textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Unsettled weather continues through tomorrow with cloudy skies and a few rounds of light rain showers through tomorrow afternoon.

- Drier weather should return for much of the weekend before more unsettled weather is likely for the first half of next week.

- Temperatures are expected to remain at least slightly above normal through the first half of next week.

SHORT TERM /Today through Thursday/

Satellite imagery reveals a large cut-off-low well off the southern coast of CA early this afternoon. This low has started to slowly progress northeastward towards our region and will continue to do so over the next 24 hours. This low will weaken as it progresses northeastward and will eventually become an open wave as it moves on shore, tomorrow night, and gets absorbed by a trough, currently moving southward out of the Gulf of Alaska. Despite being well offshore, the cut-off-low is advecting moisture into our region with PWATs around 200% of normal. As the low continues to move northeastward it will push more moisture into our region, with PWATs nearing 300% of normal this evening through early tomorrow morning. This increase in moisture can be seen both on satellite (with an abundance of clouds over our region) and on radar (weak reflectivity). Two distinct areas of reflectivity can be seen on radar: the first is is across southern AZ and the second across southern CA. The first area is likely not reaching the ground and only falling as virga, as there is an abundance of dry air at the surface. Dew points across southern AZ currently range from the mid 20s to low 30s, with temperatures in the low to mid 60s. Top-down saturation will occur through the rest of the afternoon, with rain expected to start reaching the surface by 6-8 pm MST this evening across south-central AZ. As for the second area of reflectivity, top- down saturation has already occurred and some light rain showers are starting to reach the ground out there. Rainfall amounts will be light with most locations seeing around 0.10" of rain or less.

A second round of showers is expected during the overnight hours through Thursday. One band of showers will move through southern CA, mainly over the mountains. However, some of this rain may clip portions of Imperial and Riverside counties. Another area of rain will be focused across south-central and eastern AZ, where orographic forcing will lead to periods of isolated to scattered light showers. Since the cut-off low will remain well west of our region, most of the energy associated with the low will also miss our region to the west, which will limit rainfall amounts. Additional rain amounts with this second round will be around 0.10" or less for the lower deserts and up to 0.20-0.50" for the higher terrain north and east of Phoenix. Shower activity will taper off through the evening and overnight hours.

LONG TERM /Friday through Tuesday/

Shortwave-ridging is forecast to move through the region Friday into Saturday with some modest drying occurring, mainly in the mid and upper levels. The ridging will end any lingering rain chances by Friday afternoon across higher terrain areas with much of the area seeing partly sunny skies. Another Pacific trough will also develop off the West Coast by Friday with it stalling out at least through the weekend. Several shortwave troughs are expected to move through the larger scale trough from Friday and through the weekend, but these disturbances are all expected to miss our region to the northwest. Guidance is now indicating more moisture will stick around across the Desert Southwest than previously thought and this may eventually come into play early next week once we get some energy to move through here. Some ensembles show a weak shortwave moving through late Sunday into early Monday and this could generate some light shower activity for mainly higher terrain areas. Eventually, guidance favors the larger scale trough to finally dig farther south sending a shortwave trough through our region around next Tuesday or Wednesday. The NBM is beginning to increase PoPs for this potential system, but for now it doesn't look likely to provide a widespread soaking rain either.

Temperatures for the rest of this week and through at least the first half of next week will stay quite stable. NBM guidance shows highs staying anywhere from 2-8 degrees above normal with very mild overnight lows oftentimes at least 10 degrees above normal.

AVIATION

Updated at 0540Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concerns will be gradually lowering CIGs through Thursday morning, possibly becoming MVFR (low to moderate confidence), as well as lower than usual confidence in wind directions due to virga/light showers in the vicinity. Winds will generally remain light (AOB 10 kts) through the period and favor an easterly component. Guidance has shifted the timing with best chances (30-55%) for TEMPO MVFR CIGs to late morning/early afternoon Thursday, so this is now included in the TAFs between 18-22Z, though CIGS will most likely prevail in a 4-6 kft AGL range through the period. Expect a temporary improvement in CIGS and clearing late Thursday afternoon into the evening before CIGs fill back in and lower Thursday night into Friday morning.

Southeast California/Southwest Arizona including KIPL and KBLH: The only weather issues through Thursday evening will be lower than usual confidence on winds, extended periods with VFR CIGs, and the potential for FEW-SCT low cloud decks or reduced visibilities in BR/FG at KIPL early Thursday morning. Winds will continue to favor NW for the next few hours but likely become variable for an extended period overnight into Thursday. Anticipate wind speeds to remain AOB 10 kts. One more round of VCSH/-SHRA may materialize Thursday afternoon, more likely at KBLH, but confidence in this remains low.

FIRE WEATHER

Slightly above normal temperatures will continue through at least the first half of next week. A rapid increase in moisture today into Thursday will result in some light shower activity across the region. Rainfall amounts of 0.00-0.25" are expected across the lower deserts with 0.25-0.75" across the higher terrain areas. Minimum humidity levels will increase to 25-40% today before peaking at 50-70% Thursday and Friday, with excellent overnight recovery. Somewhat drier conditions will work back into the region by the weekend, but humidities will remain elevated, with minRHs around 40-60% and excellent overnight recovery.

PSR WATCHES/WARNINGS/ADVISORIES

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