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Forecast Discussion for Phoenix, AZ

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000
FXUS65 KPSR 301517
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
815 AM MST Thu Mar 30 2017

.SYNOPSIS...
A strong low pressure system will produce windy conditions
today along with much cooler temperatures Friday. Isolated
precipitation is possible across the higher terrain of eastern
Arizona Saturday as the storm lingers. Warmer conditions will then
return Sunday and into early next week as high pressure again
builds across the region.

&&

.DISCUSSION...

A very powerful and dynamic Pacific weather system was approaching
AZ this morning. This system moved ashore in far northern CA and OR
overnight, and is forecast to track rapidly south southeastward into
northern AZ by late tonight.

We mentioned powerful because it will continue to intensify as it
tracks toward northern AZ today. However since most of the
intensification and upward vertical motion will develop over land,
and without any renewed Pacific moisture, it will be relatively
moisture starved as it muscles its way into AZ.  Additionally, as a
rule of thumb, fast and cold southward moving weather systems always
intensify due to rapid loss in planetary vorticity or rapid
latitudinal change.

And, because of how cold and dense the airmass will become following
the cold front from the north, powerful wind gusts are expected
across portions of southeast CA and southwest AZ this evening, i.e.
gusts to 50 mph possible.

Strongest 500 mb height falls of 200 meters occur between 5 pm and
midnight along the Colorado River and parts of southwest AZ, which
is a wind signature to us and very foreboding. 500 mb height falls
are even stronger in northern AZ and represent the vertical motion
part of this weather system, suggesting a chance of showers and
thunderstorms generally from Flagstaff northward, especially between
the Grand Canyon and Page AZ just before midnight.

In summary, the biggest weather threat for our forecast area, since
this system is relatively dry for southeast CA and southern AZ is
wind, lots of it and strong.  A wind advisory is already posted for
a large part of southeast CA and southwest AZ for late this
afternoon through 11 pm mst tonight.  This will perhaps be followed
a little later by a blowing dust advisory for this evening due to
the blowing dust and sand potential, especially dusty areas
emanating from the Mohave Desert since a high wind warning is posted
for that area.

Lighter winds, although still breezy from the north and northwest
will continue on Friday, along with a dramatic cool down in
the aftermath of the cold front.

For now current forecasts look ok. No updates planned other than
addressing a blowing dust advisory issuance a little later in the
shift.


.PREVIOUS DISCUSSION...603 AM MST...

Latest water vapor imagery and streamline analysis show a well-
defined trough along the northern CA coast. This system will move
steadily into the Great Basin today while deepening rapidly. Main
impact across the Desert Southwest will be an increase in wind
along and ahead of an approaching cold front. Latest NAEFS
anomalies show a widespread area of 850 mb winds above the 95th
percentile and even approaching the 99th percentile in spots.
Although the strongest winds are anticipated across southeastern
California and southwestern Arizona, gusts may still reach as high
as 30 mph late this afternoon in the Phoenix area. Latest model
runs have also hinted at some potential for shower activity across
north-central Arizona, though the latest suite of CAMs including
the HRRR show little potential for activity reaching the Phoenix
area.

The anomalous low pressure system will close off overnight before
migrating towards the Four Corners Friday. A much colder air mass
will then descend across the region and high temperatures have
been lowered further with a high of only 71 degrees expected in
the Phoenix area. If that verifies, it would be the coolest day
since March 1st, when a high temperature of 70 degrees was
recorded.

The low is expected to fill as it drifts eastward Saturday. Latest
GFS and ECMWF have trended somewhat drier, suggesting only modest
vorticity-forced ascent for shower activity across mainly eastern
Arizona. PoPs have consequently been lowered but nevertheless
remain around 20 percent across eastern Gila County. Although
warmer temperatures are anticipated Sunday, a more pronounced
warming trend is expected Sunday as a short-wave ridge emanating
from the eastern Pacific again builds across the region.

Transitory pattern will persist through early next week with
another short-wave trough likely passing by to our north Monday
night, bringing some slight cooling for Tuesday but no chance of
rain. A broad ridge will then slide eastward across the
intermountain West for the midweek period, yielding above normal
temperatures.

&&

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

A dry low pressure system will move into Arizona today bringing
gusty winds to the terminals. Mainly south winds are expected to
pick up around noon today which could create several hours of
crosswinds at the airports. However, crosswind gusts should stay
at or below 20 kts. Later in the afternoon, winds will slowly
begin veering to the southwest and will increase in strength. It
is not until late Thursday evening will winds decrease with the
westerly surface flow persisting through the night. No ceiling
concerns today with only a few high clouds expected.

Southeast California/Southwest Arizona including KIPL and KBLH:

South to southwest winds will pick up today and will become
substantially stronger by the afternoon. Southwest wind gusts of
35-40 kts will be possible at KBLH today with slightly weaker
gusts at KIPL (30-35 kts). Winds will gradually veer from
southwest to northwest at KBLH today while KIPL will stay more
westerly. No ceiling concerns expected today as only a few high
clouds are in the forecast.


Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday: After below normal temperatures on
Friday, highs will rebound on Saturday to near normal. Breezy
winds are expected once again in SE California and far western
Arizona as the low pressure system that will bring us windy
conditions today finally exits our area. Thereafter, high pressure
will follow for Sunday, resulting in a warming trend and a return to
above normal temperatures with relatively light winds. Another
passing dry low pressure system may affect our area Monday/Tuesday
which should bring another round of breezy winds although no
precipitation is expected. Its trajectory will be further north so
we are not expecting winds to be anywhere near as strong as what we
will see for today. A ridge builds in on Wednesday bringing warming
conditions and relatively light winds.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Red Flag Warning from 3 PM this afternoon to 8 PM MST this
     evening for AZZ131-132.

     Wind Advisory from 2 PM this afternoon to 11 PM MST this evening
     for AZZ020-021-025-026.

CA...Red Flag Warning from 3 PM this afternoon to 8 PM PDT this
     evening for CAZ231.

     Wind Advisory from 2 PM this afternoon to 11 PM PDT this evening
     for CAZ030>033.

&&

$$

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DISCUSSION...Vasquez/Hirsch
AVIATION...Wilson
FIRE WEATHER...Wilson

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