textproduct: NWS Flagstaff

SYNOPSIS

Deep monsoonal moisture will remain over the area through this weekend and well into next week. With this moisture comes the daily threat of thunderstorms, along with the risk for locally heavy rainfall and flooding. With the added moisture and cloud cover, most daytime temperatures will remain below normal.

DISCUSSION

Despite a night of widespread nocturnal thunderstorm activity, skies are the overall the clearest they have been in several days this morning. The increased heating and delay in convective initiation until the afternoon does increase the chance for some stronger wind gusts, especially just north of the rim. However, the main focus remains isolated flash flood potential with yet another day that features PW values and dew points about as high as the Monsoon Season can furnish. It will rain, just a question of when and where. Our current steering flow (this governs where storms go after they form) remains easterly or northeasterly for the next several days. This steering flow may transition to more of a southeasterly direction later next week, but that remains to be seen.

The current steering flow heavily favors locations just to the southwest, south and west of the central Mogollon Rim. A secondary area usually forms later each day across Apache and Navajo counties or over western New Mexico. These secondary areas are what supports nocturnal convection, which is expected at least for the next few nights. The nocturnal signal is fairly strong, largely due to the delayed convective development each day.

Compounding the issue is the arrival of an easterly wave, likely arriving Monday and Tuesday, possibly residing over central Arizona even into Wednesday. This easterly wave will increase storm motion and shear, which could strengthen storms and lead to a hail threat. While increased storm motion may lessen the flooding threat, it also leads to the possibility of training storms (additional thunderstorms forming over the same location). The bottom line, a very active and possibly historically wet period will continue until further notice.

Pertaining to the daily possibility of a Flash Flood Watch, we will again hold off, due the the complications of convective development that were outlined above.

AVIATION...Sunday 19/18Z through Monday 20/18Z

Widespread VFR conditions will prevail across northern Arizona this morning, before localized restrictions develop this afternoon. Scattered to numerous showers and thunderstorms are expected to produce brief MVFR/IFR visibilities after 21Z, with the greatest chances for reductions at higher elevation terminals. localized IFR conditions due to low clouds and BR are possible during the overnight hours. Outside of storms, surface winds are not anticipated to affect flight operations.

OUTLOOK...Monday 20/12Z through Wednesday 22/12Z...Scattered to numerous showers and thunderstorms continue through the period, with the greatest storm activity during the afternoon/evening hours. Stronger storms will produce brief MVFR and IFR conditions. localized IFR conditions due to low clouds and BR are possible during the overnight hours. Outside of storms, sfc winds are not anticipated to affect flight operations.

FIRE WEATHER...Monday and Tuesday

A medium to high grade monsoon environment continues, with generally light winds outside of outflow and thunderstorm activity. Most storms will produce wetting rain and any stronger storms may produce flash flooding. The presence of a weak low pressure system, moving from east to west, may introduce an enhanced hail threat for Monday and Tuesday.

Wednesday through Friday...Chances for showers and thunderstorms continue, generally light winds outside of outflow and thunderstorm activity. Most storms will produce wetting rain and any stronger storms may produce localized flash flooding. The continued presence of a weak low pressure system will prolong the potential for an enhanced hail threat through Wednesday.

FGZ WATCHES/WARNINGS/ADVISORIES

None.