textproduct: NWS Phoenix

UPDATE

18Z Aviation Discussion.

KEY MESSAGES

- An atmospheric river today will continue to bring widespread rain showers and isolated storms, leading to gradual rises in area washes, creeks, streams, and rivers in south-central Arizona.

- A Flood Watch is in effect for much of central and eastern Arizona through tonight.

- Isolated strong to severe thunderstorms will be possible this afternoon and evening, mostly east of the Colorado River, with the main threat being damaging winds.

- Conditions will quickly dry by daybreak Monday and remain dry through the week, along with daytime temperatures slowly warming closer to, albeit still below, daily normals during the middle of the week.

SHORT TERM /TODAY THROUGH MONDAY/

Is this the Desert Southwest of the Pacific Northwest?! The first atmospheric river of the season is moving right into the region and it is one of the strongest ARs, in terms of IVT, to hit the Desert Southwest over the last decade. Satellite TPW around midnight this morning showed the best subtropical moisture was still just west of Baja California with a favorable trajectory to work across the peninsula up into AZ. Ensemble guidance shows this slug of moisture, with IVT magnitudes pushing 1000 kg/ms, arriving in AZ by around 5 AM this morning and persisting with IVT values >750 kg/ms through around noon-early-afternoon today. So, this morning will likely be the timeframe when the bulk of the widespread soaking rain will fall. Widespread rainfall amounts of 0.5-1.5" already fell Saturday evening/night across central AZ and light to moderate rain was still falling early this morning with measured hourly rates of around 0.05-0.25". There is strong model consensus that rainfall totals the rest of Sunday will reach 0.50-1.50" or more across the lower deserts to in excess of 2.00-3.00" in the favored upslope terrain areas north and east of Phoenix. Multiple ensembles even show high probabilities (>60%) of exceeding 4.00" in the favored terrain. Rain totals across SE CA and SW AZ will be on the lighter side, however localized totals up to 0.50-1.00" are still possible with some banding. The rain that has fallen over the past two has already resulted in rises in area creeks, streams, rivers, and washes, and the Salt River Project has been releasing water into the Verde and Salt River systems. Runoff and rises is only going to continue through the end of this weekend as rain adds up, with several waterways forecast to exceed designated action stages and some that may even exceed flood stages. Even after the rain is all done the runoff may continue through the first half of this week.

While most of the rain will be stratiform and light to moderate in nature through the morning, there will be chances for thunderstorms as well, especially this afternoon behind the main band of rain and there will even be a marginal severe wind threat as the shortwave guiding the AR moves through central. Any storm today will pose more of a flash flooding risk with higher rain rates. The instability will be fairly low, CAPE values mostly under 500 J/kg), and mid level lapse rates will be poor (<6 C/km). This will hinder severe hail potential. However, there will be a strong wind maximum passing over the area, leading to effective bulk shear up to 50 kts. There will be fairly strong 925-850mb winds in south-central AZ, up around 40-50 kts, and any stronger storm that develops could mix down these stronger winds to the surface. There will also be a non-zero chance of a brief tornado in south- central AZ with sfc-1km SRH up to 100-150 m2/s2. The strong winds above the surface will likely lead to gusty winds across south- central AZ regardless this afternoon, with gusts even upwards of 30-40 mph in some areas, but there is some uncertainty how much low-level mixing will really occur. Some of the higher elevation spots east of Phoenix have better odds (~50% chance) of experiencing occasional advisory-level winds (>39 mph gusts).

By sunrise Monday, the threat of rain will come to an end and winds will die down as the trough axis pushes east of the state and much drier air is ushered in. PWATs are forecast to quickly drop to around 0.4-0.6" by sunrise Monday. Patchy fog cannot be ruled out Monday morning in sheltered valley locations, but dry air entrainment into the surface layer may preclude any formation. As mentioned already, runoff and elevated flow in many of the major waterways across south-central AZ is likely to continue through Monday. Temperatures the next couple of days will remain cool, with highs around 6-10 degrees below normal, mostly in the upper-60s to middle-70s.

LONG TERM /TUESDAY THROUGH SATURDAY/

Through the middle of the upcoming work week, following the departure of this dynamic weekend system, we can expect dry conditions, and transient upper level ridging, resulting in temperatures slowly warming closer to seasonal levels (despite daytime highs likely remaining a few degrees below the daily normals). By late week, ensembles diverge with respect to another disturbance possibly moving through the area, but guidance seem to be trending toward a relatively weak transient upper low/positively tilted shortwave that is then followed by more ridging by the end of next weekend. Regardless of the outcome, fairly benign conditions seem to be the most likely outcome. The trajectory of the late week system is not favorable for rain chances, with its residence time too short for better quality moisture to be pulled into the area, and if it remains an open wave, its positive tilt does not bode well for providing upper level support for precipitation. NBM PoPs with the latest forecast have fallen to 10% or less Friday into Saturday.

AVIATION

Updated at 1800Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT;

Considerable aviation impacts are expected to continue through early this evening as persistent SHRA activity promotes sub-VFR CIGs and VIS, which, at times, will result in MTN obscuration. The main area of moderate rainfall will migrate east of the terminals by around 20-21Z this afternoon, but another narrow band of VCSH/VCTS will push through the region within a window between 21-02Z. S/SSW winds will be common through the afternoon before they take on more of a W'rly component after sunset. Gusts around 25 kt should be present during the remainder of the daylight hours, with occasional gusts upwards of 30-35 kt not being out of the question. Although there still could be a few lingering showers through around 06Z, conditions should improve dramatically after the second band of SHRA/TSRA pushes through. W'rly flow will prevail into Monday morning, but a brief period of an E'rly component or VRB conditions may be seen within an hour or two of sunrise.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are expected over the next 24 hours. Other than some breezy W/SW'rly winds this afternoon with gusts around 25 kt, a much quieter TAF period will be seen compared to the 18-24 hours. FEW-SCT low level decks generally between 5-8 kft will linger through this evening before mostly clear skies take over for the remainder of the forecast window.

FIRE WEATHER

Widespread wetting rainfall is expected across the area today with the highest rainfall totals expected in the high terrain north and east of Phoenix. Rainfall will become more isolated this afternoon and evening, with potential for isolated thunderstorms as well. Gusty SW winds are also expected this afternoon and evening, with peak gusts up to 30-40 mph possible in the lower deserts of south-central AZ to 40-50 mph at the highest elevations east of Phoenix. Any thunderstorm may also produce locally strong to severe wind gusts. Minimum humidities will range between 55-80% areawide today and overnight maximum humidities up to 80-100%. Notably drier conditions are expected Monday through the rest of the week, but minimum humidities will still remain around 30-45% each day as temperatures will be near to slightly below normal. Winds for much of the rest of this week will return to a more typical diurnal pattern with light speeds and modest afternoon upslope gustiness.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch through late tonight for AZZ534-537>563.

CA...None.