textproduct: NWS Phoenix

KEY MESSAGES

- Another weather system clipping the area Wednesday will result in gusty winds, with increased precipitation chances over the Arizona high terrain.

- A quick moving system Thursday night into Friday will reinforce cooler temperatures with additional chances for light showers across central Arizona.

- Drier weather with much warmer temperatures will return to the region early next week.

SHORT TERM /Today through Thursday/

Pronounced longwave troughing has enveloped the western Conus early this afternoon with the strongest negative height anomalies centered along the Pacific NW coast. Over the next 84 hours, numerous shortwaves will rotate through the greater cyclonic flow, slowly carving out lower heights to the south and east. While strong jet energy is currently analyzed on the western periphery of the trough axis, this momentum should rapidly translate to the downstream jet core tomorrow in association with a progressive shortwave ensuring more zonally orientated height falls versus meridional digging.

With an initial shortwave shearing and lifting into the central Rockies this afternoon/evening, weak subsidence will dominate the majority of the forecast area through Wednesday morning though low level moisture in the form of 4-5 g/kg mixing ratios should linger in the region. Forecast confidence is excellent regarding the passage of a low amplitude shortwave across far northern Arizona Wednesday, dragging a trailing cold front through the CWA during the afternoon/evening. The greatest impact with this system will be an enhanced low level pressure gradient along the cold front with a corridor of 40-50kt H8 winds focused through SE California and SW Arizona late morning through mid afternoon. Mechanical mixing should begin partially tapping this stream of higher momentum ahead of the cold front, however the gradient should quickly weaken through the day, though a few hours of gusts in excess of 40 mph seems reasonable. Given the recent lack of rainfall in this area, areas of blowing dust (and the downstream transport of lofted dust) would be the most significant impact, especially along the I-8 corridor.

Otherwise, the bulk of ascent associated with this wave will pass north of the forecast area, however enough moisture and saturation in the H8-H7 layer juxtaposed with orographic influences should yield some scattered showers on the northern CWA border. One major limiting factor will be a tremendous midtropospheric subsidence inversion above the H7 level resulting in a very narrow saturation layer between 6-10K ft almost necessitating forced orographic ascent with the stronger H8-H7 SW winds to produce accumulating precipitation. As such, there should be a tight gradient between higher elevation foothills and lower deserts facilitating better POPs, while the limited moisture and ascent should restrict any rainfall amounts generally under 0.10". While snow levels will fall to around 5000ft with passage of this wave, dry air above 10K ft and lack of saturation in the dendritic growth zone will preclude any snowfall accumulations. Thereafter, another round of weak subsidence forcing dry weather will spread into the region Thursday awaiting the next shortwave Friday. Dampened heights and thermal fields aloft will ensure temperatures hover 5F-10F below the seasonal normals.

LONG TERM /Friday through Monday/

Broad troughing will persist across the Western U.S. on Friday as a third weather system is expected to track into northern Arizona Friday morning. Model guidance is finally coming to a consensus regarding precipitation chances Friday with the best opportunity for accumulating rainfall likely being very similar to the Wednesday system. The best moisture and forcing should be across the northern half of Arizona, however a somewhat more southerly displaced cold core should result in some scattered shower activity reaching into the south-central Arizona lower deserts Friday morning. Precipitation amounts again should be on the light side, but some uncertainty remains as far as how much and how far south the shower activity will reach. Friday's high temperatures will be very similar to Thursday, but as drier air and light winds filter into the area Friday night more efficient nocturnal cooling should lead to the coldest morning in several weeks Saturday. Lows Saturday morning very well could dip into the upper 30s for some of the Phoenix metro with rural lower desert areas into the lower/middle 30s.

Upper level ridging should then move over the region this weekend with temperatures quickly climbing to above normal starting Sunday. Guidance favors the ridge to persist into early next week with NBM forecast highs back into the lower 80s for next Monday and Tuesday.

AVIATION

Updated at 1740Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The cold front and associated band of rain is now well east of the area. However, residual SCT to BKN cloud decks with bases between 5-7 kft AGL will continue through this afternoon becoming FEW by this evening. Ceilings will lower back down to around 5-6 kft early Wednesday morning. Anticipate winds to favor W/SW through early this evening, with occasional gusts into the teens and otherwise speeds generally ranging from 8-11 kts. Guidance shows winds shifting back out of the SE and becoming elevated overnight ahead of another cold front that will move through Wednesday. This next system will present another period of gusty southerly cross runway winds by mid morning Wednesday followed by a westerly shift in the mid-afternoon. The best chances for SHRA will remain over the higher terrain NE of Phoenix.

Southeast California/Southwest Arizona including KIPL and KBLH: Extended periods of gusty southwest winds will be the primary aviation weather concern through the next 24 hours under FEW-SCT high clouds. Winds will remain generally out of the S-SW at KBLH and W at KIPL with gusts to 25-30 kts becoming common at both terminals this afternoon. Confidence is low on the amount of decoupling overnight, with some guidance showing winds becoming light and even shifting SE at KIPL, however there is a high likelihood of gusts in the 25-30 kt range continuing overnight.

FIRE WEATHER

Breezy conditions will pick up again on Wednesday as gusts once again reach above 35 mph in some areas. Another couple rounds of light shower activity will be possible across the Arizona high terrain on Wednesday, and again Friday morning as two quick moving disturbances move across northern Arizona. Afternoon MinRH values will range between 25-40% across the lower deserts to 35-60% over higher terrain areas through Friday. Overnight recovery will range from good to excellent. Drying and warming conditions are expected for the weekend with MinRHs dropping into the teens for the lower deserts and temperatures rising to above normal.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Wind Advisory from 8 AM to 4 PM MST Wednesday for AZZ530>533- 535-536.

CA...Wind Advisory from 7 AM to 3 PM PST Wednesday for CAZ565-567.

Wind Advisory from midnight tonight to 3 PM PST Wednesday for CAZ560-561-563-564-566-568>570.

Wind Advisory until 3 PM PST Wednesday for CAZ562.