textproduct: NWS Phoenix

KEY MESSAGES

- A strong low pressure system will support additional thunderstorms, some of which may be strong to severe, through this afternoon and focused more north and east of the Phoenix area.

- Thunderstorms will also be capable of producing moderate to heavy rainfall which may result in areas of flash flooding across south-central Arizona with a focus over higher terrain areas.

- Drier air will quickly move in tonight into Friday leading to more benign weather conditions into the weekend and through at least early next week.

- Below normal temperatures are forecast through the weekend before warming back into the normal range early next week.

SHORT TERM /Today and Tonight/

The environment and atmospheric parameters continue to support at least an isolated severe storm threat across south-central Arizona early this morning with the primary threat being large hail. Activity has been fairly limited since last evening, but there was one likely severe storm that formed over western Phoenix and tracked all the way through New River and another across far eastern Gila County.

The center of the strong Pacific low off the coast of southern California is slowly making its way eastward while a large baroclinic leaf stretches through Arizona into the Four Corners area. The continued moist advection out of the south southwest into Arizona has raised PWATs to between 1.3-1.5" over south- central Arizona and low level mixing ratios to 9-11 g/kg. Surface dew points are currently in the mid 60s across the Phoenix area to the upper 50s to lower 60s north and east of Phoenix. This rich moisture extends vertically through at least the mid-levels where lapse rates remain very favorable for any storms the manage to develop. MLCAPE of 500-1000 J/kg and MUCAPE of 1000-1500 J/kg at the moment will continue to support the development of any strong to severe storms within the highly sheared environment, but we are in a bit of a lull in forcing. Strong upslope low level forcing may be enough to get a few storms going through sunrise, but the modest CIN should keep the activity widely isolated.

As the low continues to progress eastward through our region later this morning into the afternoon, a cold front/dry line will likely become a better focus mechanism for storm development as early as sunrise. Pooling of moisture right ahead of this front should also result in a narrow corridor of even higher instability with HREF MUCAPEs reaching as high as 2000 J/kg from Phoenix and areas to the east of Phoenix. CAMs are quite unsure of the coverage later this morning, but they at least suggest scattered storms developing somewhere over the Phoenix metro and/or northern Maricopa County around sunrise. Once this occurs, it should put the northern half of Maricopa County within a very favorable environment for scattered strong to severe storms to develop through at least the rest of the morning hours. Severe hail of up to 2" in diameter and heavy rainfall will be the primary concerns, but wet microbursts will also support localized severe winds. The best low level shear for any brief tornado spin-ups will continue to be over the high terrain north of the Phoenix area through mid morning before transitioning over east-central Arizona by around noon and lasting through the afternoon hours. Given this potential, we anticipate another Severe Thunderstorm Watch to be issued at some point later this morning for portions of south- central and east-central Arizona.

A Flash Flood Watch remains in effect for the aforementioned areas with any stronger storms capable of producing a quick 0.5-1" of rainfall and with training of cells becoming more likely later this morning through the afternoon. Where training of storms does occur, it would not be very hard for localized 2" to fall. As the front progresses more into south-central Arizona by this afternoon, CAMs suggest most of the activity may move east of Phoenix by early afternoon before mostly ending early this evening across eastern Arizona as dry air quickly surges in from the west.

By early tonight, the low center is slated to move into Nevada with a strong trailing shortwave trough swinging through Arizona, but this will be within a much drier post-frontal environment. It may be enough to support some lingering showers over the eastern Arizona high terrain this evening, but the CAMs generally show very little activity by late evening.

LONG TERM /Friday through Wednesday/

Large scale troughing will remain situated over our region on Friday with even one or two decent shortwaves moving through the region, but little will result from their passage. Enough dry air will have already moved into the area to essentially prevent any additional rain chances across the lower deserts on Friday to at most a 10-15% chance over the eastern Arizona high terrain. The cooler air mass settling in will also keep daytime highs only in the mid to upper 70s Friday. Model guidance suggests the trough should remain over our region even through the upcoming weekend, but it will continue to weaken with our air mass slowly warming over time. NBM forecast highs for Sunday show readings into the lower 80s over the western lower deserts to near 80 degrees in the Phoenix area. By Monday, the trough is likely to finally push east of the region with modest upper level ridging prevailing over the Desert Southwest. The increased heights and continued dry conditions should push daytime highs back into the normal range (mid 80s) going into the middle of next week.

AVIATION

Updated at 0550Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concern through Thursday morning will be the potential for another round of VCTS/TS which could impact the metro terminals, result in gusty winds, brief heavy rainfall, lightning, and hail. Currently there is a lull in thunderstorm activity across southcentral AZ, however storm chances will ramp up again after midnight. The most likely period of time to see thunderstorms will be between 10Z-16Z Thursday morning. Winds will continue out of the east through much of the overnight period. Once thunderstorms develop, outflow will result in a NW shift followed by a westerly shift once storms clear east of the terminals late Thursday morning. CIGs are expected to remain around 5-7K ft AGL overnight, however there could be a period of CIGs as low as 3-5K as showers progress through the metro area.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns are anticipated through the forecast period. Winds will favor a southerly to southwesterly component at KBLH and westerly at KIPL. Gusts up to 25 kts will become common at both terminals early Thursday afternoon. FEW to SCT clouds, mostly aoa 7 kft, will prevail.

FIRE WEATHER

Strong to severe thunderstorms with heavy rainfall will remain possible over eastern districts through this afternoon before rapid drying spreads through the area this evening and tonight. The heaviest rainfall will be focused more across higher terrain areas north and east of Phoenix. MinRHs today will again be in a 30-60% range before dropping more into a 25-40% range for Friday and Saturday as cooler air settles into the region. Gusty west winds upwards of 25-30 mph will be possible across the western districts through this evening with winds becoming light starting tonight. Overall quiet weather conditions are forecast for this weekend into next week with fairly stable humidities and temperatures rebounding back into the normal range early next week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch through Thursday evening for AZZ545-547-552-556>558- 560>563.

Flood Watch through Thursday afternoon for AZZ534-537-538- 540>544-546-548>551-553>555.

CA...None.