Issue Date: 9/24/2015
Issue Time: 12:15PM
Note: Our final Flood Threat Outlook is next Monday, 9/28. Please check back then for a special winter snowpack forecast!
The last week or so has seen a rekindling of heavy rainfall activity that was a staple during the early part of summer. Thunderstorms have occurred on several days, but due to marginal atmospheric instability and fast storm motion, heavy rainfall has been more a miss than a hit. During this Flood Threat Outlook, we anticipate a continuation of the transition from summer to fall. Weather-wise, this implies that the character of rainfall transitions from thunderstorm-driven to frontal driven.
The water vapor image, above, shows the main features of the middle and upper-atmospheric circulation. A strong jet stream is found across southern Canada, extending far upstream into the North Pacific Ocean. A significant trough was located off the North American west coast. This trough will very slowly enter the continent and provide Colorado with a multi-day period of rainfall chances. Farther west, another trough/low-pressure system spins in the North Pacific. At this time, no impact is expected from this feature.
Thus, only one precipitation event is identified during this FTO. Details of this event follow.
Event #1 (Tuesday 9/29 – Friday 10/2):
No Apparent Flood Threat
A large-scale trough is expected to slowly approach the west coast of the U.S. By Tuesday (9/29) of next week, this feature is expected to impact Colorado. Atmospheric instability will once again be limited. Thus, it is not clear if there will be a potential for thunderstorm activity. Regardless, the passage of one or two cold fronts will maintain a chance of rainfall from Tuesday through Friday. As the cold front passages will be swift, rainfall will be on the light side. However, in total, between 0.5 and 0.75 inches of rain will be possible in the Central Mountains, Northern Mountains as well as the Front Range, Palmer Ridge and Northeast Plains. Thereafter, additional cold front passages will be likely during the week of Monday, 10/5. However, there are presently no indications that these events will lead to rainfall exceeding 0.5 inches.
