle of June is typically a quiet time of year for the Front Range, that won’t be the case this week with widespread showers and thunderstorms to begin the week, including the threat for large hail. The rest of the week will be warmer, but not necessarily dry either.
Wet early week
Over the weekend, there were a series of shortwaves embedded in the west-southwest moist flow, thanks to a persistent and weak low pressure in the southwestern United States. To start our week, the same pattern remains, at least through Tuesday. The mid-level pattern is shown below for 500 mb absolute vorticity. For Monday afternoon, the flow is mainly westerly with moderate shear. Numerous embedded shortwaves will traverse across Colorado this afternoon and evening coupled to a trough axis directed north-south.
On Tuesday (above), the flow turns ever so slightly to the west-northwest, with yet another shortwave system tracking through. However, at this time, you’ll notice the low pressure in California is no longer a player. This overall pattern will be the major reason for our showers and thunderstorms today and tomorrow.
In addition to the mid-level forcing Monday, surface flow will be easterly from the trough axis (bottom right). Deeper moisture is already pooling across the Front Range, approaching 1″ of precipitable water (bottom left) by this afternoon. This will spur-on the threat of heavy rainfall Monday afternoon and evening. The high moisture content will also facilitate CAPE values between 1400 and 2000 J/kg near both Boulder and Denver (bottom right).
The unstable air, moderate shear, elevated moisture will lead to a 60% chance of showers and thunderstorms to start our Monday afternoon, with action starting as quickly as early afternoon across the Denver Metro area.
Our main threats of the storms today will be heavy downpours, lightning, brief gusty winds, and large hail. Given the good shear and instability, strong updrafts will be capable of producing near 1 to 1.5 inch diameter hail across the Denver Metro. Activity should taper off around midnight.
The Storm Prediction Center has both Boulder and Denver at “Marginal Risk” for severe storms Monday afternoon. Based on the latest runs of the HRRR model, the primary window for severe storms in the Denver area will be between 1:00 and 4:00 PM on Monday.
Storm Prediction Center outlook for severe storms on Monday June 17th
Much of the same pattern will exist on Tuesday but with slightly less shear and instability. There will still be a very good chance of showers and thunderstorms for our area, but no threat of severe weather. High temperatures Monday and Tuesday will be in the 70’s.
Drier and warmer midweek, but storms return
By as early as mid-day Wednesday, drier air will filter in from the west and northwest. This is thanks to the mid-level pattern altering with a developing large trough over the Pacific Northwest. On Wednesday (bottom right), 700 mb temperatures increase to around 12°C, and to 14°C by Thursday (bottom right). In addition, the atmosphere dries out considerably by Wednesday and through Thursday afternoon. This trough in the northwestern United States will shunt the moisture into Kansas and Arizona/New Mexico. That should bring sunnier conditions to the Front Range with highs rebounding to that true summer feel…back into the 80’s Wednesday and Thursday.
In terms of late in the week Thursday night and Friday, the trough in the Pacific Northwest will start to dive southward to influence the state (bottom). A jet streak is slated to exist to our northwest from Idaho into the Dakotas. A frontal boundary will likely be embedded in this pattern change from southwest to northeast, setting up a moisture convergence region in this same area, thus slowly impacting northeast Colorado (bottom right). Uncertainty remains as to whether storms may re-enter the forecast starting Thursday evening, or if they will hold off until Friday and Saturday. We should remain dry through much of Thursday, but some storms Thursday evening and night are not out of the question. We’ll certainly keep an eye on it.
Friday will likely be a transition day between some cooler air making its way south and the warm airmass over Colorado. For certain the pattern looks to turn unsettled to end the week and begin the weekend with a better chance of storms for the region. It’s not entirely clear when the main cold front will invade northeast Colorado, but when it does, we should see better chances for rain and much cooler temperatures with highs in the 60’s to lower 70’s. It’s looking like this change will happen late Friday night or sometime Saturday.
Forecast Specifics:
Monday: Morning low clouds, then some sun, then increasing clouds with scattered to widespread thunderstorms developing by early afternoon continuing into evening. Storms will be capable of producing heavy rainfall and large hail up 1.5″ in diameter. Storms will taper off in the late evening. Highs in the mid to upper 70’s on the Plains and middle 60’s in the Foothills.
Tuesday: Mostly cloudy with widespread showers and thunderstorms developing as early as mid-day, continuing through afternoon and evening. Storms again will be capable of producing heavy rainfall but only small hail. Highs in the low to middle 70’s on the Plains and low 60’s in the Foothills.
Wednesday: Partly cloudy, drier, and warmer with only a slight chance of isolated storms, mainly across the higher terrain. Highs in the lower to middle 80’s on the Plains and lower 70’s in the Foothills.
Thursday: Partly to mostly sunny and warm with highs in the low to middle 80’s on the Plains and upper 70’s in the Foothills. There is a slight chance of late day thunderstorms.
Friday: Increasing clouds with a chance of afternoon/evening thunderstorms. Highs temperatures cooler in the 70’s for the Plains and 60’s in the Foothills.
High Country: A very active pattern will remain across the higher terrain Monday and Tuesday with likely afternoon/evening thunderstorms. Storms will have potential to produce heavy downpours, hail, and frequent lightning. The pattern mostly dries out Wednesday, but isolated to scattered storms re-enter the picture late Thursday and Friday and (of course) the upcoming weekend.
DISCLAIMER: This weekly outlook forecast was 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.
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