textproduct: NWS Phoenix

UPDATE

Updated 18z Aviation Discussion.

KEY MESSAGES

- Bands of mostly light rain will be present over the region for much of today with more isolated moderate to heavy showers and thunderstorms possible by mid to late afternoon.

- A Flood Watch has been issued for portions of the region with a focus on tonight through much of Tuesday for higher terrain areas of central and eastern Arizona. Moderate to heavy rainfall could also impact portions of the Arizona lower deserts, but it is expected to be more localized.

- Below normal temperatures are expected through the middle part of the week before warming to slightly above normal by the weekend as much drier conditions eventually spread over the region.

SHORT TERM /TODAY THROUGH TUESDAY/

Flood Watches go in effect for much of the region today and tonight with mid and upper level moisture already streaming into the region from the southwest followed by an additional influx of lower level moisture starting later this morning. Forecast moisture levels are quite good for today and tonight, but not quite full tropical moisture levels. Still, forecast PWATs reach between 200-250% of normal with the bulk of the moisture held within 725-350mb layer (8000-25000 ft AGL). We have had other events in recently memory with better moisture this time of year, but this one will involve plenty of moisture for the incoming strong Pacific trough to work with.

Current objective analysis shows the Pacific trough now digging southeastward into western portions of the region with a closed low moving into central California. The trough continues to be supported by a strong polar jet and now has become linked up with a modest subtropical jet situated over the southern tier states. Satellite imagery continues to show a stream of mid and upper level remnant moisture into the Southwestern U.S. from what used to be TC Odalys. The very light shower activity on Sunday has picked up in intensity since late evening, partially due to the improving moisture but mainly due to the increasing forced ascent from the incoming Pacific trough. Light shower activity has also begun to blossom over south-central into eastern Arizona since midnight, much farther east than the main band of rain. Hi-res CAMs seem to be handling the rain band across southeast California and southwest Arizona, but not the latest development to the east. Our expectations are for this banding light rain to continue through at least this afternoon with a few embedded moderate showers or thunderstorms at times. Rainfall amounts through this afternoon in this jet forced ascent and upslope region of the storm system should mostly remain below 0.5".

By this afternoon, we will begin to see some lower level moisture advection into Arizona from the weakening TC Polo, but the NHC track of Polo remains well to our south and southeast. Once the better lower level moisture enters the picture this afternoon, some areas across southern and central Arizona may start to see better rainfall rates, but the lack of decent instability will largely keep thunderstorm potential fairly low. CAMs generally show a more solid band of showers and embedded thunderstorms forming somewhere from around Phoenix and/or south and east of Phoenix by late afternoon before moving to the east through the evening hours. Rainfall amounts with this band could reach 0.25-0.5".

For the western lower deserts, the current rain band should move out of the area later this afternoon leaving a break in the rain and potentially some breaks in the clouds leading to CAPE building to around 500 J/kg. This increased instability should be available once the strong forcing associated with the low center moves into southern California later this evening. CAMs show scattered storms developing by early evening just west of the Colorado River and becoming more of a broken line of showers and storms as it moves into western Arizona by around midnight tonight. This broken line of storms should present the heaviest rainfall potential for tonight across western Arizona before gradually pushing eastward through central Arizona Tuesday morning. Rainfall rates upwards of an inch per hour seem likely with this convection as well as threat of some strong winds. Guidance since yesterday has been trending toward a heavy rain threat for La Paz County, so a we have added that area to the Flood Watch (in effect during the evening and early overnight hours. Localized rainfall amounts of 1-2" are possible within the Watch area.

As the Pacific low continues to slowly track east southeastward through our region on Tuesday, we expect the broken line of showers and thunderstorms to move through south-central Arizona during the morning hours and mostly during the afternoon across eastern Arizona. As the line moves east on Tuesday, it will encounter even better moisture but instability will be marginal at best (250-500 J/kg). Tuesday morning through the afternoon will present the best time frame for moderate to heavy rainfall across south-central and eastern Arizona with rainfall rates increasing to over 0.5" in some locations. This will add to the rainfall totals with average rainfall amounts between 0.5-1.0" across much of the Arizona lower deserts to 1-2" over the higher terrain north and east of Phoenix. The latest NBM (maybe too optimistic) shows 48 hour QPF probabilities of 2" or greater at around 20% in the Phoenix area to around 40% over higher terrain areas. WPC's updated Excessive Rainfall Outlooks for today and Tuesday covers most of the Arizona lower deserts in a Slight Risk (15% probability within 25 miles of a point) and a Moderate Risk (40% probability) over the higher terrain.

The low center is forecast to move right through central Arizona Tuesday afternoon and evening which may also bring a period of wrap-around showers and isolated thunderstorms a few hours after the main rain band moves through. The latest HRRR is now showing this potential focusing in on northern portions of La Paz County, Maricopa County, and through Gila County. If we manage to get any stronger storms with this part of the system, some small hail should be likely. Chances for showers and isolated storms will quickly diminish later Tuesday evening with some lingering chances persisting over eastern Arizona Tuesday night.

The temperatures forecast is mostly an afterthought for the first part of this week, but it does bring big changes beginning today. The latest NBM forecast highs show readings only reaching into the lower 80s today in the Phoenix area to the mid to upper 80s across the western lower deserts. Tuesday is little different with highs potentially struggling to top 80 degrees in the Phoenix area.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

The low is forecast to move into New Mexico by Wednesday morning and well into the Plains by Thursday with dry air advection and subsidence increasing across our area. Generally clear skies are expected for the lower deserts on Wednesday and eventually over 0808eastern Arizona by later the afternoon. Fairly strong ridging is then forecast to move into the region on Thursday with H5 heights quickly rising to between 586-590dm. Despite the quick boost in heights aloft, temperatures will be somewhat slow to recover, especially over the eastern half of Arizona. Highs should stay below 90 degrees in the Phoenix area through Thursday with the lower deserts warming back into the mid to upper 90s. As the boundary layer gradually dries out going into the weekend, we are likely to return to our typically lower humidity levels while temperatures warm to slightly above normal.

AVIATION

Updated at 1830Z.

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

The main aviation weather concerns throughout the TAF period will be the potential for periods SHRA and embedded TS activity and lower CIGs. Currently, a band of light SHRA is currently moving through the terminals and will persist through the early/mid afternoon hours with SCT-BKN CIGs as low as 6 kft and brief MVFR visibilities likely. Conditions should be mostly dry late afternoon through late tonight although areas of VCSH cannot be ruled out. Confidence is good that another round of heavier SHRA with embedded TS (25-35% chance) is likely between 10z-16z, reducing visibilities to MVFR/IFR conditions and CIGs as low as 3 kft. Wind directions through early this afternoon should be out of the NNW-N before gradually shifting out of a NE-E component by the mid to late afternoon hours with speeds aob 10 kts, although locally higher gusts upwards of 20-25 kts will be possible with the shower activity.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation concern will be the potential for TS activity at KBLH during the evening hours. In the meantime, conditions should remain dry through much of the afternoon, before scattered storms develop, mainly in the vicinity of KBLH, during the early evening hours. CIGs should remain aoa 7 kft throughout the period although could lower to 4-6 kft with any SHRA activity. Otherwise, wind directions will vary between southeast to southwest through this evening before becoming light and variable overnight. A shift to the north to northwest is expected by Tuesday morning. Overall wind speeds should range between 7-12 kts with briefly higher gusts with the shower/storm activity.

FIRE WEATHER

Elevated moisture levels and periods of showers and thunderstorms are expected through Tuesday. The peak time frame for rainfall should be today over the western districts and tonight through Tuesday afternoon for the eastern districts. Chances for wetting rain between 40-80% should greatly limit any threat of dry lightning starts. Outside of enhanced gusts associated with stronger storms and some marginal northwesterly breeziness for the western districts Tuesday, winds will follow light and diurnal trends otherwise. Drier conditions and near to slightly above normal temperatures are forecast for later this week and through next weekend.

PSR WATCHES/WARNINGS/ADVISORIES

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

Flood Watch from 6 PM MST this evening through late tonight for AZZ533.

CA...None.