Red Flag Warning 4, NWS Phoenix, AZ

357 
WWUS85 KPSR 092358
RFWPSR

URGENT - FIRE WEATHER MESSAGE
National Weather Service Phoenix AZ
458 PM MST Wed Sep 9 2020

AZZ131-CAZ231-100100-
/O.EXP.KPSR.FW.W.0004.000000T0000Z-200910T0000Z/
Yuma/Martinez Lake and Vicinity/Lower Colorado River Valley AZ-
Lower Colorado River Valley CA-
458 PM MST Wed Sep 9 2020 /458 PM PDT Wed Sep 9 2020/

...RED FLAG WARNING WILL EXPIRE AT 5 PM MST /5 PM PDT/ THIS
AFTERNOON FOR STRONG WINDS, LOW RELATIVE HUMIDITY AND VERY HIGH
FIRE DANGER FOR THE LOWER COLORADO RIVER VALLEY...

Conditions will continue to improve this evening as winds
gradually subside and relative humidities slowly
increase...reducing the fire danger levels.

$$

National Weather Service Phoenix is on the internet at

weather.gov/phoenix

Area Forecast Discussion
National Weather Service Phoenix AZ
443 AM MST Wed Sep 9 2020

.UPDATE...12Z aviation discussion.

&&

.SYNOPSIS...
A strong storm system dropping through the Rockies and Great Basin
into the Southwest will bring much cooler temperatures and breezy
conditions through Thursday. Drier and warmer conditions
gradually move into the region beginning later in the week as high
pressure rebuilds over the Southwest.

&&

.DISCUSSION...
A dry post-frontal air mass is now firmly entrenched across
Arizona and southeast California, featuring the driest low-level
air we've seen in quite some time. Temperatures are also running
10 to 15 degrees cooler across most locations, and are on track to
bottom out in the 60s over the lower deserts. Winds still remain
breezy along the lower Colorado River Valley amidst a tight
pressure gradient, with surface high pressure positioned across
the central High Plains. In the mid-levels, a cutoff low has
developed and is centered near the Four Corners. This feature will
meander erratically over the next 24-36 hours before finally
ejecting.

The most notable impact on weather across the region today will be
the much cooler temperatures. Many areas will barely creep above
the 90 degree mark during the afternoon, topping out around 4-8
degrees below normal, with cool overnight lows. Breezy to locally
windy conditions will persist across the lower Colorado River
Valley with wind speeds of 20-25 mph and stronger gusts. This will
create elevated to locally critical fire weather conditions, but
should generally remain shy of temporal thresholds for a Red Flag
Warning. There is a slim chance for isolated showers across
southern Gila County given the proximity of the upper low, but
any activity should be much less than what was observed on
Tuesday. Temperatures will start to rebound some on Thursday as
the upper low begins pulling to the northeast. This should also
result in some weakening of winds on Thursday as well, but the air
mass will remain very dry.

The upper high will rebuild over the Southwest this weekend and
bring another warming trend, with temperatures returning back to
above 100 degrees. However, unlike virtually every heat episode of
the past few months, temperatures are expected to remain shy of
excessive heat thresholds and only top out around 105 across
Phoenix to 108 over Yuma and southeast California, with cooler
lows. Relatively fast/progressive flow on the northern fringe of
the upper high will limit it's strength. A persistence forecast
for temperatures appears likely from Saturday onward given little
day-to-day change in the large-scale pattern. There is a signal
for moisture to increase early next week, and perhaps bring a
return to low-grade Monsoon moisture conditions - meaning a few
storms/buildups will be possible over the higher terrain of
south-central Arizona. However, limited impacts are currently
expected over the lower deserts.

&&

.AVIATION...Updated at 1143Z.

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

Cloud cover will be very limited today. However, there should be
some smoke with scattered to broken decks aoa 20 kft today before
conditions clear more overnight. Otherwise, there is still a
little bit of uncertainty on whether winds will switch to the east
early this morning or remain out of the west for the majority of
the TAF period. Current observations suggest that downslope winds
out of east-northeast are already coming to fruition and thus have
a little bit more confidence on light easterly winds developing
at the airports later today.

Southeast California/Southwest Arizona including KIPL and KBLH:

Look for continued breezy to windy conditions along the lower
Colorado River valley through this afternoon. KBLH should favor
the northwest with gusts to around 25-30 kts through early this
evening. Winds will favor the northwest at KIPL with gusts to
20-30 kts likely this afternoon. Cloud cover will be minimal,
however smoke from distant fires could cause scattered to broken
decks aoa 20 kft and slantwise visibility concerns.

&&

.FIRE WEATHER...
Friday through Tuesday:
Temperature will return to a few degrees above normal this
weekend, topping out in the 100-108 range each day across lower
elevations. Daily min RHs will stay in the teens through the rest
of the period. Winds will generally remain light with directions
favoring diurnal/terrain patterns. Nighttime max RH will generally
range in the 25-40% range. Some increase in moisture is possible
early next week and could bring a slight chance of thunderstorms
back to southern Gila County.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

DISCUSSION...Rogers
AVIATION...Feldkircher
FIRE WEATHER...Sawtelle