textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- An Atmospheric River tonight through early Monday will result in a prolonged, widespread rain event for portions of the region, leading to gradual rises in area washes, creeks, streams, and rivers at a minimum.

- Already saturated soils will result in efficient runoff and flooding concerns. Thus, a Flood Watch is in effect for much of central and eastern Arizona tonight through Sunday night.

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

- Conditions will quickly dry early this upcoming workweek, along with daytime temperatures slowly warming closer to, albeit still below, daily normal during the middle of the week.

SHORT TERM /TODAY THROUGH SATURDAY/

Current objective analysis reveals broad troughing across the western CONUS with a strong upper-level low spinning off the coast of the Pacific Northwest. This deep wave will be the focal point for what we can expect across the Desert Southwest through the rest of the weekend and into the start of the next work week. Guidance shows the broader disturbance splitting into two separate areas of vigorous energy, with one area being the previously mentioned coastal low, with the other setting up further south over the sub-tropical Pacific while phasing with a strong upper-level jet. The southern shortwave is currently helping to advect mid and upper-level moisture over our forecast area, which will yield considerable cloud cover through much of the afternoon and evening while also supporting a some pop-up showers, especially in supportive upslope areas. Current mesoanalysis indicate some minimal CAPE across central and eastern Arizona in association with peak day-time heating which could lead to some isolated thunderstorms as well.

These initial showers will be just the beginning of a much longer period of rain that could begin just prior to sunset this evening for parts of the region. The persistent southwesterly flow, which is typically synonymous with dry and tranquil conditions this time of year, associated with the off-shore disturbance will help pull a significant slug of moisture toward Arizona and southern California. PWAT levels will approach 1.4-1.7" which actually is not out of the ordinary as values this elevated are typically seen during monsoon season. However, this is not July or August and seeing moisture this high during the end of October/start of November is not normal. In fact, the forecasted PWATs will be around 300-350% of normal, near all-time records for this time of year compared to climatology. Projected IVT backs up the idea of how unusual the event will be as values will approach, and may even exceed at times, 750 kg/(ms), which would also be an all-time record for this time of year. It is also not as if IVTs will peak and then quickly fall off as there could be upwards of an 12 hour timeframe (primarily between 11 p.m. tonight through 11 a.m. Sunday) when climatological maximum are seen, owing to a prolonged period of moderate rainfall across the region. The most robust moisture profiles will be focused primarily from the Colorado River Valley and eastwards, as terrain-driven rain-shadow interference will likely result in some degree of drying across southeastern California, but PWAT and IVT levels there will still likely exceed the 99th percentile, albeit for a shorter duration.

With that being said, expected rain totals will vary widely across the Desert Southwest, with an unsurprising increase in QPF from west to east. The greatest rainfall amounts will be focused over the Arizona high terrain, where persistent upslope forcing could potentially yield totals between 3-5", with locally higher amounts around the most prominent terrain features. In the lower elevations of Maricopa and Pinal Counties, the extended timeframe of anomalously high moisture levels could push totals toward 0.75" to around 2.5", with the higher end of that spread favoring locations around foothill areas where orographic enhancement of showers will take place. Further west toward southwest Arizona and southeast California, totals will generally range from around 0.10" in the most heavily rain-shadowed areas on the lee side of the Coastal Range to upwards of 0.75".

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 will be in effect for South-Central AZ starting tonight and lasting into early 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.

Something that might get lost amongst the prolonged rainfall and flooding potential will be the very slight chance to see severe thunderstorms late Sunday morning through the evening. Once the the broad rain shield starts to progress further east, some minimal destabilization of the atmosphere may take place, which could increase the chances for convective initiation. However, hi-res ensemble data shows CAPE values only reaching about 100-400 J/kg, which should keep deep convection fairly isolated. However, the environment will be very conducive for thunderstorm maintenance with strong mid and upper-level ascent along with 0-6 km shear profiles near 50-70 kt, more than enough to produce rotating cells. 850-700mb winds will be consistently 40-50 kt during the above mentioned timeframe, so strong to severe winds will likely be the main threat, but even winds upwards of 40 mph may get mixed down by what appear to be unassuming showers. Low-level shear will also be quite strong, with 0-1 km SRH values rising to 150-250 m^2/s^2, levels that are rarely seen over this part of the country. Combine that with LCL heights that will be some of the lowest we see in a given year, and a few tornadoes cannot be completely ruled out. With limited instability expected, hail will not be as much of a concern, but if enough day-time heating can take place, the threat surrounding it may increase. If this outcome is realized, the greatest threat for strong storms will be focused from the Colorado River and eastward. Heavy rainfall associated with this activity could also exacerbate the flood risk.

When showers and thunderstorms begin to clear the region will depend heavily on the aforementioned convective timeframe, but at the very least, the bulk of activity will move off to the east by late Sunday night. The Pacific closed low off the Washington and Oregon coasts will serve as a quick followup behind the subtropical jet and initial trough axis, bringing a brief period of WAA and continued upslope flow for more westward-facing slopes to the east of Phoenix through the overnight hours into Monday morning, potentially yielding some further scattered showers. Beyond this, rainfall will gradually trail off as even the lower level moisture is scoured out, and flow shifts northwesterly through essentially the entire column, shutting off any upslope component.

LONG TERM /MONDAY THROUGH FRIDAY/

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 considerably with respect to another upper low/positively tilted shortwave trough possibly impacting the region. Some solutions show quasi-zonal flow taking shape over the Southwest US after the transient ridging shifts east of the area, while others show a shortwave disturbance capable of diving well south of the region and resulting in a brief moisture return. 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, 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 become introduced to the eastern AZ high terrain on the order of 15-25% Friday as a result, but we should not put too much stock in this, as the forecast remains highly uncertain at this time.

AVIATION

Updated at 0600Z.

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

Considerable aviation weather impacts are expected to continue through the majority of Sunday, with long duration SHRA and periods of IFR-MVFR CIGs and VIS. A few terminals may near alternate minimums and even airport minimums due to the low CIG & VIS by early Sunday morning. Confidence in sub-VFR conditions remains high and has been observed during periods of RA/+RA this evening already. Confidence in IFR flight categories is moderate, starting as early as 08Z-10Z with arrival of heavier bands of SHRA. The low flight categories may persist through Sunday afternoon. A break in the RA is expected tomorrow afternoon but VCTS/VCSH will hang on into the evening. Best time to see TSRA at the terminals will be between 22Z Sunday and 04Z Monday. Winds will favor SE overnight, then S by Sunday morning as speeds gradually increase. Gusts 15-25 kts are expected to develop by mid-late morning, then peak as much as 30-35 kts Sunday afternoon as directions gradually veer from S to SSW.

Southeast California/Southwest Arizona including KIPL and KBLH:

Significant aviation weather impacts can be expected tonight into Sunday morning with long duration SHRA and a period of sub-VFR CIG/VIS Sunday morning. CIGs/VIS are likely to be at their lowest between 12-16Z Sunday, favoring an MVFR range, however periods of IFR may develop at times, mainly at KBLH during heavier RA. Showers are expected to clear by late Sunday morning with cloud decks quickly scattering out behind the rain. Westerly wind gusts of 20-30 kt are expected to develop tomorrow afternoon at both terminals.

FIRE WEATHER

Spotty showers and possibly a few thunderstorms will impact primarily the higher terrain of the eastern districts today before rain chances increase areawide tonight. Widespread wetting rainfall is expected across the area tonight through Sunday night. Minimum humidities will range between 30-70% areawide today and and improve further on Sunday, following excellent overnight recoveries. Outside of any thunderstorms, winds will remain generally between 10-20 mph and favor southeasterly to southerly directions through the weekend. Conditions will gradually dry out next week as the unsettled pattern departs and temperatures moderate closer to seasonal normals, however, humidities will remain above 25% areawide, and temperatures will still remain in a slightly below normal category.

PSR WATCHES/WARNINGS/ADVISORIES

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

CA...None.