Wildfire in Arizona, September 25 to 29, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
1047 AM MST Fri Sep 25 2020

.UPDATE...
Updated 18Z aviation discussion.

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.SYNOPSIS...
Dry conditions and clear to mostly clear skies will remain in the
forecast through at least the middle of next week as strong high
pressure builds along the West Coast this weekend into early next
week. Temperatures will remain above normal through the period
with highs over the lower deserts mostly ranging from the upper
90s to up to 105 degrees.

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.DISCUSSION...
A dry weather pattern will continue across the Southwestern U.S.
for the foreseeable future with above normal temperatures
persisting likely through next week. For today into the weekend,
the flow aloft will start out fairly zonal with the upper level
jet situated across the northern tier states. This will keep the
upper level ridge across the eastern Pacific and the southern U.S.
fairly flat, but 500mb heights of 588-591dm over the Desert
Southwest will keep high temperatures several degrees above
normal. Forecast highs today of 103-105 degrees are a couple
degrees off of what the area has seen over the past couple days.
This slight cooling trend will continue into the weekend as a
weak trough dips into the Great Basin suppressing heights aloft a
bit. This should bring daily highs more into a 101-103 degree
range, but still on average five degrees above normal readings.

What was initially thought to be our next potential heat episode
for early next week, now looks more likely to be more of a
continuation of the modestly above normal temperatures. Models
still show the developing high amplitude pattern late this weekend
into next week, but have trended farther westward with the trough
over the middle part of the country. This has also shifted the
high pressure ridge westward, mainly centered along the western
coast. The ridge is still likely to influence the bulk of our
region through the middle part of next week, but now the most
likely scenario keeps the ridge axis generally west of our region.
Forecast temperatures for next week are still quite problematic
as there is a wider than normal range of possible temperatures.
Median NBM temperature guidance continues to trend cooler for the
first half of next week and we have followed this trend in our
forecast. Although it currently looks like temperatures will drop
off slightly early next week, this is not a certainty as a shift
of the ridge farther eastward would bring back the potential for
highs closer to 105 degrees. Despite the uncertain temperature
forecast, the air mass will remain very dry and skies are likely
to remain clear the majority of the time.

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.AVIATION...Updated at 1746Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT;
Southeast California/Southwest Arizona including KIPL and KBLH:

Minimal aviation concerns are expected through Saturday afternoon
as clear skies and light diurnal winds persist. Easterly or light
and variable winds at the Phoenix sites will become westerly
between 19-20Z before switching back to easterly overnight. Winds
will generally be below 10 knots with perhaps a few stronger gusts
up to 15 knots that would be most likely to occur at KDVT. South
to southwest winds will generally prevail across SE CA and SW AZ,
with 15-20 knot gusts expected this afternoon at KBLH and possibly
a period of downsloping westerlies late this evening at KIPL.

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.FIRE WEATHER...
Sunday through Thursday:
Dry conditions and above normal temperatures will continue through
the period as strong high pressure builds along the West Coast
early in the period. High temperatures across the lower deserts
will mostly range from the upper 90s to the low 100s under clear
to mostly clear skies. Daily minimum RHs will be quite low, mainly
falling to around 10% each day, while overnight max RHs mostly top
out between 20-35%. Winds will remain light through Monday before
a potential cold front moves in from the east early Tuesday
bringing breezy easterly winds and gusts up to 25 mph across
southern Arizona Tuesday morning. Winds for Wednesday and Thursday
should return to a light and diurnal pattern.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

DISCUSSION...Kuhlman
AVIATION...Hopper
FIRE WEATHER...Kuhlman