The figure above indicates the forecasted high in orange (with error bar denoting the range of ensemble forecast members), average high for this time of year in orange (dashed line), forecasted low in blue (with error bar), average low this time of year in blue (dashed line), and forecasted wind speed in purple. Note that the wind speed forecast is sustained wind (not gusts), and it is based on only one model (not an ensemble).

Weather Discussion:

What an incredible weather pattern set up late last week! With low pressure off the California coast and high pressure over the Four Corners region, a strong southerly flow set up across Arizona and a very moist and unstable airmass moved into place. The strong winds aloft created decent shearing with height to support multicellular thunderstorms and even some weak supercell thunderstorms. The big story was the rainfall totals.

Radar rainfall estimates indicate up to 6” fell on the northwest side of Prescott from 5:10 am MST July 10 – 04:09 PM MST July 16 (see the image below). There are other locations in Yavapai County on the southwest and south of Prescott that received up to 5” or 6” of rain as well. Crown King became “Drown King” as several flash flood warnings were issued for the small community and nearby drainages this weekend in response to the fact that the Gladiator Fire at Crown King greatly reduced vegetation and moisture permeability in area soils (increasing the amount of runoff).

Rain gauge measurements were lower, with 3.7"of rain at my home in Williamson Valley and 3.22" of rain at ERAU campus from 7/12 – 7/15. However, rain gauges are subject to catchment errors when we have wind-driven rain, causing the measurements to actually underestimate the actual precipitation rates.

Professor Ed Post in our College of Engineering informed me that a microburst also occurred on the ERAU campus on Friday the 13th. Apparently the only damage to the campus was two port-a-potties being toppled over near the soccer field (needless to say, that would not be a fun cleanup job). I inspected the wind record from the roof of our Academic Complex, and the peak wind gust was 30.3 m/s (or 68.9 mph) at 10:37 am on Friday July 13th. It’s rare to have wind gusts that strong here in Prescott.

Today, the vertical wind shear lingered, enabling some isolated strong thunderstorms to form. A couple of supercell thunderstorms near the Grand Canyon prompted tornado warnings. However, much drier southwesterly wind is now infiltrating the state and will reduce the potential for thunderstorms to a minimum for the next few days (Tuesday – Thursday). By Friday, high pressure will reform and build north of the Four Corners region, allowing moist southeasterly to easterly flow to bring monsoon moisture back into the state. Expect an increasing chance of thunderstorms this weekend, and a continued possibility of mainly afternoon and evening thunderstorms at least through the beginning of next week.

Rainfall totals at ERAU campus:

7/12 19.0 mm

7/13 19.4 mm

7/14 22.8 mm

7/15 20.6 mm

TOTAL = 3.22"