Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 261520
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
820 AM MST THU MAY 26 2016

.SYNOPSIS...
A broad area of low pressure will persist over the western United
States through the weekend bringing overall cooler than normal
temperatures and dry conditions. Strengthening high pressure next
week will bring a warming trend with high temperatures pushing to
near 100 degrees for the lower deserts by the middle of the week.

&&

.DISCUSSION...
The center of a seasonally deep midtropospheric wave was propagating
into NE Arizona this morning with better subsidence descending into
the rear flank of this circulation. Putting this system into
context, KFGZ 12Z sounding sampled H5 temperatures at -21.1C which
is a record low for the date; and virtually all other midlevel
height and temperature parameters fall in the lowest 95th percentile
for the end of May. This will ensure the entire forecast area will
remain 10F-15F below average and all but Gila County will be
enveloped by deep sinking motion for the remainder of the day.

Model spread is very low with respect to temperature forecasts
today, and both SREF and deterministic convective models indicate all
deep convection will remain over and NE of the Mogollon Rim.
Therefore, forecast confidence is good and only minor changes to
account for observational and satellite trends were needed in the
short term this morning.

&&

.PREVIOUS DISCUSSION...
/315 AM MST THU MAY 26 2016/
Early this morning, PWAT values were low with just 0.14 inches seen
at Flagstaff and 0.29 inches at Tucson. Surface dewpoints over the
south central deserts were up over the past 24 hours but were not
overly high mostly ranging from the mid 20s to mid 30s. Progs call
for the upper low accelerate to the northeast and move into
northwest New Mexico this afternoon, with a subsident northwest flow
aloft overspreading the deserts leading to mostly sunny skies. The
main impact from this low will be somewhat cooler high temperatures
across south central Arizona day; the greater Phoenix area will see
high temps fall into the middle 80 today.

In the wake of the departing upper low, heights will rise on Friday
and temperatures will climb back into the low 90s across the warmer
lower deserts under mostly sunny to sunny skies as drier subsident
northwest flow aloft continues across the area. Latest GFS and ECMWF
guidance as well as GEFS spaghetti ensemble members agree that over
the weekend a baggy upper trough will develop over the deserts
southwest. This will be a dry system, no precip and little if any
cloud cover is expected however it will serve to keep our high
temperatures below seasonal normal levels. Most of the warmer lower
deserts will stay in the middle 90s for the most part Saturday and
Sunday.

As we move into the first half of next week, GEFS ensemble guidance
becomes very chaotic and unsettled and solutions start to diverge
between the operational GFS and ECMWF models. However, preponderance
of the guidance suggests that the large scale troughing that has
impacted the western CONUS for some time will break down and shift
east as an upper ridge develops along the west coast. This will lead
to continued dry but warmer conditions Monday through Wednesday. By
Tuesday the warmer western deserts should reach to around 100
degrees with another degree or two of warming likely by Wednesday.
Phoenix is forecast to reach 99 on Wednesday, and that number would
still be a degree below normal, but depending on the strength of the
building ridge, we could easily see Phoenix hit or exceed 100 by the
end of the 7 day forecast period.

&&

.AVIATION...
South-Central Arizona including KPHX...KIWA and KSDL...and
and Southeast California/Southwest Arizona including KIPL and KBLH...

A weak Pacific low pressure system will continue to cover the region
today. The airmass will remain dry for mostly clear skies, however
gusty west winds will develop during the afternoon hours. Through
18z Thu, light south to southeast wind under 7 knots. From 18z Thu
to 02z Fri, increasing west to southwest wind 10 to 12 knots. Clear
skies.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday...
A series of weak low pressure systems will continue to move into the
western states. Although dry weather is forecast, the proximity of
the low pressure systems to Arizona will result in breezy
afternoons, with 15 to 20 mph winds each day. Minimum relative
humidities will range from 10 to 15 percent. Good overnight recovery
is expected. A slow warming trend will develop with afternoon
temperatures approaching seasonal normals by Wednesday.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected.

&&

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

&&

$$

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


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