textproduct: NWS Flagstaff

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 Sunday. Stay tuned.

DISCUSSION

Some high clouds will stream across the area today as a long-wave trough slowly forms over the eastern Pacific. This will continue into Thursday, although hghts will not change that much. This means 2 more days of relatively mild and dry weather, with mainly light southwesterly winds each afternoon.

This developing long-wave trough will become the focal point for our weather from Friday onward, with 2 distinct weather producers on Friday, and again on Sunday.

The first wave of rainfall begins very early Friday, likely peaking over the south-central deserts Friday afternoon. This is in response to amplifying southwesterly flow that will pick up remnant tropical moisture from the Gulf of California and northern Sonora. Some of this moisture is from the decaying T.S. Vance and some is still our remnant Monsoonal moisture pool, that has never really been swept out. A weak s/wv trough in the southwesterlies will act on this moisture, bringing about a 12 hour period of precipitation. It does look as though most of this will be showery, possibly tied closely to upslope regions. The risk of strong thunderstorms is low at this time.

This moisture is shunted east Friday night and into Saturday, with weak subsidence behind the exiting s/wv. Moist southwesterly flow persists Saturday, so some drippy showers remain possible, but not as widespread as Friday.

Later Saturday night and into Sunday, a much more significant event is possible as the main long-wave trough moves inland. As it does so, it will attempt to merge with a sub-tropical wave moving through the eastern Pacific at around 28N/29N. This may be a potent scenario and could lead to an extended period of moderate to locally heavy rainfall. Additionally, increased shear, large-scale ascent and colder air aloft raises the risk for severe weather. Due to the uncertainty with these 2 waves ability to phase, it is better to take a partially conservative approach. This still results in fairly widespread and rainy conditions, but a bit early to jump on board with severe weather and extensive flooding.

It is important to mentally prepare for the possibility of severe weather and extensive flooding, but that is not quite the official forecast yet. The combination of the 2 events could produce 1 to 4 inches of rain across the area, especially over the south-central deserts and just south of the Mogollon Rim. Stay tuned for updates in the coming days.

AVIATION...Wednesday 28/18Z through Thursday 29/18Z

VFR conditions prevail with cloud bases at or above 15kft throughout the period. However, some localized VIS reductions are possible near terminals KFLG and KSEZ due to prescribed burns. South-southwest breezes this afternoon with gusts 15-20 knots possible. Winds quickly weaken after sunset and remain light overnight.

OUTLOOK...Thursday 29/18Z through Saturday 31/18Z...Expect VFR conditions through 10Z-12Z Friday. Then, areas -SHRA/SHRA and ISOLD -TSRA producing localized MVFR for the remainder of the period. Daytime sfc winds S-SW10-15kts.

FIRE WEATHER...Wednesday and Thursday

Dry conditions and seasonable to slightly above normal daytime temperatures. Expect afternoon south to southwest winds at 5 to 15 mph.

Friday through Sunday...Expect cooler conditions Friday through Sunday, with chances for showers and thunderstorms that are likely to produce significant rainfall. The greatest potential for heavier rainfall looks to be on Sunday. Expect afternoon south to southwest winds at 10 to 20 mph each day.

FGZ WATCHES/WARNINGS/ADVISORIES

None.