Scientific Forecaster Discussion
fxus65 kpsr 202138
Area forecast discussion
National Weather Service Phoenix Arizona
235 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.
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 northwest New Mexico and a more subtle
anti-cyclone over southern/southwest Arizona. Objective analysis already
shows h2 jet winds turning westerly across the County Warning Area 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 Southeast Arizona left in the void of the departing
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 County Warning Area. 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 500 mb anti-cyclone becoming established over southern
Utah resulting in northeast flow into central and southern Arizona.
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 southeast 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
south-central Arizona including kphx, kiwa, and ksdl:
very light winds aloft and a more stable airmass will very likely
keep thunderstorm activity will off to the north and east of the
Phoenix area through the taf period. Typical westerly winds this
afternoon likely to persist until around midnight before switching
to light easterly. There is a possibility that winds could briefly
become northwesterly, or even northerly for a time early this
evening as outflows from thunderstorms well off to the north move
across the greater Phoenix area, but confidence is too low to
include this windshift in the tafs for now.
Southeast California/southwest Arizona including kipl and kblh:
no convection is expected through the taf period with any activity
that does develop remaining well off to the east in Arizona. Typical
southeasterly winds will prevail at kipl with southerly winds to
continue at kblh through the taf period.
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 southeast California 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.
Spotter information statement...
spotters should follow Standard reporting procedures.
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