textproduct: NWS Phoenix

UPDATE

Updated 12Z Aviation Discussion.

KEY MESSAGES

- Temperatures should remain in a near to below normal category for the first week of September, resulting in widespread Minor Heat Risk and localized areas of Moderate Heat Risk.

- Isolated showers and storms will be possible over the higher terrain of eastern and southeastern AZ today and Thursday

- Tropical moisture will arrive into the region late Thursday into Friday and persist through much of the weekend, resulting in a chance of heavy rainfall in some locations.

SHORT TERM /TODAY THROUGH FRIDAY/

Latest mid-lvl wv imagery and 500 mb streamline analysis reveals a deep trough of low pressure encompassing much of the West Coast while a ridge of high pressure extends from the Southern Plains into the southeastern U.S. Our forecast area is situated between both of these features under a prominent fetch of SW flow aloft. Meanwhile, TS Marie has formed over the E Pacific, well south of the tip of the Baja Peninsula. Moisture associated with this storm is already being advected well out ahead of the main circulation into N Sonora MX as evident in wv imagery. While the storm itself is expected to remain offshore and progress NW into open waters over the coming days, we will begin to see an increase in overall moisture beginning in southern AZ this afternoon and eventually encroaching on southcentral AZ by Thursday into Friday. The EPS and GEFS mean PWAT values at KPHX show a gradual rise from around 1.4-1.5" today to 1.7-1.8" by Friday with a few members reaching 2.0". As moisture advection increases each day, rain chances will follow suit. Latest NBM PoPs are picking up on widely scattered shower and storm activity across southeastern AZ this afternoon, mainly in the TWC forecast area. On Thursday, a much better push of moisture is anticipated which will bring chances for showers and storms into southcentral AZ. Currently PoPs are around 30-60% late Thursday into Friday with chances tapering down into Friday afternoon. There will likely be a sharp gradient in PoPs from W to E on Thursday and Friday as much drier air advects into SE California and SW Arizona. Therefore, rain chances across the western half of the forecast area will be slim to none initially.

Due to the presence of troughing to our west and a seasonable 500 mb hght field around 587-590 dam, temperatures will remain within a degree or two of daily normals through the end of this week. Lower desert highs will generally range from the upper 90s to around 105 degrees each afternoon, following morning lows from the upper 70s to middle 80s. The West Coast trough will also induce a compact 700-500 mb hght gradient over southeast CA and western AZ beginning today which will lead to breezy conditions across the Imperial and Lower Colorado River Valleys. Gusts could reach upwards of 25-30 mph in the aforementioned areas and potentially as high as 40 mph in far SW Imperial County.

LONG TERM /SATURDAY THROUGH TUESDAY/

The forecast becomes more uncertain heading into this weekend, but the general model consensus is showing the subtropical ridge over the southeast U.S. retrograding west into the Southern Plains. The western periphery of the ridge will likely nose into eastern AZ on Saturday. If this occurs, then the subtropical jet will likely shift farther west, setting up better moisture advection over SoCal and western AZ. This scenario is currently being reflected within the NBM showing a westward progression of PoPS from southcentral AZ early Saturday into southwest AZ and southeast CA late Saturday into Sunday. Due to an overall increase in moisture and extensive cloud cover this weekend, it is likely we will see lower instability which could limit convection and keep the threat of heavy rainfall at a minimum in our forecast region with better prospects of heavy rain and flash flooding over SoCal and S NV. There is still plenty of time for things to change and we will likely have to wait until we are within the high-res window for this weekend's forecast to become more clear.

Similar to rain chances, there is also a high uncertainty in the temperature forecast by this weekend as indicated by large (5-8 degree) spreads within the NBM inner quartile range, especially on Saturday. This is likely due to the strength of the ridge and how much cloud cover will occur over the region. It is likely that Saturday will be the coolest day with highs in the lower deserts mainly in the mid to upper 90s. However, as ridging aloft causes an overall increase in mid lvl hghts/thickness, temperatures may warm to near or even slightly above normal from Sunday into early next week.

AVIATION

Updated at 1122Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected through Wednesday afternoon. Winds will tend to follow light familiar diurnal trends with periods of VRB conditions during the overnight and early morning hours and some occasional afternoon gusts in the mid teens to around 20 kt. A slower-than-usual transition from east to west may produce a few hours of southerly cross winds at KPHX and KDVT later this morning, with perhaps a window of directions 160V230, before westerly flow takes over this afternoon. Some hi-res model guidance are hinting of TS activity moving within the vicinity of the terminals during the late afternoon to early evening hours, but no mention is included in the TAF at this time as SHRA/TSRA chances are too low ~10%. Generally clear skies tonight will give way to a FEW low-level clouds during the back half of the period before skies clear out again tomorrow night.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. Winds will favor a westerly component at KIPL, while south- southwesterly to southwesterly flow will be common at KBLH. Breezy conditions with gusts upwards of 20-25 kt are expected at each terminal. Breezy conditions are expected at KBLH during the afternoon and early evening hours and at KIPL during the early evening hours. Skies will be mostly clear through the TAF period.

FIRE WEATHER

Drier conditions will prevail through Thursday with only isolated chances for showers and thunderstorms mostly relegated to the higher terrain of southcentral and southeastern AZ. Better chances for wetting rains return to the region late this week into this weekend. Afternoon minimum humidities will range from 7-15% across Southeast CA and between 15-35% for much of Southern AZ through the next few days and will likely increase above 25% areawide this weekend. Overnight recoveries will range from poor across the western deserts and good to excellent across the eastern districts. Daytime winds will remain elevated out of the south to southwest, with the strongest gusts in Southeast CA and along the Lower Colorado River Valley peaking between 20-30 mph during the afternoon/early evening.

PSR WATCHES/WARNINGS/ADVISORIES

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