textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- A Flood Watch remains in effect for portions of south-central and eastern Arizona today.

- Showers and thunderstorms, some producing moderate to heavy rainfall, are expected today focused over South-Central Arizona. Localized rainfall amounts of 1 to 2 inches are likely.

- A few strong to marginally severe thunderstorms are possible today, especially from late morning into the afternoon, with the main hazards being large hail and damaging winds.

- A followup weather system Saturday night through Sunday night will bring even better moisture over the region and result in a prolonged period of light to moderate rainfall, leading to additional flooding concerns. Thus, a Flood Watch will also be in effect Saturday night through Sunday night for much of Central and Eastern Arizona.

SHORT TERM /TODAY THROUGH SATURDAY/

Midlevel water vapor imagery reveals an elongated subtropical jet streak, stretching from near Hawaii to just off the Baja California Coast. A broad area of upper level divergence is apparent downstream of this feature and especially to the east of a rather compact shortwave that has developed off the SoCal/Northern Baja Coast. Meanwhile, a period of southerly flow and strengthening 850-700 mb southerly jet has pulled better atmospheric moisture into the area, resulting in ensemble mean PWATs surging upwards of 200-250% of normal overnight. An area of showers and storms had already developed over Southeastern AZ yesterday evening which progressed northward, reaching the Phoenix area in the early morning hours. This area of showers and storms well out ahead of the compact shortwave is feeding on a surge of higher theta-E and coincident elevated instability (RAP analyzed MUCAPE between 500-1000 J/kg), which is easily released by the upper level divergence apparent in the upper air objective analysis. Latest CAM guidance does indicate this early shower/storm activity will be transient and clear north and east of the Phoenix area over the next few hours, however, small disturbances or strong enough outflow from this initial activity may be enough to continue redeveloping activity through the rest of the morning. The next area for shower/storm development this morning may well be over Southwest AZ, near/east of the Lower CO River and coinciding with the nose of the 850-700 mb south southeasterly jet. Some top-down moistening of the lower levels will continue to occur through the rest of the morning over portions of the forecast area, especially with any shower/storm activity that develops, setting the stage for a lower DCAPE environment that will also be conducive to higher rain rates (less sub-cloud evaporation). The highest rain rates and best coverage of showers and thunderstorms is likely to occur during the late morning through the early evening with the compact shortwave and its axis of relative vorticity moving east/northeastward across South-Central AZ. Average rain totals between 0.50-1.00" centered over and just north and east of Phoenix are still forecast. Away from the foothills/higher terrain of Central AZ, forecast rain totals drop off quickly. However, localized amounts of up to around 2" are definitely possible with any severe or training storms, including over portions of the Greater Phoenix Area. With the expected totals, a Flood Watch remains in effect for Maricopa County and portions of NW Pinal County starting this morning, then expanding into Gila County by the afternoon.

Another concern through early this evening will be the potential for marginally severe hail and damaging winds with some of the stronger thunderstorms that do develop. Forecast soundings indicate effective bulk shear upwards of 20-30 kts, even as much as 40 kts during a narrow winds of time as the shortwave moves through South-Central AZ late this morning into this afternoon. Instability will, however, be a limiting factor. MUCAPEs upwards of 500-1250 J/kg are already present this morning, and 500 mb temperatures around -12C are certainly supportive of the large hail potential, but midlevel lapse rates look like they gradually deteriorate through the rest of the day, leading to declining instability. 00Z GFS bufr sounding now only show around 250 J/kg MLCAPE this afternoon. This could also be due to poor surface insolation due to increased cloud cover over previous model runs. Nevertheless, the environment should support some rotating updrafts, which could produce marginally severe hail and localized strong to severe wind gusts with some of the strongest storms today (most likely late this morning through mid afternoon).

As the shortwave departs the area late this evening, rain chances will drop off dramatically, and much of the area should remain dry through much of the day on Saturday. Some lingering, scattered showers may persist over upslope/higher terrain areas of Central AZ, as southwesterly flow remains elevated on Saturday ahead of the broader troughing off the West Coast.

LONG TERM /SUNDAY THROUGH THURSDAY/

By late Saturday evening, conditions begin to align for a longer duration and more widespread rainfall event through Sunday. The aforementioned subtropical jet streak is shown by global guidance contracting and cyclonically curving around the base of the broader troughing off the West Coast, as it begins to progress eastward. Strong dynamical ascent mechanisms and a prolonged period of southwesterly upslope flow over the Central AZ high terrain will align with excellent, deep moisture through the atmospheric column. Impressively high PWATs are still forecast, in excess of 1.00-1.25" and near the maximum of climatology for early November. NAEFS/ENS mean IVT values are consistent with an inland penetrating atmospheric river and are also, unsurprisingly, near climatological maximum values for the time of year (peaking around 300-600 kg/(ms)). However, midlevel lapse rates will be poor and thus instability will be very limited, with MUCAPEs likely never exceeding 200-300 J/kg. We expect showers to blossom first over the high terrain northeast of Phoenix sometime in the evening Saturday before filling in across most of south-central Arizona around or just after midnight. Showers will stay somewhat scattered through the overnight hours before becoming widespread over much of Arizona by mid morning Sunday. As the main shortwave begins to track through southern California later Sunday morning, a fairly solid shield of light to moderate rain should move through southern California and then through Arizona Sunday afternoon. Upslope showers should persist over portions of south-central Arizona into the high terrain into Sunday evening, but the activity should be gradually winding down.

Expected rainfall rates on Sunday are not all that high, but rain totals will add up over time as this event will last longer than our typical weather events. Based on the record moisture levels, strong forcing, but little instability rainfall rates will generally be in the 0.1-0.2"/hr range in the Phoenix area to as high as 0.2-0.3"/hr in the high terrain north and east of the Phoenix area. Fairly steady light to moderate rainfall is expected from just before sunrise through the afternoon in the aforementioned areas with rainfall totals averaging 1-1.5" in Phoenix to 2-3" over the higher terrain. Believe it our not, the NBM is also suggesting the possibility (25%) of rainfall totals over 2" in the Phoenix to as much as 3-4" in the high terrain. Much lower rainfall totals of 0.25- 0.5" are expected over the lower deserts of southwest Arizona and southeast California due to some rain-shadowing effects and the lack of upslope forcing.

Even if the average forecast rainfall totals come to fruition on Sunday, the bulk of the rain will runoff as it will be falling on already saturated soils. In turn, another Flood Watch has been issued for a large area of south-central AZ, and the eastern AZ high terrain starting overnight Saturday night and lasting into early Monday morning. During this time we anticipate the potential for many of the washes filling or overflowing, and flooding in some of the larger creeks and rivers as all of the expected rain works through the water basins.

Dry air will begin to filter into the region from the northwest later on Sunday, overspreading the bulk of the area by Monday morning. We may have some lingering showers across the eastern 1/3rd of Arizona Sunday night into Monday morning, but additional rainfall amounts should be fairly light. The rest of Monday will bring decreasing clouds as the drier air works into the region while another follow-on shortwave trough moves southeastward into northern New Mexico. Temperatures for both Sunday and Monday will be fairly cool with highs as low as the lower 70s for the Phoenix area to maybe the mid to upper 70s across the western lower deserts. Upper level ridging is then forecast to mostly take over across the Desert Southwest going into the middle part of next week, but temperatures will be fairly slow to rise with readings staying below normal.

AVIATION

Updated at 1224Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; Periods of sub-VFR CIG & VIS and VCSH/VCTS will be the primary aviation impact today through this afternoon before conditions improve heading into tonight. Early morning showers have been mostly staying off to the E and NE of terminals. There will remain a 20-30% chance for VCSH/VCTS this morning, but best chances (50-90%) will be this afternoon, mainly between 19-02Z, and this will also be the timeframe with the highest potential for sub-VFR conditions. Lingering VCSH cannot be ruled out tonight and scattered clouds will persist with bases around 4-5K ft AGL. E-SE winds will prevail through most of the TAF period with speeds mostly at or below 10 kts. Some variability and a period of W-NW winds may develop this afternoon with VCSH/VCTS.

Southeast California/Southwest Arizona including KIPL and KBLH: No significant aviation weather impacts are expected through the TAF period. SHRA/TSRA is forecast to stay east of the Colorado River this morning, but brief VCSH/VCTS cannot be ruled out at KBLH (10% chance). At KIPL, westerly winds will prevail throughout the period while at KBLH, winds will generally fluctuate between the southeast to southwest with periods of light variability early this morning. Overall speeds will remain aob 10 kts with the exception of KIPL early this evening as temporary peak gusts upwards of 20-25 kts are likely. SCT-BKN mid to high level cloud decks along with FEW to occasionally SCT decks around 4-6K ft will be common through this afternoon.

FIRE WEATHER

Showers and thunderstorms will impact portions of the eastern districts today with heavy rainfall likely in some areas, followed by a lull in rain chances on Saturday. Minimum humidities will range between 30-70% areawide today and Saturday and improve further on Sunday, following excellent overnight recoveries. After the lull on saturday, chances for rain increase again Saturday night followed by widespread light to moderate rainfall on Sunday areawide. Outside of any thunderstorms, winds will remain generally between 10-20 mph and favor southeasterly to southerly directions through the weekend. Conditions will gradually dry out next week as the unsettled pattern departs and temperatures moderate closer to seasonal normals, however, humidities will remain above 25% areawide, and temperatures will still remain in a slightly below normal category.

PSR WATCHES/WARNINGS/ADVISORIES

AZ... Flood Watch from Saturday evening through late Sunday night for AZZ534-537>563.

Flood Watch through this afternoon for AZZ534-537>551-553>555- 559.

Flood Watch from noon MST today through this evening for AZZ552- 556>558-560>563.

CA...None.