Issue Date: Thursday, August 18th, 2022
Issue Time: 2:55PM MDT
Valid Dates: 8/19 – 9/2
The state should finally start to see some relief from monsoon rainfall next week, but not before a quick and low-end 2-day Elevated flood threat for tomorrow and Saturday. An incoming Low along with some moisture is expected to help to boost mid-level energy and rain rates for Friday and Saturday before a drying trend begins on Sunday. Without any mechanism to mix out residual moisture, afternoon mountain storms are forecast through Wednesday with the weak ridge beginning to rebuild overhead (Event #2). By the end of next week, a shortwave is likely to breakdown the ridge and drop a weak cold front through the state. Unsure about the longevity of the heavy rainfall threat, but post frontal upslope flow will likely Elevate the flood threat for a couple of days during this period (Event #3). Long-term guidance is then starting to show a true drying trend, so Event #3 should be short-lived. With the next shot at a complete dry day sometime during the last week of August, the state should easily reach 75 days of consecutive precipitation.
The quick upturn in moisture can be seen both east and west in the PW plumes below. It’s quite the turnaround in moisture return over western Colorado, which will be the focus of the Elevated flood threat tomorrow. PW then begins to drop off to more normal values for late August afternoon Saturday, which means rainfall should stick around in the forecast, but there is No Apparent threat. There is an outside chance for a low-end flood threat during Event #2, but only if a shortwave can move through during peak heating and surface moisture does not mix out. This would be most likely to occur over the central or southern high terrain earlier in the week. There is also increasing confidence that afternoon mountain storms should become more widely scattered by Wednesday with drier air moving overhead. By the end of the week, PW anomalies (not shown) are showing a slight increase to moisture again, but changes to the forecast are anticipated as Event #3 gets closer.
Event #1: Friday – Sunday (8/19 – 8/21)
Mid-level lift and an increase in moisture will return the flood threat through Saturday, and an Elevated threat has been issued for high terrain over western, central and southern Colorado.
More widespread rainfall should return to western high terrain and mountains on Friday with some spillover into the Urban Corridor possible with northwest steering flow. Max 1-hour rain rates up to 1.25 inches and 3-hour totals up to 1.75 inches will be possible along with small hail and strong winds from the thunderstorms that develop. Heading into Saturday, the heavy rainfall threat shifts to the southern high terrain, including the Raton Ridge, with steering flows becoming more southerly. Similar 1-hour rain rates are anticipated with some longer duration rainfall helping to increase totals over the southern and central mountains. The Elevated threat for both events are on the low-end, but debris slides, mud flows and road flooding will be possible over steep terrain.
Event #2: Monday – Wednesday (8/22 – 8/24)
Residual moisture and some mid-level lift should keep scattered to widely scattered storms over the mountains in the forecast, but at this time there is No Apparent threat.
Not much to mix out the moisture and mid-level energy trapped under a building ridge, so this will likely lead to mountain showers developing each afternoon. Max 30-minute rain rates up to 0.75 inches will be possible at the beginning of the event but decreasing surface moisture should reduce rain rates by Wednesday. If a well-defined shortwave moves through the flow at peak heating during the first part of the week, there may be a low-end flood threat issued for a particular area. However, the likelihood of this happening is low enough that flooding is not expected at this time. Afternoon high temperatures should also begin their upward trend during this event with 90°Fs returning to the forecast across the lower elevations of the Grand Valley and Northeast Plains by Wednesday.
Event #3: Thursday – Tuesday (8/25 – 8/27)
Quick Elevated flood threat issued for the end of the week before a drying trend takes place.
There will likely be a 1- or 2-day Elevated flood threat at the end of the week as a passing trough to the north pushes a cold front through the state. Before that front pushes through, afternoon temperatures should get quite warm with 90°Fs forecast for most of the lower elevations (below 6000ft). Currently, long-term guidance has the front moving through Thursday evening or Friday. Depending on the timing of the frontal passage, some rainfall may be possible over the eastern plains. Post-frontal upslope flow during this active monsoon season will likely keep the flood threat elevated for at least a day. Details with this event are sure to evolve, and due to low confidence in precipitation placement, no map has been drawn. Be sure to tune back into the FTO next Monday and Thursday for the latest.
