Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 210401

Area Forecast Discussion
National Weather Service Phoenix AZ
900 PM MST Thu Jul 20 2017

The best thunderstorm chances will be limited primarily to higher
terrain communities of central and eastern Arizona through the
weekend before expanding westward somewhat Monday. While some
thunderstorms could affect a few lower elevation areas, coverage and
intensity will be less than the past week. And despite the reduction
in thunderstorm activity, temperatures should remain near the
seasonal average into next week.


It`s been another rather interesting day, convectively speaking,
over the lower deserts east of the lower Colorado River valley. Huge
upper high still in place over most of the CONUS, centered over nrn
OK; weak southeast flow aloft on the western periphery of the high
was present across Arizona. Moisture values remain elevated with
PWAT values in Phoenix/Tucson around 1.6-1.7 inches. Surface
dewpoints have not changed much, still running in the 60s across the
lower central deserts. Weak disturbances continue move to the north,
rotating around the high and adding support for thunderstorms.
Despite the presence of the embedded disturbances, one change of
note in the flow has been the decrease in the strength of the
steering flow - 700-300mb mean winds have dropped below 10kt and is
near calm over much of south central AZ. With very weak steering
flow, storms will not move very quickly and will typically be bound
to the higher terrain where they form. Storms over the deserts will
pulse up and then dissipate with little or no storm-relative flow to
move them along. Thus the main drivers for convection this evening
have been outflow boundary interaction. We saw isolated to scattered
storms form north of the CWA this afternoon, and over SE AZ this
evening. Boundaries have since moved into the central deserts from
the north, the northwest, the south and southeast. New storms and
showers have then formed where boundaries interact. Due to cool
afternoon highs (Phoenix only reached 103), instability was lowered.
Very moist profile in the low layers (as seen in the 00z RAOBs)
limited potential for strong outflows due to poor evaporational
cooling). All this combines to limit both strength of convection and
their corresponding outflow.

All this said, previous forecasts underplayed the areal coverage of
storms in the lower deserts this evening and have been updated to
show 10-20 POPs over much of the lower desert thru midnight. The
storms that have developed have proven to be garden-variety,
typically well below severe levels. Due to the high moisture,
locally heavy rains have occurred but we have yet to see amounts
needed for flash flooding. We have issued a couple of SPS and FLS
products to cover the evening convection and do not expect further
intensification of convection overnight. Will continue to monitor...

Numerous MCV`s continue to float around the SW Conus this afternoon
within a weakening flow pattern characterized by a pronounced
easterly wave lifting north through NW New Mexico and a more subtle
anti-cyclone over southern/southwest AZ. Objective analysis already
shows H2 jet winds turning westerly across the CWA in response to
the aforementioned easterly wave ejecting to the northeast and
prevailing strong jet core cutting through central CA/NV. As a
result, divergence fields are lifting into northern AZ, as well as
developing over far SE Arizona left in the void of the departing
easterly wave.

The net result later this afternoon will be more confluent flow over
the forecast area largely unsupportive for better convective complex
organization and sustenance. Adding to the reduced thunderstorm
activity (at least compared to the past week) is weaker outflow
boundaries given the moist environment and lower T/Td spreads. While
distinct outflows are likely to form with higher terrain storms and
flow downhill, cold pools will likely be more shallow and probably
insufficient to spark strong newer updrafts. Aside from showers
already building along the typical terrain features of central AZ,
Gila County is most under the gun for storms and locally heavy rain
through early evening.

Convective activity may be very muted Friday as the combination of
confluence aloft and stronger anti-cyclonic subsidence in the
midlevels spreads through the forecast area. Naturally, a few storms
will likely form over the mountains, yet steering flow would deflect
any storms away from the bulk of the CWA. It`s probably a better bet
that very little of the area experiences rain Friday, and have
really limited much in the way of POPs to terrain areas east of
Phoenix Friday afternoon/evening.

Through the weekend, midlevel flow reconfigures slightly with the
center of the H5 anti-cyclone becoming established over southern
Utah resulting in northeast flow into central and southern AZ.
However as this happens, there is no change in the mostly
unsupportive westerly jet winds, so thunderstorm activity may
struggle somewhat to maintain integrity trying to propagate into
lower elevations (not to mention outflow boundaries that may still
be weaker than experienced earlier in the monsoon season). Ensemble
probabilities are not very optimistic about any widespread storms
over the weekend, and its look more like a typical areal and
temporal monsoon storm coverage.

Monday through Wednesday...
A transition may begin through next week with a potential turn to a
more southwesterly component to the flow and a gradual drying trend.
However, there is significant uncertainty in this trend as flow may
hold at a southerly component with lower level trajectories still
from the SE and importing moisture. At some point next week, there
will likely be a shortwave partially absorbed in the southerly flow
and supportive for organized storms. Pinpointing such a day this far
in advance is fruitless, and have continued with modest POPs for the
time being.


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

Earlier we were not expecting convection into the greater Phoenix
area due to very weak steering flow confining storms to the higher
terrain. However, outflow boundaries proved stronger and more
persistent than expected; they have moved into the area from the
northwest, north, south and southeast and have allowed isolated
showers/storms to form and affect the TAF sites. Variable winds up
to 25 kt have resulted due to various merging boundaries. Have
updated TAFs to add VCTS this evening and even mentioned TSRA
earlier at KDSL. Expect isolated convection affecting the TAFs thru
around 05z this evening before dissipating and leaving SCT-BKN mid
to high decks which will persist into the morning hours. Expect less
convection in the central deserts/greater Phoenix area tomorrow due
to continued weak and somewhat unfavorable steering winds and a
slightly more stable environment.

Southeast California/Southwest Arizona including KIPL and KBLH:

No convection expected in the western deserts for the next 24 hours
as the atmosphere remains relatively stable with very weak winds
aloft. Will keep TAFs short as any cloud decks will typically be few-
sct with bases mostly aoa 12k feet. Winds continue to favor the SE
at KIPL and the south at KBLH with speeds below 15kt. No real
aviation concerns over the western deserts or the TAF sites thru
Friday evening.

Aviation discussion not updated for amended TAFs.


Sunday through Thursday:
Afternoon and evening thunderstorms will continue next week,
although the most concentrated storms will be confined to the higher
elevations north and east of Phoenix. Lower elevations of Arizona
could still see some storms, but less coverage than the past week
while SE CA will struggle to receive any rainfall. Otherwise,
minimum afternoon humidity levels will remain generally around 20-
40% range with good to excellent overnight recovery. Typical
summertime breezes can be expected except near thunderstorms where
erratic gusty winds will be likely.


Spotters should follow standard reporting procedures.





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