textproduct: NWS Flagstaff

SYNOPSIS

A series of Pacific storm systems will tap into more tropical moisture beginning Friday, with widespread showers and isolated thunderstorms moving across the state. A Flash Flood Watch is in effect for Yavapai and Gila counties. Shower chances will diminish a bit for Saturday, with a more significant storm system possible for late Saturday night through early Monday. A Flash Flood Watch is in effect for all of northern and central Arizona during this time.

DISCUSSION

Dry and mild weather continues across our area this afternoon, but this will soon change with a couple of significant weather systems due to impact our region over the next few days. The first system is due after midnight tonight through Friday evening. A shortwave over the Pacific will move northeastward across Arizona, drawing northward some deep moisture with precipitable water values close to the 99th percentile for late October. This first system will also bring increased wind shear and instability, peaking over areas of Yavapai and Gila counties and perhaps up to the Mogollon Rim during the day Friday. A few strong to severe thunderstorms are expected, and this area is in a Marginal Risk for severe weather on the latest SPC outlooks. The main threats with these storms will be large hail and damaging winds. A Flash Flood Watch is also in effect Friday into Friday evening for Yavapai and Gila counties where a general 0.50" to 1.00" rainfall is forecast, locally higher amounts to 2.00" in any training thunderstorms. Outside of the watch area, more isolated instances of flooding will still be possible, including the Mogollon Rim up to the Grand Canyon/Kaibab Plateau. A lull in activity is still forecast beginning later Friday evening and continuing through mid evening Saturday as we are in between systems. Mid and upper levels dry out, with just enough low level moisture lingering for an isolated shower, but this is looking minor for any Halloween related outdoor activities.

Late Saturday night through Monday morning - the latest guidance continues to show a major rainfall event across much of our forecast area. Deep moisture from tropical origins is forecast to move into our area from the southwest, with even higher precipitable water values of 0.75"-1.25" nearing the maximum observed for this time of year. Deep low pressure diving down the west coast is forecast by most guidance to at least partially phase with a low off the Pacific, resulting in an extended period of large scale ascent, locally enhanced by strong southwesterly upslope flow. Rainfall amounts are looking impressive, with NBM mean values of 2-3" widespread over the Mogollon Rim, White Mtns, and Gila/Yavapai counties during this period. Given the duration of the event and moisture anomalies, locally higher amounts are not out of the question in the most favored upslope locations. NBM 90th percentile rainfall in these areas is 4-5" in the latest run. North/east of the Mogollon Rim the orographic effect will not be as significant, though some enhancement will occur along the Black Mesa/Chuskas as well as the Kaibab Plateau. Mean rainfall in these areas is still 0.75"-1.50" with locally higher amounts. Given the current degree of saturation of soils due to previous storm systems, we have opted to include all of our forecast area in a Flash Flood Watch beginning at 11 PM MST Saturday and continuing through 5 AM Monday. Flash flooding and small stream flooding will be the initial threat, transitioning to elevated streams and possibly flooding on the larger creeks/rivers even after the precipitation ends. Several of the latest stream/river forecasts are rising to near or above flood stage by Sunday into Monday, including streams coming off the Mogollon Rim into Yavapai/Gila, as well as Moenkopi and the Little Colorado River. Improving weather conditions are forecast from west to east through the day Monday, with generally fair weather expected most of next week.

AVIATION...Friday 30/06Z through Saturday 31/06Z

Mainly VFR conditions overnight, with conditions gradually deteriorating after 12Z along and west of a KGCN-KPAN line. After 18Z, expect that -SHRA/-TSRA and areas of MVFR conditions will spread eastward across Arizona through the day, reaching the New Mexico border by sunset. Gusty/erratic winds possible in/near showers/storms, with southerly winds 5-15 kts expected otherwise.

OUTLOOK...Saturday 31/06Z through Monday 02/06Z...Isolated to scattered -SHRA/-TSRA and areas of MVFR conditions likely through 12Z Saturday. A brief break between storms is forecast from 12Z Saturday to 00Z Sunday, with mainly dry weather but with local MVFR conditions lingering over the mountains. Afterwards, numerous SHRA and isolated -TSRA forecast for central/northern Arizona, where widespread MVFR/IFR conditions are be anticipated. South- southwest winds 5-15 kts on Saturday, strengthening to 10-25 kts on Sunday.

FIRE WEATHER...Friday and Saturday

Look for increasing chances for showers/thunderstorms early Friday, spreading southwest to northeast. Heaviest rain accumulation will be along and south of the Mogollon Rim, where amounts could range between 0.25-1.00 inch. Flooding will be the main concern, but severe thunderstorms are not out of the question. We'll see a brief lull in activity on Saturday, before additional activity ramps up in the evening. Gusty/erratic winds likely near/in showers/storms, with south-southwest winds 5-15 mph otherwise.

Sunday through Tuesday...Numerous showers/storms anticipated on Sunday and continuing into the early morning hours on Monday. Heavy rain leading to flash flooding/flooding will be the main threat. Severe thunderstorms will also be possible, causing gusty/erratic winds, hail, and frequent lightning. Look for south-southwest winds 10-25 mph Sunday, northwest winds 10-20 mph on Monday, becoming light north-northeast winds on Tuesday.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch from Saturday evening through late Sunday night for AZZ004>018-037>040.

Flood Watch from 10 AM MST Friday through Friday evening for AZZ008-018-037-038.