Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 091821

Area Forecast Discussion
National Weather Service Phoenix AZ
1121 AM MST Sun May 9 2021

.UPDATE...Updated Aviation Discussion.


Dry conditions and generally clear skies will prevail over the
next few days as a weak weather system remains situated well
north of our region. Temperatures through early week are forecast
to be slightly above normal while daytime breezy conditions will
be common. Warmer temperatures for the latter half of the week are
expected as daytime highs are likely to top the 100 degree mark
across the lower deserts by Thursday.


Broad upper level troughing is now dominating the western half of
the CONUS as part of a Rex Block that has developed over the past
24 hours. This flow pattern will keep the trough in place at least
through Monday providing our region with near to slightly above
normal temperatures and continued dry conditions. The main weather
concern through Monday will be the breezy daytime winds and low
RHs contributing to elevated fire weather conditions across much
of the region. Temperatures will essentially be steady state
through Tuesday with highs mostly around 95 degrees across the
lower deserts.

Model ensembles remain in good agreement showing the Rex Block
breaking down Tuesday into Wednesday allowing the trough situated
to our north to migrate eastward into the central part of the U.S.
This shift will allow the upper level ridge that has been parked
just off the West Coast to spread eastward over the Western U.S.
on Wednesday. This will mark the first day of the warming trend as
highs are forecast to warm into the upper 90s. By Thursday, the
ridge axis is likely to be over our region with 500mb heights
topping out around 584dm, or around 90% of climo for the period.
This modest ridge and higher heights should bring high
temperatures on Thursday, and likely on Friday too into a 100-103
degree range. PWATs through the latter half of the week will
remain quite low, averaging 0.25-0.40" as little if any clouds
will be seen. For next weekend, model spread is still quite
large as a trough is expected to develop off the West Coast, but
there is no apparent agreement on when the trough may approach
our region. For now, the most likely scenario is for the ridge to
mostly remain over our region into next weekend, but with
gradually lowering heights. This should lead to some slight
cooling, but it may hold off until next Sunday or even the
following Monday if the trough ends up stalling out off the West
Coast. Either way, models do agree seasonably dry conditions will
prevail over our region for the foreseeable future.


.AVIATION...Updated at 1820Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

No major aviation concerns are expected through the TAF period.
Southerly winds this morning below 10 kts will continue to veer
toward the west to southwest this afternoon. Speeds will increase
this afternoon as sustained winds climb to around 10-12 kts along
with a few occasional gusts upwards of 18-20 kts. Breezy
conditions will subside this evening before winds transition to
the east overnight. Mostly clear skies are expected to persist.
While skies will remain clear in terms of cloud cover, nearby
wildfires may create slantwise visibility issues for north and
east approaches.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are expected through the next 24 hours.
Southeast winds will be favored at KIPL through the remainder of the
morning before veering toward a south to southwest component this
afternoon and evening. Breezy conditions will increase this
afternoon with a few occasional gusts into the teens possible at
KIPL. At KBLH, winds will favor a southerly component through the
period with gusts around 20 kts expected this afternoon. Clear
skies will persist through the TAF period.


Tuesday through Saturday:
Temperatures will start trending upward beginning Tuesday. By
Thursday, high temperatures at most lower desert locations will be
at, or near, 100 degrees with little change through Saturday.
There will be a bit less breeziness in the Tue-Thu time frame
compared to the preceding weekend followed by an uptick in winds
Friday and Saturday. Minimum humidities will be quite low on
Tuesday over south-central AZ with readings in the 5-10% range
(around 10% higher terrain) with little change thereafter. Over
southeast CA and southwest AZ, MinRH values will generally be in
the 10-15% range then decline to the 5-10% range by Thursday with
little change through Saturday. MaxRH values will be at their
highest Tuesday morning with readings in the 25-35% range over
south- central AZ and 30-60% over southeast CA and southwest AZ.
By Thursday morning, MaxRH readings decline to the 20- 30% range
most places with little change through Saturday.


Spotters should follow standard reporting procedures.




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