textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- A Flood Watch has been issued for portions of south-central and eastern Arizona for Friday.

- Showers and thunderstorms, some producing moderate to heavy rainfall, are expected on Friday centered over south-central Arizona. Localized rainfall amounts of 1 to 2 inches are likely. - A second weather system will result in a rainy Sunday for the entire area with light to moderate rainfall much of the day.

- Sunday will again bring flooding concerns as additional rainfall amounts of 1 to 2 inches are expected from Phoenix and areas to the north and east of Phoenix.

SHORT TERM /Today through Friday/

Wet weather in on its way with two weather events Friday through Sunday, both of which could produce heavy rainfall. Current objective analysis shows a complex synoptic set up with a deep closed low centered off the coast of Washington and several shortwaves along a near climatological record strong west to east subtropical jet stretching from just northeast of Hawaii to just west of Baja. This jet will play a pivotal role in how much rainfall we can expect over the next several days, especially on Sunday as it will drive a strong atmospheric river into our region.

For today, the large scale troughing along and just off the West Coast will gradually get more organized with a weak shortwave moving through California into the Great Basin, followed by another shortwave farther to the southwest which will eventually track through our region on Friday. The passing disturbance to our northwest will bring some thicker higher level clouds today while also helping to shift our flow out of the south. Guidance shows strong southerly moist advection throughout the day today and into Friday with PWATs quickly rising from just above normal currently to 200-250% of normal by Friday morning. Forecast soundings and the latest HREF guidance also show good elevated instability developing across south-central Arizona tonight into Friday, potentially leading to MUCAPEs of over 1000 J/kg in a small corridor from central Pima County through western Pinal into northern Maricopa Counties. Modestly strong jet forced ascent will also develop over our area within the left exit region of the subtropical jet by Friday morning and last through the afternoon hours. This upper level forcing will combine with 25-30 kts of H8-H7 upslope southerly flow pointed directly at south-central Arizona.

Hi-res CAMs are showing showers and isolated thunderstorms developing somewhere over or near the Phoenix area as early as 4-6 AM Friday morning, but coverage is likely to remain fairly scattered through a good portion of the morning. By late morning or early afternoon, shower and thunderstorm coverage is expected to become more widespread across south-central Arizona with the morning elevated storms potentially becoming rooted within the boundary layer. Given the forecast MUCAPE may stay near 1000 J/kg into Friday afternoon and 0-6km shear will still be a respectable 25-30 kts, a few strong to severe storms look possible. Even if storms do not turn severe, any storms will be capable of producing heavy rainfall on Friday and training of storms will be possible. Average forecast rain amounts on Friday are still hovering between 0.5-1.0" centered over and just northeast of Phoenix, but localized amounts of up to around 2" are definitely possible. A Flood Watch has been issued for the bulk of south-central Arizona on Friday with the peak potential of heavy rainfall from late morning through the afternoon. As the second shortwave trough moves to the east into New Mexico late Friday evening, any lingering showers or thunderstorms across eastern Arizona should come to an end.

Temperatures today will be very similar to the last few days with highs mostly in the mid 80s across the lower deserts, despite the periods of thicker high clouds. The increased clouds and the periods of rain on Friday should hold daytime highs to at most 75 degrees in the Phoenix area, while the western lower deserts will only dip down to around 80 degrees for highs.

LONG TERM /Saturday through Wednesday/

A short drying period will move in on Saturday, but higher level moisture and clouds will be increasing throughout the day. We can't rule out a few light showers on Saturday, but with little to no instability and drier air in the mid levels little if any additional accumulating rainfall is expected through at least the afternoon. Some guidance is showing shower activity to begin already in the evening, but for most areas it should hold off until after midnight.

Attention then turns toward one of our stronger ARs that can affect the Desert Southwest moving in Saturday night and lasting through much of Sunday. The AR will also coincide with a fairly strong shortwave trough developing west of northern Baja on Saturday before tracking right through the southern half of Arizona during the daytime hours Sunday. This weather event is shaping up to be a classic El Nino season AR event which would normally occur from late December through February. All of the ingredients for a prolonged moderate rainfall event look to be there on Sunday with an impressive 18 hours of 35-50kt southwesterly upslope H8-H7 flow and at least 12 hours of strong jet forced ascent. These impressive forcing mechanisms will also line up with record or near record moisture levels for this time of year with PWATs rising to 300-350% of normal. Instability will be lacking with MUCAPEs at most 200-300 J/kg, but the long duration of the rain will make up for the lack of higher rainfall rates.

We expect showers to blossom first over the high terrain northeast of Phoenix sometime in the evening before filling in across most of south-central Arizona around or just after midnight. Showers will stay somewhat scattered through the overnight hours before becoming widespread over much of Arizona by mid morning Sunday. As the main shortwave begins to track through southern California later Sunday morning, a fairly solid shield of light to moderate rain should move through southern California and then through Arizona Sunday afternoon. Upslope showers should persist over portions of south-central Arizona into the high terrain into Sunday evening, but the activity should be gradually winding down.

Expected rainfall rates on Sunday are not all that high, but rain totals will add up over time as this event will last longer than our typical weather events. Based on the record moisture levels, strong forcing, but little instability rainfall rates will generally be in the 0.1-0.2"/hr range in the Phoenix area to as high as 0.2-0.3"/hr in the high terrain north and east of the Phoenix area. Fairly steady light to moderate rainfall is expected from just before sunrise through the afternoon in the aforementioned areas with rainfall totals averaging 1-1.5" in Phoenix to 2-3" over the higher terrain. Believe it our not, the NBM is also suggesting the possibility (25%) of rainfall totals over 2" in the Phoenix to as much as 3-4" in the high terrain. Much lower rainfall totals of 0.25-0.5" are expected over the lower deserts of southwest Arizona and southeast California due to some rain-shadowing effects and the lack of upslope forcing.

Even if the average forecast rainfall totals come to fruition on Sunday, the bulk of the rain will runoff as it will be falling on already saturated soils. We anticipate a more expansive Flood Watch will be needed for Sunday with many of the area washes filling or overflowing, potentially for a good chunk of time. We may also have to worry the flooding of some larger creeks and rivers later Sunday into Monday as all of the expected rain works through the water basins.

Dry air will begin to filter into the region from the northwest later on Sunday, overspreading the bulk of the area by Monday morning. We may have some lingering showers across the eastern 1/3rd of Arizona Sunday night into Monday morning, but additional rainfall amounts should be fairly light. The rest of Monday will bring decreasing clouds as the drier air works into the region while another follow-on shortwave trough moves southeastward into northern New Mexico. Temperatures for both Sunday and Monday will be fairly cool with highs as low as the lower 70s for the Phoenix area to maybe the mid to upper 70s across the western lower deserts. Upper level ridging is then forecast to mostly take over across the Desert Southwest going into the middle part of next week, but temperatures will be fairly slow to rise with readings staying below normal.

AVIATION

Updated at 1100Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; No major weather issues will exist through early Friday morning. Wind behavior will be similar to the past 24 hours. Another extended period of mid/late afternoon south cross runway winds (140v240) is expected around the Phoenix metro before becoming light W in the evening. Extended periods of light and variable to nearly calm conditions will be common.

Dry conditions are expected through the next 24 hours. Around or shortly before sunrise on Friday morning showers and storms are expected start popping up in/around the Phoenix Metro. Due to this VCSH/SHRA has been introduced starting around 10Z Friday, with a PROB30 for TSRA at KPHX starting mid morning. While cloud bases will drop to between 4-7K ft AGL, lower CIGs will likely not fill in completely until late morning/Friday afternoon. Any heavy rainfall could also lead to briefly lowered VIS. Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues will exist through Friday morning under periods of increasing thick cirrus decks. Wind behavior will be similar to the past 24 hours. KIPL will see winds fluctuate between S/SE and W. Winds at KBLH will become S late this morning or early this afternoon and generally fluctuate between SW and SE through the remainder of the period. Winds speeds will be AOB 8 kt. Extended periods of variable winds to nearly calm conditions will continue to be common.

FIRE WEATHER

After another overall dry day today, unsettled weather conditions are expected starting Friday. Showers and thunderstorms will impact much of the eastern districts on Friday with heavy rainfall likely in some areas. Minimum humidity levels today will improve slightly, but still fall into a 20-40% range before rising substantially on Friday with widespread 45-65%. The unsettled weather and cooler temperatures will last through the weekend with widespread rainfall on Sunday. Outside of any thunderstorms, winds will remain on the lighter side through the weekend.

PSR WATCHES/WARNINGS/ADVISORIES

AZ... Flood Watch from Friday morning through Friday afternoon for AZZ534-537-538-540>551-553>555-559.

Flood Watch from Friday afternoon through Friday evening for AZZ552-556>558-560>563.

CA...None.