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FXUS65 KPSR 161233 
  Area Forecast Discussion 
  National Weather Service Phoenix AZ 
  533 AM MST Sat Dec 16 2017 
  .UPDATE...To Aviation and Fire Weather Discussions... 
  Quiet weather is expected today before the next weather system moves  
  in. This one still looks to bring chances for rain and mountain snow  
  to the eastern half of Arizona Sunday and into early Monday.  
  Temperatures will also be cool with this system, potentially  
  bringing the coolest temperatures of the winter so far. Tuesday and  
  beyond will see a return of dry and gradually warming weather before  
  another cooling trend sets in for the latter part of next week. 
  Early this morning, high pressure aloft continued to dominate  
  Arizona`s weather pattern, keeping a very dry airmass in place  
  across the area; latest Tucson sounding indicated just 0.2 inches of  
  PWAT and the 2 am central desert surface dewpoints remained very low  
  and mostly in the teens to low 20s. High pressure aloft will be  
  weakening and shifting off to the east today as a rather cold but  
  not overly wet Pacific upper trof approaches from the northwest. 
  IR imagery early this morning showed a shield of considerable  
  mid/high clouds dropping into the area and spreading into northwest  
  AZ and far SE CA; this moisture is ahead of a developing trof that  
  will eventually bring scattered showers mainly to the eastern  
  portion of Arizona on Sunday. We still do not expect this system to  
  be overly wet and QPF values remain low; latest SREF plumes data  
  only indicated a median of less than 0.05 inches of rain in Phoenix  
  on Sunday and WPC rainfall totals for the event are well below one  
  quarter of an inch over any of the lower deserts. A look at latest  
  blended total PWAT satellite imagery showed very low moisture  
  content off of northern Baja /southern CA and no atmospheric rivers  
  are expected to be available to import moisture into the area ahead  
  of this system. 
  For days now, operational models and ensemble guidance have  
  struggled to resolve this approaching trof; as it dives  
  southeastward out of the Pacific northwest and into the desert  
  southwest, progs have tried to close off a portion of the low but  
  the track of the closed low remains very uncertain - even at this  
  late date. Models have waffled quite a bit on how much moisture will  
  be pulled into southern AZ ahead of the low and POP forecasts have  
  varied wildly from run to run. Latest 00z forecasts have taken a  
  huge step back with POP forecasts for the deserts; one example is  
  the latest MAV guidance from the GFS which now calls for just a 3  
  percent chance of rain in Phoenix Sunday afternoon compared to 50  
  percent or more from earlier runs. ECE MOS from the European now  
  calls for less than 50 percent rain chances in Phoenix for Sunday,  
  down from about 90 percent just one run ago. Most of the latest  
  operational runs, and most of the GEFS ensemble members now want to  
  close off an upper low by Sunday morning, but keep the center over  
  northern Baja/gulf of CA and they then track the low to our south,  
  mostly through northwest Mexico giving best forcing/moisture and  
  rain chances to southeast Arizona and not the central/southcentral  
  Despite being only a day or so away from this event, guidance is  
  still struggling and due to the poor consistency we opted to keep  
  forecasts pretty much as is - and so did our neighboring offices.  
  So, we still call for a chance of showers developing late tonight  
  and continuing into Sunday afternoon before tapering off Sunday  
  night as the low shifts slowly off to the east. Higher POPs, into  
  the likely category, remain over higher terrain areas to the east of  
  Phoenix Sunday. This looks to be a relatively high POP but low QPF  
  event for our area. Latest model trends have also correspondingly  
  raised our high temps for Sunday and we have made a nod to these  
  trends by pushing our highs on Sunday up several degrees; Phoenix is  
  now forecast to rise to near normal with high of 65 on Sunday.  
  Little change in these temps is expected Monday. 
  On Monday, as the low moves off to our east, some wrap around  
  moisture will linger over the higher terrain of southern Gila County  
  and we will keep a slight chance of morning showers in the forecast.  
  Should mention that latest trends keep snow levels a bit higher than  
  earlier runs and as such - given the low expected QPF - there will  
  be little if any accumulating snow in the forecast and any that  
  falls should stay generally above 5000ft. By Monday afternoon we  
  expect mostly sunny or sunny skies along with desert highs in the  
  mid to upper 60s. 
  Upper ridging will build into the area Tuesday into Wednesday for  
  mostly sunny days, clear nights and a modest warming trend allowing  
  the warmer deserts to reach into the lower or even middle 70s by  
  Wednesday afternoon. 
  For the latter portion of the extended, Thursday into Friday, latest  
  guidance has become more insistent that another large and rather  
  cold upper trof will drop into the area from the north and gradually  
  develop with a closed low eventually forming somewhere over southern  
  Arizona or northwest Mexico. GFS and ECMWF differ on details and  
  moisture, but we have sufficient confidence to add some slight  
  chances to the forecast mainly over southern Gila County on Friday.  
  This deepening trof will usher in another bout of much cooler air so  
  a marked cooling trend has been introduced, with desert highs  
  falling into the low to middle 60s over the deserts by Friday.  
  Phoenix is now expected to see a below normal high of just 63 next  
  Friday under mostly sunny skies. 
  South-Central Arizona including KPHX, KIWA, and KSDL:  
  Clouds will increase and thicken from the west today, varying 
  15-25kft in height and coverage SCT thru to OVC. Typical easterly 
  AM winds will become light and variable into the afternoon, speeds 
  less than 6kts. Easterly winds will return into the evening. Upper 
  trough moving through the region later tonight into Sunday will be 
  capable of producing lower cloud layers down between 8-10kft 
  (possibly lower in the more aggressive model forecasts) for early 
  Sunday, along with slight rain chances. Introduced VCSH for KPHX  
  midday Sunday and other area terminals could see at least VCSH  
  mention including in TAF packages later today. 
  Southeast California/Southwest Arizona including KIPL and KBLH: 
  Light wind regime to continue for KIPL and KBLH with headings 
  varying from southeasterly to northwesterly. High clouds will 
  persist between SCT thru OVC coverage today with ceilings 18-20kft 
  possible. Another storm system will drop into the region from the 
  north overnight and into Sunday, switching surface winds to 
  northerly headings and supporting breezy conditions. Strongest 
  winds will be over area ridgetops to the west of the terminals, 
  but elevated gusts are possible for KBLH (more so than KIPL) near 
  20-30kts through the day Sunday. 
  Aviation Discussion not updated for amended TAFs. 
  Monday through Friday... 
  Upper low pressure feature that moved into the region on Sunday 
  will be slow to exit, however end most precip chances across the 
  eastern Districts. In the wake of the exiting system, winds will 
  be light and variable with subtle trends toward drainage slopes 
  and diurnal patterns. Dry northwest flow will linger for Tuesday 
  and Wednesday before another colder and more potent storm system 
  sweeps in from the northwest. Breezy west-northwest winds will 
  develop overnight Wednesday and into Thursday, impacting area 
  ridgetops first then mixing into the lower desert elevations 
  during the daytime. There could be some precipitation that 
  develops over Arizona with this next system, however forecast 
  model differences are really noticeable after Thursday with how 
  deep the system is and how long it sticks around the area. Daytime 
  humidities will be range higher than the last few weeks, 15 to 20 
  percent for the lower elevations and 30 percent or greater for the 
  higher terrain. 
  Spotters should follow standard reporting procedures. 
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National Oceanic and Atmospheric Administration
National Weather Service
Phoenix Weather Forecast Office
P.O. Box 52025
Phoenix, AZ 85072

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