textproduct: NWS Phoenix

UPDATE...Updated Aviation Discussion

KEY MESSAGES

- Above normal temperatures will cool into a below normal category during the latter half of the week.

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

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

SHORT TERM /Today through Monday/

An intensifying subtropical jet streak and attendant upper low approaching from the East Pacific will impact the area over the next 48 hours, and herald the beginning of a period of unsettled weather through the middle of the week. This feature will displace shortwave ridging today with modest height falls arriving Monday, while also allowing deep southerly flow and placement of the forecast area within a very favorable left front quadrant of the upper jet. The resultant flow pattern will force the import of higher moisture values in the H7-H5 layer by this afternoon, though forecast soundings still limit mixing ratios at 5 g/kg through the tropospheric column. Impressive ascent mechanisms will move into southern Arizona this afternoon acting on modest elevated instability, and HREF membership continues to advertise isolated deep convection around the Tucson area. A deep sub-cloud dry layer with T/Td spreads better than 50F will promote strong, gusty outflow winds being absorbed in the general southerly flow. While recent HRRR iterations are more aggressive than its HREF counterparts, there is a consensus depicting quasi-organized outflow surging into northern Pinal County late afternoon/early evening which could conceivably import some lofted dust towards the Phoenix metro. A residual isolated shower would not be inconceivable, but would tend to become focused over higher terrain areas to the east of Phoenix.

As the core of the upper low draws overhead Monday, lapse rates will steepen more noticeably resulting in MUCape ~200 J/kg materializing over the eastern part of the CWA. 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 1120Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concern through tonight will be the potential for gusty southeasterly outflow winds early evening. Until then, expect typical diurnal wind trends and clear skies. Expected convection starting near Tucson late afternoon may send gusty winds toward at least KIWA and potentially well into the Phoenix area, though confidence is still somewhat low. Also can't rule out a few showers or even weak thunderstorm making their way toward southeastern Phoenix early to mid evening with the main concern being erratic wind shifts and a brief period of gusty winds of 20-30 kts. Outside of the potential for outflow southeasterly winds, wind directions will favor the southwest from this afternoon through at least midnight Sunday night.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather impacts are expected through the next 24 hours. Winds will favor a SE'rly component at KIPL, until a W-SW shift this evening, and S'lry component at KBLH through the TAF period. Afternoon wind gusts up to 20-25 kts are expected again at KBLH during the afternoon, with less gustiness at KIPL. Skies will be clear to mostly clear through tonight.

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.