textproduct: NWS Phoenix

UPDATE

Updated 00Z Aviation Discussion.

KEY MESSAGES

- Lingering rain chances mainly across Arizona high terrain this afternoon before ending completely this evening. - Below normal temperatures are forecast through Wednesday before creeping back into the normal range late week.

- Breezy to windy conditions will impact the western deserts through Thursday before spreading across the rest of southern and central Arizona by Friday. Advisory level winds will be possible by Friday, particularly across southeast Arizona.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Scattered light to moderate rain showers were continuing to fall this morning across south-central AZ and a lot of the area was locked under thick low-level clouds. Soundings from the Phoenix area this morning showed a nearly fully saturated layer from the surface to 700mb. Above 700mb there is a lot of dry air. This moisture has been trapped below 700mb for the past couple days and the persistent S-SW flow has continued to support modest upslope ascent for showers to continuously develop. Most hi-res models have showers shifting east of the Phoenix area for the afternoon and evening, focusing over the higher terrain areas, but an isolated shower still cannot be ruled out across Phoenix through 4-6 PM this evening. Heading into this evening and tonight the layer below 700mb will finally dry out as a dry cold front moves through, associated with a low pressure system that will pass through northern AZ (currently over southern NV). The rain and clouds have stuck around a bit longer than initially thought and as a result temperatures are a bit cooler. Temperatures in the Phoenix area at noon were still in the mid and upper 60s across eastern areas to low 70s across western areas. Most of Phoenix will likely fail to reach 80 degrees this afternoon, especially the longer the clouds can stick around.

As mentioned, a dry front will sweep through the area this evening and tonight, which eliminate shower chances across the entire local area by midnight. This front and the passing low pressure system will also bring some cooler air that will be most noticeable with the temperatures the next couple mornings. Low temperatures tomorrow morning are forecast to be in the 50s across most lower desert areas with some low-lying exposed areas and some rural areas dipping into the 40s. Some localized valley fog cannot be fully ruled out tomorrow morning following the wet period, better potential in sheltered valleys NE and E of Phoenix, but there will be a lot of dry air entrainment above the surface. Skies will be clear Wednesday and temperatures will warm into the 70s across south-central AZ and reach 78-83 degrees across some western desert locations. With strengthening surface pressure to the north behind the upper level low pressure system, breezy to locally windy northerlies (25-35 mph wind gusts) will develop across southeast California and southwest Arizona from this afternoon through tomorrow.

LONG TERM /THURSDAY THROUGH MONDAY/

Forecast confidence is increasing for advisory level winds developing later this week across eastern and southern Arizona. Guidance continues to show a fast moving dry shortwave trough diving southwestward into the western part of the large scale trough lingering across the Southwestern U.S. Wednesday night into Thursday. This disturbance is then expected to form a cut- off low just to our southwest at the same time strong ridging edges into California and the Great Basin.

Thursday looks to be a transition day with a continuation of enhanced northerly winds down the Lower CO River Valley while temperatures warm closer to normals as northerly downslope warming spreads through much of Arizona. These northerly winds should quickly shift out of the east late Thursday night/Friday morning as the cut-off low forms and drifts southwestward off the coast of Baja. A strong pressure gradient will then develop across Arizona in response to strong surface high pressure building over the southern Rockies and the Plains to the east and low pressure to our west. There may even be a distinguishable backdoor cold front boundary that will move through Friday morning. Guidance shows 30-45 kt H8 winds setting up across southern Arizona Friday morning which should translate to widespread 30-40 mph easterly gusts at times to as high as 45 mph across portions of Pinal and southern Maricopa Counties. A Wind Advisory is expected to be needed for these areas on Friday and possibly into Saturday morning before the gradient begins to relax.

The strong easterly winds on Friday will also usher in some cooler air from New Mexico, keeping daytime highs in the low to mid 80s across the Phoenix area. The western lower deserts will be less affected by this back-door front with highs closer to 90 degrees.

Model uncertainty increases for the weekend as there is some question of how long the cut-off low will sit just to our south before ejecting back to the east and whether or not the remnants of TC Simon will play any role in our weather. As of right now, guidance shows the cut-off weakening and shifting east northeast across northern Mexico later Saturday into Sunday while the remnants of Simon mostly likely bypasses our region to the southeast. Before this occurs, the positioning of the cut-off low and the increased easterly flow is expected to bring good mid and upper level moisture advection into Arizona and southern California on Friday, but likely little if any low level moisture. The most likely scenario calls for increased higher level clouds on Saturday and a 10-20% chance of showers focused more toward southeast Arizona. However, there are some ensemble members (maybe 20%) that try to pull better low level moisture (even potentially from Simon's remnants) into at least southeast Arizona Saturday into early Sunday. If this were to occur, it would bring much better rain chances as far northwest as the Phoenix area. Although still unlikely, it will be something to watch over the next few days. By early next week, we should have another ridge with dry air advection moving in from the west leading to quieter weather conditions.

AVIATION

Updated at 0000Z.

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

No major aviation weather concerns are anticipated through the TAF period. CIGs will continue to gradually increase and scatter out through the evening becoming mostly clear skies overnight. Some occasional gusts into the upper teens are possible through the early evening. Otherwise, winds speeds will be aob 10 kt. Wind directions will tend to follow their typical diurnal tendencies. However, at KPHX winds will likely go light and variable overnight/tomorrow morning instead of developing the typical easterly component.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation weather concerns are anticipated through the TAF period under mostly clear skies. Winds at KIPL will fluctuate between NE'rly and W'rly with speeds aob 10 kt. At KBLH, winds will be northwesterly the whole TAF period with gusts up around 20 kt through the early evening tonight then again tomorrow afternoon and early evening.

FIRE WEATHER

Isolated showers will persist across the eastern districts, mainly high terrain areas, through this afternoon before drier air sweeps through the area behind a passing cold front this evening and tonight. Elevated moisture levels will keep MinRHs today between 25-35% for the western districts to 40-70% for the eastern districts before lowering down to 20-40% Wednesday. Northerly breezy winds will develop across southeast CA and southwest AZ today and strengthen each day through Thursday before strong easterly winds spread through the region by Friday. Below normal temperatures through Wednesday will eventually return to near normal temperatures late week. Some moisture return Friday into the weekend may bring low- end rain chances by Saturday.

PSR WATCHES/WARNINGS/ADVISORIES

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