textproduct: NWS Flagstaff

SYNOPSIS

Shower and thunderstorm chances will increase through tonight, with widespread showers and embedded thunderstorms Monday. This is associated with an incoming trough from the West Coast that will interact with tropical moisture and potentially bring heavy rain, strong thunderstorms and flash flooding. A Flood Watch has been issued for Monday and Tuesday which is the time period when the heaviest rain is likely to occur. Shower and thunderstorm chances will then decrease Wednesday and Wednesday night, with mild and dry conditions to start off October.

DISCUSSION

Moisture from tropical cyclones Odalys and Polo is beginning to overspread the area, starting in the high and mid levels at first which has increased cloud cover today, with some light rain trying to reach the surface late this afternoon. The potential for showers and thunderstorms will continue to increase overnight as moisture surges across AZ.

By Monday, a deep West Coast trough approaches with a closed upper-level low expected to develop at its base, then slowly move across the region Tuesday. This trough will act upon the tropical moisture already in place and lift from the approaching trough will generate some embedded thunderstorms that could become strong to severe at times with heavy rain, strong winds and some large hail. WPC continues to message a moderate risk for excessive rainfall for most of the region for Monday and then shifting focus for the eastern part of the state Tuesday as the trough and closed upper-level low move eastward. A Flood Watch remains in effect for Monday and Tuesday. Widespread rainfall amounts of 1 to 2 inches forecast across the high deserts of northeast Arizona have an average return interval of 5 to 10 years, making this at least an uncommon event there. High temperatures across the region Monday and Tuesday will be quite cool due to the cloud cover and rainfall, 10 to 20 degrees below average. By Wednesday night, precipitation chances start winding down in the far eastern parts of the state as the trough sweeps east and takes the moisture plume with it.

One thing to note: Despite the fact the tropical moisture is coming from two tropical cyclones to the south, neither one of these circulations will actually make it up into Arizona. Odalys is looping south and will likely die in the open ocean, Polo is expected to make landfall in Baja California, cross the Gulf of California and break apart to our south when it makes landfall again in mainland Mexico. The strong trough and upper-level low moving in from the west and acting on that tropical moisture drawn north will be the source of the active weather Monday and Tuesday.

By Thursday onward dry air from the west and north will take over and limit the potential for precipitation through the rest of the week. Some ridging will moving in towards the end of the week look for a pleasant start to October as high temperatures recover to a few degrees above average.

AVIATION...Monday 28/00Z through Tuesday 29/00Z

VFR conditions to start the period, lowering to MVFR between 09Z-12Z. Showers and thunderstorms will become more numerous as the night progresses becoming area wide during the day Monday. Winds south 5-15 kts daytime, light and variable overnight. Gusty and erratic winds near storms.

OUTLOOK...Tuesday 29/00Z through Thursday 01/00Z...Expect scattered to numerous showers and thunderstorms area wide through Tuesday afternoon with the focus of precipitation then shifting towards eastern Arizona on Wednesday. Look for areas of MVFR conditions with brief IFR conditions in heavier showers and thunderstorms. Daytime winds variable at 5-15 kts Tuesday, west to northwest at 5-15 kts Wednesday. Light and variable at night.

FIRE WEATHER...Tonight through Tuesday

Precipitation chances will ramp up overnight tonight with widespread wetting rain and embedded thunderstorms on Monday, presenting a moderate flood and flash flood risk. Widespread precipitation expected Tuesday with flash flood chances continuing. Precipitation will begin to focus more over eastern Arizona Tuesday evening. Winds S 5-15 mph on Monday, N-NE 10- 20 mph on Tuesday.

Wednesday through Friday...Some lingering precipitation chances in far eastern Arizona Wednesday, drying out elsewhere. Dry with no precipitation across the state Thursday and Friday. Winds NW-N 5-15 mph on Wednesday, light and variable 5-10 mph Thursday, light and variable most areas Friday with the exception of some winds E 5-15 mph along the Rim.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch from 11 PM MST /midnight MDT/ this evening through Tuesday evening for AZZ004>018-037>040.