Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
1005 PM MST Tuesday Jul 28 2015
shower and thunderstorm chances will slowly increase through
Thursday. High pressure building over the region will then reduce
rain chances somewhat into the upcoming weekend...but at least
slight chances will remain over south-central Arizona. Temperatures
will remain near normals overall with a slight dip below normal for
Wednesday and Thursday due to widespread cloud coverage.
so far this evening...infrared satellite imagery/lightning shows
thunderstorm activity and large convective complexes that persist
around Nogales and across Sonora Mexico. Debris clouds are streaming
northward across the region from this activity...and outflow
boundaries from earlier thunderstorms near Tucson have been moving
northward through northwest Pinal County...currently into the
greater Phoenix area with local gusts to 35 miles per hour and some reduced
visibility in blowing dust. The outflow brought rapid 8-10 degree
jumps in dew points as well...back into the 60s at many sites across
the lower deserts.
The gusty winds will diminish by midnight...however our attention
now turns toward the inverted trough that continues to move into
Southeast Arizona. The outflow will trigger isolated showers and
thunderstorms in its wake...but the potential exists for this
isolated activity to continue overnight. Most current hi-res models
still indicate some light convective activity across south central
Arizona late tonight. Chances are slight...but any storm that forms
will have the potential to producer some brief heavy rain...
especially given the heavy rain and flash flood issues that occurred
earlier this evening near Tucson.
Both Wednesday and Thursday still look like the best potential for
showers and thunderstorms...but confidence in how widespread rainfall
will be is still on the low side. During the day Wednesday...showers
and thunderstorms will likely be focused near the inverted trough
that moves through southern Arizona during the day Wednesday and into
southeast California Wednesday evening into Wednesday night. Typical
high terrain monsoon activity may be somewhat limited due to the
potential for extensive cloudiness...but with expected cape values
near 500 j/kg and some breaks in the clouds...we should still see at
least isolated to scattered thunderstorms Wednesday afternoon and
evening. Threats of blowing dust and localized flooding will be
possible during this event. Conditions Thursday will be similar to
Wednesday likely minus the inverted trough. Instability will be
somewhat limited due to cloud cover and warmer mid-levels...but
moisture will be plentiful with precipitable waters remaining near 1.5-1.7 inches.
Probability of precipitation for Thursday are more broad-brushed due to a lack of a trigger
but favoring higher terrain areas. High temperatures both Wednesday
and Thursday will likely fall a bit below normal due to the increased
moisture and cloud cover.
Beyond Friday...although there will still be enough monsoon
moisture to keep up some activity through the weekend and into early
next week ... combination of less available moisture (pwats only
in the 1.3-1.4 inch range across south-central az) and warmer middle-
level temperatures...as the main the upper high positions itself
pretty much overhead during this entire period...will likely limit
thunderstorm activity...with pretty much near-climatology probability of precipitation loaded into
the grids at this time.
south-central Arizona including kphx...kiwa...and ksdl...
Expect scattered to broken cloud decks at or above 10kft overnight into
Wednesday...with shower/thunderstorm activity remaining isolated
over the central deserts. Outflow winds from distant thunderstorms
will remain elevated to around 25kt...eventually diminishing by 07z.
After 12z Wednesday...thunderstorm chances increase...as well as
areal coverage...with strong outflows and heavy rain possible
impacts to terminals into the afternoon and evening hours.
Southeast California/southwest Arizona including kipl and kblh...
Winds will generally remain light and somewhat variable through
tonight. Scattered cloud decks at or above 10kft will begin to overspread
the area...and an outflow boundary may bring an increase in wind and
moisture late tonight...as well as higher chances of thunderstorms
across the area Wednesday.
Aviation discussion not updated for amended tafs.
Fire weather...Friday through Tuesday... low-level moisture should
decline somewhat through the weekend...though conditions will remain
favorable for at least isolated thunderstorm activity across much of
the region. Less activity is then anticipated early next week as the
monsoon high shifts westward over Arizona. Winds will remain fairly light
each day aside from afternoon breeziness and the potential for
stronger thunderstorm outflow winds.
Spotter information statement...
spotters are encouraged to report conditions according to Standard
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