Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 241755 AAB

Area Forecast Discussion
National Weather Service Phoenix AZ
1055 AM MST Thu Jun 24 2021

Updated Aviation discussion.


Isolated showers and a few storms will continue to be possible
through this afternoon, but most locations will be dry today.
Increasing high pressure and drier conditions starting Friday are
expected to push high temperatures back to around 110 degrees
across the lower deserts by this weekend. Unsettled weather
conditions are likely starting next Monday or Tuesday as
increased moisture is likely to bring even better shower and
thunderstorm chances.


Yesterday`s showers and thunderstorms were actually quite unusual
for June as Phoenix recorded its highest daily rainfall amount
(0.17") during the month of June since 1984. The combination of
moisture which streamed out of northern Mexico starting Tuesday
and upper level forcing associated with the trough just to the
west of the region helped to bring fairly widespread showers and
thunderstorms to the region. Rainfall amounts overall were fairly
light with generally less than 0.20" across the Phoenix area to
as much as a half an inch across southern Maricopa and western
Pinal Counties and likely more than an inch in localized spots in
western La Paz County and eastern Riverside County.

For today, significant drying will occur aloft as the upper level
trough spinning just off the central California coast quickly
weakens as it gets flattened from the building ridge just off the
Pacific Northwest. Despite the drying aloft, plenty of boundary
layer will still be present for some isolated showers and
thunderstorms through this afternoon. Daytime heating should get
this isolated convection going again by early this afternoon with
the most favored areas across La Paz County, Table Top south of
Phoenix, and the high terrain north of Phoenix. A modest 15-20 kts
southwesterly steering flow could conceivably carry this activity
off their originating higher elevation areas, but the drying
aloft will limit instability and the upper level support that
aided storms yesterday will be waning or gone altogether. The
chances for any showers or storms within the Phoenix area today
will be quite low or less than 15%.

Friday and Saturday will return to a more typical late June hot
and dry pattern with the upper level trough to our west shearing
out completely while drifting southeastward through our region on
Saturday. The drying trend in combination with typical late June
500mb heights of 586-590dm will lead to warming temperatures
through the weekend with forecast highs near normals by Friday and
then likely around 110 degrees for the Phoenix area and 111-114
degrees across the western deserts for Sunday. These readings for
Sunday are borderline high HeatRisk, but will hold off on any
Excessive Heat products for now due to the localized risk and the
fact the past several NBM model runs has been trending cooler
with this next round of heat.

Next week is looking more and more interesting with each passing
day as confidence is increasing in deep monsoon moisture streaming
into the Desert Southwest from the east and south during the first
half of next week. Easterly flow is likely to develop on Monday
within a highly amplified upper level pattern with a strong ridge
center across the Pacific Northwest and a deep trough extending
throughout the Plains States. There may even be enough marginal
moisture present as early as Sunday to bring some isolated
convection across the eastern Arizona high terrain and likely more
activity but still limited to the high terrain on Monday. By
Tuesday or Wednesday, models indicate at least one inverted trough
may progress westward through northern Mexico into southern
portions of the Desert Southwest. Even though this favorable
monsoon pattern is a bit unusual this early in the season, it is
hard to dismiss as models have been consistent in showing this
over the past few days. Deep monsoonal moisture for scattered to
possibly widespread showers and thunderstorms across much if not
all of the area may occur by the middle of next week.


.AVIATION...Updated at 1755Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

For this afternoon, high-res models are indicating the development
of isolated showers/TS over parts of south-central AZ, mainly well
north and west of the Phx metro. At the present time, there is
actually an isolated TS that has developed over SW Maricopa County
as well. Although there is a remote chance (less than 10%) that
showers/TS could impact one of the PHX area sometime this afternoon,
confidence remains far too low to reflect SHRA/TS in any of the the
TAFs at this time. Otherwise, anticipate more shower/storm activity
west of the metro area (e.g. La Paz County) and even more so to the
north (southern Yavapai County). Otherwise, anticipate a lot less
cloud cover than yesterday. Current southeasterly winds to become
light s-ly crosswinds for at time around midday before finally
becoming westerly with a bit of gustiness this afternoon. Westerly
winds to continue will into tonight, then become light easterly
towards morning on Friday. Friday to feature mainly CLR skies with
westerly winds returning during the afternoon hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

An upper low centered just west of KSFO is producing unseasonably
strong southwesterly flow aloft (AOA FL120) that will trend toward
westerly this afternoon and continue through Friday. This has
helped import moisture/humidity into the region. Anticipate
shower/storm activity this afternoon to once again be favored over
eastern Riverside and La Paz Counties. But, coverage is not
anticipated to be as much as yesterday. Confidence in TSRA or VCTS
occurring at KBLH too low at this time to reflect anything more than
VCSH in the TAFs but it bears watching. Apart from shower/storm
related clouds in the aerodrome later this afternoon, anticipate
some scattered altocumulus with bases roughly around FL100. Surface
winds will favor familiar diurnal patterns - including a shift to
gusty westerly winds developing late afternoon/early evening at
KIPL. Drier air moving into the region from the west to lead to
mainly CLR skies at both TAF sites tonight and on Friday.


Saturday through Wednesday:
Expect drier conditions and a warming temperatures this weekend
with highs climbing to around 110 degrees. Min RH values will
generally be in the single digits through this weekend with
overnight recoveries only to 15-25%. RHs then improve through the
first half of next week with Min RHs around 10-15% Monday and
15-25% Tuesday and Wednesday. Overnight recoveries improve to
30-50% early next week. With the increased moisture, rain and
storm chances will increase as well, primarily across eastern
districts on Monday likely followed by more widespread activity
late in the period. Aside from any thunderstorm outflows next
week, winds will follow typical diurnal patterns with afternoon


Spotters should follow standard reporting procedures.




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