Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
510 AM MST Fri Aug 4 2017

.UPDATE...
Updated Aviation and Fire Weather sections.

&&

.SYNOPSIS...
Thunderstorm chances will rapidly dwindle through the weekend and
into early next week as dry air spreads into the region. Any
rainfall will be relegated mainly to high terrain locations through
the middle of next week. As the atmosphere dries, temperatures will
also rebound towards near and slightly above average readings.

&&

.DISCUSSION...
In the mesoscale short term, one prominent MCV was lifting north
towards the Las Vegas area while another probable convectively
enhanced vorticity lobe was exiting Arizona into western New Mexico.
This scenario was present under a shifting synoptic pattern where
more amplified Conus flow was becoming dampened, and elongated
subtropical ridging was becoming re-positioned south of the
international border. The resultant deeper westerly flow will
gradually scour away some of the better moisture while also forming
a more laminar, subsident (and convectively unfriendly) pattern
aloft into at least the beginning of next week.

While Thursday evening regional soundings were starting to show some
of this westerly flow and drying aloft, substantial boundary layer
moisture still lingers to the tune of Pwats hovering in the 1.50-
1.75 inch range. With wet soils and standing water from recent
rainfall, near surface moisture will likely hold during the day (sfc-
H8 mixing ratios 12-13 g/kg) while dry air eats away from above
bringing total column Pwats closer to a 1.25-1.50 inch range.
Satellite imagery was already showing good clearing into central
Arizona and afternoon heating should provide good thermal upslope
flow tapping this abundant sfc moisture.

Forecast soundings indicate minimal inhibition by peak heating this
afternoon, but modeling also suggests neutral to subsident flow with
limited convective coverage. Feel scattered thunderstorm coverage
over high terrain areas is a good bet today with only isolated pop
up showers/storms towards lower terrain communities. The biggest
question is the potential presence and depth of outflow boundaries -
with little inhibition, some stronger boundaries could be sufficient
to force more numerous storms given the high near sfc moisture.
However given the paucity of model support, could not get too
aggressive with POPs today.

More pronounced dry westerly flow will envelop the entire southwest
Conus Saturday and Sunday as positive H5 height anomalies become
stationed over northern Sonora. Once again like Friday, near surface
moisture may linger over parts of the forecast area; however, a more
substantial push of dry air aloft coincident with a warming
midtropospheric profile should effectively shut down deep convection
within the forecast area for several days. In fact, some model
output indicates sfc-H7 mixing ratios falling to 7 g/kg with H5
temperatures increasing to -3C by Sunday (net result = Pwat under 1
inch, zero Cape, and no chance at storms).

This drier synoptic pattern should hold into at least Tuesday as the
entire flow configuration upstream and north of the region will stay
blocked. In fact, it could be until the end of next week before
moisture seeps back into the region and the stronger deep westerly
flow relinquishes it influence. In the meantime, there is excellent
operational and ensemble support that central H5 heights near 594dm
will build into SE Arizona during the first half of next week. With
the drier airmass, these heights aloft would result in temperatures
rebounding back into a normal or slightly above normal range.
Certainly looking like nothing record breaking or excessive, but a
distinct change from the cooler more humid conditions of late.

&&

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

Far less cloudiness and storm activity today compared to past
couple days as drier air begins moving into the region today.
However, a weak disturbance combined with residual moisture will
keep a slight chance of storms going over the lower
deserts (mainly east of the Lower Colorado River Valley). Better
chances north and east of metro Phoenix. Surface winds will tend
to follow familiar diurnal patterns...except for more westerly
winds for the Imperial Valley as compared to recent weeks.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday:
Dry conditions prevail Sunday through Tuesday with minimum
humidities in the teens on the lower deserts. Overnight humidities
decline but remain good. Max temperatures increase somewhat but
only slightly above normal values. Storm chances return to the
higher terrain of south-central Arizona Wednesday and Thursday
humidities begin a modest upward trend during this time. Winds
next week will follow familiar warm season patterns.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

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

Visit us on Facebook, Twitter, and at weather.gov/phoenix

DISCUSSION...MO
AVIATION...AJ
FIRE WEATHER...AJ

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