177 FXUS65 KPSR 221031 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 331 AM MST Fri Dec 22 2017 .SYNOPSIS... A warming trend will begin today, though below normal temperatures will persist across the region. High pressure building across the Western states will result in a return to above normal temperatures Saturday and will continue through next week. && .DISCUSSION... Latest water vapor imagery shows the vort max that produced the breezy and chilly conditions Thursday continues to weaken while becoming elongated east of the Four Corners. With the mid-level trough axis already located east of the area and moving steadily eastward, thickness rises will prevail today resulting in a warming trend. However, in the wake of the trough, some of the coldest air of the season is currently affecting the Desert Southwest. Several hours of sub-freezing conditions are likely across portions of La Paz County, Pinal County and Gila County where a Freeze Warning remains in effect for this morning. With conditions starting out relatively cool this morning, temperatures will struggle to reach the 60 degree mark across the lower deserts this afternoon, given the lack of late-December insolation. Below-normal temperatures are expected again tonight and the latest blended guidance indicates that spots along the Colorado River Valley including Parker and Blythe may see a few hours of sub-freezing temperatures, due to the lighter winds and more favorable conditions for radiational cooling. A more discernible warming trend is expected by Saturday as the eastern Pacific ridge migrates eastward. Positive height anomalies will return to the region, heralding a period of westerly flow and benign weather along with above normal temperatures through at least Wednesday. Thereafter, operational models diverge with the latest GFS suggesting a short wave trough diving southward through the southern Rockies will bring a slight cooling trend later in the week. Considerable uncertainty is evident in the ECMWF ensembles and the official forecast represents a consensus, with temperatures generally dropping back a few degrees but remaining above normal. However, this pattern is not favorable for significant moisture transport and precipitation is unlikely the remainder of the year. && .AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL: Southeast California/Southwest Arizona including KIPL and KBLH: An upper low centered near the Four Corners as of 00Z will continues to track southeastward and move onto the southern Plains during the day Friday. Expect moderate to strong northerly flow through much of the troposphere across the forecast area tonight and Friday but with local variations in direction at the surface. For metro Phoenix, westerly directions early this evening will trend toward northeast and east by late evening - except for PHX which will favor westerly surface winds through about 09Z- 10Z. Foothill areas may see some localized northeasterly gustiness overnight. For far southwest AZ and southeast CA, winds in the lower levels above the surface will be fairly strong but conditions are anticipated to remain below Low Level Wind Shear thresholds. Expect substantially lighter winds during the day Friday. Skies will be clear except for some localized ceilings near FL100 this evening over the higher terrain of north and east of metro Phoenix. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Saturday through Wednesday: Temperatures will steadily rise through early next week as high pressure builds over our area with above normal temperatures returning by Sunday. Afternoon humidity minimums will range between 15-25% for most days while overnight recoveries will generally be poor to fair as dry air remains in place. Winds now look to remain generally light which will help keep fire danger to a minimum despite the relatively dry conditions. The one exception may be that locally breezy conditions could develop Saturday night into Sunday mainly over elevated locations of south- central AZ. No precipitation is expected through the forecast period. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Freeze Warning until 9 AM MST this morning for AZZ533-552-553- 556>558-560>562. CA...None. && $$ Visit us on Facebook, Twitter, and at weather.gov/phoenix DISCUSSION...Hirsch AVIATION...AJ FIRE WEATHER...Wilson