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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 210549

Area Forecast Discussion
National Weather Service Phoenix AZ
1049 PM MST Fri Sep 20 2019

Updated Aviation section.


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 and thunderstorm 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.


A dry broad trough remains in place over the western US resulting
in cooler temperatures today and through the weekend. Current 
water vapor imagery shows the long anticipated Pacific trough 
currently over the northeastern Pacific while Hurricane Lorena 
approaches the mouth of the Gulf of California. Interestingly,
that storm seems to be favoring a more northerly track which would
keep it over warm gulf waters...and potentially advecting even 
more moisture into our area. Upper level moisture from Lorena is 
already sending bands of high clouds into northern Mexico and 
southwest Texas. By Sunday, the aforementioned Pacific trough will
drop along the west coast towards Arizona providing a favorable 
set up for an increase of rain chances across Arizona. Most 
thunderstorm activity should remain over southern Arizona Sunday 
but we may see an isolated storm or two approach the forecast area
that evening. Otherwise, moisture will increase throughout the 
rest of Arizona very late Sunday.

There seem to be three general scenarios for how this event may 
unfold with plenty of variability inbetween. The first scenario 
has the trough and moisture lining up somewhere in time and space
producing an environment conducive to heavy rain. The second 
scenario is similar but one in which we see a large increase in 
instability before the heavy rain which would favor more 
strong/severe storms than we might see otherwise. This outcome 
could also be enhanced if a cutoff low develops. The third 
scenario has the trough and moisture misaligned completely and 
leaving much of the forecast area dry.

The forecast currently favors a close blend of the first two
scenarios with some chance for strong storms. There is 
uncertainty regarding the exact timing of arrival and location of 
the trough, such as how far east or west the trough axis sets up, 
which will affect where the deeper plume of moisture lines up. 
Subtle shifts in either timing or direction may make huge 
differences in the outcomes. There is also no clear answer yet as 
to whether the trough progresses quickly through the region or 
develops more like a cutoff low, possibly favoring longer lasting 
rain chances as moisture wraps around the low. Both the GFS and 
Euro ensembles seem to favor the latter. If this does occur, 
severe weather with gusty winds and large hail may be possible as 
cold air aloft produces an enhanced period of instability. As we 
approach the event, Monday still looks like the best day for the 
most/heaviest rain with lingering activity into the week.

To further muddy the waters, the two main ensemble model packages
continue to struggle with the details and evolution of the system
as evidenced by the model spread in QPF. The GEFS plumes rainfall
totals continues its downward trend but also with an increasing 
spread. The mean is now close to 0.60 inches for Phoenix with the 
most aggressive member at 1.40". The EPS is still wetter but also 
with a wide spread in values. NAEFS QPF probability maps a large 
swath of 40-60% chance of greater than 0.10 inches across the 
state and into portions of southeast California. However, that 
guidance suggests scant possibility of 1.00" or greater anywhere 
in Arizona. Even if the chances for widespread heavy rainfall 
might be down trending, orographic effects would suggest higher 
rain totals over any elevated terrain features, including any of 
the mountains around Phoenix and the Rim. Stay tuned to this 
developing potentially significant and high impact weather event!


.AVIATION...Updated at 0550Z 

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

Expect surface winds to follow familiar diurnal trends through
Saturday evening. Any afternoon gustiness will be minor. Expect
skies to remain clear through Saturday morning before possibly
seeing a few altocumulus in the late afternoon and evening. 

Southeast California/Southwest Arizona including KIPL and KBLH: 

Flow in the lower levels of the troposphere will favor
northwesterly directions through Saturday. At the surface, winds 
will be more variable but speeds will be light. 

Aviation Discussion not updated for amended TAFs.


Monday through Friday: 
No major changes to the overall forecast...A low pressure system
will lead to cooler temperatures and an increase in rainfall
potential, particularly Monday and Tuesday, for most locations
east of the Colorado River. Lingering storms and showers may occur
throughout the week. In addition, minimum RH values will improve 
as well as overnight recoveries. Otherwise, expect some stronger 
afternoon breezes, especially on Monday, with gusts as strong as 
20-30 mph.


Spotters should follow standard reporting procedures.