textproduct: NWS Flagstaff

SYNOPSIS

Deep monsoonal moisture will remain over the area through at least mid week, with a slight decrease later in the week. With this moisture comes the daily threat of thunderstorms, along with the risk for locally heavy rainfall and flooding. Daytime temperatures will remain at or below normal much of the week, with hotter temperatures possible toward the weekend.

DISCUSSION

An active monsoon pattern is forecast to remain in place across northern Arizona for the next several days as a ridge of high pressure centered over Colorado continues to pump deep sub-tropical moisture into the region. Precipitable water (PW) values are exceptionally high for mid-July, with guidance showing an inch or more across northern Arizona, surface dew points in the 50s at higher elevations, and dew points in the 60s at lower elevations.

High-resolution model guidance indicates unusually high instability today, with CAPE values, a measure of atmospheric instability, ranging from 1000 to 2500 J/kg across the area. The highest instability is concentrated along and south of the Mogollon Rim. Coincident with the areas of highest instability, pockets of 0-6km wind shear between 20 and 30 knots are forecast. These shear values are higher than typically observed in mid- July, and is expected to enhance storm organization and the severe weather threat. As a result, we will be monitoring for flash flooding alongside a moderate risk for powerful storms capable of producing large hail and damaging wind gusts. Convective development is expected to increase over the higher terrain around noon before storms drift westward at 5-10 mph.

The monsoon high pressure cell is forecast to remain relatively stationary over the next few days, ensuring that deep moisture stays locked in place across northern Arizona. On Tuesday, an inverted trough will move along the western side of the ridge and track across Arizona, passing overhead on Wednesday. This feature may provide additional dynamical lift to aid convective development.

From Thursday onward, a shift in the synoptic pattern is anticipated as the majority of model guidance shows the monsoon high pressure cell strengthening and migrating westward. This retrograding position will bring warming aloft, resulting in more stable conditions. Under this scenario, we would see a downward trend in storm activity and a gradual warming trend through the end of the week. However, this shift will not bring a complete shutdown in monsoon activity. The extended forecast maintains probability of precipitation values of 30-50% over the higher terrain, and 30% or less for the lower elevations.

AVIATION...Tuesday 21/00Z through Wednesday 22/00Z

Scattered to numerous SHRA/TSRA will continue through 06Z. KPGA has remained dry and will continue to stay dry through 05Z. Stronger storms this evening will be capable of producing wind gusts 30-45 kt, hail, and brief MVFR and IFR conditions. After 06Z, activity becoming isolated to low-end scattered. Localized IFR conditions due to low clouds and fog are possible during the overnight hours. Outside of storms, winds light and variable. Another active day is expected of monsoon storms is expected Tuesday. Expect scattered to numerous SHRA/TSRA with isolated +TSRA from late Tuesday morning (~17Z) through the afternoon hours (~00Z). The primary threat will be MVFR to IFR conditions in heavy rain. Gusty outflow winds up to 45 kts and hail are also possible.

OUTLOOK...Wednesday 22/00Z through Friday 24/00Z...Expect scattered to numerous SHRA/TSRA and isolated +TSRA Through Wednesday, then as monsoon moisture decreases SHRA/TSRA coverage will become scattered, but will continue to develop during the late morning through late afternoon hours Thursday and Friday. Any stronger storms will produce brief MVFR to IFR conditions. Some IFR low clouds and fog possible, mainly for overnight hours. Outside of storms, expect light winds around 5-10 kts.

FIRE WEATHER...Today and Tuesday

A medium to high grade monsoon environment continues, with generally light winds outside of outflow and thunderstorm activity. Most storms will produce wetting rain and any stronger storms may produce flash flooding and hail, especially through Tuesday evening.

Thursday through Saturday...Chances for showers and thunderstorms continue, though coverage will decrease somewhat. Generally light winds outside of outflow and thunderstorm activity. Most storms will produce wetting rain and any stronger storms may produce localized flash flooding. Expect a warming trend and minimum RH back to around 20% for the lowest elevations.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch from Tuesday morning through Tuesday evening for AZZ004>009-012>018-039-040.