Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000 FXUS65 KPSR 201445 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 745 AM MST THU OCT 20 2016 .SYNOPSIS... Dry weather with mostly clear skies will continue across the Desert Southwest the rest of the week and into the weekend. Temperatures will continue to be unseasonably warm, and breezy weather at times will last into Friday. By Sunday afternoon, a large Pacific low pressure system will move into the western states. This will draw a considerable amount of subtropical moisture into the region from the southwest direction, with a slight chance of showers Sunday afternoon persisting through Tuesday, mainly over south central Arizona. && .DISCUSSION...
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A high pressure system is still forecast to build into the region today through Saturday. East to northeast breezes will continue to develop today over portions over southwest and south central AZ today, diminishing late tonight. Otherwise clear skies and above normal temperatures expected through Saturday. Current forecasts look good. No updates planned. .PREVIOUS DISCUSSION...522 AM MST... As of 1 am this morning, high pressure aloft centered to our west was providing dry subsident northwest flow aloft into the desert southwest; the latest FLG sounding showed just 0.17 inches of PWAT and TUS was at just 0.42 inches. IR imagery showed clear skies area wide and surface dewpoints over the central deserts at 1 am ranged from the mid teens to near 40 at Phoenix. Over the next few days, from today through Saturday, model guidance as well as GEFS ensemble guidance, calls for a broad upper ridge to gradually progress inland, then eventually shift east of Arizona. With H5 heights approaching 590dm in the center of the high, we can expect the string of 90+ degree high temperatures to continue into the weekend, with the warmer central and western deserts reaching into the mid to upper 90s through Saturday with the warmest day in the Phoenix area likely to be on Friday with a high near 97 forecast. Although highs will be 5-10 degrees above normal, we are not looking at records at this time. The record high in Phoenix through Saturday is at least 102 degrees each day. Of course, we can expect generally sunny days and clear nights through Saturday. It appears that the dry and unseasonably warm weather will come to an end starting on Sunday, as a large area of low pressure developing along the west coast starts to tap subtropical moisture; southerly low level flow begins to spread considerable mainly mid and high level cloudiness across the area during the day on Sunday as as UVV fields start to increase - areas of mid level Q- convergence develop over the western deserts during the day -chances for light precipitation start to increase. POPs have been raised into the slight chance category over areas west of the greater Phoenix area Sunday although most numbers are below about 15 percent. Expect a bit of cooling as well under mostly cloudy skies, although warmer deserts will likely remain in the low to mid 90s. Depending on how thick and persistent the cloud cover is, temps in places may fall into the upper 80s. Sunday night through Monday night we see a progressive series of short waves move out from the main upper trough and pass through the area, leading to increasing chances of precipitation, especially over south central Arizona. By Monday afternoon, the most significant QG forcing moves into south central AZ and as the atmosphere wets up from above, rain chances climb into the 15-25 percent ballpark over the deserts, reaching into the chance category over higher terrain areas to the north and east of Phoenix. High temps Monday will fall into the mid 80s to low 90s with a high right at 90 now forecast for Phoenix Sky Harbor. Modest cooling aloft will support weak instability Monday and we will look for a slight chance of an afternoon thunderstorm mainly from central Phoenix eastward. On Monday night the last in a series of short waves starts to exit into New Mexico, and rain chances will primarily focus over the higher terrain east of Phoenix with POPs in the 30 percent ballpark forecast, and 20 percent called for in the greater Phoenix area. By Tuesday afternoon expect just a lingering slight chance of showers over southern Gila county with skies becoming mostly sunny over the central and western deserts. Overall with this precip event, POPs have followed the trends of the NAEFS guidance and have been raised a bit from the previous forecast package. High pressure aloft then builds into the area from the west on Wednesday for mostly sunny skies and above seasonal normal high temperatures.
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&& .AVIATION... South-Central Arizona Including KPHX, KIWA, and KSDL: Moderate to strong east-northeast winds can be expected today and tonight in the lower levels. Before the inversion begins to break after 15Z, surface winds will be light and variable - even south or southwest (except near foothill areas where northeast will be favored). Expect east-northeast surface winds gusting 15-20 kts after 17Z before weakening late afternoon/early evening. Otherwise, skies will be clear. Southeast California/Southwest Arizona Including KIPL and KBLH: Moderate to strong northeast and east winds can be expected today and tonight in the lower levels. Before the inversion begins to break after 15Z, surface winds will be light and variable except near the Lower Colorado River Valley where north will be favored. Expect north surface winds to prevail after 17Z (locally gusting 15-20 kts LCR Valley) before weakening late afternoon/early evening. Otherwise, skies will be clear. && .FIRE WEATHER... Saturday through Wednesday... Temperatures start to decline over the weekend but will remain above normal. Humidities will increase as well. A Pacific low pressure system will lead to a slight chance of showers beginning Sunday over portions of southwest Arizona and southeast California. Slight chances will expand to the whole forecast area Sunday night and Monday before trending down from west to east Monday night and Tuesday. Wetting rains, if any, would be quite spotty and most likely be limited to the higher terrain of south-central Arizona. Dry conditions are expected Wednesday. Minimum humidities peak on Monday with values of 20-30% at most locations before slowly trending down. Overnight recovery will be good to excellent. No strong winds are anticipated through the period. && .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed this week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ Visit us on Facebook...Twitter...and at DISCUSSION...Vasquez/CB AVIATION...AJ FIRE WEATHER...AJ is the U.S. government's official web portal to all federal, state and local government web resources and services.