textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion

KEY MESSAGES

- Lingering rain chances this morning will push into the eastern 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/

Lingering moisture associated with a deep trough currently centered over Nevada is allowing for a continuation of isolated showers across central and eastern Arizona early this morning. The trough has hung around slightly longer than previously advertised which in turn has allowed for a bit more shower activity than forecast yesterday. Additional rainfall amounts over the past 12 hours have ranged from up to around 0.10" across much of Maricopa County to as much as 0.25-0.50" in far eastern and northern areas of Phoenix and into the foothills north and northeast of Phoenix. Another round of showers is expected to move through the Phoenix area through sunrise with showers persisting across the eastern Arizona high terrain through at least early afternoon. Additional localized amounts up to around 0.25" will be possible.

Drier air is gradually working into western portions of the area and this should work through the rest of Arizona by this evening. The cooler air mass will keep the bulk of the area in the 70s today with overnight lows dipping into the 50s tonight under clear to mostly clear skies. Similar highs are expected for Wednesday, but with some western desert locations topping 80 degrees as ridging begins to work into the area from the west. The exiting of the deep trough will also bring locally windy conditions across southeast California and southwest Arizona from this afternoon through Wednesday.

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 between the cut-off low to the southwest and the ridge to our north resulting in increasing easterly winds Friday morning. Guidance shows 30-45 kt H8 winds setting up across southern Arizona Friday morning which should translate to widespread 30-40 mph 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 cool 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 1123Z.

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

The main weather concern will be lingering isolated showers moving through the metro over the next few hours along with VFR CIGs between 5-7k ft AGL. Due to the sparse coverage and quick moving nature of these showers, VCSH has been left in the TAFs, however periods of direct impacts are possible. Winds may be erratic, but should maintain a mostly southerly component until the activity passes, then to resume typical diurnal tendencies. Wind speeds will generally be aob 12 kt through the period, with the exception of some occasional gusts in the upper teens this afternoon. VFR CIGs will begin to clear the airspace late this morning into the afternoon before clearing to FEW mid level clouds by the late evening hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

Gusty winds will continue to be the main aviation concern through the TAF period. Southwest winds at both terminals are expected to shift out of the NW-NNW by late this morning (12-19Z). Current gusts between 20-25 kts will continue at KIPL through most of the morning period before tapering off heading into the afternoon, although sustained winds will remain aoa 10kts up near midnight tonight. Winds at KBLH will be aob 10 kts through the morning before picking up, with gusts between 20-25 kts by this afternoon. FEW mid level clouds will clear by mid morning and remain clear for the remainder of the TAF period.

FIRE WEATHER

Isolated showers will persist across the eastern districts for the first part of today before drier air sweeps through the area behind a passing cold front. 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.