textproduct: NWS Flagstaff

SYNOPSIS

An active weather pattern will continue through the end of the week. A strong weather system will bring widespread showers and thunderstorms beginning Wednesday morning and continuing through Thursday. The primary concerns are flooding and flash flooding, along with the potential for severe thunderstorms.

DISCUSSION

A very active weather pattern is expected across the state through the next week. A strong low pressure system just off the California coast today, will move inland and through the Desert Southwest through Thursday. This storm is unusually strong for this time of year and with that will come a variety of hazards. The low eventually lifts through the Great Basin on Thursday, dragging a cold front through the region late Thursday into Friday. A piece of energy breaks away from the departing low, then becomes a closed low over Arizona and lingers through the weekend.

Late Tonight though Thursday...A very active couple of days are expected for Wednesday and Thursday across the entire forecast area. Let's break it down by hazard.

Rainfall: Strong moisture advection from the south will continue through Thursday. There is a good plume of moisture being brought up from the western coast of Mexico and through the Gulf of California. Some of this moisture is left over from post-tropical cyclone Trudy and is enhanced by the continued active tropical activity in the eastern Pacific. Showers and thunderstorms will develop by late tonight and spread northward through the state and become widespread in coverage on Wednesday and Thursday. PWAT values have risen slightly over the past several model runs and range from 0.8-1.25", which are anomalously high and above the 95 percentile for the region this time of year. Rainfall amounts will vary across the region but expect widespread 1.0-3.0" amounts to occur. Some locally higher amounts between 3-5" are certainly possible, especially in areas that see continued convective storms and multiple rounds of precipitation. Models continue to show the highest amounts along and south of the Mogollon Rim. Additionally, areas across the Navajo Nation may see 1-2" of rainfall.

Flooding/Flash Flooding: The expected rainfall combined with the fact that we have wet antecedent conditions, will result in a flood and flash flood risk. The convective nature of storms over the next two days could result fairly high precipitation rates, which may lead to flash flooding. Be advised that area creeks and streams along with main stem rivers may see 2-3 foot rises. The Flood Watch is in effect for the entire forecast area from early Wednesday morning through late Tuesday evening.

Severe Weather: A strong mid-level jet is expected to move over the region through the next couple of days. Strong southwesterly winds will strengthen and combine with increasing moisture through the mid levels, with steepening lapse rates. Shear is high with this system (50+ kts) and CAPE values are higher than usual for mid/late October and in the 500-1000 J/kg range. A limiting factor for stronger storms would be the increasing cloud cover but we look to have enough shear and lingering instability to keep storms propagating through the evening and overnight periods. Storm relative helicity remains high with modeled helicity updrafts showing support for supercell activity and rotating storms. The Storm Prediction Center has placed parts of our area in an Enhanced Risk (category 3 out of 5) of severe weather for Wednesday and in a Slight Risk (category 2 out of 5) for the much of eastern Arizona for Thursday. All severe hazards (damaging winds, large hail, a few tornadoes).

Be prepared and have a way to receive warnings!

Friday and onward...The main low pulls off to the east but some of its energy separates from the flow and may become a closed low over Arizona through the weekend and into early next week. For now, shower and thunderstorm activity decreases by late Thursday into early Friday with only isolated coverage for Friday through the weekend. Cooler temperatures are expected to continue with generally light winds.

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

VFR to MVFR, especially near heavier showers or thunderstorms. Expect scattered to numerous showers and thunderstorms to develop after 06Z and spread northward through Wednesday morning. Numerous showers and thunderstorms throughout the day Wednesday. There is potential that some storms could become severe on Wednesday, producing large hail or damaging wind. Winds light and variable or terrain driven overnight, becoming SE-S 10-20kts Wednesday. Gusty and erratic winds near thunderstorms.

OUTLOOK...Thursday 22/00Z through Saturday 24/00Z...VFR to MVFR conditions, especially near heavier showers and thunderstorms. Numerous showers and thunderstorms Thursday, some may be severe with damaging winds and large hail. Activity diminishes from west to east late Thursday night. Winds S-SW 10-20 kts with gusts to 30 kts Thursday, SW-W 10-20kts Friday.

FIRE WEATHER...Tonight through Thursday

Increasing showers and thunderstorms late tonight become widespread on Wednesday and Thursday. Some storms may produce very heavy rainfall leading to flooding or flash flooding both days. Some stronger storms may produce strong winds and hail. Outside of storms, expect south winds 10-20 mph with gusts 30-40 mph on Wednesday, becoming south through southwest on Thursday. Daytime temperatures near to below normal.

Friday through Sunday...Isolated showers and thunderstorms are expected for Friday through Sunday. Southwest through west winds 10- 15 mph with gusts to 25 mph on Friday, then light northerly winds 5- 15 mph on Saturday and Sunday.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch from late tonight through Thursday evening for AZZ004>018-037>040.