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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 191824
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1124 AM MST Thu Sep 19 2019

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS... 
Dry and mostly clear conditions will affect much of the region
through Saturday with temperatures within a few degrees of
seasonal normals. As early as late Sunday, likely lasting through
the first half of next week, rain chances greatly improve,
especially across Arizona. This next weather system will have the
potential of dropping heavy rainfall across portions of the area 
early next week.

&&

.DISCUSSION...
Everything still looks on track for a potent transition type
weather system to affect the Desert Southwest Sunday into
Wednesday. Until then, a dry airmass will remain in place across
our region while a fairly dry Pacific trough passes through the 
Great Basin today and tonight. Southern California and Arizona 
will see increased height packing later today resulting in a 
breezy day as widespread gusts of 20-25 mph are expected this 
afternoon. Upper level heights will also lower across our region 
today into early Friday before the trailing trough axis exits to 
the northeast late Friday. High temperatures today will begin to 
reflect the cooling aloft with readings in the upper 90s across 
the western deserts to around 100 degrees in the Phoenix area. 
Temperatures will drop a bit more into Friday and Saturday with 
highs just below normal readings. Overnight lows tonight and 
Friday night should end up being the coolest thus far this season 
as temperatures dip into the low to mid 60s across the western 
deserts to the upper 60s across the south-central Arizona deserts.

Attention then turns to what could be a very substantial weather 
event late Sunday through at least Tuesday. Overall, model 
guidance is still showing abundant low and mid level moisture 
advecting northward into Arizona beginning Sunday, some of which 
will likely be associated with the remnants of Hurricane Lorena. 
Lorena is still forecast to dissipate well south of Arizona, but 
we will most likely see moisture from the tropical system. What 
is more concerning is what could become of the remnant moisture 
as models are still consistent showing a deep Pacific low diving 
south out of the Pacific Northwest early Sunday into the Desert 
Southwest by midday Monday.

As usual, there is still quite a bit of uncertainty with the exact
track of the upper level low, but for the most part both the GFS
and European ensemble means have been tracking it as far south as 
southern California before stalling out for a day or two. The 
placement of the upper level low will be crucial as to what will 
transpire (and where) late Sunday through Tuesday, and possibly 
even Wednesday depending on how long the low sticks around. The 
strength of this low and the associated upper level jet will 
likely be significant enough to promote a large area of strong 
vertical ascent just to the east of the low center. The 
combination of the anticipated tropical moisture in place and 
persistently strong vertical ascent will promote bands of showers 
and thunderstorms well east of the low center where the deeper 
moisture is likely. Just to east of the low center and in an area
of lower moisture content (but still sufficient for storms) will 
likely be the area of most concern for severe storms. Widespread 
heavy rainfall, significant flooding, and severe storms with 
damaging winds and large hail will be possible if the low center 
deepens enough and stalls out over southern California or western 
Arizona. If the low is not quite as strong, it could shift farther
to the east passing through northern/eastern Arizona into New 
Mexico resulting in much less impacts. We will continue to 
monitor this upcoming significant weather system over the next 
several days and hopefully nail down the potential impacts for 
specific areas as soon as possible. Stay tuned for the latest 
updates!

&&

.AVIATION...Updated at 1815Z 

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

Upper low strengthening to the northwest of Arizona will result 
in increasing dry southwest flow aloft over the next 24 hours. 
Although winds in the greater Phoenix area will maintain typical 
diurnal direction trends, wind speeds will likely be a bit 
stronger during afternoon hours with some gustiness likely. Look 
for occasional gusts over 20 kts after 21z and continuing through 
02z. There may be a protracted period of mostly southerly winds 
now through 21Z with sustained speeds around 10 kts at times and 
maybe a gust or two into the teens. Winds should swing more to the
southwest by 21z. Winds will then taper off after 02z and 
eventually return to the southeast after midnight tonight. Clouds 
should not be too much of an issue, look for FEW-SCT decks near 
10-kft today, with skies becoming generally clear after 02z with 
a few upper-level clouds aoa 25 kft.

Southeast California/Southwest Arizona including KIPL and KBLH: 

An upper low strengthening to the northwest of Phoenix will 
result in increasing southwest flow aloft over the next 24 hours. 
This will result in elevated southwest to west winds across the 
western deserts. Winds should favor the west at KIPL, with 
strongest gusts between 22Z and 06Z tomorrow. Winds will favor the
southwest at KBLH with similar gustiness during the afternoon and
early evening. At BLH and IPL winds will switch to the northwest 
after midnight with lighter wind speeds. Skies to be generally 
clear next 24 hours.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday: 
Drier conditions and lighter winds will be seen Saturday and most
of the day Sunday with near normal temperatures and humidity
levels between 5% and 15%. Another low pressure system seems 
likely by late Sunday night, potentially bringing decent rain 
chances, higher humidities, and breezy to windy conditions through
potentially Wednesday. Strongest winds right now look to be on 
Monday with gusts up to 20-30 mph.

&&

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

&&

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

$$

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