Issue Date: Monday, June 22nd, 2020
Issue Time: 2:15PM MDT
Valid Dates: 6/23 – 7/7
Event #1 begins tomorrow with an Elevated flood threat issued. Moisture looks to increase somewhat under the ridge as the axis slides slightly to the east. Additionally, without any upper level jet over the state, storms should become more stationary. This will help to increase totals over eastern Colorado, and slow moving storms could be dangerous if they form over the Spring Creek or Decker burn area. To the west, more of the same (dry conditions) with only isolated high storms forecast along the Continental Divide.
There’s a lull in the flood threat on Wednesday and Thursday, but a shortwave will likely combine with increased moisture on Friday afternoon. This should help return rainfall to portions of western Colorado as well. With more westerly flow forecast aloft as the shortwave passes overhead, some heavier rainfall may be possible as storms make their way to the eastern plains. Thus, the Elevated flood threat. Residual moisture will likely produce afternoon storms over the northern high terrains on Saturday, before it begins to dry out behind the trough.
Note that the y-axis has been increased to 1.5 inches in the PW plumes below. There is good consensus that PW will remain at or above 1 inch over Denver tomorrow, and with slower steering flows this will mean a chance for higher rain rates and rainfall totals. Since 1 inch is a good proxy for heavy rainfall, the Elevated flood threat has been issued – although mostly for the burn areas. PW looks to decrease today to around climatology average over western Colorado. The GEFS is showing quite a bit of uncertainty in regards to moisture later this week across the state, but there looks to be an upwards trend over eastern Colorado. It is likely by Friday that moisture over western Colorado will increase across the northern high terrains with the passing shortwave, so PM storms could possibly return to the forecast.
I always like to point out the strong upward trend in PW over western Colorado that begins around the summer solstice each year (bottom image). You see the same increase over eastern Colorado as monsoon season arrives, but the slope of the line isn’t quite as steep. Despite PW values increasing in climo during this time of year, the weather pattern looks to dry out early next week and pair with an increase in southwesterly surface winds. So be on the lookout for critical fire weather to return to the state anytime from Sunday to Tuesday.
Event #1: Tuesday (6/23) – Saturday (6/27)
Elevated Threat/No Apparent Threat as ridging and shortwaves combine with on and off low-level moisture over the state.
Not a lot of upper air support during this period, which means there is a low chance for severe storms. The best chance for a severe storm or two will be on Friday or Saturday as shear looks to increase over the eastern plains. By how much is the real question as the GEFS is still having a hard time pinning down the next system’s details. The precipitation prediction below has quite a few days of rainfall in it, and also expect the totals to be more patchy in coverage than widespread. The isohyets also assume the storms will make it into the eastern plains (higher moisture) on Friday with an increase in westerly flow as the shortwave passes overhead (at peak heating). So, a lot of ingredients still need to come together, and it’s hard to know get the details correct more than a couple days in advance. The Palmer Ridge, Front Range, Wet Mountains and southern Raton Ridge are specifically included in the 1.50 inch ring due to these regions precipitation climo/pattern in June.
