Dense Fog in Arizona, September 20, 2018

Name: Dense Fog in Arizona, September 20, 2018
Start: 2018-09-20T05:00
End: 2018-09-20T09:00
Location: Arizona

September 20, 2018 · NWS PSR · 3 events in 3 counties and zones

Where it happened

Every event with a place to put it; 3 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.

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The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 3

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS PSR wrote it up.

High morning humidity combined with generally clear skies and light winds to allow for the formation of dense fog during the morning hours across the lower deserts southeast of Phoenix. The fog only lasted for two to three hours, and began around 0500MST across portions of northwest Pinal County as well as areas around Queen Creek and Apache Junction. Most of the dense fog reports were actually readings from automated weather stations such as the AWOS station in Casa Grande, which registered a visibility at one quarter of a mile. The lowest visibility reported, 1/16 of a mile, was from a NWS employee about 5 miles south of Apache Junction. No accidents were reported despite the sharply reduced visibility and associated hazardous driving conditions.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Phoenix, AZ wrote its last area forecast discussion before the first event at Sep 19, 2:30 PM MST, 14 hours 30 minutes earlier.

Area forecast discussion, Sep 19, 2:30 PM MST, 14 hours 30 minutes before

Area Forecast Discussion
National Weather Service Phoenix AZ
230 PM MST Wed Sep 19 2018

...Updated Aviation and Fire Weather...

.SYNOPSIS...
Showers and a few thunderstorms will persist across mainly the
eastern half of Arizona through tonight. For Thursday and into
next week, the region can expect dry weather. Temperatures will be
near normal Thursday then remain above normal into the middle of
next week.

&&

.DISCUSSION...
Very moist airmass resides over most of our CWA this morning. The
19.12Z KPSR sounding had a PWAT of 1.85" which is at record levels
for the back half of September. Not surprising, the sounding
exhibited a moist adiabatic profile above 700 mb. MRMS and
GOES-17 data depict the players we've been watching on the move
this morning. Furthest south, a tropical depression remains with
convection flaring up. That is all feeding the moisture plume that
extends northward into Arizona, and another weak low embedded
toward the north end of the GoC. Swinging into CA/NV, a pronounced
trough with enhanced southwesterly flow and much drier air. The
19.12Z KVEF sounding had just 0.3" of PWAT. Thus we have a
convergence air of moist flow, oriented SSW to NNE broadly from
Ajo to Phoenix to Flagstaff.

The radar and satellite depiction so far today has generally
played out as expected, though certainly more stratiform than
expected. Deep but weak isentropic lift in the very moist
environment is yielding a fairly solid rain shield at this point
in central Arizona. Further to the south where the FFA is broadly
in effect, there is a little more sunshine and cyclonic rotation
noted, thus heavier rain is still likely there with more
instability.

Expect given trends to continue into the evening hours, with
precip intensity increasing south/east of Phoenix with a gradual
west-to-east translation of the precipitation. By early Thursday
morning, everything should be fairly wrapped up.

Much drier air will be working in Thursday from west-to-east and
top-to-bottom. Most areas, from the Imperial Valley through
Phoenix and into the Globe area, should wake to still humid and
muggy morning. As the boundary layer warms and mixes through the
day though, dew points should drop markedly. Temperatures will end
up being near normal under abundant sunshine.

For the weekend and into next week, ridging with above normal
heights will persist across the Southwest CONUS yielding dry and
warm weather. A weak trough moving through the PacNW may briefly
suppress the ridge Sun/Mon, resulting in very modest cooling.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Light to moderate rain and visibilities nearing 6 miles with
obscuration of distant mountains will begin to clear by late
afternoon and only a few lingering showers in the vicinity through
the evening. Confidence is good that winds will shift to an easterly
heading with the end of the rain by early evening. Otherwise, Winds
may shift to a westerly direction shortly after sunrise Thursday
while some lower sct deck could briefly result in partial mountain
obscuration through the morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
Very minimal aviation concerns through Thursday afternoon as
thunderstorm activity should remain east of terminal locations.
Winds will favor a southerly direction through tonight and could
obtain gusts up to 20kt through tonight before becoming lighter and
more variable Thursday morning. A very remote chance exists for
ground fog and lower visibilities to develop at KIPL around sunrise,
but chances do not dictate any mention in the TAF package.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Temperatures will rebound over the weekend and climb several degrees
above seasonal normals next week. A pronounced drying trend will
exist with little to no chance of showers or storms. Minimum
humidity levels in a 15-30% range over the weekend fall more solidly
into the teens next week with some readings into the single digits
over far southeast California by Monday. Overnight recovery will
vary widely from fair to excellent. Winds will remain mostly light
and quite typical for late summer/early autumn.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch until 11 PM MST this evening for AZZ548>555-
     558>563.

CA...None.
&&

DISCUSSION...Iniguez
AVIATION...MO
FIRE WEATHER...MO/CB

The radar

The national radar mosaic for the moment the first event began.

Open the radar archive at this moment

Event by event

3 dense fog. Local standard time, as filed.

Dense Fog, Northwest Pinal County

When: September 20, 5:00 AM MST to 9:00 AM MST
Where: Northwest Pinal County forecast zone
Reported by: AWOS
Coordinates: 32.904, -111.345 (the zone’s center; the record gives no point)

High morning humidity in combination with generally clear skies and light winds allowed for the formation of patchy dense fog southeast of Phoenix during the morning hours on September 20th. According to the AWOS automated weather station located in Casa Grande, at 0536MST visibility lowered to one quarter of a mile in dense fog. No accidents were reported due to the low visibility and associated hazardous driving conditions. A Dense Fog Advisory was issued at 0643MST and remained in effect through 0900MST.

Dense Fog, Aguila Valley

When: September 20, 5:30 AM MST to 9:00 AM MST
Where: Aguila Valley forecast zone
Reported by: NWS Employee
Coordinates: 33.349, -112.491 (the zone’s center; the record gives no point)

High morning humidity in combination with generally clear skies and light winds allowed for the formation of patchy dense fog to the southeast of Phoenix on September 30th. According to an NWS employee located about 5 miles south of Apache Junction and near the intersection of Elliot Road and Meridian Road, dense fog resulted in visibility lowering to 1/16 of a mile at about 0600MST. At about the same time, a trained spotter 2 miles southeast of Apache Junction reported dense fog. A Dense Fog Advisory was issued for the area at 0643MST and it remained in effect through 0900MST. Fortunately there were no reports of accidents as a result of the low visibility and associated hazardous driving conditions.

Dense Fog, Southeast Valley/Queen Creek

When: September 20, 6:00 AM MST to 9:00 AM MST
Where: Southeast Valley/Queen Creek forecast zone
Reported by: AWOS
Coordinates: 33.349, -112.491 (the zone’s center; the record gives no point)

High humidity in combination with generally clear skies and light winds allowed for the formation of patchy dense fog southeast of Phoenix during the morning hours on September 20th. According to the official AWOS weather station located at the Phoenix Mesa-Gateway Airport, at 0700MST the visibility lowered to one quarter of a mile in dense fog. A Dense Fog Advisory was issued for the area starting at 0643MST and ending at 0900MST. Fortunately there were no reports of accidents due to the low visibility and associated hazardous driving conditions.

What these are

Dense Fog: Fog reducing visibility to a quarter mile or less, filed where it caused accidents or disruption.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in Arizona in 2018 Every state in 2018 This day in weather history Warnings in Arizona, September 2018

Sources

NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-10-01. Each event’s entry at NCEI: 780043, 780049, 780047.