Issue Date: Monday, June 18th, 2018
Issue Time: 2:40PM MDT
Valid Dates: 6/18 – 7/3
This next FTO period will begin with an Elevated flood threat as Event #2 continues from Thursday’s FTO (now Event #1). This upper trough will linger west and north of Colorado through Wednesday evening until is slowly slides east and breaks apart by Thursday morning. At this time a ridge begins to build to the west of Colorado. The ridge will pull in dry air over the majority of the state as seen in the water vapor imagery below. This will bring warm and dry weather to most of the state, but there still is a chance for some thunderstorms Thursday over the far eastern plains. At the start of next weekend, another trough drops into the Great Basin, which will give eastern Colorado and the northern high country another chance of more widespread thunderstorms and rainfall on Saturday and Sunday. The latest run of the GFS shows the upper low spinning over Colorado through Monday pulling in high dew points over the eastern plains. Starting Tuesday, the ridge beings to build back over Colorado drying out the atmosphere and limiting afternoon showers and thunderstorm development.
Precipitable Water (PW) values are not that impressive this next week for western Colorado. Although the forecast is hinting at some moisture return over the weekend, the chances of PW remaining below average are high. This should limit heavy rainfall chances and may cause some fire danger over the Central and San Juan Mountains as the trough drops into Colorado next weekend bringing stronger winds. East of the Continental Divide, tomorrow afternoon looks rather active over the Palmer Ridge/eastern plains with some severe thunderstorm action. Heavy rainfall chances also remain high with PW values close to or above 1 inch helping create high rainfall rates. Upper level dynamics will support widespread thunderstorm development tomorrow afternoon, and it is expected to continue through the early morning hours.
Below we describe each of the two identified precipitation events in more detail.
Event #1: Tuesday (6/19) – Thursday (6/21)
Elevated Flood Threat/No Apparent Threat as moisture remains high over the eastern plains and combines with favorable upper level dynamics to bring severe weather and heavy rainfall on Tuesday.
PW values over 1 inch are a good indicator of heavy rainfall potential. Tomorrow afternoon, the GEFS moisture plumes are indicating values greater than 1 inch over most of the eastern plains. With upper dynamic support, widespread thunderstorms (some severe) are forecast. Dew point values near the KS/CO border are forecast to reach 60-65F, which will cause flash flooding for storms that track over this area. Max 1-hour rain rates up to 1.8 inches/hour will be possible. Threats tomorrow include large hail, wind gust up to 60 mph and local heavy rainfall. Tune back into the FTB tomorrow morning for more details. Wednesday into Thursday, thunderstorm coverage is expected to decrease with the entrainment of more dry SW flow. Heavy rainfall chances will be confined to the eastern Colorado border.
Event #2: Saturday (6/23) – Tuesday (6/26)
No Apparent Threat as the next trough drops into the Great Basin and spins over Colorado.
Confidence in the strength, timing and moisture associated with the next upper level trough is still fairly low. However, both global models indicate another trough sliding through the region at the end of this week. At this time, expect a general increase in thunderstorm coverage on Saturday and Sunday over northern and eastern Colorado. It is a bit early to tell how much moisture will accompany this system, but PW values hint at increases both east and west of the Divide. If PW is not able to substantially increase west of the Divide, expect Red Flag Warning and critical fire danger again this weekend as this trough passes through. The best chance for moisture west of the Divide will be over the Northern Mountains.
