textproduct: NWS Phoenix

UPDATE

Updated 00Z Aviation Discussion.

KEY MESSAGES

- Enhanced moisture aloft will surge over the region today and Sunday resulting in increasing clouds and eventual chances for light rain during the latter half of the weekend.

- Rain is likely across much of Arizona to start off the workweek with the possibility of some strong to severe thunderstorms by Tuesday. Moderate to heavy rainfall will also be possible, particularly across the Arizona high terrain.

- Temperatures will cool to below normal during the first half of next week with highs only in the eighties for much of the lower deserts.

SHORT TERM /Today through Sunday/

Relatively flat ridging is currently in place over the Desert Southwest as the sub-tropical high sits over Southern Texas and Northern Mexico. Meanwhile, Hurricanes Odalys and Polo continue their residence in the Eastern Pacific. WV imagery shows a sheared off moisture plume from the former of these Tropical Cyclones currently progressing toward the Baja Peninsula and Southern California. This enhanced moisture aloft will eventually spread over the Desert Southwest this evening into tonight, leading to a noticeable increase in cloud cover. This will be the first indication of a drastically changing pattern for the region as we go through the next few days. Before we get to those changes though, mostly sunny skies and temperatures in the upper 90s to around 105 degrees will be common across the lower deserts this afternoon.

By Sunday morning, saturated moisture profiles above the 600mb level and ascent associated with an approaching Pacific cold- core will support the presence of light shower activity across the region, beginning in Southeast California before spreading eastward toward South-central Arizona later in the day. However, a decently dry sub-cloud layer will be in place, with dew point depressions near the surface around 20-25 degrees, so pinpointing exactly where rain may reach the ground will be easy task. Not to mention the added uncertainty of knowing exactly where bands of rain will setup and progress. Nonetheless, the region is blanketed in PoPs between 10- 30% as moisture flux will be strong and persistent enough that seeing shower activity appears all but certain. As mentioned above, the spatial uncertainty of what locations will actually see measurable rainfall is why PoPs are not any higher. For the areas that do measure, it likely will not amount to much. QPF projections indicate that few hundredths is the most likely outcome, with some isolated higher amounts around 0.10" at most if showers track over the same areas. The increase in clouds will drop temperatures compared to what is expected this afternoon, but a high degree of exists that lower elevations readings will max out near normal in the middle to upper 90s regardless.

LONG TERM /MONDAY THROUGH FRIDAY/

Model guidance is still pointing toward the potential for a significant weather event occurring early next week as a strong Pacific low pressure system moves into the region at the same time moisture from TC Polo is forecast to advect northward into Arizona. There have been some changes within the model runs over the last 24 hours, noticeably with the low deepening further on Monday and potentially becoming a slow moving closed low for Tuesday into early Wednesday. The latest NHC track of Polo still keeps it well to our south and southeast with its eventual remnant circulation missing Arizona. However, it is still expected to heavily influence our low and mid level moisture which will be one of the key ingredients needed for a big rainfall event.

Model clusters heavily favor a closed low forming to our northwest going into Monday with further strengthening possible as it moves into our region on Tuesday. This deepening would result in a slower moving system which has pushed back the timing of the best rain chances, now most likely occurring late Monday night and/or on Tuesday. Scattered showers and isolated thunderstorms are still expected during the first half of Monday, but they should favor the central and eastern Arizona high terrain. Pockets of moderate rainfall will also be possible Monday, but the heavy rainfall threat seems to have pushed back to Monday night and Tuesday. WPC still maintains a Slight Risk for excessive rainfall for Monday and Monday night for most of Arizona, minus the southwestern portion.

Assuming no big changes with the timing and the strength of the low, Tuesday now looks to hold our best chances for widespread rainfall, likely moderate to heavy at times, and potentially a few discrete severe thunderstorms. As the Pacific low slowly moves into our region, strong 35-55 kt mid level south to southwesterly flow will support very good upslope forced ascent into central and eastern Arizona. At the same time, we are expecting to fall under strong difluence aloft from as early as Monday evening through Tuesday evening. One of the more uncertain ingredients for the severe storm potential is the amount of instability that will be available. Although model guidance suggests CAPE will struggle to top 500 J/kg, it seems very possible a narrow corridor of better instability closer to 1000 J/kg may develop later on Tuesday. If all the ingredients come together, a few severe storms producing damaging winds and large hail will be possible.

The latest QPF amounts have risen over the south-central Arizona lower deserts with average amounts closer to 1.0" in the Phoenix area to anywhere from 0.15-0.5" across the western lower deserts. The highest storm total rainfall amounts are expected over the high terrain north and east of Phoenix with current forecast average amounts of 1-2". Locally higher amounts will occur due to training of showers and storms with some areas potentially reaching 3" for the two day period. The latest NBM guidance shows probabilities of 2" as high as 55% for the higher terrain north and east of Phoenix to 20% within Phoenix. NBM probabilities of getting 3" of rainfall lower to 10-20% for higher terrain areas. We will have to wait and see if these rainfall probabilities hold, although even at this point it seems quite likely Flood Watches will be needed for later Monday and Tuesday.

Due to the likely deeper and slower progression of the Pacific low, rain chances now extend through Tuesday night for the eastern half of Arizona and we anticipate PoPs to increase on Wednesday if the timing holds. Any rainfall Tuesday night and Wednesday should be more scattered, but we can't rule out a few thunderstorms producing additional localized moderate rainfall. If the cold core of the low hangs out over our area into Wednesday afternoon, we could easily see some afternoon storms that produce small hail. For the western portions of our area on Wednesday, drier air is expected to quickly move in leaving clearing skies and quiet conditions.

Temperatures during the first part of next week will definitely cool off with highs falling into the 80s for the south-central Arizona lower deserts to the upper 80s and lower 90s for the western lower deserts. After the Pacific low finally ejects to the east late week, upper level ridging is favored to move in from the west leading to gradual warming with highs potentially reaching 100 degrees again by next weekend.

AVIATION

Updated at 0025Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major weather concerns are expected over the next 24 hours. However it looks like VCSH/SHRA will start to move in around or after 00Z tomorrow and has been added to the KPHX TAF with this package. Overall winds will tend to follow their typical diurnal tendencies with extended periods of light and variable winds during the diurnal transitions. Wind speeds will generally remain aob 10 kt for at least the next 24 hours. BKN-OVC high clouds will be common through the period with increasing mid level clouds (with bases aoa 10-15 kft) moving in starting tomorrow morning. VCSH/SHRA will start to move in around 00Z tomorrow and currently have VCSH beginning at KPHX at 23Z, with a TEMPO for -SHRA from 03-06Z. Periodic light showers are expected to continue into the overnight hours. While there will be light showers around the Phoenix Metro, most of the activity may fall as virga, especially initially. Some periodic gusts into the upper teens are also possible with the showers.

Southeast California/Southwest Arizona including KIPL and KBLH: The possibility of VCSH/SHRA will be the main aviation concern in the upcoming TAF period. Current SE'rly winds at KIPL will go W/SW'rly this evening and overnight before settling out of the S early tomorrow morning. At KBLH, winds will continue to fluctuate between S'rly and SW'rly, with a brief period of NW'rly possible early tomorrow morning. Wind speeds at both terminals will generally be aob 10 kt through the period. VCSH/SHRA will start to move into KIPL's area by mid-morning and therefore VCSH has been introduced into this TAF package beginning at 15Z. Confidence in -SHRA is low (less than 30%) so that has been left out of the TAF for now, in fact most rain may fall at virga. At KBLH, any rain in the morning and afternoon is expected to remain to the south and east of the terminal with better chances for VCSH/SHRA at the terminal arriving after 00Z tomorrow. BKN/OVC high clouds will continue with FEW-SCT mid level clouds moving in tomorrow morning.

FIRE WEATHER

Dry conditions will prevail today before higher level moisture moves in on Sunday resulting in chances for mostly light rain. MinRHs today will bottom out today at 15-20% across the lower deserts to 20-30% over the eastern Arizona high terrain. Slow improvement in RHs is expected on Sunday before much higher moisture levels move in on Monday as rain chances quickly increase and expand. Winds will generally follow the typical diurnal tendencies with limited to no afternoon gusts this weekend. Rain and much cooler weather are expected for Monday and Tuesday with heavy rainfall a decent possibility across the eastern districts. Conditions should eventually dry out later next week with temperatures returning to near normal.

PSR WATCHES/WARNINGS/ADVISORIES

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