textproduct: NWS Phoenix

UPDATE...Updated 00z Aviation Discussion

KEY MESSAGES

- A Flood Watch remains in effect 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.

- 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.

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

SHORT TERM /Today through Friday/

Today will be the last dry and calm weather day ahead of the next couple of weather systems that will bring rainy weather starting Friday and expected to last through the holiday weekend. A large trough is currently positioned off the western coast, with a cut- off low developing along the coast of Washington. Additionally, a near climatological record strong west to east subtropical jet is stretched from northeast Hawaii to just west of the Baja coast. 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.

As mentioned the rest of today will continue to be quiet, with increasing cloud coverage throughout the rest of the day as the southwesterly flow aloft is already beginning to be advected into the region ahead of the large scale trough over the west coast. By Friday PWATs will be near 200-250% of normal, and while the troughing feature will remain positioned off the west coast, a shortwave will track through the Desert SW Friday afternoon. This shortwave will aid in providing enough ascent for showers and thunderstorms to develop starting as early as Friday morning. Forecast soundings and the latest HREF guidance continue to point towards 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. Latest Hi- res CAMs show show showers developing as early as 5-7AM however activity is expected to be light early Friday morning. The main window for widespread showers and thunderstorms won't be until late morning/ early afternoon, where a few severe storms will be possible, as SPC did upgrade the region from general thunderstorms to a Marginal risk for severe weather Friday for south-central and southeastern AZ. Storm totals Friday are forecasted to generally be between 0.5-1.00" centered over and just northeast of Phoenix. However, localized amounts of up to around 2" are definitely possible with any severe or training storms. With the expected totals a Floor Watch remains in effect for Maricopa County, portions of NW Pinal County starting Friday morning, then expanding into Gila County by the late morning hours.

Otherwise, 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 Arv 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. In turn another Flood Watch has been issued for a large area of south-central AZ, and the eastern AZ high terrain starting Sunday, into Monday morning. During this time we anticipate the potential for many of the washes filling or overflowing, and flooding in some of the larger creeks and rivers 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 2345Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; Thunderstorm activity with gusty and erratic winds as well lowered CIGs and VIS due to heavy rainfall Friday afternoon will be the main aviation weather issues throughout the TAF period. In the meantime, quiet conditions will prevail through at least early Friday morning. Winds early this evening will be quite variable before an easterly component establishes later this evening into the overnight period with overall speeds aob 7 kts. VCSH is possible Friday morning before strong to locally severe thunderstorm activity develops starting around midday and continuing through much of the afternoon hours with major operational impacts expected due to gusty and erratic winds and temporary reductions in VIS and CIGs to MVFR/IFR conditions. Activity will move east of the terminals after 00z with some VCSH possible through the early evening hours. SCT-BKN mid to high level cloud decks will prevail through tonight before lowering to as low as 5-6 kft by Friday morning, temporary lowering to as low as 3 kft in any heavier TSRA Friday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather impacts are expected throughout the TAF period. At KIPL, westerly winds will prevail throughout the period while at KBLH, winds will generally fluctuate between the south to southeast with periods of light variability early Friday morning. Overall wind speeds at both terminals will remain under 10 kts. SCT-BKN mid to high level cloud decks along with FEW to occasionally SCT decks around 4-7 kft will be common throughout the period.

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 are between 20-40% before rising substantially starting 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 Saturday evening through late Sunday night for AZZ534-537>563.

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

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

CA...None.