textproduct: NWS Flagstaff

SYNOPSIS

Northern Arizona will remain in an active weather pattern through the week. A few isolated showers and thunderstorms are possible on Tuesday, then a strong weather system will bring high chances for showers and thunderstorms beginning Wednesday morning and continuing through Thursday. The primary concerns are flooding and flash flooding, especially across eastern Arizona. There is a secondary concern for severe thunderstorms, though there is lower confidence in this hazard.

DISCUSSION...Today

Current mesoanalysis and satellite imagery reveals a subtle shortwave and a weak 40kt jet at 500mb over eastern Arizona. This has combined with marginal instability (100 J/kg of MUCAPE) to allow for very isolated and weakly rotating thunderstorms across the White Mountains as of 2 AM MST this morning. This is expected to end within a few hours as the mid- level pattern shifts northeastward. Into today, a deep upper-level low will continue to rapidly dive southward along the West Coast while ridging over the central CONUS amplifies. This will help lift the weak shortwave fully out of Arizona, and brief subsidence aloft will lead to suppressed daytime shower activity. Still a few weak showers or even thunderstorms will remain possible especially towards the Kaibab and the White Mountains.

Late tonight through Thursday...Beginning late tonight, the West Coast low will begin to push eastward and carry a deep surge of subtropical and tropical moisture to northern Arizona. This moisture surge is somewhat enhanced due to continued active tropical cyclone activity across the Eastern Pacific. PWAT values (a measure of atmospheric moisture) are forecast to rise up to 0.8-1.1 inches, well above normal for mid-October. As a result, there will widespread showers and thunderstorms bringing a primary hazard of Flooding/Flash Flooding through the period. The current forecast calls for the heaviest rainfall accumulations along/south of the Mogollon Rim, with a 1.0-3.0 inches expected. Another area of concern will be the Navajo Nation, where widespread rainfall accumulations of 1.0-2.0 inches are forecast. Localized heavy rainfall will remain possible even outside of the regions. As a result, a Flood Watch has been issued for all of Northern and Central Arizona from 5 AM MST Wednesday-11 PM MST Thursday (6 AM MDT Wednesday-12 AM MDT Friday). Have a way to receive Flash Flood Warnings and be prepared to take action should warnings be issued.

There also remains a Severe Thunderstorm threat. A strong 500mb jet streak moves overhead through the period, and 50+ kts of shear are likely. There still is a degree of uncertainty in the available instability, however mesoscale models remain adamant that we will see CAPE values 500-1500 J/kg from high surface dewpoints and steepening mid-level lapse rates. If the higher instability manifests, there will be severe hail and wind threats. The shear profile also suggests potential for rotating storms, and even an isolated tornado threat. The SPC maintains a Marginal risk (category 1 out of 5) of severe weather across all of Northern and Central AZ on Wednesday and Thursday, and even a Slight risk (2 out of 5) for severe storms across the Eastern Mogollon Rim and White Mountains on Wednesday. Besides heavy rain and thunderstorms, expect gusty daytime winds up to 30-40 mph on Thursday, though likely remaining below Advisory criteria.

Friday and onward...Activity is expected to trend downward by Friday into after the main trough axis pushes past Northern AZ and drier air moves in. Some slight chances for showers look to linger each day, but there is low confidence during this period. Expect lighter winds and temperatures remaining below normal otherwise.

AVIATION...Tuesday 20/12Z through Wednesday 21/12Z

Primarily VFR, periods of MVFR/IFR in heavier showers and storms. Isolated showers and thunderstorms possible over the White Mountains and Kaibab Plateau after 18Z. Numerous to widespread showers and thunderstorms begin moving in from the south after 06Z Wednesday. Winds S-SW 5-15kts gusts to 20kts daytime Tuesday, light and VRB overnight. Gusty and erratic winds expected near storms.

OUTLOOK...Wednesday 21/12Z through Friday 23/12Z...Widespread shower and thunderstorm activity bringing periods of MVFR/IFR conditions. Activity diminishes from west to east late Thursday evening/night. Winds S 10-15kts with gusts to 25kts Wednesday, S-SW 10-20kts with gusts to 30kts Thursday.

FIRE WEATHER...Today through Wednesday

Isolated showers and thunderstorms are possible during the daytime today, then activity will increase in coverage late Tuesday and become widespread on Wednesday. Some storms may produce very heavy rainfall leading to flooding or flash flooding on Wednesday. Winds are south 10-15 mph with gusts to 25 mph on Tuesday, then 10-20 mph with gusts 25-35 mph on Wednesday. Daytime temperatures near to below normal.

Thursday through Saturday...Widespread showers and thunderstorms continue on Thursday, becoming isolated for Friday and Saturday. Some storms may produce very heavy rainfall leading to flooding or flash flooding on Thursday. South through southwest winds 15-25 mph with gusts 30-40 mph on Thursday, becoming southwest through west 10- 15 mph with gusts to 25 mph on Friday, then light northerly winds 5- 15 mph on Saturday.

FGZ WATCHES/WARNINGS/ADVISORIES

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