Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/02/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 447 PM MST Wed Sep 1 2021

.UPDATE...Updated 00Z Aviation Discussion.

.SYNOPSIS...
Scattered to widespread showers and thunderstorms are possible
this afternoon and evening east of the lower Colorado River area
before mainly being confined to southwest and south central
Arizona tonight into Wednesday. Locally heavy rainfall capable of
producing flash flooding is possible, but localized severe weather
such as damaging winds cannot be ruled out. A Flash Flood Watch
is in effect through late tonight for portions of La Paz,
Maricopa, Pinal and Gila Counties until late tonight. A drying
trend will begin on Thursday, but lingering chances for isolated
storms will remain over the Arizona high terrain into the weekend.
Well below normal temperatures are expected through Thursday and
warming back near or just below normal by the weekend.

&&

.DISCUSSION...
Surface temperatures are running 5-8 below seasonal normals with
another sub-100 degree day underway for Phoenix. Overhead, visible
satellite imagery shows an expansive field of cumulus that stretches
from the Colorado Plateau all the way past Yuma. Meanwhile, the
center circulation of a midlevel low is easily spotted over
northwestern AZ by the cyclonic flow of showers on the multi-
radar/multi-sensor system. This low will progress slightly east before
lifting towards the NNW tonight while providing large-scale
forcing supportive for another afternoon and evening of showers
and thunderstorms across many portions of Arizona.

The convective environment is interesting as clearing skies and modest
daytime heating are destabilizing the atmosphere. Satellite derived
instability indicates 200-500 j/kg of CAPE over the southwestern
quadrant of AZ with the highest instability west of Maricopa County.
Strong flow through the midlevels may support strong updrafts capable
of producing an isolated severe weather threat as 15-30 kts of 0-6 km
shear remains ahead of the main low circulation. Hi-res guidance does
hint at warming aloft with H5 temps going from -8C to -6C
which is less favorable for convection. The subtle warming but may
only serve to limit, not eliminate, convection as -6C temps are
still adequately cold. The severe weather threat is still
relatively low and very isolated, but cannot be ruled out with
thunderstorm(s) that develop this afternoon with all severe
threats in play.

As for those storms, convection has initiated over southern
Mohave/SW Yavapai/La Paz County where those higher cape values
were detected. Activity is anticipated to progress eastward this
afternoon and evening as the midlevel low moves closer. Isolated
to scattered thunderstorms will mostly stay east of the Colorado
River and push through the central portion of the state. Storm
chances are generally 15-30% across La Paz and the northern half
of Maricopa County, and 25-40% chances across Gila County. Most
higher terrain locations within those areas, such as the southern
fringes of the Bradshaws and Mazatzals, (i.e. from Wickenburg to
Cave Creek) have the best chances at around 30-50%. Storm coverage
and impacts will not be ubiquitous with many able to carry-on
without too much notice.

The flood threat is lower than previously forecast as stronger
flow will encourage faster storm motions and modest drying aloft
over the last 18 hours has reduced atmospheric moisture.
Precipitable water have dropped from a high of 1.99 inches on
yesterdays 00z KPSR sounding down to 1.72 inches as of this mornings
12z launch. Recent aircraft soundings have shown additional drying
too but a slight uptick in moisture will occur as the southern tail
of the low swings through this afternoon. Overall, moisture is
still adequate for an elevated flash flood concern, particularly
with any thunderstorms that do develop, as rain rates with storms
could exceed 1.00 in/hr with 1-2 inches of new rainfall. However,
occurrences of these types of rates/amounts will be more isolated,
with most places more likely to see lower rates and rainfall
amounts 0.50 inches or less,. Many places will remain completely
dry. Despite the reduced overall risk, the Flash Flood Watch
remains in effect through tonight until the potential for those
higher rainfall rates ceases.

By tomorrow, the area will see a general warming and drying trend,
with isolated storms only over the higher terrain of eastern AZ.
Afternoon temperatures will warm up from the mid to low 90s today
and into the 100-105 degree range by Saturday. Those kinds of
temperatures are anticipated to stay with us through next week as
the WPC Cluster tool indicates confidence for a ridge building
over the west coast. However, we could be looking at some periods
with increased rain chances as minor embedded upper level disturbances
cycle around the ridge.

&&

.AVIATION...Updated at 2347Z

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

Primary aviation impact today through the TAF period will be
persistent SCT to BKN low clouds with bases generally around 5-6
kft. Isolated showers and storms that have now develop across
extreme portions of northern Maricopa county will continue through
this evening with a slight chance (15-25%) for storms in the metro
area, favoring the northern reaches of the metro. At this point,
there is not enough confidence to include VCSH/VCTS in any of the
Phx area TAFs, with the exception of VCSH at KDVT and KSDL. Any
showers or storms should shift to the high terrain east of Phoenix
by 9-10Z. Winds will remain light, aob 10 kts, through much of the
TAF period. Directions will be variable at times, but generally
follow typical diurnal trends. More erratic winds will be possible
in the vicinity of any storms. A few showers/TS could develop again
Thursday afternoon, but confidence is far too low at this point to
include VCSH/VCTS in any of the TAFs.

Southeast California/Southwest Arizona including KIPL and KBLH:

Skies to remain mainly CLR through the TAF period, aside from FEW to
SCT cumulus through the afternoon/evening hours today. Winds will
favor typical diurnal trends with periods of variability. KBLH will
favor the south with speeds up to 10-12 kts, while KIPL see light
southeasterly winds through most of the period aside from brief
westerly sundowner winds up to 10-14 kts this evening.

&&

.FIRE WEATHER...
Friday through Tuesday:
Rain chances will be near zero for all but the far eastern
mountains of Arizona Friday. Those rain chances will slowly creep
back west through the weekend and remain into next week. For now,
minimum humidity values will drop back into the 15-35% for this
weekend and into early next week with modest overnight recovery.
Winds will generally follow diurnal trends aside from stronger
gusts in thunderstorm outflows.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through late tonight for AZZ533-534-537-538-
     540>558-560>563.

CA...None.
&&

$$

DISCUSSION...12
AVIATION...Percha/Benedict
FIRE WEATHER...Iniguez