Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000 FXUS65 KPSR 201600 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 900 AM MST Thu Jul 20 2017 .SYNOPSIS... Thunderstorm chances will be limited to Arizona through the weekend before expanding into southeast California Monday. As is typical, chances will be best over the higher terrain. A gradual decrease in storm chances begins next week as drier flow develops. && .DISCUSSION... Numerous MCV`s are floating around the SW Conus this morning encapsulated in a weakening flow pattern characterized by a pronounced easterly wave lifting north through New Mexico and a more subtle anti-cyclone over southern AZ. Of nearly equal importance is increasing westerly flow aloft impinging into California and Nevada which will act to reconfigure the flow pattern over the next 24 hours. As the H2 easterly wave lifts further north into Colorado later today, the confluent westerly flow at jet level will begin to spread into much of Arizona limiting the development of more organized thunderstorm systems beginning as early as this afternoon. Otherwise, one consequence of all the small vorticity centers collected with ample moisture in the H7-H5 layer is the amount of accas lingering over the region. The flat nature to the clouds is an indication of the higher static stability; and the resultant modest reduction in insolation may also have additional negative influence on storm development later today (not to mention impacts on temperatures). Convection will be favored through far SE and NE Arizona this afternoon with more of a subsidence background covering the bulk of the forecast area (sans Gila County where numerous slow moving storms are likely this afternoon). Thus, have strongly limited POPs to the eastern foothills and mountains of the CWA and would only expect weaker outflow boundaries elsewhere around the Phoenix metro. && Today and Friday... A larger cyclonic feature lies over New Mexico. While the axis of the associated inverted trough will track through eastern AZ, it looks like the vorticity advection will mainly affect the Colorado Plateau. On the larger scale, flow aloft weakens for this afternoon and evening in the wake of the inverted trough and the main high pressure region over the CONUS shifts emphasis a bit further east. With weak steering flow, storms will be less likely to advect off of the higher terrain. Blended TPW imagery indicates that precipitable water is starting to trend down. GFS and NAM show a downward trend for today and again Friday. The ECMWF less so. The declining moisture will also tend to lead to less storm coverage. However, it is not a sharp drop off. Saturday and Sunday... Steering flow becomes more distinct with an anticylonic center taking position near Las Vegas. This would be favorable scenario for storms to reach metro Phoenix. Northeast or easterly component to the steering flow looks to continue through Sunday. Storm chances spread further west as well. Monday through Wednesday... A transition begins during the next work week with a turn to a more southwesterly component to the flow and a gradual drying trend. Thus tried to depict a gradual downtrend to the PoPs. && .AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL: East winds will continue at terminals until the usual switch to westerly occurs around noon. Thunderstorm chances decrease for today as relatively light steering winds will have difficulty moving storms from the mountains down into the valley. A weak outflow generated from thunderstorms in the higher terrain may possibly move through the TAF sites this evening but chances look low. For now, will continue with a persistence forecast. Southeast California/Southwest Arizona including KIPL and KBLH: No convection is expected today as any activity should remain well off to the east in Arizona. Typical southeasterly winds will prevail at KIPL with southerly winds at KBLH. Aviation discussion not updated for amended TAFs. && .FIRE WEATHER... Saturday through Wednesday: Afternoon and evening thunderstorms will continue into early next week, although the most concentrated storms will be confined to the higher elevations north and east of Phoenix. Occasionally heavy rain will be possible at any location impacted by storms. Otherwise, minimum humidity will remain around 20-30% in the lower deserts and between 40-50% for the higher elevations of Gila County. Typical summertime breezes can be expected except near thunderstorm outflows. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ Visit us on Facebook, Twitter, and at DISCUSSION...MO/AJ AVIATION...Wilson FIRE WEATHER...MO/Wilson is the U.S. government's official web portal to all federal, state and local government web resources and services.