textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Temperatures near to slightly above normal through early next week will cool into a below normal category during the latter half of the week, especially for the western deserts.

- Typical afternoon breeziness will prevail through the beginning of the upcoming work week followed by increasing winds across the region during the middle of the week.

- A slight chance of thunderstorms will exist over the eastern Arizona high terrain Monday, albeit with limited rainfall potential.

SHORT TERM /TODAY THROUGH MONDAY/

Sensible weather conditions and/or changes through the next 48-60 hours can be tied to an intensifying subtropical jet streak and attendant upper low developing over the Eastern Pacific, far off the Baja Peninsula early this afternoon. In the near term, this developing system over the Eastern Pacific will help pump heights aloft downstream, forming a transient shortwave ridge over the Southwest US that will slide to our east by Sunday but nonetheless bring positive height anomalies overhead. RAP analysis already shows H5 heights near or just above 580 dam over southern AZ at this hour, an increase of 2-4 dam from early this morning. Ensembles continue to advertise H5 heights peaking around 581-584 dam on Sunday, representing a range between the 60th-80th percentile of climatology for this time of year. Despite the notable positive height anomalies, they will be short-lived, and so lower level thermal profiles will have little time to respond. As a result, lower desert highs will likely remain in the middle to upper 90s daily through Monday, with some of the warmest spots touching 100F on Sunday.

Deep southerly flow will be imparted over much of the state as the shortwave ridge slides east of the forecast area Sunday, importing a modest amount of moisture in the mid-upper levels. Meanwhile, global guidance remains in excellent agreement that the subtropical jet streak and attendant upper low (which is shown keeping in pace with the "left exit" region of the jet) will push onshore along SoCal/Northern Baja California Sunday night into Monday. Given this progression, expect synoptic ascent mechanisms to be maximized over the state Sunday night into Monday morning. However, forecast soundings suggest shallow, transient saturated layers above 700 mb, with a deep, dry near-surface layer exhibiting large T/Td spread. As a result, and consistent with latest CAMs, scattered light showers/virga may develop over Southeastern AZ late Sunday afternoon and push northward Sunday evening/night, resulting in spotty (if any) measurable precipitation primarily east/southeast of the Greater Phoenix Area. As the core of the upper low draws overhead Monday, lapse rates will steepen more noticeably, and instability is expected to develop primarily over the high terrain of Southern Gila County, which could lead to a few thunderstorms. The very dry sub- cloud layer will mean the primary threat from any storms will be dry lightning and gusty outflow winds, and the threat will be short- lived for our forecast area as dry air should quickly arrive from the southwest Monday afternoon.

LONG TERM /TUESDAY THROUGH FRIDAY/

An overwhelming preponderance of ensemble members are now advertising a western Conus pattern previously preferred by human forecasters with developing blocking, and deepening slow moving features affecting the western Conus. As a result, confidence has improved that pronounced negative height anomalies entering the Pacific NW will descend into the Great Basin, then stall in the form of a large cutoff low, perhaps as far south as southern UT/northern AZ. Given this evolution, temperatures will retreat into a below normal category during the middle of the week, and possibly persisting through the remainder of the week. Automated NBM/WPC output has started reflecting this trend, though it would not be unexpected that future forecasts trim a couple more degrees closer to the lower quartile of the numerical distribution during the latter half of the week.

Otherwise, the greatest impact with this system should be increased wind speeds with a seasonally strong jet and aggressive height falls entering the region. The initial surge will enter SE California late Tuesday, then sweep across the entire CWA on Wednesday. Deep mechanical mixing should tap higher momentum 20-30kt winds through the boundary layer with the most intense speeds in downsloping regions of SE California and around terrain features of the eastern CWA. Recent mandated NBM forecasts have started catching up on this increased wind speed paradigm, however some of the deficiencies in the ensembling process are likely still restricting the full extent of wind speeds. Regardless, advisories in the most wind prone areas of SE California are looking more likely, while low relative humidity and dry fuels may foster an enhanced fire danger across much of the region. Depending on the positioning and progression of the attendant closed low, these impacts may persist into Thursday before relaxing late in the week.

AVIATION

Updated at 2355Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Winds over the next 24 hours will follow a similar pattern to the previous 24 hours, with extended periods of 140v220 during the afternoon hours and rather late shift - 23-01Z - to predominant W-SW. There is high confidence a gulf surge boundary will move through this evening around 05Z and lead to a brief, 1-2 hr, uptick in SW winds. There is low confidence of an outflow boundary moving through the terminals from the SE Sunday evening, after 01Z, with a wind shift and brief uptick in winds. Mostly clear skies will prevail with FEW high-based cumulus in the afternoons and evenings.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather impacts are expected through the next 24 hours. Gusty S-SE winds will subside through this evening at both terminals. Winds will favorSE'rly component at KIPL and S'lry component at KBLH through the TAF period. Afternoon wind gusts up to 20-25 kts are expected again at KBLH Sunday afternoon, with less gustiness at KIPL. Skies will be mostly clear through Sunday.

FIRE WEATHER

Temperatures will remain near to slightly above normal through early next week before modest cooling arrives during the middle of the week. Minimum afternoon humidity levels will fall into a 10-20% range with poor to fair overnight recovery of 25-40% improving closer to a fair to good range next week. A marginal increase in moisture will result in a slight chance for afternoon thunderstorms and a few lightning strikes over high terrain of eastern districts Monday. Little to no accumulating rainfall should be anticipated, and the threat for new wildfire starts due to lightning strikes will be heightened. Typical afternoon upslope gustiness of 15-25 mph will be common through early next week with speeds increasing markedly during the middle of the week. Stronger gusts over eastern district high terrain and through the lower Colorado River valley Wednesday combined with low RH and dry fuels may result in near critical conditions.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.