FTO 08-15-2019: Elevated Flood Threat Issued for Eastern Colorado & Critical Fire Weather Forecast over Western Colorado

Issue Date: Thursday, August 15th, 2019
Issue Time: 12:25PM MDT
Valid Dates: 8/16 – 8/30

Event #1 of this FTO begins tomorrow and the Elevated flood threat lasts only a day as dry air and subsidence moves back into the area on Saturday. The flood threat tomorrow would be over the eastern plains with severe thunderstorms possible along a boundary that forms due to lee troughing. Dynamics in the area would allow these storms to produce large hail (greater than 2 inches), damaging winds and heavy rainfall. Storms will be moving moderately fast, so not thinking this will be more than a Low flood threat at this time. However, if the shortwave from Wyoming is moving fast enough to arrive during the afternoon or early evening, this threat may need to be upgraded. Please tune into the FTB tomorrow.

Break in heavy rainfall for a couple of days and very hot temperatures before a weak PW surge occurs over the eastern half of Colorado. The passing shortwaves to the north set up a nice ridge pattern that puts this area in a favorable position for subtropical moisture advection and post frontal upslope flow. Shortwaves moving around the ridge will likely enhance coverage of storms, but only if they are timed correctly. Higher PW values are kept east and south of the state at the end of next week before the next statewide monsoon surge begins. Thus, the Elevated threat at the end of Event #2. Still lower confidence this far out in the actual flood threat, but an active pattern is back in place starting Tuesday.

Not much change in the GEFS moisture plumes from the last FTO. PW values looks to increase back to the west over eastern Colorado temporarily tomorrow before dropping off again. The return of subtropical moisture over eastern Colorado can be seen Tuesday and Wednesday in the graphic on the left (last couple of days). Over western Colorado, enhanced jet activity over the northern border and very low moisture will likely cause critical fire weather Friday and Saturday. A Red Flag Warning is in place for Friday, which will likely be issued again for Saturday. Please see NWS Grand Junction for more up to date information. Slight high moisture for the next passing trough may keep fire issues at bay, but still quite a bit of spread in the model members (gray lines).

Event #1: Friday (8/16) – Saturday (8/17)

Elevated Threat/No Apparent Threat for a lee trough and associated severe thunderstorms in a moisture rich environment.  

While the storms with this setup have been moving fairly quickly the last couple of days, the amount of rain they have produced has been quite impressive. That is why there is an Elevated flood threat issued for tomorrow. As storms tomorrow move from the Northeastern Plains into the Palmer Ridge/northern Southeast Plains, steering winds should slow down a little, so more accumulation is likely in the higher dew point region. We’ll still have to see where the dry line sets up tomorrow and how tight the gradient is, but there is a very good chance for a couple severe thunderstorms capable of heavy rainfall. The main threats from the storms will be damaging winds and, once again, large hail (greater than 2 inches). Not a great sign for the crops, so hopefully coverage is isolated (looks to be this way now). Saturday, a couple scattered storms may be possible with residual moisture over the mountains and adjacent plains. The far eastern plains look to remain capped, so that should keep the severe threat minimal.



Event #2: Tuesday (8/20) – Tuesday (8/27)

Elevated Threat/No Apparent Threat as an active weather pattern sets up with on and off moisture east and west of the Continental Divide.

Elevated threat is first for this event over eastern Colorado, and then the next Elevated threat is for the full state. There looks to be heavy mountain precipitation on Tuesday and Wednesday following the passage of a cold front (return of low level moisture). Not thinking it will be a large threat area for flooding, but burn areas will need to be watched especially closely over the Southeast Mountains. The flood threat decreases for a bit before the next statewide monsoon surge sometime near next weekend. Not yet sure of the strength of the surge, but worth mentioning as this could bring some much needed rainfall to western Colorado. The majority of the area (minus the eastern San Juan Mountains) is 0.75 to 1.5 inches below normal. The Southwest Slope and northwest corner of the state has been put back into the D0 drought category, which means it is Abnormally Dry. Hoping for a good monsoon surge as this could easily be the last one of the season and help get some of the areas out of drought troubles (as minimal as they are).