Prescott, AZ 86301 Weather

Synopsis

Dry weather and seasonable temperatures will continue through Thursday. A series of Pacific storm systems will tap into more tropical moisture beginning Friday, with widespread showers and isolated thunderstorms moving across the state. Shower chances will diminish a bit for Saturday, with a more significant storm system possible for late Saturday night through early Monday.

Discussion

The proverbial calm before the storm gives us another relatively mild and dry day today, then everything changes by late tonight and into Friday.

A deepening long-wave trough over much of the eastern Pacific will begin to draw sub-tropical moisture and remnant Monsoonal moisture northward by tonight. A weak s/wv around 30N will then move into Arizona Friday morning, providing marginal vertical ascent, shear and instability. The instability will peak early, as lower-level moisture moves into the central deserts, before fading by Friday evening as the column becomes saturated.

This moisture and associated forcing will bring about a 12-18 hour period of showers and thunderstorms to our area, especially to the south of the Mogollon Rim and into the foothills of Phoenix Metro. Some hi-res models indicate rainfall totals of over 2 inches by Friday evening, driven locally by training thunderstorms. Due to already saturated conditions and recent flooding, we have issued a Flash Flood Watch for Yavapai and northern Gila counties on Friday. This may need to be pushed northward, but we will focus on the higher confidence regions for now.

Later Friday night, the instability fades and weak subsidence develops behind the exiting wave. High moisture values will stick around into Saturday, but without the forcing and instability, we expect just isolated showers.

By late Saturday evening, a significant tropical moisture plume originating around 24N will take aim at Arizona, as the now fully developed west coast trough starts to move inland. The main trough will pick up another s/wv around 29N by Sunday morning, attempting to phase all this moisture and the s/wv with the main trough passage Sunday afternoon.

This is a somewhat crazy situation and although there is still some timing/phasing issues Sunday, confidence is growing for yet another major precipitation event. Additionally, the main trough will drive a cold front through our area late Sunday and into early Monday. This will provide additional lift, shear and instability, as well as lowering snow levels.

The QPF from early Sunday through early Monday will range from about 1 to 3 inches. Locally higher amounts are possible over our southern CWA, closer to the tropical moisture plume. The increasing shear and colder aloft Sunday afternoon may also lead to severe weather. It is quite likely we will need additional headlines for Sunday and Sunday night, with flooding becoming a serious concern. Let us get through Friday first.

Finally, high pressure returns from Tuesday onward next week, for warming and drying conditions.

Aviation

Thursday 29/18Z through Friday 30/18Z: Mainly VFR conditions expected, but could see conditions gradually deteriorate after 12Z primarily along and west of a KGCN-KPAN line. Expect -SHRA/-TSRA to move from the southwest to northwest through Friday, with heaviest showers and greatest coverage for storms to be along and south of higher terrain. Gusty/erratic winds possible in/near showers/storms, with southerly winds 5-15 kts expected otherwise.

OUTLOOK: Friday 30/18Z through Sunday 01/18Z: VFR/MVFR conditions likely through 12Z Saturday from scattered to widespread -SHRA/-TSRA. A brief period of calm/dry conditions expected between 12Z Saturday and 00Z Sunday. Afterwards, numerous showers/thunderstorms forecast for central/northern Arizona, where widespread MVFR/IFR conditions can be anticipated. Gusty/erratic winds likely near/in showers/storms, with south-southwest winds 5-15 kts on Saturday, strengthening to 10-25 kts on Sunday.

Fire weather

Thursday and Friday: Dry conditions continue for Thursday, with seasonable temperatures. Increasing moisture Thursday night will lead to widespread showers and thunderstorms Friday, with chances for flooding south of the Mogollon Rim. Daytime winds will continue to be generally light, ranging from south to southwesterly at 5 to 15 mph.

Saturday through Monday: Shower coverage becomes isolated much of Saturday, before a significant moisture event develops late Saturday night through early Monday. Heavy rainfall, colder temperatures and even severe weather is possible, especially Sunday. Southerly winds at 10 to 15 mph continue Saturday, before increasing to 15 to 25 mph on Sunday. Winds shift west to northwest Monday, 10 to 20 mph with gusts from 20 to 30 mph.

Watches, warnings and advisories

Flood Watch from Friday morning through Friday evening for AZZ008-018-037-038.