Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000 FXUS65 KPSR 211136 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 436 AM MST Thu Sep 21 2017 .UPDATE...
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To Aviation and Fire Weather Discussions...
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&& .SYNOPSIS... An area of low pressure and associated cold front will sweep through the region over the next two days. The end result will be substantially cooler temperatures along with occasionally breezy conditions. Precipitation chances will be neglible and limited to only some sprinkles over the higher terrain locations. Temperatures will only slowly warm back to near average next week. && .DISCUSSION... A positively tilted, yet seasonally deep negative height anomaly through the NW quarter of the Conus will nearly stall over the Great Basin and Southwest through early next week as downstream high amplitude ridging temporarily blocks the progression of the westerly flow pattern. H5 heights below 570dm will hover over the northern half of Arizona through the weekend easily providing the coolest weather of this burgeoning autumn season. The blocked flow regime will be stubborn to relinquish its hold on the country even into the middle of next week with somewhat lower heights likely lingering over the forecast area towards the end of the month. Despite this blocking pattern and larger anomaly framework, ensemble output displays remarkable agreement through early next week yielding better than expected forecast confidence. Though the first vestiges of height falls and cold advection aloft are already impinging on SE California early this morning, the more pronounced impacts will be experienced late this afternoon and evening with a cold front and much lower theta-e airmass crossing the San Diego mountains. Fropa timing looks quite favorable for tapping and transferring momentum in the H8-H7 layer (40 kt above the shallow windward marine layer) and enhancement through the formation of stronger mountain rotors across the western half of Imperial County. The wind advisory was expanded slightly east towards terrain features into central Imperial County though impacts will be most evident along the I-8 corridor from El Centro westward. Otherwise further east into central Arizona, there is model evidence of enhanced sfc pressure packing along a pre-frontal trough such that some localized blowing dust coincident with 30 mph gusts along I-8 from Gila Bend to Casa Grande is also possible. The aforementioned cold front will forge through the state Friday, finally clearing into the New Mexico on Saturday. Moisture availability along this boundary will be tenuous, at best with only a few model iterations suggesting a narrow moist tongue present within a frontal based orographic upslope regime (albeit before more supportive cooler temperatures aloft advect eastward). Based on forecast soundings, little more than a brief shower/sprinkle looks possible along this front. Winds will remain occasionally gusty Friday afternoon though begin to weaken after fropa and sunset Friday evening. More notable through the weekend will be the unusually cool temperatures. Ensemble means depict heights and temperatures aloft within the trough core some 3 normalized standard deviations below normal, or below the 2nd percentile for the end of September. Closer to central Arizona, anomalies will be closer to the lowest 10th percentile, that is, historically equating to highs only in the middle/upper 80s. Even more amazing will be overnight lows in the 50s and lower 60s and these readings some 10F-15F below average will persist into early next week. The overall troughing structure will be maintained over the SW Conus into the middle of next week, however heights will slowly rise as weakening and filling of the trough begins. Downstream eastern Conus ridging will also weaken allowing for a pattern shift/adjustment by the end of next week. At this point, operational and ensemble output diverge with respect to how much vorticity/circulation remains centered over Arizona or gets displaced by high pressure building from the east Pacific. Regardless, temperatures will undoubtedly moderate much closer to the seasonal normal beyond the middle of the week. && .AVIATION...
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South-Central Arizona including KPHX, KIWA, and KSDL: Typical east-southeasterly AM winds will turn southerly midday while beginning to pick up in speed. A continued swing to west- southwest headings will occur between midday and early afternoon with gusts 20-25kts possible. Westerly oriented winds will stay longer in the terminals overnight than usual, switching early Friday AM to easterly headings for a brief period. Conditions will remain dry with limited thin high clouds possible early Friday AOA 25kft. Southeast California/Southwest Arizona including KIPL and KBLH: Strong cool season frontal passage will generate windy conditions for the terminals today and into the overnight. Wind gusts 25-35kts will possible, more likely at KIPL than KBLH, from west- southwest headings by the afternoon. Skies will remain mostly clear and winds will subside into the overnight Thursday. Aviation Discussion not updated for amended TAFs.
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&& .FIRE WEATHER...
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Saturday through Wednesday: Post-frontal weekend temperatures will fall below seasonal normals for the districts with periodic breeziness possible over area ridgetops. Conditions will largely remain dry and under mostly clear skies as troughing holds over the region. Some moisture from the east may begin to be wrapped into trough circulation mid next week, raising area dewpoints and introducing some very slight chances for precipitation over southeast AZ and the White Mtns. Overall, humidities will fall mainly into the 15-30 percent range with good overnight recoveries.
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&& .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES...
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AZ...None. CA...Wind Advisory from 11 AM this morning to 1 AM PDT Friday for CAZ560>568.
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&& $$ Visit us on Facebook, Twitter, and at weather.gov/phoenix DISCUSSION...MO AVIATION...Nolte FIRE WEATHER...Nolte

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