Tornado in Arizona, July 24, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
142 PM MST Fri Jul 24 2020

.SYNOPSIS...
Chances for showers and thunderstorms will generally remain
confined to south-central Arizona through Saturday, with periods
of locally heavy rain possible across southern Gila County through this
evening. Southwest Arizona and southeast California will
generally remain dry as the moisture remains further east.
Rain chances decrease for Sunday into the first half of next
week. Temperatures will be below normal for most locations through
the weekend before warming back up above normal for the first
half of next week.

&&

.DISCUSSION...
A mid-level moisture plume was evident on water vapor imagery this
afternoon across the eastern half of Arizona. Showers and
thunderstorms have been developing within this plume, with the
most widespread activity northeast of Tucson occurring in response
to a weak impulse spreading northward. An axis of precipitable
water is nosing into the Phoenix area from the southeast,
featuring magnitudes of 1.5-1.6 inch. Observed soundings from this
morning show very weak flow in the lower-mid troposphere, with
stronger southerly flow aloft. As a result, should see slow-moving
storms with anvils rapidly spreading northward through this
evening. Flash flood potential appears to be heightened across
eastern portions of southern Gila County this afternoon in
response to the aforementioned upper impulse. A Flash Flood Watch
has been issued here, which includes the Blue River 1 and 2 burn
scar areas. Other short-term guidance, including the HRRR, HREF,
and experimental WarnOnForecast (WOF), show isolated to scattered
thunderstorms extending into the outskirts of the Greater Phoenix
area this afternoon and evening. Locally heavy rain will be the
main threat, along with strong wind gusts. Blowing dust potential
appears to be tempered by the recent rainfall southeast of the
Phoenix area, and the lack of strong/widespread outflow generation
across the lower deserts. Given the tight moisture gradient from
west to east, southwest Arizona and southeast California will
remain mostly dry, with exception to perhaps a few isolated
thunderstorms over La Paz County.

Not much change in the environment on Saturday with the level of
activity likely to be dictated by the amount of cloud cover and
convection from the previous night. Most short-term ensemble
guidance favors scattered convection across southern Gila County
and along the northern parts of Maricopa County. A slight
downtrend on Sunday is expected with modest drying, keeping the
highest thunderstorm chances across southern Gila County. The
large-scale pattern should remain fairly consistent through much
of next week, with strong southwesterly flow featuring weak
shortwave troughs remaining positioned to the northwest, which
will keep a tight moisture gradient near the Colorado River Valley
and preclude exceptionally rich moisture from moving northward.
One potential wildcard is the remnants of Tropical Storm Hanna
(currently near the Texas coast), and if that moisture will
eventually move northward across northwestern Mexico.

&&

.AVIATION...Updated at 1812Z.

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

As Monsoon disturbances continue to move north through the region
expect partly to mostly cloudy skies with variable FEW-BKN decks
from 7-15 kft. Light winds should start to favor the south during
the early afternoon around 5-10 kts. The main concern will be the
chance for outflow winds and isolated showers/storms during the
late afternoon or the early evening. An outflow boundary looks
likely from the east, with gusts near 20 kts possible around 1-3Z.
If storms were to redevelop near terminals, the best time frame
would be from 01-07Z. Reduced visibilities around 4-5 sm in areas
of patchy blowing dust or showers will also be possible. Although
uncertainty exists on the exact timing and position of storms and
their outflow winds, the northern and eastern terminals, like
KIWA, KDVT and KSDL appear to have better better storm/wind
chances.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect dry conditions under clear skies through the TAF period.
Light to light and variable diurnal winds will be favored through
the much of the period. The exception will be breezy southerly
winds at KBLH during the afternoon with some gusts into the teens.

&&

.FIRE WEATHER...
Monday through Friday:
A low-grade Monsoon pattern is forecast to take hold for most of
the week, with the relatively highest chances for isolated
thunderstorms confined to southern Gila County. Temperatures will
also warm to a few degrees above-normal by mid-week. Dry air will
remain firmly entrenched across southeast California and southwest
Arizona for most of the period. Minimum relative humidity values
will generally range from the low teens across southeast
California to the 20s over south-central Arizona. Overnight
recovery above 30% is expected for all locations with values above
50% over the higher terrain of Gila County. Diurnal winds are
expected aside from any gusty outflows associated with
thunderstorms.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch until 10 PM MST this evening for AZZ560-563.

CA...None.
&&

DISCUSSION...Rogers
AVIATION...Feldkircher
FIRE WEATHER...Smith/Hopper