Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Flagstaff Arizona
250 PM MST sun Oct 4 2015
Synopsis...a storm system will slowly move from Southern California
to Southeast Arizona over the next couple days. Expect breezy
conditions with scattered showers and thunderstorms through early
Wednesday. A few severe storms are possible...especially on Monday
and Monday night.
Discussion...deep closed low continues to slowly move across
Southern California this afternoon. This system will drive our
weather for the next couple days.
Boundary layer moisture continues to increase across central and
northern Arizona...with dew points now in the 40s to the low 50s in
most areas. Warm temperatures aloft...between 500 and 600mb...have
kept the atmosphere capped so far this afternoon...and only in the
past hour have a few lightning strikes been reported across the
forecast area. Rain in Yavapai and Coconino counties have resulted
in 0.25 to 0.5" accumulations in favored areas.
With the moisture in place...focus is now on identifying
destabilizing factors that may trigger convection. With recent
clearing across portions of Arizona...we may see enough additional
surface heating for convective activity to pick up later this
afternoon and early this evening. This will have to be watched
closely...sfc-based cape is forecast to increase over the next
Likely more significant...it appears that at least two destabilizing
factors may come to play on Monday/Monday evening. As the upper
level low approaches northern Arizona...we'll see significant
cooling in the capping 500-600mb layer during the day on Monday.
Additionally...there appears to be support for a weak shortwave
rotating around the closed low which may impact the area by Monday
afternoon. Forecast soundings are consistent in showing
marginal/modest instability...but copious shear/helicity for
organized...possibly severe convection Monday and Monday evening.
Other than the convective threat...the strong slow-moving closed low
will bring relatively widespread showers and thunderstorms and cool
temperatures through early Wednesday...with the highest peaks
receiving their first snow of the year. Several inches are likely
above 10,000 feet by Wednesday morning.
Once this system moves out of the area late on Wednesday...we can
expect fair weather for the rest of the week as a ridge of high
pressure builds over the southwest.
Aviation...for the 00z package...an approaching weather system will
bring significant chances for rain showers/thunderstorms and rain across northern Arizona over
the next 24 hours. Expect MVFR conditions in showers...with brief
reductions to IFR/LIFR in heavier bands. Generally light south to
southeasterly breezes are anticipated during the period. Aviation
discussion not updated for taf amendments.
For northern Arizona weather information visit weather.Gov/Flagstaff