textproduct: NWS Phoenix

UPDATE

18Z Aviation Discussion.

KEY MESSAGES

- Dry and warm conditions, under passing high clouds, are expected today with highs mostly in the mid to upper 70s across lower deserts.

- A strong weather system arriving Monday and Tuesday will bring rain chances, breezy to windy conditions, and noticeably cooler temperatures.

- Another weather system clipping the area Wednesday will bring additional breezy to windy conditions and rain chances, and yet another drier system Thursday into Friday will help keep temperatures cooler through the end of the week.

SHORT TERM /TODAY THROUGH TUESDAY/

A transient ridge is currently passing over the Desert Southwest, with the ridge axis having already shifted east into New Mexico early this morning. A large trough is seen on wv satellite off the west coast of the U.S.. This is leading to deep southwesterly flow over the Southwest, with a fetch of mid and high clouds streaming into the region. Despite the clouds today, temperatures this afternoon will warm into the middle to upper 70s across the lower deserts, with a few pockets of 80F. Pressure falls across the region ahead of the coastal trough will lead to some minor breeziness today, with afternoon and evening gusts up to 15-20 mph.

The primary talk of the forecast the past few days has been the anticipated adverse weather impacts with the strong coastal trough and its embedded shortwaves moving through Monday and Tuesday. The most significant impacts, from rain, snow, and wind, will mostly miss the southern AZ and southeast CA deserts, in favor of the CA coastal areas and ranges and northern AZ up into southern NV and UT. This is largely due to the PVA forecast tracks staying just north of the area. Regardless, impacts are still expected across the lower deserts. Perhaps most notably from wind, with the base of the trough setting up over the area and a very strong jet streak developing overhead, with 250mb wind speeds up to 160-170kts. Global models, and now hi-res models, continue to show strong low level winds as well, with 850-700mb winds reaching 40-50kts Monday night through Tuesday morning. These strong low level winds will translate to breezy to locally windy surface conditions. Wind gusts up to 25-40 mph are anticipated. Although, there may be a limit to how much winds are able to mix down during the overnight hours. A Wind Advisory will be in effect for portions of southeast CA late Monday through Monday night.

The coastal trough will draw a relatively narrow, but considerable fetch of subtropical moisture into the Desert Southwest Monday, with EC ENS and NAEFS supporting IVT values up to 400-500 kg/ms (around 99th percentile of climatology) and PWATs up to 0.75-0.90" (150-200% of normal). This moisture should support scattered showers Monday night through early Tuesday morning, with highest PoPs (80%) right around midnight in south- central AZ. The highest PoPs due line up with the leading shortwave trough passing just to the northwest, of the lower deserts. Despite the energy with the shortwave just missing, guidance continues to show a low-mid level front sliding across the lower deserts, with potentially a broken line of showers along the front. Global ensembles do support a limited amount of low level instability (100-200 J/kg of SBCAPE). So, a stronger solid line, with embedded thunder, cannot be ruled out. This front and any stronger showers may also be able to mix down stronger, 30-40 mph, winds in the middle of the night. With the strong low level winds, showers will be moving quickly, which will likely greatly limit rainfall amounts. Latest QPF forecast ranges from 0.00-0.20" across the lower deserts to 0.25-0.50" north and east of Phoenix. Highest odds for >0.50" will be northeast of Phoenix at around 80% in Payson, compared to only 10% in Phoenix. There will be pretty quick drying post-front, through mid to late morning Tuesday and PoPs drop off through the afternoon, but a few lingering spotty weak showers cannot be ruled out, especially in the terrain east of Phoenix.

Temperatures cool down with the Monday-Tuesday system, but with the core of the low staying northwest of the area, there will not be a dramatic cool down. Forecast highs are still in the low to mid 70s Monday and right around seasonal normal Tuesday with upper 60s to lower 70s. Mornings will be chilly, mostly in the middle 40s to middle 50s. With the temperatures staying warmer, snow levels will be up around 7000-8000'. However, should a strong line of showers develop along the front, dynamic cooling could rapidly lower snow levels down to 6000'.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Another couple of troughs look to follow the vapor trails of the Mon-Tue system, passing through Wednesday and again late Thursday into Friday. These follow-on troughs will move through the region very quickly, with the strong jet streak expected to hold in place over head through the end of the week (great if you are flying east and want a quicker flight!). These troughs will also likely stay just north of the lower deserts, largely due to the jet streak, and pass through on more of a west-to-east trajectory. After already drying following the front at the beginning of the week (PWATs down to around 0.40"), the trajectory of these troughs as well are not ideal for rain in the southern AZ and southeast CA deserts. Latest NBM continues to scale back PoPs across lower elevations, down mostly to 10-20%, with increasing PoPs in the high terrain of northern AZ and on the windward side of the SoCal mountains. Showers that do develop will be moving quickly with strong west-to-east winds still in place with the strong jet overhead. So any showers at lower elevations likely will not produce much if any rainfall. Snow levels will be lower with the follow-on systems, to where some very light snow may fall down to 4000-4500', but do not expect the snow to stick. There is a very low chance (5-10%) for a dusting to an inch in western Joshua Tree NP Wednesday morning.

A couple things to expect across the southern deserts with the next troughs is more wind and cooler temperatures. Strong westerly winds are expected across the area Wednesday, perhaps stronger than with the early-week trough, due mostly to timing. The Wednesday trough will pass more during the day, which will fair better for momentum transfer to the surface. The west-to-east gradient orientation will also be more favorable for downsloping enhancements into the lower elevations of southeast CA. Widespread wind gusts up to 25-40 mph are currently in the forecast, with localized spots up to 45-50 mph in southeast CA. Below normal temperatures are in the forecast Thursday and Friday, which has not happened much this winter. H5 heights are forecast to fall to around 560dam and 850mb temperatures to around 1-2C for the the end of the week. This will translate to lower desert lows mostly in the 40s to mid and upper 30s in some rural and valley locations, and sub-70F highs are forecast for the afternoons. There is some uncertainty in the synoptic outlook heading into next weekend this far out, but over 50% of global ensembles favor a return of positive height anomalies and thus warming temperatures. Latest NBM forecast has high temperatures pushing back toward the middle to upper 70s by Sunday.

AVIATION

Updated at 1750Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns are anticipated through much of the forecast period, however an elevated southerly cross-runway wind component will become established at KPHX late Monday morning. Southeasterly winds will prevail over the next few hours before shifting more out of the south late this afternoon with no westerly wind shift anticipated today. Due easterly winds will return after sunset. Overall wind speeds will remain aob 8 kts throughout this evening. Beginning around 17Z-18Z Monday, winds will become more elevated up to 11-13 kts out of the S-SE at all terminals. Periods of SCT mid-high clouds will overspread the region through tonight.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns under passing mid to high-level cloud decks can be expected throughout the TAF period. Winds will transition from SE to W at KIPL and S to SW at KBLH. Some gusts into the mid to upper teens will be possible at both sites late afternoon into early this evening.

FIRE WEATHER

Warm and dry conditions will last through today with MinRH values around 20-30%. There will be occasional breeziness this afternoon, with S-SE gusts up to 15-20 mph, mostly in the western districts and higher terrain. Multiple weather systems will then impact the region through the work week, with greatest impacts expected Monday through Wednesday. The weather systems will bring a chance for scattered rain showers, mainly Monday night through Tuesday morning and on Wednesday. Breezy to locally windy conditions are also expected, with gusts up to 25-40 mph and locally higher in mountains and east-facing downslope areas. Winds will be strongest Monday night - Wednesday. A cold front Monday night may lead to sudden strong westerly wind gusts and there is a low chance for lightning with showers along the front (10%). MinRH values are only forecast to improve to 25-35% through the first half of the week, with gradual drying during the second half. Overnight recoveries will be good at around 60-80%.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...Wind Advisory from 1 PM Monday to 1 AM PST Tuesday for CAZ560- 561-564-568-570.

Wind Advisory from 1 PM Monday to 10 AM PST Tuesday for CAZ562.