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Area forecast discussion
National Weather Service Phoenix Arizona
1035 PM MST sun Apr 23 2017

Update...updated aviation discussion...

high pressure will remain situated over the region today keeping
desert high temperatures well above seasonal normals. Some of the
warmer deserts may approach 100 degrees today. After today, a
gradual cooling trend will begin lasting through the end of the
week. Periods of mainly high clouds can be expected along with
breezy or even windy conditions during the afternoon hours each day.
There will be little if any chance for rain this week outside of
higher terrain areas mainly east of Globe.



Close, but not quite sums up this afternoon, as the high of 99 at
Sky Harbor fell just short of the first triple-digit day of the
year. A patch of high cloudiness that moved over the region this
afternoon likely kept US from reaching 100. Global model output
continues to show a shift to a broad upper-level trof over the
western Continental U.S. This week, with a series of embedded shortwaves
bringing a trend towards significantly cooler weather as the week
progresses. Thus, it looks more-and-more likely that the calender
will roll into may before Sky Harbor (and most of the lower deserts
of our cwa) will hit 100. For the short term, the main concern for
the rest of the night will be gusty winds over parts of southeast CA, with
the latest NAM 3km forecast soundings showing 30 kt winds at 850mb
continuing until early tomorrow morning. Thus, will allow our Wind
Advisory for extreme southwest Imperial County to continue. Outside
of some minor modifications to the hourly temp/dewpoint/wind grids,
inherited forecasts are still looking good.


Previous discussion...
upper ridge heights have transitioned eastward into New Mexico
this early afternoon, with cirrus trailing as far south as the
Baja Peninsula ahead of the upper vort Max swinging through the
Great Basin this afternoon. Regional 500mb heights are trended a
bit lower than morning model forecasts, with aircraft soundings
indicating heights may have already fallen into the mid to upper
570s over the 580s still drawn across the length of the forecast
area. Broken to overcast high level cirrus (trailing cirrus id'd
above) bisects the forecast area with clear skies either side of
SW Arizona and the lower Colorado River valley. Cirrus field having
little effect on surface temperatures so far today, with readings
3 to 5 degrees warmer than obs this time Saturday across southeast
California as well as the south-central Arizona deserts. Still some limited
potential to see daytime mixing peak temperatures (especially at
the ASOS sensor at kphx) at the 100f mark, but an occurrence of
it would be short-lived with many desert sites ambient warmed
into the upper 90s. By this evening, winds will begin to increase
through the lower atmo layers with 20 to 30 kts at 925mb. This
will result in locally windy conditions through the Superstition
Hills and Coyote Mountains in southeast Imperial County, California. Gusts
through the early evening and into the early morning may crest
40mph at times and as such, a Wind Advisory has been issued for
that corner of Imperial co.

Operational runs of the GFS and European model (ecmwf) as well as gefs ensemble
output continue to agree that we will see the development of a very
large and broad upper trof over much of the western and central
Continental U.S. This week. Initially the flow aloft becomes stronger and
rather zonal, and with time flow becomes more northwesterly, as a
series of disturbances move through the flow and across mainly
northern Arizona. All of this will result in a cooler airmass,
allowing high temperatures to fall off early this week, with even
more significant cooling to occur starting Friday and continuing
into the weekend. Most of the disturbances that pass by the area
will stay mostly to the north of the lower deserts and any chance
for precipitation will be confined to higher terrain areas north and
east of Phoenix - especially areas to the east of Globe. One such
disturbance will race southeast and across the 4 corners area
Tuesday, bringing the first shot of much cooler air to the area and
lowering high temps into the upper 80s over the central deserts.
Behind the exiting wave Wednesday into Thursday high temps will
rebound and return to the low to mid 90s.

The final push of cool air begins Thursday night as the upper trof
deepens even further and a closed low center dives south towards the
4 corners area, and a cold front swings into southern Arizona. As this
low center moves through The Four Corners, a slight chance for
showers will develop over the higher terrain east of Phoenix with
best rain chances (such as it is) occurring on Friday. High temps
over the south central deserts will lower into the mid 80s for the
most part with the cooling persisting into the weekend. The far
western deserts (including far southeast california) will stay a bit
warmer, mostly in the upper 80s and low 90s. There will be breezy
conditions over the area during the afternoon hours pretty much
every day this week as low/mid level gradients tighten, but some of
the strongest wind will likely occur Thursday and Friday associated
with the continued deepening of the main upper trof. It is possible
that wind advisories may be needed at some point and there may be
fire weather impacts as well, especially if humidities drop low
enough. We will be monitoring the potential for dangerous fire
weather during the days to come.


south-central Arizona including kphx, kiwa, and ksdl:

Current light westerly winds are expected to switch to typical
diurnal easterly at a time later than usual tonight (in the 8-10z
timeframe). Westerly winds to return by mid-morning on Monday with
stronger gusty winds developing by afternoon, then persisting well
into the night. Cloudiness to be confined to just some high cirrus
passing overhead from time-to-time.

Southeast California/southwest Arizona including kipl and kblh:

Current westerly winds at kipl and southwesterly winds at kblh to
diminish to the 6-10 kt range by daybreak before increasing again
during the afternoon hours. Sustained wind speeds will rise up to
near 20 kts with gusts as high as 30 kts during the late
afternoon/evening period. Cloudiness to be confined to just some
high cirrus passing overhead from time-to-time.

Aviation discussion not updated for amended tafs.


Fire weather...
Tuesday through saturday:
temperatures will continue to hold above seasonal averages, but not
at readings as warm as the weekend. After Wednesday, a gradual
cooling trend will settle into the region as mostly dry storm
systems sweep through the Great Basin and down through the
southwest. Outside of a remote chance at some light showers over
high terrain of Gila County Thursday, widespread dry weather will
cover the districts. Dry afternoons with 5-15% will be common with
overnight humidities varying between mostly fair to good. Active
storm track through the west will keep afternoons breezy for the
district, with upslope west winds and occasional gusts possible
through week's end. While widespread areas of fire danger due to the
low humidities and winds look unlikely, some localized pockets of
elevated fire danger may be realized Thursday.


first hundred degree day facts for Phoenix and yuma:

average first 100f (30 yr normal period 1981-2010): may 2nd
earliest (period of record 1895-2016): March 26, 1988
latest first occurrence (1895-2016): June 18, 1913

average first 100f (30 yr normal period 1981-2010): April 29th
earliest (period of record 1876-2016): March 12, 1916
latest first occurrence (1878-2016): June 13, 1971


Spotter information statement...
spotter activation will not be needed through the middle of next


Psr watches/warnings/advisories...
California...Wind Advisory until 5 am PDT Monday for caz562.



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