Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231627

Area Forecast Discussion
National Weather Service Phoenix AZ
925 AM MST Tue May 23 2017


High pressure building in from the west will continue
to warm temperatures across the region through the middle of the
week, with high temperatures rising well above 100 degrees at many
lower desert locations. Cooler temperatures will return by the
latter portion of the week and early weekend as low pressure once
again moves into the western US.



The path towards warmer temps continues across the region this
morning, with Phoenix Sky Harbor recording a low of 77 degrees, 4
degrees above what was seen yesterday morning. Temperatures aloft
have also warmed further, with the 500mb temp on the Tucson balloon
sounding showing a rise from -13C to -10C in the last 24 hours.
Given the further warming aloft, it still seems reasonable to expect
warmer highs today, especially over SE CA and parts of SW AZ, where
a few locations can expect temps to approach 110F, with Heat Risk
levels rising into the "red" range. For the short term, outside of
some minor adjustments to the hourly grids to better reflect current
trends, inherited forecasts are still looking good.


Weak upper shortwave heights have all but washed out this evening
in model evening streamline plots, with the only indicator of its
presence in the ML water vapor imagery showing an area of
increased moisture spanning much of southern Arizona and
California this early morning. Expansive low pressure remains over
the eastern two-thirds of the CONUS with amplified Pacific high
pressure finally moving inland to the West Coast. This area of
high pressure, while within seasonable parameters in terms of
height and thermal profile, will string together 3 to 4 days of
warm temperatures (starting on Sunday) for the lower desert
forecast area locales. The greatest thicknesses and warmest
thermal profile will fall near and west of the Colorado River
Valley, where daytime highs will range 105 to 110 today. As such,
Heat Risk rises to widespread moderate to areas of high risk
across the Imperial Valley as well as the Yuma area and portions
of the Lower Gila River heading towards Yuma. An Excessive Heat
Warning has been issued for those locales for today. Highs across
south-central Arizona will range from 100 to 105 today.

Upper troughing entering the Pacific Northwest has deepened
slightly on last evening`s model runs. 500mb height forecasts have
fallen into the low 580s by Wednesday aftn on the southern fringe
of the trough. Warming for Wednesday may be muted across the
western CWA and only achieve an additional degree - in spite of
better mixing and increasing winds with this new forecast
solution. So instead of a peak of temperatures on Wednesday, it
may be more a plateau between today and Wednesday before a cool
down drops temperatures off for the end of the week.

Overnight Wednesday into Thursday, ridge heights will be flattened
as strong Pacific low pressure moves into the Pac NW. Trough
heights will continue to trail southward through the
Intermountain West, introducing height falls as early as Wednesday
but most noticeably late Wednesday into Thursday on the order of
3 to 5 dam. High pressure ridge will move to our east drawing a
local enhancement of the sub-tropical jet across southern CA and
into the Southwest late Wednesday into Thursday. Forecast 250mb
jet winds sample 80-90kts with surface winds forecasts nudging
into the 90th to 95th percentile GEFS reforecast ranges. Advisory
conditions may develop as early as Wednesday, but look likely for
Thursday over the Peninsular Ranges between San Diego and El
Centro with gusts of 45 mph or greater possible. Ridgetop winds
will eventually mix and expand down into the lower elevations
during the day Thursday with gusts for the western deserts ranging
30 to 35 mph. Gusts across southwest and south-central Arizona
could range 20 to 30 mph. Visibility and air quality impacts are
anticipated in the dust/sand prone areas, especially across
southeast California. Elevated winds and dry advection into the
region could also add localized fire weather concerns to the mix
for Thursday. With all the wind and height falls transitioning
into the area, high temperatures cool into the upper 90s to near
100 for Thursday and fall further for the end of the work week.

Temperatures look likely to stay below triple digit readings
through the weekend as longwave troughing holds over the region,
keeping elevated thicknesses and warmth removed from the area. A
gradual warming trend will begin Sunday and into early next week
and Pacific ridging returns warmth to the area from the west.


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

Mostly clear skies and light winds following typical diurnal wind
patterns can be expected through Wednesday morning. No aviation
concerns through the TAF period.



Friday through Tuesday:
Breezy southwesterly winds, with sustained speeds ranging from
10-20 mph, can be expected area-wide on Friday. A few gusts to 30
mph will also be possible. Temperatures will be slightly below-
normal as highs only reach the low 90s, but dry conditions will
still result in minimum RH values of 10-15 percent. As a result,
at least low-end elevated fire weather conditions look likely.

High pressure will build over the Great Basin this weekend into
early next week, and will result in weakening winds and warmer
temperatures. High temperatures of 100-105 are forecast across the
lower deserts on Monday and Tuesday. A weak low pressure system
may also develop off the southern California coast early next
week, and advect modest amounts of moisture into eastern Arizona.
Will have to monitor for the potential for isolated thunderstorms
across the higher terrain of southern Gila County on Tuesday
afternoon, but at this point lightning potential appears low.


Spotter reports should not be needed.


AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST this
     evening for AZZ532-536.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT this
     evening for CAZ566-567.



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