textproduct: NWS Flagstaff

SYNOPSIS

A very active period of weather is ongoing and a Tornado Watch is in effect through 6 PM MST for Coconino, Navajo, and Yavapai counties. A strong weather system is bringing widespread showers and thunderstorms this afternoon, and the threat will shift more toward eastern Arizona 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 continue to evolve across Arizona through Thursday evening, with threats of flooding/flash flooding, severe thunderstorms, and tornadoes. A strong low pressure system sliding south-southeastward along the coast of California. This pushing a deep moisture surge, with tropical and subtropical origins, and providing an environment favorable for severe weather. The low will eventually lifts through the Great Basin on Thursday, shifting the area of heavy weather and strong to severe thunderstorms more eastward, across eastern AZ, dragging a cold front through the region late Thursday into Friday.

It as already been an active morning with severe thunderstorm and tornado warnings as mesoanalysis showed 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 with MLCAPE values around 250-500 J/kg across most of the region. This transport of warm air and high dewpoints is continuing and MLCAPE is now around 500 with pockets of 1000 J/kg across the region. This is still coupled with bulk shear around 50kts and 1km storm relative helicity 100-250 m2/s2, we are still seeing scattered discrete supercell thunderstorms initiating along and north of the higher terrain. Latest SPC mesoanalysis does shows a slight straighting of the hodograph, which is supportive of both left and right moving supercells, producing large hail, while still maintaining a tornado threat. There is a Tornado Watch in effect for Coconino, Navajo, and Yavapai counties through 6 PM MST. This is expected to a slight decline in extent for a couple hours after 6 PM as conditions become slightly unfavorable, however the overall environment will continued to support thunderstorms overnight with severe weather still a possibility. Severe thunderstorms bringing 1-2 inch hail have remained the primary threat thus far, with an isolated tornado risk. A secondary threat of flash flooding is also in play given the deep moisture. However, fast storm motion with localized heavy rainfall rates in thunderstorms will help to limit the areal coverage of flash flooding. Greatest risk for flash flooding will be sensitive burn scars, washes, and areas susceptible to water running off onto roads and nearby property.

Thursday and Thursday evening...Threats of flash flooding and severe thunderstorms continue through the period, though the main area is expected to shift more towards eastern Arizona as the strong upper-level low moves over Arizona and then starts to push off to the east Thursday evening. 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 continues 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 on Thursday, it is expected to shift a bit further east towards the Eastern Mogollon Rim and White Mountains with potential for strong storms along the propagating cold front. SPC for Thursday as a Marginal risk (level 1 out of 5) for most of northern AZ, with a small Slight risk (level 2 out of 5) for portions of Gila and far southern Navajo and Apache counties. The 500 mb jet is expected start weakening Thursday afternoon and instability may be more limited for Thursday, though breaks in the clouds would increase instability due to daytime heating. Continue to monitor the weather and for any updated information.

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...Thursday 22/00Z through Friday 23/00Z

Areas of SHRA and TSRA through the period, along with ISOLD +TSRA. Large hail and possible tornadoes with severe storms. Gusty winds near any storms. Winds generally light until 18Z-02Z Thursday, when S-SW10-20g25kts develops. Expect frequent MVFR/IFR.

OUTLOOK...Friday 23/00Z through Sunday 25/00Z...Areas of SHRA and TSRA moving east Thursday night, along with ISOLD +TSRA. Any areas of precipitation will produce MVFR/IFR. Coverage becoming more ISOLD by Friday and especially Saturday. Sfc winds S10-15kts Friday and light for Saturday.

FIRE WEATHER...This afternoon through Friday

Scattered showers and thunderstorms are becoming more widespread this afternoon and will continue through Thursday. Some storms may produce very heavy rainfall leading to flooding or flash flooding today and Thursday. Some stronger storms may produce strong/severe winds, large hail, with isolated tornadoes possible. Outside of storms, expect south winds 10-20 mph with gusts 30-40 mph, becoming south through southwest on Thursday. Daytime temperatures near to below normal. The potential for heavy rainfall and strong thunderstorms is expected to decrease Thursday night and Friday with southwest to west winds 10-15 mph with gusts to 25 mph.

Saturday through Monday...Isolated showers and thunderstorms are expected Saturday and Sunday, with a slight chance for Monday. Expect light northerly winds 5-15 mph on Saturday and Sunday, becoming more northeast to east Monday but remaining light.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch until 6 PM MST /7 PM MDT/ this evening for AZZ004>018-037>040.