FTO 07-04-2019: Elevated Rainfall and Riverine Flood Threat for the Holiday Weekend

Issue Date: Thursday, July 4th, 2019
Issue Time: 1:45PM MDT
Valid Dates: 7/5 – 7/19

There are two events identified for this next FTO. Event #1 is broken down into two features, which are marked in the water vapor imagery below. Today into tomorrow, the upper level low will progress eastward and breakdown the ridge. This will allow some moisture to move in from the Pacific over eastern Colorado. Post frontal upslope flow will also help moisten the lower atmosphere tomorrow and mid-level energy associated with the trough will help provide more widespread coverage of storms. The vorticity max (orange “X”) then moves slightly to the west and deepens off the Pacific Northwest coast. This will allow the ridge to begin to build over Colorado again. The axis of the ridge will be in a favorable location for another PW surge both east and west of Colorado. This should return showers to at least the southern high terrain over the western slope and reduce critical fire weather this weekend.

Tuesday expecting a break in heavy rainfall as the trough full pushes to our east and the ridge positions itself in a way that allows the atmosphere dry out a bit (westerly flow aloft). Event #2 begins by Wednesday as the next upper trough moves over the Pacific Northwest. As the trough slowly traverses to the east, it will push the ridge with it allowing the next PW surge to occur. By next weekend, models build another ridge, but further to the west, which should reduce the chances of heavy rainfall. There is lower confidence in the forecast this far out for Event #2, so please tune back into the FTO next Monday.

Looking at the GEFS PW plumes below, you can see some nice moisture surges that are associated with the ridge axis movement. Over eastern Colorado, the front passing through overnight on Thursday should help increase low level moisture for more widespread storms tomorrow. There is another PW surge on Saturday, which occurs both east and west of the Divide reinforced by northerly surface flow Friday night. With the high over the New Mexico area, better moisture can be pulled into western Colorado. This should increase the chances for rainfall on Saturday over the southern high terrains (west) and increase the threat over recent burn areas (west and east). With long spells of moisture over eastern Colorado, expect some fog, cloud cover and light rain Friday and Saturday morning as well. This could decrease the chances for afternoon storms over the plains by capping the area (preventing instability from forming). After this weekend, quite a bit of disagreement between the model members, but there does seems to be a trend for an increase in PW values by mid next week. This would mark the beginning of Event #2.


As anticipated, there has been another peak in flow this week from warmer temperatures (snowmelt), dam releases and multiple days of rainfall at the beginning of the week. There is an Elevated threat through Monday due to the dam releases and high flows. Minor flooding is anticipated to over low lying areas, but banks will also become unstable from the swift currents. Please use extra caution with water activities as dam releases and high flows can dislodge debris and cause waters to rapidly rise. Beginning tomorrow, most rivers show a decrease in streamflow, which should start to drop gages below the Action stage. However, if a storms on Saturday and Sunday track over the same area around a high flow, there may be an increase flood threat. There is currently only one Flood Warning near the Avondale gage on the Arkansas River downstream of Pueblo Dam. This is due to a recent release from the dam. This will likely push the La Junta gage back into Action stage as early as this afternoon with a peak in flow forecast on Saturday afternoon. For the latest on the riverine flood threat, check out the real-time AHPS gage heights and follow your local NWS office for all Flood Warnings and Advisories. Note: all NWS Flood Advisories and Warnings will be mentioned in the daily FTB that are active at the posting time.

Event #1: Friday (7/5) – Monday (7/8)

Elevated Threat as low level moisture surges over Colorado as the ridge is broken down by a trough moving to the east.  

Friday morning will begin a little bit cloudy and foggy for eastern Colorado due to extra moisture available from the passing of a front overnight. This should reduce temperatures over eastern Colorado, although it will still be quite hot by tomorrow afternoon. Post frontal upslope flow is forecast to initiate widespread showers and thunderstorms tomorrow afternoon and overnight. Burn areas over the Southeast Mountains look to stay mostly rain free and the far eastern plains looked capped, so highest rainfall accumulates are expected to be over the Front Range, Palmer Ridge and Urban Corridor.

Saturday, storms are forecast to return to the southern high terrains of western Colorado with widespread thunderstorm and rainfall activity over eastern Colorado as well. Storms will begin in the morning with stronger thunderstorms possible over the Raton Ridge and Southeast Plains during the afternoon and evening. Flooding may also be possible over the Junkins, Hayden Pass and Spring Creek burn areas, so tune back in to the FTB on Saturday morning to monitor the threat. Sunday and Monday, afternoon storms over the mountains and adjacent plains will be possible again. With high PW values through this period, the entire event has an Elevated flood threat. Decent steering flow should keep storms from dropping too much rain over one place, but back building storms may cause increased accumulations as well as longer duration moderate rainfall. We will be watching high flows on rivers and creeks closely as storms that track of the same area 2 or more days in a row may cause some minor flooding issues, which would be similar to last week.


Event #2: Tuesday (7/9) – Sunday (7/14)

No Apparent Threat/Elevated Threat as the next trough pushes the ridge axis to the east and creates another moisture surge from the Pacific and Gulf of Mexico.

Still have lower confidence in this event as the next trough sets up over the Pacific Northwest and allows the ridge to rebuild over the state. There will be a break in heavy rainfall on Tuesday between the two systems, but there will likely be enough residual moisture under the ridge for more isolated showers and thunderstorms during the afternoon that are diurnal driven. With the potential placement of the high over New Mexico, this would also return the chance of rain and thunderstorms to western Colorado. Wouldn’t say the monsoon has set up just yet, even though we are getting these PW surges. Models are (fairly consistently) starting to set up the 500mb high in the correct place by mid-July. So the waiting game begins.