734 
FXUS65 KPSR 161018
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
315 AM MST Sun Jul 16 2017

.SYNOPSIS...
Temperatures will moderate during the early parts of this week as 
moisture and thunderstorm activity increases across the region. 
Areas of blowing dust and gusty winds will likely accompany any 
thunderstorms during the afternoon and evening hours. Increasing 
moisture levels will also lead to greater storm coverage as well as 
elevating the threat for flooding rains the next several days. 

&&

.DISCUSSION...
The flow pattern across the SW Conus continues to become 
increasingly favorable towards general ascent supporting convective 
development and sustenance through much of the forecast area over 
at least the next 48-72 hours. The H5 anti-cyclone center over 
southern NV this morning will slowly translate into Utah later today 
while a well defined inverted trough bisecting Chihuahua and New 
Mexico squeezes under the southern periphery of the ridge. The 
juxtaposition of the midlevel inverted trough and upper divergence 
fields from a deformed H2 layer appear almost optimal this evening 
and overnight (and continuing into Monday and Tuesday for that 
matter) for persistent deep layer ascent.

One of the primary forecast uncertainties today deals with the 
ability for the atmosphere to recover from extensive convection 
Saturday evening/overnight. 03Z KPSR sounding sampled a somewhat 
contaminated environment, but nothing completely overturned or 
prohibitive of rapid destabilization later today. However, there is 
still question towards the magnitude of clearing today as the 
convective complex over northern Sonora was only recently decaying 
with its associated thick cirrus shield. On the other hand, 
subsidence and clearing were spreading west from New Mexico which 
should aid in cloud erosion later this morning. Given the evolution 
of the setup yesterday and recent trends in satellite imagery, feel 
sufficient clearing and destabilization will occur today though 
convective initiation may be delayed somewhat versus the past 
couyple days. 

One major benefit of the thunderstorms last evening/overnight was 
the boost in low level mixing ratios as objective analysis shows 
solid 12-14 g/kg levels now settled through the entire southern half 
of Arizona. And despite vertical mixing this afternoon, well mixed 
sfc-800mb 12 g/kg amounts should hold yielding MUCape at least 1500 
J/kg in a weakly capped environment. Where as yesterday a subtle 
shortwave diving SW helped erode capping around H7, no such wave is 
seen today in WV imagery; and it may be solely on the inverted 
trough and divergence fields to create ascent, remove capping 
inhibition, and maintain thunderstorm mode. High resolution models 
are not necessarily unanimous in congealing storms over the eastern 
half of the CWA this evening, however SSEO neighborhood 
probabilities (along with other non-SSEO members) are extremely 
bullish in bringing mature storms to lower elevations. And many 
signs remain present indicating this evening might be one of those 
extremely active monsoon evenings with multiple high impacts for the 
area. 

Assuming this evening and tonight are indeed the larger convective 
periods, Monday would prove to be a more inactive day with a worked 
over atmosphere. Have kept POPs for most of the daylight hours 
towards a lower end range for mid July though cannot rule out 
showers at any point given the greater amounts of moisture advecting 
into the region. While PWATs near 1.75 inches (per NAEFS mean)are 
only towards the 90th percentile, actual moisture amounts closer to 
2 inches may actually come to fruition. Thus, a better flood threat 
will begin to materialize starting today and a flash flood watch has 
been issued through Monday (and could be extended through Tuesday 
when all is said and done). 

With the moisture increase and lapse rate reduction, storms will 
tend to transition more into a heavy rain and flash flood threat 
(although strong winds will remain a substantial threat). While 
still relatively low in confidence, there is evidence among 
operational models that Tuesday morning may provide the ingredients 
necessary for a nocturnal/morning convective event associated with a 
more robust shortwave embedded in the easterly flow. The ECMWF has 
been consistent in presenting this solution and every monsoon season 
there are 1 or 2 days with this scenario. Will need to maintain a 
close watch on this period given the potential high impact during an 
uncommon time of day. 

The forecast for the second half of the week is still somewhat 
uncertain, though the trend among operational and ensemble members 
remains towards maintaining strong positive height anomalies over 
the central plains with slight retrogression towards the Southwest. 
Medium range models also still indicate as this transition occurs, 
drier air will infiltrate and advect westward into the midlevels. 
This would effectively increase inhibition at lower elevations and 
return thunderstorm activity/coverage back to a more typical monsoon 
season level. Therefore, have attempted to depicts POPs closer to a 
more standard temporal and spatial distribution while also warming 
temperatures back into a near to slightly above normal range. 

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL: 
Another active monsoon afternoon/evening looks to be in store
across the region, with scattered-numerous showers/thunderstorms
once again developing across the higher terrain to the north and
east of the Greater Phoenix Area. Hi-res computer models are in
strong agreement that at least a solid outflow boundary will be
pushed down into the Phoenix area this evening, with the most
likely time of arrival at the terminals being in the 01Z-02Z
timeframe. Peak gusts may exceed 30kt. Some blowing dust will
likely accompany the winds, with at least minor reductions in
visibilities likely. Lighter easterly winds are then expected to
prevail through the rest of the night and into Sunday morning. As
far as the impacts from the thunderstorms themselves, there is
still not enough confidence to mention anything more then VCTS is
the tafs at this time. A few lingering showers are possible past
midnight and into the early morning hours Sunday and we should see
BKN to OVC mid/high cloud decks from the mid evening into the
morning hours Sunday with most bases aoa 10k feet.

Southeast California/Southwest Arizona including KIPL and KBLH: 
Thunderstorm chances over the far western deserts, and at the TAF
sites, still appear rather low thru the taf period, but we will be
seeing more mid and high level cloudiness at the terminals with
mid decks 12-15k feet along with variable amounts of cirrus. Winds
will continue to favor the south at KBLH and southeast at KIPL
over the next 24 hours, with speeds genly below 15kt at both taf
sites.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday: 
The early portion of the week looks to be the most active, 
convectively speaking, as deep moisture will be in place allowing 
for scattered mainly afternoon and evening showers and thunderstorms 
across much of the area, especially over south-central Arizona. 
Minimum RH values will be very high Monday and Tuesday, ranging from 
25 to nearly 40 percent. High temperatures will fall well below 
seasonal normals with some of the central deserts dropping into the 
90s. For Wednesday through Friday, convective activity will fall off 
slightly, becoming more isolated over the lower deserts with the 
best chances of thunderstorms focused over the higher terrain areas 
north and east of Phoenix. Minimum RH values will drop slightly but 
remain elevated, ranging mostly from 20 to 30 percent over the 
deserts each day. High temperatures will rise but for the most part 
will stay slightly below seasonal normals. Winds will change little 
from day to day following typical diurnal patterns with winds 
favoring the south over the western deserts and the southwest across 
south central Arizona during the afternoon and evening hours.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch from 2 PM MST this afternoon through Monday 
     evening for AZZ534-537>563.

CA...None.

&&

$$

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DISCUSSION...MO
AVIATION...Percha
FIRE WEATHER...CB