textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Better moisture and more favorable conditions for showers and storms are expected by this evening, lasting into Thursday morning. - A Flood Watch has been issued for the high terrain across central and portions of eastern Arizona from late afternoon through tonight. Locally heavy rainfall will also be possible into the south-central Arizona lower deserts.

- Near to below normal temperatures are forecast through Friday before warming back up over the weekend into early next week.

SHORT TERM /TODAY THROUGH THURSDAY/

Today and tonight are shaping up to be quite busy as the subtropical ridge continues to shift eastward out of the region while an upper level trough moves in from the west. The main forecast concern will be the threat for heavy rainfall and localized flooding mainly over the high terrain north and east of the Phoenix area. A Flood Watch has been issued for all of Gila County as well as over far northern Pinal and northeast Maricopa Counties from late this afternoon through tonight.

The mostly over-turned atmosphere that resulted in lower convective activity on Tuesday is recovering while increasing south southwesterly flow today will help to pull even better moisture northward throughout much of the area. The incoming trough will provide the extra boost to pull moisture farther to the north northeast, while also providing broad (albeit weak) upper level forcing. The typical strong subsidence that holds back convection from forming, especially over the lower deserts, will disappear later this afternoon with forecast soundings showing little to no CIN. The cooler daytime highs will however limit how much instability that will develop with MUCAPEs mainly between 500-1000 J/kg from late afternoon into the evening hours before dropping after midnight. Moisture levels will improve through the afternoon hours with low level mixing ratios around 11 g/kg over the lower deserts to as high as 12-14 g/kg over the high terrain. A south to southwest steering flow will also increase to around 10-15kts by this evening, but that is not fast enough to prevent heavy rainfall from occurring.

Latest hi-res CAMs are in good agreement showers and storms will begin to develop over at least the high terrain mid to late afternoon and then even over the lower deserts by early evening. The generally uncapped atmosphere should allow for good areal coverage of showers and storms during the evening hours with the best area of focus across northern Maricopa County into Gila County. Additional pockets of showers and storms are expected with some guidance hitting La Paz County and over the lower deserts of Maricopa and Pinal Counties. The activity is likely to be more short-lived, but some redevelopment will remain possible throughout the evening hours. This area will need to be watched for a potential expansion of the Flood Watch. The main concern for heavy rainfall potential stems from a high likelihood of localized rainfall rates of 1-2"/hr and training of storms as depicted by the CAMs over the high terrain just to the north and east of the Phoenix. The activity is likely to continue past midnight before winding down and gradually pushing to the east by around sunrise Thursday morning. Additional isolated showers and thunderstorms will again be possible Thursday afternoon, but with the atmosphere drying out and forcing waining there will be very little chance of any localized heavy rainfall.

Temperatures will fall further today into Thursday, especially over eastern and central Arizona. NBM forecast highs for the Phoenix area show upper 90s to around 100 degrees both days with highs more in a 100-104 degree range across the western lower deserts where mostly dry conditions will prevail.

LONG TERM /FRIDAY THROUGH TUESDAY/

Dry air will continue to spread northeastward through Arizona on Friday with low level mixing ratios dropping to around 8 g/kg over the lower deserts to 9-10 g/kg over the eastern Arizona high terrain. These moisture levels during the summer monsoon season typically prohibit convection across the area with at most a 10-20% chance of isolated showers or weak storms across the far eastern Arizona high terrain. However, even with the drying conditions surface dew points well into the 50s will still keep that slight muggy feeling in the air. The trough will continue to influence the region through Friday keeping temperatures a few degrees below normal under partly cloudy skies.

Saturday looks to be quite similar to Friday with periods of higher clouds persisting, but temperatures will begin to tick upward again as the subtropical high begins to nudge back into our region. Guidance shows H5 heights gradually rising into a 592-595dm range by early next week, or back to around the 90th percentile of climatology. Moisture levels look to stay fairly steady for the rest of the weekend into early next week keeping at most low grade monsoon storm chances in place. The increased heights and somewhat dry conditions will help to push daytime highs to above normal by Monday with readings right around 110 degrees for next Monday and Tuesday.

AVIATION

Updated at 1844Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary forecast challenge through the TAF period continues to be timing out the best potential for showers and thunderstorms across the metro area. For this afternoon, can't rule out an isolated shower or thunderstorm developing, though confidence in this occurring is too low to reflect in any TAFs. Heading into this evening, timing of shower and thunderstorm activity becomes more tricky. Latest guidance supports isolated VCSH/VCTS developing as early as 01Z/02Z with better chances potentially occurring later in the evening getting closer to midnight. Chances for SHRA/TSRA may extend into the overnight and early morning hours Thursday. Heavier showers and storms may lead to temporary MVFR visibilities. FEW-SCT, and at times BKN, cloud bases down to ~7-8 kft are expected through the period. Winds will favor the W/SW this afternoon, with gusts at times upwards of 20 kts, before confidence in directions decreases this evening as winds may become more erratic with convection. Thunderstorms may produce some gusty, erratic outflows with a 30-40% chance of wind gusts exceeding 30 kts.

Southeast California/Southwest Arizona including KIPL and KBLH: There will be a slight chance (10-20%) for VCSH/VCTS at both terminals later this afternoon and early evening, mainly between 22-01Z at KIPL and 23-03Z at KBLH. Otherwise, a typical SW-SE wind component will prevail through the TAF period. Wind speeds up to 7-12 kts will be common at both terminals with gusts up to 20-25 kts at KBLH during the afternoon. Locally stronger gusts will be possible with stronger thunderstorms with a 50-60% chance for outflow winds to exceed 30 kts. FEW to SCT clouds will be common over the area with lowest bases down to around 10K ft AGL.

FIRE WEATHER

Elevated humidity levels, cooler temperatures, and good chances for wetting rains are anticipated through Thursday morning over at least the eastern districts. The best rain chances will be during the evening and overnight hours tonight with lingering chances through Thursday afternoon. Afternoon MinRHs will range between 25-35% across the lower deserts and 40-50% for the eastern high terrain, while overnight recoveries will remain excellent. Winds will predominately be out of the south southwest much of the period with the exception of any modest outflows from thunderstorm activity. Drying conditions will move into the region Friday and through the weekend dropping MinRHs to between 15-25% while temperatures gradually rise back to above normal.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from 4 PM MST this afternoon through late tonight for AZZ541-545-547-549-552-555>558-560>563.

CA...None.