FTO 08-01-2019: 2019 Monsoon Causes an On and Off Heavy Rainfall Threat through Next Week

Issue Date: Thursday, August 1st, 2019
Issue Time: 2:50PM MDT
Valid Dates: 8/2 – 8/16

There is an Elevated flood threat for both events of this FTO with breaks in heavy rainfall forecast every couple of days. Monsoon season is well underway, and since we are at the peak of climatological PW values, the active pattern is not surprising. By tomorrow, the subtropical high shifts to the west over the four corners regions, which essentially cuts off the PW flow over western Colorado limiting afternoon rainfall to scattered showers over the higher terrains. We will watch for any subtropical moisture sneaking into the southwest corner of the state (dependent on the placement of the high) as this could cause high enough rain rates for flooding issues over the 416 burn area. Post frontal upslope flow behind the front over the southeast corner of the state tomorrow may cause some flooding issues for recent burn areas over the Southeast Mountains, thus the Elevated flood threat. From Sunday into Monday, the ridge weakens with a passing trough. This will push a weak cold front through the state on Monday, which could trigger some heavy rainfall and a few severe thunderstorms over eastern Colorado. The eastern plains look to remain capped on Tuesday minus over the mountains where storms will initiate again with upslope flow. Not seeing a flood threat at this time, although details could easily change over this weekend.

For Event #2, the next northward monsoon push begins on Thursday. Not looking quite as strong as the current one, so at this time thinking coverage of the heavy rainfall threat will be less. Nonetheless, there will be an Elevated flood from Friday into Saturday. The low begins to move onshore next weekend, and once it moves through, more zonal flow is anticipated. This pattern typically pushes the high moisture to the east and suppresses the monsoon surge.

The GEFS moisture plume page hasn’t been updated yet today, so the image below is a little old. You can see the current PW surge, which peaks this evening. After this event, it looks like average PW through next Wednesday. Note the scale on the left (DEN) is slightly different than on the right (GJT). The slightly elevated levels of PW over eastern Colorado tomorrow may cause some heavy rainfall over the Southeast Mountains/Raton Ridge, but there will overall be a downtick in coverage and intensity through next Wednesday when compared the last couple of days. The burn areas over the Southeast Mountains will be watched carefully tomorrow for a flood threat, but elsewhere this time, thinking rain rates will be gradual enough to avoid flooding issues.

Below is a recap of the July’s temperature and precipitation. It was noted in the last couple of FTOs that the onset of the monsoon was a little slow this year. However, the majority of eastern Colorado received more than average rainfall with the 200-300% percent of normal area equating to about 2-3 inches. A bit drier back to the west, although it is important to note that August is their big rainfall month. Their 25-50% of normal precipitation equates to no more than 1 inch below normal. This below average rainfall did cause the D0 (Abnormally Dry) drought to be placed back on the drought monitor map over the northwest and southwest corners of the state. As far as temperatures, quite the turnaround from May and June, which were very cool. Most of the state was 1 to 3°F above normal with a couple of notable stretches of upper 90°Fs to 100°Fs. This put the cooling degree days (base 65°F) between 60-80% higher than normal along the Urban Corridor and the adjacent plains of the Southeast Mountains.

Event #1: Friday (8/2) – Wednesday (8/6)

Elevated Threat as the subtropical high shifts into the Four Corners Region and a cold front drops through the state on Monday night.  

On and off again moisture, but the threat for Event #1 is mostly over eastern Colorado and the mountains. There may be an Elevated threat over the 416 burn area on Sunday, but the placement of the high/strength of the ridge will be key (determines if the moisture can meander north). Monday to Wednesday, the threat moves into the eastern plains. This is especially true on Wednesday when the passing shortwave will help support thunderstorms over the plains and cause steering flows to have more of a westerly component as it breaks down the ridge. Not seeing any huge moisture surges, so an Elevated threat should suffice with a lee trough setting up from Monday to Wednesday. Of course if a passing shortwave teams up with peak heating while moisture is elevated, there may need to be a Moderate flood threat issued.



Event #2: Thursday (8/8) – Sunday (8/11)

Elevated Threat as the ridge begins to rebuild before being shifted east causing the next moisture surge.

The ridge begins to build again during Event #2, and the axis will event slide east causing the next monsoon moisture plume to move northward when a shortwave moves through. The PW values look to peak on Friday and Saturday right now, thus the Elevated flood threat during this time frame. Lower confidence in this solution over western Colorado than eastern Colorado as well. Should be a fairly rapid event with the just brushing the northern border. This will also keep steering flows faster, which should limit rainfall accumulation.