textproduct: NWS Phoenix

UPDATE

Updated 00Z Aviation Discussion.

KEY MESSAGES

- A chance of showers and a few thunderstorms will exist over areas south and east of Phoenix tonight and Monday, albeit with limited rainfall potential.

- Showers and storms that form late this afternoon into the evening will be capable of producing locally strong, gusty winds and blowing dust, with the greatest risk over Pinal County.

- Increasing wind speeds with occasionally stronger gusts will develop across parts of the area through the middle of the week.

SHORT TERM /TODAY THROUGH MONDAY/

Early afternoon water vapor and visible satellite imagery indicate an increase in midlevel moisture across eastern/northern portions of AZ aided by southerly flow aloft, as cumulus decks continue to expand and begin to vertically develop over and downstream of ridgetops and other prominent terrain features. This southerly flow has developed ahead of a subtropical jet streak, intensifying as it punches onshore along the Baja Peninsula, with an attendant upper low approaching SoCal/Northern Baja at this hour. As the system draws near, the forecast area will fall under the left front quadrant of the jet streak with excellent upper level divergence noted, providing for strong ascent (likely maximized sometime tonight into early Monday morning). However, moisture will be a huge limiting factor for accumulating precipitation through the next 24- 36 hours. In GFS bufr soundings for KTUS, KPHX, and even the Globe, AZ area, only a relatively shallow saturated layer above 650-700 mb can be seen at times, with a deep, dry sub-cloud layer. Latest HREF membership continues to show showers and storms forming across Santa Cruz and Eastern Pima Counties over the next few hours, with activity pushing northward with time and sending a quasi-organized outflow into northern Pinal County late afternoon/early evening which could conceivably import some lofted dust towards the Phoenix metro. HREF neighborhood (within 25 miles) probabilities also indicate a 50% chance for wind gusts in excess of 30 kts near storms, focused over Pinal County between 4-8 PM MST today. A few residual isolated showers would not be out of the question across the Greater Phoenix Area this evening, but these would tend to become focused over higher terrain areas to the east of Phoenix later on.

As the core of the upper low draws overhead Monday, lapse rates will steepen more noticeably resulting in better instability. Latest HREF membership and GFS bufr soundings for Globe, AZ indicate MUCAPEs peaking over the eastern CWA mid-late Monday morning upwards of 200- 500 J/kg. With a lesser sub-cloud dry layer over high terrain locations and the best ascent occurring overnight, the opportunity for rainfall will increase through Monday morning in Gila County. However for the most part, HREF mean QPF barely eclipses 0.01" though persistent robust moist ascent in the midlevels will ensure several opportunities for minor accumulations. Additional deep convection may erupt later Monday afternoon with peak heating and lingering midlevel moisture with dry lightning and gusty outflow winds the greatest threat. However, any activity should be short- lived as dry air and subsidence arriving behind a passing trough axis shuts down the threat by Monday evening.

LONG TERM /TUESDAY THROUGH SATURDAY/

There is now resounding agreement among the full suite of ensembles that deep negative height anomalies entering the Pacific NW will descend into central/southern Nevada midweek as the North American flow pattern evolves into a high amplitude block. This evolution will favor the development of a large cutoff temporarily stalling just NW of the forecast area before filling and lifting into the northern Rockies late in the week. Temperatures may cool as much as 5F-10F below normal during the middle of the week, and narrowing numerical guidance spread yields improved forecast confidence. Prevailing deep westerly flow will ensure dry weather through this period despite the cold core aloft skirting the northwest parts of the CWA.

The most impactful aspect of this system should be increased wind speeds as a strong jet core and seasonally impressive height falls surge into the region. The initial round of height falls and leading edge of stronger jet winds will punch into southern California late Tuesday, then sweep across the entire CWA on Wednesday. This setup along with the deepening windward marine layer and potential passage of a midtropospheric front will favor strong sundowner winds and possible mountain rotors and hydraulic jumps across western Imperial County Tuesday and Wednesday where advisories may be necessary. Otherwise, deep mechanical mixing should tap higher momentum 20-30kt winds through the boundary layer across the entire CWA Wednesday afternoon resulting in an enhanced fire danger given fairly low humidity levels and very receptive dry fuels. As midlevel heights start filling and the cold core slowly lifts north Thursday and Friday, wind speeds will gradually relax while temperatures edge higher.

AVIATION

Updated at 2350Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Main aviation weather impact and forecast uncertainty is the potential for brief elevated wind gusts and a sudden wind shift from an outflow boundary this evening. Guidance continues to support an outflow boundary arriving from the south around 03-04Z this evening, however guidance has backed off on the intensity and thus the boundary may only lead to a brief S-SE wind shift. Wind directions may go from W-SE-W within an hour window. Odds of visibility restrictions from blowing dust is low, but hazy skies cannot be ruled out. Winds should otherwise follow typical diurnal trends, similar to the previous few days.There is a low chance (10%) for a few weak vicinity showers early Monday morning, mainly between 09-12Z. Additional showers and storms Monday, through the afternoon, will stay east and northeast of terminals. FEW to SCT clouds around 10-12K ft AGL will be common through Monday morning.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under mostly clear skies can be expected throughout the TAF period. At KIPL, winds will continue out of an easterly direction ahead of a westerly shift this evening. Westerly will prevail through the night and then may trend toward a S-SE component during the day Monday. At KBLH, winds will generally fluctuate between the south and southwest, with periods of light variability. Overall wind speeds will range between 6-12 kts, with some occasional afternoon and evening gusts upwards of 20 kts.

FIRE WEATHER

Slightly above normal temperatures will briefly cool into a below normal category during the middle of the week. Minimum afternoon humidity levels will fall into a 10-20% range with mostly fair to good overnight recovery of 30-50%, though some areas will occasionally only reach poor recovery near 20% at times. An increase in midlevel moisture late this afternoon will result in a slight chance for showers/thunderstorms in southern Arizona leading to locally gusty outflow winds and the potential for a few lightning strikes moving towards southern Gila County. A 20% thunderstorm threat will continue in far eastern districts Monday though limited 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 early this 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 will result in an elevated fire danger, though cooler temperatures and higher humidity level may preclude critical conditions.

PSR WATCHES/WARNINGS/ADVISORIES

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