Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000 FXUS65 KPSR 271041 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 341 AM MST TUE SEP 27 2016 .SYNOPSIS...
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A low pressure system over northwest Mexico will slowly move north through the Desert southwest the next few days. This will lead to variable cloudiness, cooler temperatures, and a threat of showers and thunderstorms. Best chances for precipitation look to be today and tonight. Dry and stable weather will return Friday through next Monday.
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Today and tonight... Upper low centered roughly over the northern Sea of Cortez. One of the vort lobes associated with the low is approaching the Mexico border. It is helping to produce a small batch of thunderstorms. The NCAR ensemble picked up on this development. It goes on to show thunderstorm activity moving northward and nibbling at the Phoenix metro by mid morning but mostly stays south of there. Latest HRRR however keeps it south of the Mexico border but then redevelops activity over Pima County by midday spreading it into northern Pinal and southwest Maricopa County in the afternoon. There is some support for this scenario from other hi-res models (from NCEP and UofA). The main reason for the convection is cooling aloft (given that we`ve already had moist advection) as the low drifts northward. Interestingly, despite the deep southeast and southerly flow, precipitable water is advertised by multiple models to decline over southern AZ the rest of the morning (especially portions of southeast AZ). Despite having similar PWAT values, there are significant differences in the 1000-700mb mean mixing ratios between the GFS and NAM. The NAM is about 2 g/kg more moist and thus it has more CAPE. The RAP agrees with the NAM. The SREF shows increasing CAPE today but median values stay below 500 j/kg over southern AZ. There will be some CIN to contend with though and thus PoPs are relatively conservative. SSEO reflects the presence of the CIN and keeps emphasis of convection south of metro Phoenix today. There will be enough vertical wind shear for longer lived storms with more coherent motion than we often see in the Monsoon. SREF shows median values of 0-6km bulk shear remaining below 30 kts for our area (but close). Best shear mainly over Cochise and Graham counties where there will be less moisture. Anticipate some blowing dust issues with storms moving from Pima County into Pinal and southwest Maricopa this afternoon. Before the afternoon activity though, we may see some weaker shower activity as there have been batches of weak echoes overnight - most noticeably over northern Gila County. With the approaching vort lobe expect we will see more as the morning progresses. Wednesday through Friday... During the middle and latter part of the week, models show amplification of the flow pattern over North America and the northeast Pacific such that anticyclones over the interior West and northwest Mexico phase up over the Plains while troughing deepens over eastern North America and the northeast Pacific/western Canada. In the process, the upper low currently to our south will move slowly northward and weaken into a short wave and be followed by another short wave. Lingering moisture and the passing waves keep precip chances going through Thursday with only very slight chances for our easternmost areas Friday. The preponderance of the 00Z (plus the 03Z SREF) indicate that storm activity might be most potent on Thursday but will mainly be limited to the eastern third of Arizona. Highs stay near to below normal. Saturday through Monday... Dry southwest and westerly flow aloft is in place over the weekend as the Pacific trough slowly moves inland. Looking at breezy to windy conditions but nothing overly strong looking yet. Near normal high temps Saturday then decreasing several degrees by Monday.
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&& .AVIATION... South-Central Arizona Including KPHX, KIWA, and KSDL: Easterly winds to persist for the Phoenix area terminals through the evening. Another round of elevated and periodically gusty winds are possible tomorrow. Initial band of clouds/virga/wind continue to clear to the west-northwest with a narrow pocket of clearing that should transition through the area over the next few hours resulting in FEW to SCT skies briefly. Additional mid-level cloud decks will then transition into the area towards the early morning hours with additional potential for virga/very light rainfall. Earlier lofting of dust from regionally strong easterly winds may impact slantwise vsbys during the morning hours Tuesday. Southeast California/Southwest Arizona including KIPL and KBLH: Easterly winds will persist into the evening and make a gradual turn to more northerly headings during the early Tuesday morning hours. BKN to OVC mid and high level decks will also persist, along with some lofted dust moving in from the east which may impact slantwise/long-range vsbys in the morning. Some virga/sprinkle may present overnight but be brief enough in nature that even VC mention in the TAFs would be a bit much. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Thursday through Monday...A low pressure system across the Desert Southwest will bring an increase in moisture that will persist through Thursday. The chance for showers and thunderstorms will be highest across northeastern Arizona and across the higher terrain north and east of Phoenix. However, some rain is possible in the lower deserts, particularly Thursday. Drier conditions are anticipated Friday and through early next week as southwesterly flow develops ahead of an area of low pressure in the Pacific Northwest. Winds will follow typical diurnal headings and remain on the lighter side, with the exception of some afternoon breezes up to 15 to 20 mph in southwest Arizona and southeast California Saturday and Sunday. && .SPOTTER INFORMATION STATEMENT... Skywarn radio Net activation will not be needed. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ Visit us on Facebook...Twitter...and at DISCUSSION...AJ AVIATION...Nolte FIRE WEATHER...Hernandez is the U.S. government's official web portal to all federal, state and local government web resources and services.