Weather Service Report


616 
FXUS65 KPSR 221206 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
505 AM MST Mon Oct 22 2018

.UPDATE...Updated aviation and fire weather discussions.

&&

.SYNOPSIS... 
Periods of isolated showers and thunderstorms for Phoenix and 
scattered showers and storms to the south and east of Phoenix will 
prevail through Tuesday, although only modest rainfall is 
anticipated for most areas. Temperatures will also cool slightly 
with readings closer to seasonal normals. Drier conditions will move 
into the region by the middle of the week with temperatures 
remaining near seasonal normals before a warming trend into next 
weekend.

&&

.DISCUSSION....
The upper level trough over the Southwestern U.S. will continue to
provide chances for showers and thunderstorms to the northeastern
2/3s of Arizona through Tuesday. Current water vapor imagery 
shows the low center is situated across western Nevada while 
several embedded shortwaves rotate through the trough. 
Atmospheric profiles remain conducive for thunderstorms northeast 
of a line from Tucson to Phoenix to Las Vegas. This led to a few 
severe thunderstorms yesterday with severe hail being the main 
result. Current radar shows the bulk of the shower and 
thunderstorm activity along the Colorado River Valley north of La 
Paz County, but the last hour has shown isolated activity 
beginning again across Pinal and central Maricopa Counties. Fairly
cold air aloft will continue to allow for steep lapse rates and 
relatively unstable atmospheric conditions while shear profiles 
remain favorable for organized storms. A jet streak moving 
through Baja early this morning should give support to more 
showers and thunderstorms to Phoenix and areas east of Phoenix. 
This activity is supported by the bulk of the CAMs with it lasting
into at least early afternoon. These storms should remain fairly 
isolated to scattered and sub-severe, but we may see some small 
hail with the strongest storms.

Later this afternoon, there should be a lull in the activity due 
to less favorable jet positioning, but there may still be a few 
storms that form over higher terrain areas. Tonight also seems 
quiet for the most part, but slight chances will remain in the 
forecast as all it would take is a hardly noticeable shortwave to 
trigger further activity. Temperatures today should drop a couple
degrees from yesterday with readings right around seasonal
normals.

A more significant shortwave is still forecast to affect the region
on Tuesday, but the latest model runs have shifted the timing a 
bit later with it moving through mainly during the daytime hours 
Tuesday versus during early to mid morning. Under nearly the same
atmospheric conditions as we are currently seeing, this should 
create a better chance for showers and thunderstorms. 
Environmental conditions should again be conducive for some strong
to severe thunderstorms, much like what we had yesterday. Shear 
profiles do look a bit weaker later on Tuesday, but it should 
still be sufficient for some discrete longer lasting storms. High
res CAMs essentially all agree there will be scattered to 
widespread showers and thunderstorms Tuesday afternoon across the 
eastern half of Arizona. This activity should diminish by late 
Tuesday evening with a few lingering showers or storms into 
Tuesday night. The shortwave is shown exiting to the east 
Wednesday morning with drier air beginning to filter in from the 
west and rain chances coming to an end.

Weak troughing will still remain across the Desert Southwest on
Wednesday before a building Pacific ridge off the West Coast 
begins to nudge into the region on Thursday. As this ridge moves 
over the region Friday into the weekend, temperatures will rise 
above normals with some deserts likely topping 90 degrees. There 
is quite a bit of model spread at this time, especially among the 
GFS members, but for now we are pushing temperatures more toward 
the more consistent and warmer European. 

&&

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

Short wave disturbance rotating northeast through the area this 
morning will result in isolated showers and thunderstorms. Better 
bet is for showers but thunderstorms have already been occurring. 
For now will mention VCTS only at KIWA which is closer to current 
storms but may have to amend if storms continue and approach the 
other terminals including KPHX. As wave moves off to the east this 
morning, shower threat will diminish so VCSH mention will end after 
18z. Expect largely BKN decks this morning mainly around 8k feet 
with cloud decks becoming more scattered during afternoon and 
evening hours. Cannot rule out CIGs developing again overnight 
tonight. Winds will favor east/southeast through the afternoon, 
becoming light variable or light north/northwest late this afternoon 
into early evening before returning back to the east after midnight.

Southeast California/Southwest Arizona including KIPL and KBLH: 

Generally light winds and virtually no precip chances expected in 
the western deserts next 24 hours. Winds to be largely light 
variable at both TAF sites much of the time. Skies genly clear but 
as the day progresses could see increasing cirrus decks becoming BKN 
overnight tonight. No real aviation concerns at this time.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday: Dry conditions and gradually lowering 
humidities will return during the Wed-Sun period as winds aloft 
become more westerly. Winds should remain at 15-20 mph or less area-
wide. Minimum humidities will be on the decrease through the 
weekend, with values in the 15-50 percent range Wed (lowest over SE 
CA) dropping into the teens and 20s by the weekend. Expect a warming 
trend with near normal high temperatures Wednesday rising to above 
normal readings this weekend. Warmer lower deserts reaching into the 
low to mid 90s over the weekend.

&&

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

&&

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

$$

DISCUSSION...Kuhlman
AVIATION...CB
FIRE WEATHER...Hernandez/CB

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Western Regional Climate Center, wrcc@dri.edu