789 
FXUS65 KPSR 081154
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
454 AM MST Mon May 8 2017

.SYNOPSIS...
An unusually strong low pressure storm system for early May will 
bring unsettled conditions for the Southwest United States for the 
first half of the week. This system will bring a chance of scattered 
showers and isolated thunderstorms through Wednesday morning. 
Afternoon highs will only reach into the 70s to near 80 the next 
several days with periods of mostly cloudy conditions. Clearing 
skies with a warming trend will develop through the second half of
the week.

&&

DISCUSSION... 
The current water vapor and IR imagery is showing the 
unseasonably strong closed low pressure system digging slowly but 
surely SSE just off the coast of northern Baja CA. Radar indicated 
very light showers and sprinkles from NW Mexico to SE CA and SW 
AZ. This system, with about a H5 551dm center will fill only 
slightly as it continues to dig as far south as almost central 
Baja CA by late afternoon today. This trek will allow for 
increased entrainment of moisture, from the abundant moisture of 
the marine layer along the Baja coastal Pacific up to H7 and H5, 
into the reaches of the system. Meanwhile this southern journey 
will help provide temporarily rising heights at H5 and partly 
cloudy to mostly sunny conditions over S central AZ well into 
Monday afternoon. 

Beginning very late this afternoon the low pressure system will 
begin to lift NNE back and into AZ with a strong vort max, high 
H8-H5 moisture fields, lifting and falling heights leading the 
way. This will increase the chances of rain showers and slight 
chances of isolated thunderstorms beginning this evening over S 
central AZ, however, with the main forcing, vorticity, height 
falls and lifting really moving in AFTER midnight, the best 
chances of precipitation will begin then, with good chances 
expected through Tues night. Wind gusts in the vicinity of 
isolated thunderstorms may produce gusts up to 30 kts. Modest CAPE
values near 200 j/kg will gradually increase to near 400 j/kg. on
Tues. I increased POPs just slightly, mostly for Monday night and
Tues. Storm total QPF's, as supported by the ensemble model 
spread, will be mostly on the modest side ranging from about 
0.10-0.25 in. in Phoenix and the lower deserts, but slightly more 
over the high country to the north and east. 

The low will continue to lift NE into NM Tuesday night and Wed. 
along with the return of rising heights Tuesday evening which will 
result in a steady decrease in POPS, although wrap-around 
moisture and spokes of vort., and cyclonic flow aloft on the W 
side of low will keep chances of precipitation in the area through
Wednesday.

A high pressure ridge will build into the area during the later half 
of the week with H5 reaching near 581dm by Fri., along with the 
possibility of a run on 100 degrees again by Fri. and Sat. 

&&

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

A very large Pacific low pressure system will continue to develop 
over the region, centered over northern Baja today, then tracking 
toward Phoenix late tonight and Tuesday. Through 23z Mon, sct-bkn 
clds 8-12 thsd agl. Slowly increasing south wind becoming 8-12 kts by 
21z Mon. From 23z Mon to 03z Tue, bkn-ovc clds 8-12 thsd agl, 
isolated rain showers. South wind 8 to 12 knots.
 

Southeast California/Southwest Arizona Including KIPL and KBLH: 

A very large Pacific low pressure system will continue to develop 
over the region, centered over northern Baja today, then tracking 
toward Phoenix late tonight and Tuesday. Through 03z Tue, bkn clds 8-
12 thsd agl. Isolated rain shower. North to northwest wind 7 to 12 
knots.
 
Aviation discussion not updated for amended TAFs. 

&&

.FIRE WEATHER...
Wednesday through Sunday... 

Wednesday...the large cold Pacific low pressure system over AZ will 
begin to move east into New Mexico by Wednesday afternoon. A chance 
of showers can be expected from Phoenix east on Wednesday, with 
clearing elsewhere. Below normal afternoon temperatures are still 
expected. Minimum relative humidities will range from 25 to 35 
percent. 

Thursday through Sunday...drier and warmer conditions are expected 
with above normal temperatures. Minimum relative humidity Thursday 
will range from 15-20 percent, then fall to near 10 percent Friday 
through Sunday. Afternoon southwest winds 10 to 20 mph, but 
increasing Saturday and Sunday into the 15 to 25 mph range. Fair 
overnight recovery. 

&&

.CLIMATE...

Record cool high temperatures Sunday through Tuesday:

Date        Phoenix         Yuma
----        -------         ----
May 7th    70 in 1965     72 in 1964
May 8th    66 in 1930     64 in 1912
May 9th    67 in 1912     74 in 1922

&&

.SPOTTER INFORMATION STATEMENT... 
Spotter reports of gusty winds and blowing dust may be needed 
early this week.

&&

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

$$

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DISCUSSION...Sawtelle
AVIATION...Vasquez
FIRE WEATHER...Vasquez 
CLIMATE...Deemer