Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 152241
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
341 PM MST Mon Oct 15 2018

.SYNOPSIS...
Another strong low pressure system will develop across the Desert
Southwest through Tuesday, with beneficial rainfall expected
across portions of Arizona, mainly from Maricopa County eastward.
Drier air will move in from the west on Wednesday to allow for
dry conditions to persist until the weekend. Rain chances may
return by this weekend as another low pressure system affects the
Desert Southwest. Temperatures will remain below normal this week,
but a gradual warmup may bring them closer to normal by the
weekend.

&&

.DISCUSSION...
Latest water vapor imagery shows an upper trough across northern
Arizona beginning to close off. This has induced a strong northerly
flow down the CO River Valley, where gusts have occasionally reached
35-40 mph. The windy conditions have also generated areas of blowing
dust across southeastern CA, and a blowing dust advisory is in
effect through this evening. Visibilities could drop as low as 1
mile in these areas.

Above normal moisture remains in place across the Desert Southwest
in the wake of Saturday`s rainfall. SPC mesoanalysis indicates
PWATS near six tenths of an inch across the Phoenix area this
afternoon, which is above average, but less than the NAEFS 90th
percentile.

Latest models remain in good agreement, depicting robust
vorticity-forced ascent overnight across AZ as the low deepens and
becomes more negatively tilted. Consensus from the CAMs is that a
band of precipitation will develop this evening from Maricopa County
eastward. Outlier appears to be the U of A WRFs, suggesting some
stronger cells may clip the Phoenix area overnight. However, latest
trends in the HRRR suggest the bulk of the activity will remain
east of the Phoenix area.

As the cutoff low migrates across south-central AZ late Tuesday
morning, the band of precipitation will pivot around Maricopa County,
with most of it relegated to the higher terrain. QPF has trended
downward somewhat, with the latest multi-model median and National
Blend only projecting a few hundredths of an inch of precipitation
for Phoenix by mid Tuesday morning, when precipitation is expected
to taper off across the lower deserts. Meanwhile, it will feel
considerably cooler between the clouds and breezy conditions.
Forecast high of 70 degrees in Phoenix would be the coolest day
since March 18th.

&&

.PREVIOUS DISCUSSION...
Guidance calls for the upper low to start shifting towards the
north Tuesday night into Wednesday, with the low center moving
into far northwest Arizona by Wednesday afternoon. Rain chances
continue to diminish as this occurs as a more subsident westerly
flow moves into the deserts and by Wednesday afternoon POPs will
lower into the single digits across the area. Another cool day is
in store Wednesday as highs stay in the mid to upper 70s over the
central deserts and reach into the lower 80s over the far
southeast California deserts.

We look to have a couple of days of relatively calm and dry weather
Thursday into Friday as the upper low stays to our north and a drier
west flow aloft - on the south side of the low - remains in place
over the area. High temperatures will rise a bit with the central
Arizona deserts climbing into the low to mid 80s by Friday although
readings stay below seasonal normals despite generally sunny skies.

Over the weekend we will see a return to more unsettled conditions
as a high over low blocking type pattern sets up over the western
CONUS. Another closed low forms over the desert southwest with the
center setting up west of Phoenix and again, this will wrap moisture
back into the area leading to a slight chance for showers and the
possibility of isolated afternoon/evening storms. POPs have been
raised into the slight chance over the deserts Saturday and Sunday
with chance values over the higher terrain east of Phoenix. High
temperatures stay a bit below seasonal normals and in the 80s over
the lower deserts. There is still some uncertainty as to how the low
evolves over the weekend but many of the GEFS ensemble members do
suggest this pattern developing so there is sufficient confidence to
add the showers back into the forecast.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Gusty east-northeast winds are expected this afternoon and evening
as the low pressure system continues to deepen over the Colorado
River Valley. Winds will generally be sustained to 10-15 knots
with gusts to 25 knots. Skies should remain mostly clear this
afternoon as our moisture has been scoured out behind a weak
front, but clouds and shower chances should increase this evening
due to moisture advecting back into the area as the low slowly
ejects to the northeast. For now, we have included broken to
overcast 5 kft ceilings with light showers during this time
without including thunderstorms as instability appears to be
limited particularly in the overnight hours. Shower activity
should push east of the terminals sometime tomorrow morning,
persisting longest at KIWA until 15Z. Winds will remain easterly
tomorrow with gusts gradually subsiding by the afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

A deepening low pressure over the Colorado River Valley will
continue to cause gusty northerly winds KIPL and KBLH with gusts
of 30-35 knots possible this afternoon at the terminals with 40-50
knots possible at higher elevations. Localized areas of blowing
dust may also reduce slantwise visibility with localized areas of
surface visibilities below 3SM at times. Wind gusts will begin to
decrease this evening at KIPL, but may remain sustained to 25
knots at KBLH. Skies should generally remain mostly clear through
the period with perhaps a few ceilings to 10 kft before the low
pushes far enough east of the region tomorrow.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday: Drier conditions are expected late in the
week as the persistent low pressure system moves eastward into the
Plains. Minimum relative humidity values will drop into the
teens over southeast California, but remain in the 20-40 percent
range elsewhere with good overnight recovery. Winds will favor
north and east directions for most of the week. Rain chances
will return to the forecast this weekend as yet another upper
level disturbance affects the Desert Southwest. Temperatures are
expected to remain at or below normal through the period.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory until 8 PM MST this evening for AZZ530.

CA...Wind Advisory until 8 PM PDT this evening for CAZ560>564-566-
     568>570.

     Blowing Dust Advisory until 6 PM PDT this evening for CAZ562>567.

&&

$$

DISCUSSION...Hirsch
PREVIOUS DISCUSSION...CB
AVIATION...Hopper
FIRE WEATHER...Hopper/Hirsch


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