Dust Storm in Arizona, April 19, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
935 AM MST Thu Apr 19 2018

.SYNOPSIS...
A strong, but mostly dry weather system will affect the Desert
Southwest into Friday bringing a return of windy and critical
fire conditions across the eastern half of Arizona. A significant
warming trend is expected this weekend into early next week with
high temperatures eventually returning into the middle to upper
90s over the lower deserts.

&&

.DISCUSSION...
Dry southwest flow aloft continued across Arizona this morning,
ahead of an approaching Pacific upper level low pressure system.
Surface cold front was moving thru central portions of Imperial and
E. Riverside county, and the front will be quickly moving east and
into the western deserts later this morning. Tightening gradients
associated with the low have resulted in strong gusty west/southwest
winds over portions of Imperial county; areas of blowing dust have
spread through the Imperial valley and the lofted dust will move
into the AZ deserts during the day today. Gusts over 40kt have been
seen in Imperial, with visibility lowered to near 1 mile. Greater
Phoenix area will stay ahead of the front into the mid afternoon,
and increasing breeziness along with low level warm advection should
allow the high temps to reach into the low (and maybe mid) 80s
today. Due to strong winds occurring out west, a Wind Advisory was
issued for the Imperial Valley and it will remain in effect thru 7
pm this evening. Current forecasts look to be in good shape - will
continue to monitor winds/dust in case additional wind advisories or
blowing dust advisories are needed as the day progresses.

.PREVIOUS DISCUSSION...
An approaching Pacific trough will move in from the northwest
later today and slowly move through the Desert Southwest tonight
into early Friday. Current water vapor imagery shows the low
center just off the central California coast with weak moisture
advection ahead of the low. Southerly flow across the Desert
Southwest will continue to increase through this morning with
winds becoming gusty initially across southeast California and
southwest Arizona by mid morning. Gusty winds will become
widespread this afternoon peaking late this afternoon with gusts
25 to 35 mph. Winds will likely hit advisory level across Imperial
County and southern Gila County late this afternoon with gusts
peaking at 40 to 45 mph. This should lead to at least some
patching blowing dust and possibly very localized dense blowing
dust this afternoon. The strong winds and existent dry boundary
layer will also lead to a period of critical fire weather
conditions this afternoon and early evening. Red Flag conditions
are anticipated during this time for the eastern 2/3s of Arizona.

As the low center enters southern Nevada late this afternoon, a
weak cold front will also be progressing through central Arizona,
eventually moving through the rest of Arizona by midnight tonight.
The cold front will hold temperatures mostly in the middle 70s
today west of Phoenix, but the bulk of the metro Phoenix area
should still reach into the lower 80s. Winds will eventually
diminish into tonight as the low center moves east through
southern Utah and the surface gradient relaxes. The upper trough
will linger across the region on Friday allowing a return to
breezy conditions, but substantially lower winds than today.
Highs Friday will rise back to near normals for southeast
California and southwest Arizona, but end up mostly around 80
degrees for the Phoenix area. Models are still showing a chance of
showers or even a remote chance of a thunderstorm across far
east-central Arizona Friday evening, but PoPs remain right around
10% due to limited moisture and overall weak upper level support.

Upper level ridging will take hold behind the exiting trough for
Saturday with 500mb heights quickly rebounding to around 580dm.
Models are still showing a weak shortwave coming off the Pacific
through Baja Saturday night, eventually into northern Mexico on
Sunday. This feature will bring a period of high clouds into the
Desert Southwest Saturday night into Sunday, but it should not
disrupt the warming trend over the region. Highs Saturday should
fall around 90 degrees while Sunday looks to be closer to 95
degrees.

Weak southwesterly flow aloft will remain across the Southwest
U.S. into early next week with 500mb heights hovering around
580-582dm. This will keep temperatures around 7-10 degrees above
normal through at least Tuesday. Model spread increases for the
middle of next week with another potential Pacific low approaching
the West Coast. This feature may help to amplify the ridge over
the Interior West if it becomes mostly cut-off from the northern
jet stream as indicated by the warmer European. The GFS is farther
north with the trough, more progressive and thus keeps
temperatures a bit cooler. Given this system is a week away, a
blend was used for forecast temperatures yielding highs mostly in
the middle 90s.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

Expect moderate easterly winds this morning to switch around to the
southwest and west by late morning and become very strong and gusty,
especially during the afternoon hours. By mid afternoon westerly
winds of 15-20kt with gusts as high as 26-30kt can be expected.
Areas of lofted dust will also develop during the afternoon,
possibly reducing slantwise visibilities. Breezy westerly winds will
also continue into the evening and nighttime hours near 10kt with
gusts of 15-18kt.  Skies will be partly cloudy with high cloud CIGS
near 25kft moving in by midday to be replaced by a few low clouds
near 7kft this evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

Very windy conditions and areas of blowing dust are expected to
develop this morning across the region and persist into the nighttime
hours. KIPL will have southwesterly to westerly winds at 16-20kt
with gusts of 26-32kt along with reduced visibilities and CIGS in
blowing dust. Visibilities as low as 3sm and CIGS near 6Hft are
possible. KBLH can expect west to southwest winds at 17-20kt gusting
to 24-32kt today and continuing into tonight. Skies will also be
partly cloudy with high cloud decks near 25kft this morning to be
replaced by FEW low clouds near 7kft this afternoon and evening.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday...

As high pressure begins to build back in from the west expect a
warming trend together with very dry conditions. Above normal max
temps near 90 on Saturday will climb to the mid to upper 90s by
Tuesday. Min RH values near 6-10 percent are also expected to last
throughout the period. Breezy to windy conditions will develop
late in the period as a low pressure system approaches from the
west. Overnight recovery is expected to be poor to fair.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters are encouraged to report dense blowing dust.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Red Flag Warning from noon today to 8 PM MST this evening for
     AZZ132-133.

     Wind Advisory from 2 PM this afternoon to 7 PM MST this evening
     for AZZ558-560>563.

CA...Wind Advisory until 7 PM PDT this evening for CAZ562-564-566-
567.

&&

DISCUSSION...CB
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Sawtelle
FIRE WEATHER...Sawtelle