October 1st: Hurricane Rosa, heavy rains and burn scars

Many of you probably heard about the remnants of Hurricane Rosa (the last advisory from the NHC for Rosa was issued on October 2nd) impacting areas of the southwestern United States earlier this week. I have been aware of the risk of debris flows and flash flooding on burn scars in mountainous areas with steep slopes. See for example, see this presentation by the National Weather Service Weather Forecast Office Los Angeles about the January 9, 2018 debris flows on the Montecito CA burn scar.

I am just learning that the risks posed by heavy rains causing debris flows and flash flooding are not just limited to California. While California is known for debris flows on burn scars, other areas of the west with step mountainous terrain are also susceptible. See this short article about why burn scars are susceptible to flash flooding/debris flows from the National Weather Service Western Region Headquarters.

Knowing that Hurricane Rosa impacted areas of the southwestern United States with heavy rains, I looked at a couple of National Weather Service (NWS) Weather Forecast Offices (WFO)  that appeared to impacted by flash flooding from Rosa to see what is and was going on: NWS WFO Salt Lake City Utah and NWS WFO Grand Junction Colorado. Both WFOs had issued flash flood watches with wording about the possibility of debris flows and flash flooding on recent burn scars, I noted graphics on flash flood watches on the Facebook page for both offices.

• The NWS WFO Grand Junction Colorado shared a graphic about a flash flood watch on their Facebook page on October 1st, with wording about the possibility of debris flows/flash flooding on burn scars.
• The NWS WFO Salt Lake City Utah posted a flash flood watch on their Facebook page on September 30th with wording about the possibility of debris flows/flash flooding on burn scars. Some of you might be interested in a briefing from the WFO Salt Lake City Utah on Facebook on October 2nd around mid-day. Please note that the briefing is for a certain point in time and will not reflect conditions that happened after the briefing. 

I don't know if the rains led to debris flows/flash flooding from burn scars in either the Grand Junction or the Salt Lake City WFO. I have not heard about any debris flows/flash flooding on burn scars, but it is entirely possible that I don't have current information. The point here is that it is important that these WFOs were watching for the possibility of debris flows/flash flooding on burn scars in their forecast area so that their citizens could be prepared to take action.