11 AM, July 30, 2014.
Steady rain has been falling in Boulder this morning at the rate of about 0.20″ per hour, although that intensity varies a bit on the short time scale. The day will stay wet with long periods of light rain and drizzle punctuated by heavier showers and possibly a thundershower.
The intensity of the heavier showers should be lower than yesterday’s thunderstorms when 2-4 inches accumulated quickly in parts of Larimer and Weld counties and out on the northeastern Colorado plains. Today’s rain is likely to bring another widespread 0.25- 0.50″ this afternoon and evening, and some local areas will probably get 1-2 inches. The ground is getting pretty wet, so watch out for areas of minor flooding and possibly a few locations with more serious debris flows.
Today’s record low maximum temperature for Boulder is a remarkable 57 set on a cool, damp July 30th in 2009. We are already warmer than that. The record low maximum in Denver is 64 set in 2009 and that one is within reach. The wettest July 30th in Boulder is 1.74″ set in 1998 and it is very likely that we are well over that as of 11 AM.
The somewhat moist monsoon pattern is likely to continue into early next week, although it should be warmer and less wet Thursday. But be prepared for some afternoon showers/thunderstorms. Whether we get another round of widespread heavy rain after tomorrow is uncertain, but I’ll update as needed.
For the Front Range and High Plains of Colorado and southeastern Wyoming, significant rainfall is likely later today through Wednesday for much of the area, with some localized areas of heavy rainfall. Minor flooding is likely in places, and the possibility exists for localized areas of damaging flooding or slope failures.
Numerous slow-moving showers and thunderstorms should develop Tuesday afternoon and evening. Some localized areas may receive 1-3 inches in just a few hours. Then, widespread somewhat steady rain is likely to effect many locations during the night and into Wednesday, with another 1-2 inches over a large area. By Thursday morning many parts of the region will likely have storm totals of over an inch, and some locations will probably have totals in the 3-5 inch range.
Comparison with September 2013:
The moisture content of the atmosphere today is likely to reach about 150% of average for about 12-24 hours, compared to over 200% of average for 60 hours in the storm last Sept 10-12th. So the magnitude today is not quite the historic proportions that we saw last September. But it appears likely that the rain later today through tomorrow may be heavy enough to cause some problems.
The general pattern today, last September, and in other big rain events share some basic similarities. 1) A weak atmospheric disturbance is enhancing mid-level “monsoon” moisture from the tropical Pacific, 2) humid easterly flow in the low levels will bring in Great Plains moisture, and 3) the upslope flow combined with instability will help to turn that moisture into clouds and precipitation.
We will have a break from the 90-degree heat this week, and just about everyone should receive some rain. It appears likely that some areas will receive locally heavy rainfall, so watch out in flood prone areas. The greatest risk for locally heavy rainfall will be in the Tuesday-Wednesday time frame, with rain possible even in the late night and morning hours on Tuesday-Wednesday.
It’s not a classic monsoon setup that we sometimes see this time of year. But above-average amounts of atmospheric moisture and a disturbance from the northwest might enhance the easterly flow and instability enough to result in soaking rains for parts of the Front Range and high plains of Colorado and southeastern Wyoming.
Many of you have been reading my weather updates for the last few years. I decided to go with a blog. I’ll be posting this week about the somewhat cooler weather with some risk of locally heavy rain, especially in the Tue-Wed time frame. Enjoy!