Scientific Forecaster Discussion
fxus65 kfgz 221115
Area forecast discussion
National Weather Service Flagstaff Arizona
415 am MST sun Sep 22 2019
Synopsis...today will feature dry weather and generally light
winds. Showers and thunderstorms are expected Monday through
Wednesday. A few stronger thunderstorms will be possible Monday
afternoon and evening. An unsettled pattern is forecast into next
Discussion...early morning temperatures have cooled well below
seasonal averages for many locations across northern Arizona
today. Light surface winds and very dry air both aloft and near
the surface helped to produce these cold morning temperatures.
Temperatures will increase today as near-surface mixing commences
with diurnal heating this morning.
Today will be the last day within a quiet weather pattern for at
least the next three days, but likely through next weekend.
Increasing moisture aloft will create some afternoon and evening
sky cover as winds will be limited to local afternoon breezes.
A deepening trough is currently located offshore of the Pacific
northwest region this morning. This trough will continue to dig
towards the southwest through today and tonight. By early Monday
morning, a closed low will form and moisture will have been
transported northward over Arizona. This increasing moisture will
interact with the system and produce some rainfall for much of
northern Arizona from Monday through Wednesday. The highest
rainfall totals are currently forecast along and south of the
Mogollon Rim. With this system, there will be increased
instability and strong shear, so concern exists for a few severe
storms Monday afternoon and evening. Although, thunderstorms
embedded within more widespread stratiform bands are still
possible at any other time with this event too.
Guidance continued to become more in alignment on the placement
and strength of the closed low over the last two model runs. One
fairly large uncertainty that persists is the placement for the
band of highest precipitation totals. This uncertainty was one
factor that kept any products from being issued related to the
upcoming weather pattern.
The closed low will move into southwestern Arizona Tuesday and
slowly shift to the south and east through Wednesday. This
placement of the closed low will put northern Arizona on the
northern fringe of the moisture. The result will likely be more
isolated to scattered showers or storms during this time.
There is moderate agreement among the forecast models through the
remainder of this week after Wednesday. The low will become
absorbed into the west-southwesterly mean flow and track
northeast across Arizona Wednesday night into Thursday. There are
chances for more organized showers and storms within this period.
A longwave trough is progged to develop across much of the
western Continental U.S. By Friday and be nearly-stationary into Saturday. A
slight chance for showers and thunderstorms remains in the
forecast through this cooler and windy period.
Aviation...for the 12z package...look for VFR conditions and
generally light surface winds at 5-15 kts over the next 24 hours.
Aviation discussion not updated for taf amendments.
Fire weather...expect dry weather and locally breezy afternoon
winds today. On Monday, an area of low pressure approaching Arizona
will Shepherd in an increasing chance for showers and
thunderstorms through the day, lasting into Monday night. A few
storms could become strong. The best chance for wetting rains will
be along and south of the Mogollon Rim.
Tuesday through Thursday...unsettled weather continues with a
chance for showers and thunderstorms each day. Temperatures will
will remain near to a few degrees below normal.
Short term...no layers