textproduct: NWS Phoenix

UPDATE

Updated 00Z Aviation Discussion.

KEY MESSAGES

- Periods of isolated to scattered showers will continue today through Tuesday morning. - Temperatures will continue to cool through the first half of the week with highs dipping into the seventies, while overnight lows easily reach into the fifties for Tuesday and Wednesday.

- Dry weather will settle in by Wednesday before breezy to windy conditions return later in the week and temperatures slowly return to near normal.

SHORT TERM /TODAY THROUGH WEDNESDAY/

The local area remains downstream of a large Western CONUS trough and under a persistent strong SSW flow. Isolated to scattered showers continue to develop across portions of south-central AZ. Mesoanalysis shows this is a high shear - low CAPE environment and with warm southerly flow and a lot of dry air aloft, above 700mb, there has hardly been any lightning with the activity so far today. Regardless, the showers have been strong enough at times to produce high rain-rates, with some instantaneous rates observed up to 1- 2"/hr. Unfortunately the showers are zoomin', moving NE at roughly 30 kts (35 mph). This has inhibited the potential of receiving any significant rainfall from any one shower. However, with the uniform shear, several showers have formed into linear segments or have developed along convergent cloud streets and this allowed for a few areas to pick up a quick 0.25-0.50". Shower chances are expected to continue through this afternoon and evening, but they will may become more isolated across the lower deserts and focused mostly over the AZ high terrain to the north, as model soundings show LCLs lifting (decreasing the cloud depth) and dry air continuing to slowly filter in.

Heading into tonight a shortwave trough at the base of the larger Western CONUS trough is expected to swing through the area with a weak surface frontal boundary. PoPs go up again in response to this, up to 40-70% between 10 PM and 5 PM - with the latest NBM - across Maricopa and Pinal Counties. Showers should then shift completely east of the Phoenix area by the afternoon tomorrow as even more dry air filters in aloft. Despite the shortwave and front, rainfall is expected to remain on the light side, especially with showers still moving very fast. Most areas are likely to receive less than 0.25" (this includes receiving nothing at all), with latest HREF only showing around a 10% chance of receiving >0.25" across the lower deserts with tonight's activity. Highest rainfall potential will be across the higher terrain areas east and northeast of Phoenix. A few isolated spots of up to 1" or more will still be possible where stronger showers train over the same area.

A much stronger cold front will sweep through the area tomorrow night through Wednesday morning as the deep low pressure at the center of the large Western CONUS trough digs through the northern half of AZ. This will be a dry front that will clean out much of the lingering low level moisture. Dew point temperatures are forecast to go from the mid to upper 50s Tuesday afternoon in the Phoenix area to dipping below 50 degrees by Wednesday afternoon. Mean mixing ratios will also drop to 3-5 g/kg across the area by Wednesday afternoon. So, no additional showers are expected heading into Wednesday as the dry air surges in. In addition to the drying conditions midweek, the low Wednesday will lead to the development of a N-S pressure gradient leading to increasing northerly winds across southeast California and far western Arizona early Wednesday likely peaking late Wednesday morning before the gradient relaxes. Wind gusts of 25-30 mph can be expected across much of the Lower CO River Valley Wednesday morning into the afternoon.

Temperatures today will not get much warmer than they currently are early this afternoon. Most lower deserts are in the 70s to low-80s. With the shortwave trough tonight and then stronger upper low moving through tomorrow night, the next couple days are forecast to be cooler. The latest NBM forecast has just about all lower desert locations failing to reach 80 degrees tomorrow and again for Wednesday across south-central AZ. Perhaps more notable will be the much cooler low temperatures. Morning low temperatures will be dipping into the 50s across the lower deserts; coolest on Wednesday. Some areas will even dip into the 40s, like up around Wickenburg, and portions of Joshua Tree NP will fall into the 30s. This will be the coldest push of air since the Spring. The last time Phoenix Sky Harbor reported a low below 60 degrees was March 11. Don't let the snowbirds know!

LONG TERM /THURSDAY THROUGH SUNDAY/

Models are finally coming into better agreement for late this week into the weekend showing shortwave energy and an upper level jet max diving southwestward through the region on Thursday. At the same time a strong upper level ridge just off the West Coast will move inland. For now, it seems this set up will result in a weak cut-off low forming somewhere to our southwest by Friday with ridging settling in just to our north. To complicate things even more, a polar air mass will quickly dive through the Plains States Thursday potentially bringing a back-door cool front into eastern Arizona Friday morning. Even though this is a complex pattern, it really won't amount to much for our area as dry air will remain entrenched enough to keep rain chances below 10%.

However, it is looking increasingly likely a strong pressure gradient will develop across Arizona by Friday leading to breezy to locally windy easterly winds developing Thursday night into Friday morning. NBM guidance does show the potential for Advisory level winds, but from past experience these winds tend to be overdone. We should at least expect occasional wind gusts of 25-35 mph to maybe some 35-40 mph possible across far southern Gila County, Pinal County, into southern Maricopa County. These gusty easterly winds will be more pronounced during the latter half of the morning before diminishing some during the afternoon. Guidance suggests a repeat of these gusty winds on Saturday before the gradient begins to relax into Sunday. Moderating temperatures are forecast for late week with highs warming into the 80s by Thursday and likely staying at or just below normal through next weekend.

AVIATION

Updated at 2320Z.

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

Gusty winds, periods of VFR CIGs between 5-7K ft AGL, and periods of VCSH/SHRA will be the main aviation weather concerns through tomorrow morning. A band of showers is moving through the east valley now with no lightning, brief heavy rainfall, gusty winds, and lower CIGs (around 4-5 kft). This should be exiting the airspace within the next hour, with some lingering showers behind it. Periodic periods of showers cannot be ruled out through the evening so VCSH has been added through the overnight hours. Although, not much activity is expected until 9-14Z overnight when a more defined area of showers is expected to develop and move through the Phoenix Metro. A PROB30 line remains in the TAFs with SHRA BR, reduced VIS, and CIGs lowering to 5 kft. Showers may linger through 16Z before fully clearing the area. Wind speeds will generally be aob 12 kt through the period, with the exception of some strong 25-35 kt gusts possible with some of the shower/storm activity and some occasional gusts in the upper teens tomorrow afternoon. Wind directions will generally favor a typical diurnal pattern, but may favor more of a southerly direction than southeasterly overnight/tomorrow morning. VFR CIGs will begin to clear the airspace late Tuesday morning into the afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

Gusty winds will be the aviation concern through the TAF period. Winds will mainly be out of the west at KIPl and out of the SW at KBLH through most of tomorrow morning. By late tomorrow morning/early tomorrow afternoon winds will go more NW'rly at both sites. Current gusts up around 25 kt will continue at KIPL through most of the overnight before tapering off. However, the gusts at KBlH will taper off over the next couple of hours with gusts up around 20 kt returning tomorrow afternoon. FEW-SCT decks aoa 7 kft will gradually clear through the evening becoming mostly clear by during the overnight hours.

FIRE WEATHER

Periods of isolated to scattered showers are expected through tonight before a weak cold front moves through and drier air begins to filter in tomorrow. A stronger and drier front will follow tomorrow night into Wednesday. Elevated moisture levels will keep MinRHs mostly above 40% through tomorrow before lowering down to 20-35% Wednesday and Thursday. Breezy conditions are expected just about every day. A northerly gradient wind will develop across southeast CA and southwest AZ tomorrow and strengthen each day through the middle of the week, with morning and afternoon gusts up to 20-30 mph. Stronger breezes will spread to the rest of AZ by Friday with breezy to windy easterlies developing. Below normal temperatures are expected through at least the middle of the week. Moisture return late week into the weekend may bring a return of rain chances.

PSR WATCHES/WARNINGS/ADVISORIES

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