textproduct: NWS Phoenix

UPDATE...Updated 12z Aviation Discussion

KEY MESSAGES

- Fairly stable temperatures around 4 to 8 degrees above daily normals will result in widespread Moderate Heat Risk, with isolated pockets of Major Heat Risk at times, through the middle of next week.

- An increase in moisture by the end of the week into the weekend will lead to slight chances for showers and storms, with better chances currently focused during morning and overnight periods.

- Overnight low temperatures will be noticeably warmer as a result of increased humidity, with the typically warmer lower desert locales struggling to cool below 80 degrees by the weekend.

SHORT TERM /TODAY THROUGH SATURDAY/

Early morning midlevel water vapor imagery and 500 mb RAP analysis reveals a couple striking deformation axes draped across the region and intersecting over the forecast area. These areas of deformation have come about in between to two separate geopotential high/low couplets: one northern couplet larger in scale and formed via curvature in the polar jet stream, with an upper level ridge off the West Coast and downstream mean troughing over the Intermountain West and North-Central CONUS, and a second, southern couplet is comprised of a cutoff upper low off the Baja Coast and a subtropical anticyclone centered approximately over far Northern Mexico/Western Texas. The southern couplet will play a more prominent role in our local weather, as ensembles remain in good agreement that the Baja low will meander northward over the next 24-36 hours and the subtropical high will edge east northeastward as the northern stream/mean troughing retreats slightly northward. This evolution will allow a period of moist southerly flow to establish over the forecast area, importing anomalously high PWATs (up to around 200- 250% of normal, 1.1-1.5") and leading to slight chances for showers and thunderstorms as early as Friday morning.

Upper level divergence and some modest PVA from the Baja low will provide for good ascent Friday, however, the main limitation will be moisture as the atmosphere moistens throughout the day. By Saturday, ascent mechanisms are less well-defined or will have shifted away from the forecast area, but decent moisture (near- surface mixing ratios as much as 9-10 g/kg, PWATs peaking around 1.5") and instability (GFS bufr soundings indicate MUCAPE up to 500-750 J/kg) will be present. As a result, only slight (15-20%) chances for showers and storms enter the picture both days, and the coverage and exactly where they will form remains fairly uncertain. Current CAMs that extend out 60-84 hours seem to support an area of isolated to scattered showers developing over Southeast CA and/or Southwest AZ Friday morning and drifting north/northeast heading into the afternoon. CAMs currently show very little in the way of lower desert shower/storm development Saturday but more activity focused over the AZ high terrain and far Southeast AZ/Eastern Sonora during the afternoon. However, things such as subtle vorticity lobes or gravity waves from distant convection the previous day could be sufficient to trigger elevated convection Saturday morning over the lower deserts. This forecast will bear close watching, as cutoff lows are famously tricky for models to handle, and subtle differences in the evolution described here could lead to significant changes in the coverage/location of showers and storms.

Heights aloft will increase very slightly today and generally stabilize Friday onward, with ensemble mean H5 heights fluctuating between 588-592 dam (65th-90th percentile of climatology) over the forecast area. This will translate to afternoon highs commonly 103F-110F across the lower deserts through Saturday, around 5F above daily normals. One notable difference will be warmer overnight lows, thanks in large part to the increase in moisture and some influence from cloud cover Friday-Saturday. Dewpoint temperatures are forecast to increase relatively quickly from their current observed values between 25F-35F into a 50F-60F range across much of the area over the next 24-48 hours. As a result, overnight lows ranging from the middle 70s to middle 80s will become common as early as Friday morning.

LONG TERM /SUNDAY THROUGH WEDNESDAY/

A few changes are noted in the latest ensembles and global model runs: measures of moisture over the forecast area (e.g., PWATs, mixing ratios) are peaking at slightly lower values, however, moisture is not effectively scoured out and will likely linger into early next week, providing further chances for afternoon AZ high terrain thunderstorms. The limited instability with large T/Td spreads and dry sub-cloud layer will favor dry lightning and gusty outflow winds versus heavy rainfall, as is typical with convective activity in June. These changes come as ensembles now show the cyclonically curved polar jet, and any attendant shortwave troughs rotating through the base of the mean trough over the North-Central CONUS, remaining further east of the region. While flow aloft will turn out of a drier westerly or northwesterly direction Sunday, the more distant influence of polar jet will mean that the flow stays fairly weak over region, much less effective at scouring out lingering moisture. Positive midlevel height anomalies remaining fairly stagnant through the middle of the week will allow for steady temperatures in a 4 to 8 degree above normal range. Temperatures will be slightly higher in the western deserts, but regardless, lower desert highs will peak near or just above 110F for the typically hotter spots.

AVIATION

Updated at 1110Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns under gradual increase in mid/upper level cloud decks can be expected throughout the TAF period. The overall wind pattern will exhibit the typical diurnal tendencies with speeds generally aob 10 kts. A period of gusty westerly winds close to 20 kts is possible this evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns under a gradual increase in mid/upper level cloud decks can be expected throughout the TAF period. At KIPL, winds through this afternoon will be out of the southeast before shifting out of the west for a period early this evening. At KBLH, winds will fluctuate between the south to south- southwest. Overall wind speeds will generally be aob 10 kts.

FIRE WEATHER

Temperatures 3-6 degrees above normal will prevail with lower desert highs near 110F in some of the hotter spots, especially by the weekend. Dry conditions will continue today with relative humidity bottoming out around 8-15%. Low level moisture will improve beginning Friday and through this weekend with minimum humidity largely above 15% and overnight recoveries generally improving from poor to fair. With the increasing moisture will also come increasing chances for at least isolated thunderstorms across the AZ high terrain over the weekend, with gusty outflow winds and dry lightning potentially leading to natural fire starts being the main hazards. The overall wind pattern will remain diurnal with speeds at or below 15 mph and limited afternoon upslope gustiness.

PSR WATCHES/WARNINGS/ADVISORIES

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