textproduct: NWS Flagstaff

SYNOPSIS

A very active period of weather is anticipated over the next two days. A strong weather system is bringing widespread showers and thunderstorms this morning, and will continuing through Thursday. The primary concerns are flooding and flash flooding, severe thunderstorms, and even a few tornadoes. Drier conditions with lingering slight chances for showers are expected by Friday into the weekend.

DISCUSSION

A concerning situation will evolve across Arizona over the next two days, with threats of flooding/flash flooding, severe thunderstorms, and tornadoes. A strong low pressure system off the California coast is beginning to move inland. This is well positioned to carry a deep moisture surge of tropical and subtropical origins, as well as provide an ideal environment for severe weather. The low eventually lifts through the Great Basin on Thursday, dragging a cold front through the region late Thursday into Friday.

This morning...The first surge of tropical moisture has arrived, and activity is ramping up quickly early this morning. Current mesoanalysis shows a 50-55kt jet overhead at 500mb, as well as a strengthening southeasterly low-level jet advecting warm air and high dewpoints into northern AZ. This has allowed for increasing instability south of the Rim during the overnight hours, with MLCAPE values currently around 250-500 J/kg in Yavapai and Gila counties. Coupled with bulk shear around 50kts and storm relative helicity 100- 200 m2/s2, we are seeing scattered discrete supercell thunderstorms initiating along the terrain as of 2 AM MST. This is expected to decline in extent over the next few hours as conditions become slightly more unfavorable, however continued thunderstorms activity is expected especially further west. Severe thunderstorms bringing 1- 2 inch hail have remained the primary threat thus far. A secondary threat of flash flooding is also in play this morning given the arrival of deep moisture. However fast storm motion and scattered coverage will allow rain accumulations to likely remain below flash flooding concerns outside of sensitive burn scars.

Today through Thursday evening...Threats of flash flooding and severe thunderstorms continue through the period. For flooding, we will remain in a continuous fetch of deep tropical/subtropical moisture with modest instability and enhanced low-level flow. Precipitable Water (a measure of atmospheric moisture) will rise to 0.8-1.2 inches across most of our area. This is highly anomalous for mid-October. Despite the fast expected storm motion, periods of training or back-building storms are possible and could lead to locally heavy rainfall rates with flash flooding concerns. Also several rounds of heavy rain are expected for most, and longer duration flooding of rivers, streams, normally dry washes, drainage basins, etc is possible. This is exacerbated further by very saturated surface conditions for all of northern AZ. As a result, a Flood Watch will be in effect beginning at 5 AM MST today and continuing through 11 PM MST Thursday. The threat of flooding ends on Thursday evening as a cold front sweeps from west to east and brings much drier air in its wake.

For the severe threat, an unstable environment with an impressive shear profile will be maintained through the period. The jet at 500mb will continue to strengthen on Wednesday as the low pressure system slowly approaches. Despite cloud cover to limit destabilization, hi-res guidance remains adamant that CAPE values up to 1000-1500 J/kg will develop by Wednesday afternoon with help from low-level moisture advection and steepening mid-level lapse rates. Several rounds of severe weather are possible, with the peak threat window beginning late Wednesday afternoon through evening. Stronger supercell storms will be capable of large 2+ inch hail, damaging wind gusts, and even a few tornadoes. SPC maintains an Enhanced risk (level 3 out of 5) of severe weather centered along the Mogollon Rim on Wednesday. Into Thursday, the severe threat looks to shift a bit further east towards the Eastern Mogollon Rim and White Mountains with potential for strong storms along the propagating cold front.

Be prepared and have a way to receive warnings!

Friday and onward...Activity is expected to trend downward by Friday into the weekend after the main trough axis pushes past Northern AZ and drier air moves in. A closed low may linger across western AZ, and slight chances for showers and thunderstorms remain in the forecast each day. Expect lighter winds and temperatures remaining below normal otherwise.

AVIATION...Wednesday 21/12Z through Thursday 22/12Z

Scattered to numerous showers and thunderstorms this morning, becoming widespread by the mid-to-late afternoon. Drops to MVFR/IFR, especially near heavier showers or thunderstorms. Occasional severe storms with large hail or damaging wind are expected. Gusty and erratic winds near thunderstorms, else winds are SE-S 10-20kts gusting to 30 kts after 18Z Wednesday.

OUTLOOK...Thursday 22/12Z through Saturday 24/12Z...Periods of widespread showers and thunderstorms continue Thursday, then activity diminishes from west to east late Thursday night. Some severe storms with large hail or damaging winds expected. Drops to MVFR/IFR, especially near heavier showers or thunderstorms. Much drier by Friday, with slight chances for lingering showers. Winds S-SW 10-20 kts with gusts to 30 kts Thursday, SW-W 10-20kts Friday. Gusty and erratic winds near storms.

FIRE WEATHER...This morning through Thursday

Increasing showers and thunderstorms this morning becoming 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 large 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 through Thursday evening for AZZ004>018-037>040.