Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 07/08/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 500 PM MST Wed Jul 7 2021

.UPDATE...Corrected Aviation Discussion.

.SYNOPSIS...
Drier conditions will continue today before a slight increase in
moisture Thursday into Friday likely leads to increased storm
activity over the Arizona high terrain, but chances are fairly low
for lower desert locations. Above normal temperatures will also
affect the area through the rest of the week with highs likely
peaking Saturday, mostly in a 108 to 115 degree range across the
Arizona lower desert and mostly between 112 to 118 degrees over
southeast California and the Lower Colorado River Valley. For
early next week, storm chances are likely to increase while we see
a bit of a reprieve from the above normal temperatures.

&&

.DISCUSSION...
Another hot day is well underway as the morning clouds have all
but cleared out. Temperatures are on the upswing, as current
temps are already in the 103-109 degree range with another 3-5
degrees of warming anticipated by late this afternoon.

Unfortunately, today may turn out to be the coolest day in the
near-term as the warming trend continues. Our weather will be
largely dominated by a strong ridge of high pressure that remains
centered near the AZ/UT border according to current objective
analysis and infrared imagery. Little movement, other than minor
longitudinal shifts of the center, is anticipated by the ridge
over the next few days. However, the overwhelming majority of
guidance shows the ridge strengthening the next few days with H5
heights climbing to 594-598 dm by Friday. This will undoubtedly
lead to more heat, with afternoon high temperatures forecast to
approach 115 degrees east of the Colorado River Friday/Saturday,
and upwards of 115-120 degrees west of the river Friday-Sunday.
NBM probabilities for excessive heat are highest across the board
Saturday afternoon when numerous locations currently have a greater
than 70% chance for reaching 110 degrees and around a 50% chance
or greater for reaching 115 degrees. Some places may be spared a
few degrees here and there depending on potential moisture increases
and/or overnight cloud cover resulting from distant thunderstorms.
Regardless of whatever the exact temperatures may be, the extra
heat will increase the HeatRisk for everyone, thus supporting the
upgrade to Excessive Heat Warnings for many lower desert areas
beginning Friday.

Aside from the strengthening ridge, part of the heat up is connected
to a drier atmospheric column. This morning`s KPSR and recent
aircraft soundings indicate precipitable water values are just over
1.00". Water vapor imagery shows a dry airmass draping across the
northern two-thirds of Arizona but there are hints of lingering
midlevel moisture farther south in the decay of last night`s MCS
over Sonora, MX. For this afternoon, convection is underway over
the higher terrain in far eastern Arizona, and will likely move
towards southern AZ due to northeasterly flow aloft. The latest
HREF shows this evolution well with strong agreement between the
many hi-res models. A few storms may attempt to enter far eastern
Gila County but drier air will temper development farther west.
The preponderance of guidance keeps storms east of our CWA on a
southward track through Graham/Greenlee Counties towards Tucson
and Nogales. Closer to home, those storms may send easterly
outflows towards the lower desert areas of Pinal County, but any
outflows are most likely to remain too far east for immediate
consequence. If those storms are able to become strong enough
and/or develop slightly farther west, there is a low chance, 20%
or less, for outflow winds strong enough (30+mph) to reach the
dust prone areas of Florence-Casa Grande. Overall, the threat for
blowing dust today is low.

Trends for tomorrow`s thunderstorm potential continue to intrigue.
Despite our drier conditions, hi-res ensembles favor a quick
return of some boundary layer moisture, as last night`s
aforementioned southern AZ moisture uptick will likely be enhanced
by today`s convection and an overnight gulf surge. HREF and global
ensembles (GEFS and ECMWF Ens) show PWs increasing to 1.20-1.60"
across south-central AZ and the Rim Country Thu/Fri. The added
moisture (and heat) are typically enough to destabilize the
atmosphere for mountain storms north and east of Phoenix with
slight potential for storms to drift into the lower desert. Right
now, chances for storms are best in eastern Gila County, around
15-30%, and 10-20% for lower desert areas of Maricopa County. The
main threat for any storms that develop will be lightning strikes,
strong gusty winds, locally heavy rain, and outflow winds capable
of blowing dust.

We could see similar chances for storms again on Friday but trends
through the weekend are likely to diminish with models showing some
northeasterly drying flow and a fairly strong capping inversion
likely being in place. We should still see some afternoon isolated
storms over the high terrain, but no realistic chances into the
lower deserts.

Model ensembles show the upper level ridge slowly weakening early
next week and becoming a bit elongated from southwest to
northeast. At the same time, there is a fairly good signal for a
large inverted trough tracking westward across northern Mexico.
This feature along with the weakening of the ridge may allow for
deeper moisture to begin working back into our region at some
point during the first half of next week. NBM PoPs also reflect
this with chances increasing from east to west early next week.
Temperatures should also continue their downward trend next week
with forecast highs dropping back to around normal readings
starting Tuesday.

&&

.AVIATION...Updated at 1755Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT Monsoon
impacts will generally remain well outside of the Phoenix area
through Thursday morning. Current westerly winds to continue through
the evening hours with a switch to a more southerly direction likely
later this evening before finally becoming easterly overnight. CIGS
to remain in the FEW-SCT range through early Thursday with cloud
bases remaining mainly AOA 12k feet.

As far as the forecast is concerned for Thu afternoon/evening, a
typical switch to westerly winds is expected during the mid/late
afternoon hours. There are indications that TS activity will be
noticeably greater over the higher terrain well north and east of
Phoenix than what we are seeing today as moisture levels increase
across the region. These storms will likely attempt to move into the
lower deserts during the late afternoon/evening hours. The main
impacts from these storms will be outflows the move into the Phx
area from an easterly to new-ly directing along with areas of
blowing dust. Have gone with VCTS and a windshift to ene-ly in the
Phx area TAFs to account for this possibility.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns anticipated. Winds will generally
retain a southerly component at both KIPL and KBLH. At KBLH, there
is also the potential for occasional gusts up to 20-25 kts this
afternoon and again on Thu afternoon.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Relatively drier conditions will relegate most storms to the
higher terrain north and east of Phoenix through the weekend.
However, increasing moisture will bring better chances for storms
and wetting rains later in the week. Storms that form will be
capable of producing lightning strikes, locally heavy rain, strong
wind gusts, and long-traveling outflow winds that cause sudden
changes in wind directions. Otherwise, Saturday will be the
hottest day of the week with widespread temps in excess of 110
degrees but temps do cool closer to seasonal normals as the
moisture increases. Min RH values will generally fall into the
15-20% range for the lower desert and 25-45% for higher elevations
and agricultural areas. Overnight recoveries will exceed 35% most
places with recoveries potentially exceeding 50% for higher
elevations. Outside of thunderstorms, winds will follow typical
diurnal/topographic influences with afternoon gusts near 15-20
mph.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM Friday to 8 PM MST Saturday
     for AZZ530-532>534-537-538-540>544-546-548>551.

CA...Excessive Heat Warning from 10 AM Friday to 8 PM PDT Saturday
     for CAZ569.

     Excessive Heat Warning from 10 AM Friday to 8 PM PDT Sunday for
     CAZ560>563-566>568-570.

&&

$$

DISCUSSION...AD/Kuhlman
AVIATION...Percha/Hirsch
FIRE WEATHER...Kuhlman/Hirsch