Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 271453
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
750 AM MST Thu Apr 27 2017

.SYNOPSIS...
Deepening low pressure over the Four Corners area on Friday will
bring windy conditions across Arizona including high fire danger.
There will be a chance of showers and possibly a thunderstorm
during the day Friday across the high terrain east of Phoenix.
Much cooler conditions will then move in behind the system on
Saturday, but high temperatures will quickly climb back well into
the 90s for next week.

&&

.DISCUSSION...

Dry northwest flow aloft replete with variable high clouds are
forecast today and tonight. Today will be the last 90 deg day on the
deserts for a few days as a very strong cold front is forecast to
plunge south into AZ Friday afternoon.  Again, very strong
atmospheric height falls over central AZ Friday afternoon portend
wind, and lots of it. This upcoming wind episode peaks between 6-9
pm MST Friday evening over the south central AZ deserts and
mountains, and will be slightly stronger than the one last Tuesday.
Red Flag Warnings for high fire danger have been posted for Friday
afternoon and evening from the Colorado River Valley eastward to
Phoenix.

Additionally, there is also a chance of afternoon showers and
thunderstorms over the mountains of southern Gila County where the
strongest vertical motions are forecast, including a low level
moisture surge from Mexico ahead of the front.

Current forecasts look for now. No short term updates planned.


.PREVIOUS DISCUSSION...515 AM MST...

Broad troughing remains a prominent feature across much of the
U.S. with a building Pacific ridge off the West Coast. As the
Pacific ridge amplifies through tonight, a strong upper level jet
max will dig southward along the western periphery of the trough
allowing for the intensification of the upper low as it pushes
southward into the Four Corners area late Friday. Before this
occurs, the Desert Southwest will remain under northwest flow
today with gradually falling heights aloft starting today. Today`s
highs will gain a couple degrees over Wednesday`s highs, but
cooler air will be heading our way starting Friday.

As the closed low digs south toward the area on Friday, surface
pressure will rapidly fall over New Mexico creating a strong west
to east pressure gradient. The main impacts on Friday will be the
windy conditions and the dangerous fire danger conditions
developing late in the afternoon across much of southern Arizona.
Strong mixing to above 700mb will likely result in wind gusts
reaching 40 mph across portions of south-central Arizona, but
more likely over eastern Arizona. Some blowing dust across the
lower deserts is likely, but much of it will remain fairly lofted.
As the cold front sweeps southward Friday afternoon and evening,
we should see some scattered showers and possibly a few
thunderstorms east of Phoenix, but limited moisture will keep most
areas dry. Very dry air will usher in behind the cold front Friday
afternoon allowing for surface dew points to crater, likely
bottoming out in the single digits early Friday evening. The
windy conditions and the very dry surface conditions will promote
high fire danger which has prompted widespread Red Flag Warnings
across southern Arizona Friday afternoon through the middle of
the evening.

The closed low will slowly progress southeastward through New
Mexico on Saturday leaving a breezy but cooler day for the area
on Saturday. The strong Pacific ridge off the West Coast will
begin sliding into our region on Saturday, but especially going
into next week. Models have finally come into better consensus
showing this ridge slowly sliding right through the Western U.S.
next week. The previous model runs showing another potential
trough digging into the Desert Southwest now have this trough much
farther to the northeast completely bypassing our region. This
will result in a warmer forecast for next week as highs will
climb into the middle 90s as early as Monday followed by some of
the warmest deserts likely reaching 100 degrees for the middle of
next week.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:

Moderately strong dry northwest flow aloft will continue to result
in breezy conditions during the afternoon and evening hours at the
TAF sites with winds favoring a westerly direction. Expect gusts
over 18kt at times at each site, mostly after 20z, with gusts
diminishing by 03z. Winds to favor the west past midnight as it will
take some time for winds to return to normal diurnal directions.
Skies to consist of variable mainly high thin cirrus during the TAF
period with SCT-BKN decks aoa 25kt for the most part.

Southeast California/Southwest Arizona Including KIPL and KBLH:

Winds to favor the north along the lower CO river valley next 24
hours including KBLH with speeds mostly below 12kt during the day
today. Expect a period of time later this afternoon and thru the
evening for winds to swing more southwest before strengthening and
turning back towards the north after midnight. Wind to favor the
west for the most part at KIPL but there will be a period this
afternoon where winds swing more north/northeast but stay genly
below 10kt. Clouds to consist mainly of thin high cirrus with FEW-
SCT decks for the most part but occasionally thin BKN.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday:

Saturday will pose one more day of fire weather concern as breezy
conditions will persist across the western deserts, especially from
the lower Colorado river westward. With very dry conditions
persisting and minimum RH values into the single digits, critical
fire weather thresholds may be reached from the river west mainly
during the morning hours before winds start to abate. It will be
quite cool however, with high temperatures down to near 80 degrees
across the south central deserts so that may abate the fire weather
concerns slightly. After that, we can expect high pressure to slowly
build across the area from the west leading to a steady warming
trend along with continued dry conditions. Minimum RH values should
remain in the single digits over most of the lower deserts each day
and only into the teens over the higher terrain areas. High
temperatures over the warmer deserts will reach mid to upper 90s by
next Tuesday. We can expect breezy conditions during the afternoon
and early evening each day with winds favoring the west or northwest
and one of the breeziest days could well be next Tuesday across
south central Arizona.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation will not be needed through the middle of next
week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Red Flag Warning from 11 AM to 9 PM MST Friday for AZZ132.

     Red Flag Warning from 3 PM to 9 PM MST Friday for AZZ133.

     Red Flag Warning from 10 AM to 3 PM MST Friday for AZZ131.

CA...Red Flag Warning from 10 AM to 3 PM PDT Friday for CAZ231.

&&

$$

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DISCUSSION...Vasquez/Kuhlman
AVIATION...CB
FIRE WEATHER...CB


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