textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- The combination of abundant moisture and a weather system moving into the region will help to bring periods of showers and thunderstorms across south-central and eastern Arizona through Wednesday.

- The main forecast concern will be heavy rainfall and localized flash flooding from around the Phoenix metro into the higher terrain north and east of Phoenix.

- Shower and thunderstorm chances will greatly diminish by Thursday and generally come to an end starting Friday.

- Near normal temperatures through Thursday are expected to warm to slightly above normal by the weekend.

SHORT TERM /Today through Wednesday/

Much of eastern and central Arizona will remain under the threat of moderate to heavy rainfall through Wednesday with the best chances likely late tonight through Wednesday afternoon or evening. The main impetuses for this potential will be a Pacific trough/upper level jet positioned just to our north and northwest, a mid-level disturbance eventually moving through Arizona on Wednesday, and a consistent plume of subtropical moisture positioned over the eastern half of Arizona.

Current objective analysis shows a lull in activity as drier air temporarily pushed eastward through all of southwest Arizona into western portions of Maricopa County leading to PWATs as low as 1.4-1.6" in the Phoenix area. However, a strong Gulf surge is starting to push its way northward into Yuma County and Pima County with the expectation of this pushing surface dew points to between 65-70 degrees and low level mixing ratios of 12-13 g/kg as far west as the Lower CO River Valley. Broad upper level ascent over the region will continue to improve today into tonight likely peaking early Wednesday just as PWATs reach their max of 1.8-2.0" across south-central Arizona. CAMs had been showing shower and storm redevelopment somewhere near to northeast of Phoenix early this morning, but satellite imagery is giving no signs of a trigger for this to happen. The latest CAM runs have also backed off or completely eliminated this convective potential. The most likely scenario for today is for only isolated showers and maybe a few storms through early afternoon before the convective development gets going with the heating of the day, focused more across the eastern Arizona high terrain. Forecast soundings do show some support for a few strong storms this afternoon, but given the somewhat moist lower levels, downdraft winds should stay sub-severe. However, given forecast CAPE of up to around 1500 J/kg this afternoon, it would not be surprising to have one beefy storm capable of producing severe wet microbursts. Chances remain fairly good (40% chance) for some storms will develop as far west as Phoenix or even just west of Phoenix later this afternoon through this evening, but chances will be better after midnight.

Attention then turns to what should be the peak of this event as PWATs increase and reach their peak late tonight through Wednesday afternoon. Guidance shows rather good upper level ascent developing over south-central Arizona after midnight and likely persisting through much of Wednesday. We are also watching a mid- level shortwave that is currently over the southern Baja area as this is forecast to move into southern Arizona at some point from mid morning to late afternoon Wednesday. The combination of the upper level ascent and this mid-level disturbance is likely to bring a period of widespread showers and isolated to scattered thunderstorms. With PWATs anywhere from 1.6-2.0" and low level mixing ratios of 12-14 g/kg late tonight through much of Wednesday, it is likely some areas will receive moderate to heavy rainfall at times. Both the GEFS and EPS mean QPF show average amounts of 0.5-1.0" over the Phoenix area to 1.0-1.5" over much of the higher terrain north and east of Phoenix. HREF guidance also shows pockets of higher QPF amounts of up to around 2" possible. The potential for moderate to heavy rainfall will be the highest from just before sunrise to around sunset on Wednesday and this is the likely time frame for a Flood Watch that will likely be needed soon. We also can't rule out a strong or severe storm Wednesday afternoon due to improving winds aloft and bulk shear increasing to 25-30kts, but given the likely extensive cloud cover these chances remain fairly low. Although a low probability of less than 10% or so, the best chances for a severe storm should be over western Pinal and Maricopa County Wednesday afternoon. Scattered showers and storms may persist into the overnight hours Wednesday night, but the activity should be diminishing as upper level ascent shifts more into New Mexico.

LONG TERM /Thursday through Monday/

Drier air will gradually spread eastward on Thursday, with lower desert rain chances mostly coming to an end. However, lower level moisture will still remain in place likely allowing another round of showers and isolated thunderstorms on Thursday across the eastern Arizona high terrain. Given the drying aloft on Thursday, the threat for heavy rainfall is expected to be much lower than Wednesday. The synoptic flow is forecast to shift more out of the west northwest late this week into the weekend and this will help to bring a continuous flow of drier air through the region. By Friday, PoPs are at most 10-15% in eastern Gila County to near 0% in the Phoenix area. Lower level moisture should also eventually get scoured out with surface dew points dropping into the mid 30s to lower 40s across the western lower deserts to the upper 40s/lower 50s in the Phoenix area starting Saturday.

Temperatures will mostly be an afterthought for the next several days with highs likely failing to reach 100 degrees in the Phoenix area through Thursday to right around normal for the western deserts. Once the drier air moves through the region later this week, temperatures will begin to warm with lower desert highs climbing up to around 105 degrees by Saturday.

AVIATION

Updated at 1005Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The potential for SHRA/TSRA and associated impacts will be the primary weather concern through Wednesday morning under frequent cloud decks 7K ft and above. Confidence is good that E/SE winds will prevail through the morning. The majority of SHRA/TSRA this morning is favored just east of the metro area. However, a few SHRA/TSRA could redevelop near terminals prior to sunrise.

Forecast confidence deteriorates Tuesday afternoon and evening contingent on activity in the morning. Moderate confidence exists that winds remain E/SE or become southerly during the afternoon with any additional TS holding off until the evening. Development and coverage of storms Tuesday evening remains highly uncertain, however somewhat better confidence exists regarding more widespread SHRA/TSRA and significant operational impacts late Tuesday night into Wednesday morning.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather concerns will exist through Wednesday morning under mostly clear skies. Winds will continue to behave similarly to the past 24 hours with predominantly southeast at KIPL, and southerly at KBLH with local afternoon gusts 20-25 kt.

FIRE WEATHER

Periods of scattered showers and thunderstorms are expected through Wednesday across the eastern districts supporting a heavy rainfall and localized flood threat. Moisture levels will remain elevated except for southeast California. Expect minimum humidity levels to still fall into a 15-25% range for the western districts while the eastern districts will only dip to 35-50% through Wednesday. Winds will favor the south to southeast over much of the area through Wednesday with gusts of 20-30 mph at times across southwest Arizona. By Thursday into Friday, much drier air is forecast to spread eastward across the region, eventually ending rain chances for all but the far eastern Arizona high terrain and lowering humidities into the teens across the lower deserts.

PSR WATCHES/WARNINGS/ADVISORIES

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