textproduct: NWS Flagstaff

SYNOPSIS

Increasing chances for showers and thunderstorms are expected through this weekend, along with cooler temperatures, as monsoonal moisture increases over the area. Flash flooding will be possible Saturday through Monday. Daily chances for storms look to potentially continue into much of next week.

DISCUSSION...Today

High pressure over Arizona begins to gradually weaken and move eastward today. With less subsidence aloft, coverage of afternoon showers and thunderstorms looks to increase today. Latest hi-res guidance has the greatest coverage mainly from the Mogollon Rim and northward, given the southwesterly flow. However, a few storms also look possible across Yavapai County, as they initiate off the Bradshaw Mountains.

Flash flooding will be the primary concern with today's storms, with 14Z RAP guidance showing PWATs around 0.80-1.10 inches. 0-6km shear does look to gradually increase (~20-25 kts) through the day across northwestern Arizona ahead of a trough from the west. However, MLCAPE looks to largely remain at most 500-1000 J/Kg across western Arizona. Therefore, the threat for widespread severe weather is low, bit an isolated strong to severe storm can not be totally ruled out.

There is the potential for some overnight convection to move northward this evening from the Lower Deserts. However, hi-res guidance this morning seems to be backing off on this potential. Nevertheless, some slight chance were maintained with the forecast into the overnight hours. The best chances for this look to mainly be near the I-17/US-89 corridor, potentially into Navajo and Gila counties.

Saturday through Monday...An active period of monsoonal conditions is expected this weekend into Monday. As the ridge gradually shifts eastward, a trough over the Pacific Northwest begins to dig into the West Coast. Increasing monsoonal flow will bring deep monsoonal moisture northward, with PWATs exceeding an inch of the higher terrain to near 1.5 inches across central Arizona. Additionally with the trough, added dynamics will help promote nocturnal activity into both Saturday night and Sunday night.

While storm motions of 15-25 kts will limit the overall residency time of any one storm, the likelihood of numerous coverage will lead to a high probability of training storms. Thus, flash flood will remain a concern through much of the weekend. This will especially be true over area fire scars, slot canyons, and other rain sensitive locations.

Given the added dynamics, a few strong to severe storms will be possible during the afternoon hours each day. The exact locations at greatest risk will largely be dependent upon clearer skies during the morning and early afternoon hours, to allow for destabilization from daytime heating. The primary threat from any severe storm would be large hail, with some potential for gusty winds in the lower elevations.

Outside of the precipitation chances, cooler and breezier conditions are expected. Afternoon highs by Monday are forecast to be nearly 5- 10F below normal for this time of year, with highs in the low to mid 70s over the High Country and mid 80s to low 90s in the Lower Deserts.

Tuesday through Thursday...A fast-moving trough looks to move through Great Basin near the middle of next week. At the same time, Tropical Storm Karina looks to initiate a surge of sub-tropical moisture. The extent of the moisture push still remains in question, given how far the storm looks to be from Baja. Increased southerly flow with the trough however should help pull the moisture into at least central Arizona. Most guidance also indicates the potential for another tropical cyclone to develop near the end of the week behind Karina.

As a result, continuing period of at least semi-active monsoonal conditions looks probable through the coming week. Uncertainty still remains however, with many of the GEFS, EPS, and CMCE members split between PWATs remaining high and ticking downward. The operational guidance does have heights tick upwards slightly, so a slight warming trend seems possible, but overall seasonable to slightly cooler than normal temperatures look to continue.

AVIATION...Friday 28/18Z through Saturday 29/18Z

Mainly VFR conditions, localized/brief MVFR possible in storms. Isolated to scattered showers/storms this afternoon, becoming mainly isolated after 04Z into Saturday morning. Winds SW-W 5-15kts this afternoon. Light and variable or terrain driven overnight. Gusty and erratic winds possible near storms with the potential for gusts up to 40 kt.

OUTLOOK...Saturday 29/18Z through Monday 31/18Z...VFR conditions with brief MVFR/IFR possible in heavier storms. Scattered to numerous showers/storms Saturday afternoon across all of northern Arizona with some of these storms lasting overnight Saturday into Sunday morning. Scattered to numerous showers/storms Sunday afternoon into Monday, including the overnight period. Winds SW-W 10-20kts daytime, light and variable or terrain driven overnight. Gusty and erratic winds near storms.

FIRE WEATHER...This afternoon through Sunday

Isolated to scattered showers and thunderstorms this afternoon and into the evening hours. Humidity and storm coverage increase significantly this weekend, with storms likely persisting through the overnight hours both Saturday night and Sunday night. Outside of gusty and erratic storm driven winds, expect daytime southwest to west winds around 10-15 mph with gusts 20-25 mph. Variable winds at night.

Monday through Wednesday...Scattered to numerous showers and thunderstorms Monday, with coverage decreasing to isolated/scattered Tuesday and Wednesday. High temperatures cool to slightly below normal. Outside of storms, expect southwest to west winds around 10- 15 mph with gusts 20-25 mph each afternoon, becoming variable at night.

FGZ WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until 8 PM MST this evening for AZZ006-037.