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FXUS65 KPSR 192106
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
206 PM MST Thu Jul 19 2018

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Chances for isolated to scattered showers and thunderstorms will
continue for the next several days before a decrease in activity
is expected for Sunday through early next week. High pressure is
expected to increase across the region late this weekend ushering
in a strong warming trend and excessive heat for early next week.

&&

.DISCUSSION...
Good afternoon all, looking at the 12Z sounding from Phoenix we
recorded an enormous amount of ML CAPE. All in all, 1936 J/kg was
observed which is a 12Z record for the month of July (per the SPC
sounding climatology)! A bit like yesterday, we are expecting
this profile to possibly be capped this afternoon with about 50
J/kg of convective inhibition. One difference this afternoon is
about half of the convective resolving models show some activity
moving through the Phoenix area this evening whereas yesterday
none of them had activity in the lower elevations. In addition to
this, the environment should have about 25 kts of effective bulk
shear this afternoon which is a lower minimum used for organized
convection. If storms do develop, this could bring more organized
outflows possibly bringing dust and strong to severe winds.
Finally the steering flow will be out of the east southeast this
afternoon, so we will be watching for storms moving down from Gila
County and the Tucson area. Again, if the convective cap proves
to be too strong, storms will dissipate as they move to the lower
elevations but we will just have to wait and see what develops.

Looking towards tomorrow, high pressure currently over the Great
Basin will build over Texas and New Mexico. Convective wise,
drying in the lower levels will likely reduce ML CAPE over the
Phoenix area to around 1500 J/kg to 1000 J/kg Friday and
Saturday, respectively. The persistent midlevel cap will once
again inhibit activity for the lower elevations but cannot
completely rule out storms for the low deserts of south-central
Arizona.

By the time we reach Sunday, the aforementioned high pressure will
reposition itself over the AZ/NM border pushing up 500 mb heights
and 850 mb temperatures. Consequently, temperatures will soar to
near 110 F for the lower deserts on Sunday while the lower levels
continue to dry. Model consensus points to around 1.25 inches of
PW by Sunday afternoon effectively shutting off convective
activity for all lower desert locations. The ridge will continue
to build Monday and Tuesday pushing 500 mb heights to around 600
dm! During this period, excessive heat is expected throughout our
entire CWA except for the highest elevations. Currently, we
continue our excessive heat watch but will certainly need to be
upgraded to a warning if this forecast persists. By the time we
get to Wednesday, all models begin to weaken the ridge but by how
much remains to be seen. Given the possibility of continued heat,
we have extended the heat watch for the lower elevations through
Wednesday. Some relief and perhaps a return of monsoon moisture
may come towards the end of next week.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Low confidence forecast this evening for thunderstorm activity
across the Phoenix area. Thunderstorms are developing well to the
east of Phoenix this afternoon, and should gradually spread
westward across Gila County. It is very unstable across south-
central Arizona, so the potential is certainly there for
thunderstorms to develop across the lower deserts. Capping still
remains a concern, so it is uncertain if storms can sustain
themselves into the lower deserts, and how widespread they will
be. Moderate to high confidence in an outflow boundary affecting
area terminals after 03Z, with a wind shift from the east-
northeast expected. The strength of any outflow is more uncertain
and will be dependent on how far west storms can sustain
themselves. Otherwise, prevailing west-southwest winds should
continue with scattered to broken mid-high clouds overnight.

Southeast California/Southwest Arizona including KIPL and KBLH:

Little in terms of aviation concerns for the next 24 hours as no
thunderstorms are expected to impact the terminals. Winds will
typically favor the southeast at KIPL and the south at KBLH with
speeds mostly 12kt or lower. A few high based afternoon cumulus
are possible otherwise just variable amounts of mid/high cloud
decks.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday: Monsoon moisture will gradually decrease
this weekend and into next week bringing only a slight chance of
thunderstorms to only the higher elevations by Sunday. Daytime
minimum relative humidity values will also fall closer to 15%
while recoveries will be fair to poor. A significant increase in
heat is advertised for Monday through Wednesday, with high
temperatures above 115 degrees expected over the lower deserts.
The winds will be generally light and follow typical diurnal
trends. Only very isolated afternoon and evening thunderstorms are
expected early next week over higher terrain east of Phoenix,
mainly east of Globe.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Watch from Monday morning through Wednesday
evening for AZZ532-534>544-546-548>551-553>555-559.

Excessive Heat Watch from Tuesday morning through Tuesday
evening for AZZ545-547-552-556-560>562.

Excessive Heat Watch from Tuesday morning through Wednesday
evening for AZZ530-531-533.

CA...Excessive Heat Watch from Tuesday morning through Wednesday
evening for CAZ560>570.

&&

$$

DISCUSSION...Wilson
AVIATION...Rogers/CB
FIRE WEATHER...Wilson/CB
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