textproduct: NWS Flagstaff

SYNOPSIS

Warm temperatures continue with isolated to scattered showers and thunderstorms today. Cooler temperatures and more widespread showers and thunderstorms are expected beginning Monday and lasting through at least Thursday.

DISCUSSION

Strong high pressure will be centered near the four corners today, with slight weakening and an eastward shift for Monday. For today, precipitable water values are near normal for early August with surface dewpoints in the 40s along/north of the Mogollon Rim and 50s to the south. With daytime heating we will see scattered thunderstorm development by early afternoon along the higher terrain, with storm motion toward the west/northwest toward the lower elevations of Gila/Yavapai later in the afternoon and evening. While the guidance doesn't indicate high coverage of precipitation in these areas (PoP 30-50%), there is a good chance of gusty outflow winds impacting much of this region this evening. Isolated severe gusts (58+ mph) will be possible. With the high still in place, an Excessive Heat Warning remains below 4500 feet for the Grand Canyon and Marble/Glen Canyons until 8 PM this evening. With slight weakening of the high, and additional cloud cover, we opted not to extend the warning into Monday.

By Monday, we are anticipating an increase in storm activity that should last through at least Thursday. High pressure will shift slowly eastward as a weak trough approaches the west coast. This will induce more of a south/southeast flow and allow for higher moisture values to advect into our region. Forecast precipitable water during this time is near 1.00" along the Mogollon Rim/WHite Mtns and up to 1.50" for the lower elevations of Yavapai/Gila Co. With this additional moisture, storms will be more efficient rainfall producers and the risk of at least localized flash flooding will be on the increase. In addition, storms may continue later into the evening and even overnight, especially Tue/Wed nights. A disturbance is also shown moving up from the south by several of our models late Wednesday into Thursday which could increase storm activity further. All in all, things are looking pretty active Monday through Thursday, then some drier air is forecast to move in from the southwest by Friday or Saturday and remain through the weekend. This would likely not shut off convection completely, but result in less daily coverage.

AVIATION...Sunday 09/18Z through Monday 10/18Z

Mainly VFR. ISO- SCT SHRA/TSRA 20Z-03Z, most likely INVOF a KGCN-KFLG-KJTC line. Brief MVFR conditions and gusty outflow winds in excess of 40 kts possible in and around TS. Additional SHRA/TSRA possible INVOF KPRC/KPAN through 12Z. Winds W/SW 5-15 kts, becoming light and VRB after 03Z.

OUTLOOK...Monday 10/18Z through Wednesday 12/18Z...Mainly VFR. SCT- NMRS SHRA/TSRA each day from 19Z-03Z, especially over the higher terrain. Brief MVFR/IFR conditions and gusty outflow winds in excess of 40 kts possible in and around +TS. Otherwise, daytime winds W/SW 5-15 kts, becoming light and VRB overnight.

FIRE WEATHER...Monday and Tuesday

Cooler temperatures, along with increasing chance for showers and thunderstorms. Greatest chances will mainly be over the higher terrain. Heavy downpours, gusty winds, and hail will be possible in any stronger storm. Outside of storms, expect variable winds around 5-15 mph.

Wednesday through Friday...Below normal temperatures, with continued daily chances for scattered to widespread showers and thunderstorms. Outside of storms, winds will be west/southwest 5-15 mph.

FGZ WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until 8 PM MST /9 PM MDT/ this evening for AZZ005-006.