textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- An Extreme Heat Warning remains in effect for portions of the western deserts today and for the Phoenix Metro area through Tuesday. - Today will be the hottest day of the week with lower desert highs reaching 105-112 degrees, followed by a gradual cooling through the remainder of the week.

- Dry conditions will prevail this week with periods of passing high clouds. Breezy conditions are anticipated along the Lower Colorado River valley on Tuesday and the AZ high terrain on Wednesday.

SHORT TERM /TODAY THROUGH TUESDAY/

A robust upper-lvl ridge has expanded over our forecast region this afternoon with H5 heights now peaking between 588-590dm, or near the 99th percentile of climatology for mid-May. These positive hght anomalies coupled with clear skies and very low relative humidity will result in highs of 106-112 degrees this afternoon, or around 15 degrees above normal. The forecast high in Phoenix is still around 109 degrees today which is only one degree shy of the daily record.

On Tuesday, a weak upper level disturbance will move up the Baja coastline into SE California. As this occurs, mid-lvl winds will become more southerly and draw some moisture into our region. Overall, PWATs will rise to around 0.6-0.8" and promote at least scattered afternoon cloud cover and potentially isolated shower activity across southeast Arizona to as far north as eastern Pinal and southern Gila Counties. Even if some showers do manage to develop, we are not anticipating any measurable rainfall in our area. Instead, much of the precipitation will fall as virga and could result in localized wind gusts up to 30 mph. An increased pressure gradient associated with the passing shortwave will help to drive a decent push of southerly winds across far southwest Arizona and southeast California. Temperatures Tuesday will react to the falling heights mainly across southeast California and southwest Arizona where highs will fall back to between 102-106 degrees. The Phoenix area will only dip a couple of degrees from today's highs with readings topping out 104-107 degrees. Thus, the Extreme Heat Warning will continue into Tuesday for only the Phoenix Metro.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Wednesday will be a pattern transition day for the Desert Southwest as upper level troughing from a much larger Pacific trough nudges southeastward into the region while a sub-tropical upper level jet comes on shore across northern Mexico into Arizona. The jet will drive increased higher level clouds across at least the eastern half of Arizona for much of Wednesday, while the lowering heights will allow for cooling conditions. Forecast highs Wednesday range from the upper 90s to around 100 degrees across the western deserts to around 100 degrees in the Phoenix area.

The main Pacific shortwave is set to miss our region well to the north on Thursday, but the broader upper level troughing is still favored to move through Thursday into Friday. This will drive heights even lower while dropping highs more into a 95-99 degree range for both Thursday and Friday. The pattern for the upcoming weekend is likely to support a weak ridge moving through giving a slight boost in temperatures as readings may top 100 degrees again for some of the lower deserts. The westerly dry flow late this week and through next weekend should provide for mostly clear skies.

AVIATION

Updated at 0550Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Occasionally gusty SSW-W winds Tuesday afternoon/evening will be primary weather issue through the TAF period. Winds will eventually settle out of familiar E'rly drainage directions tonight (already established at KSDL, KDVT). By 18-20Z Tuesday, confidence is moderate to good that SSW-SW winds will become favored across the terminals, with occasional gusts to around 15 kts during the rest of the afternoon. During the evening, confidence is currently high enough to include TEMPO stronger SW-W gusts, with Hi-Res guidance showing a signal similar to a gulf surge. However, virga to the south/southeast of the Phoenix airspace could conceivably cause some erratic wind behavior and gusts from a more southerly direction around the same time, with a higher likelihood of that impact at KIWA.

Southeast California/Southwest Arizona including KIPL and KBLH: Periods of gusty winds varying between SSE-SSW at KBLH and between SE and W at KIPL will be the primary aviation concern through the next 24 hours. A period of VRB winds late this evening into the overnight hours will precede a shift out of the S at KBLH and SE at KIPL between 08-11Z Tuesday. Speeds will increase through the morning hours, with gusts likely developing by the afternoon to around 20 kts at KIPL and 25 kts at KBLH.

FIRE WEATHER

Hot and dry conditions will prevail with lower desert highs topping out 10 to 15 degrees above normal through Tuesday. Temperatures will cool closer to average by the end of the week. Minimum humidity levels will bottom out around 10% or lower each afternoon with overnight recoveries remaining poor to fair, averaging 20-35%. There will be increased breeziness, particularly across the western districts Tuesday afternoon where gusts up to 25-30 mph will be possible. These stronger winds coupled with very dry conditions will result in elevated to near critical fire weather. The breezy to locally windy conditions will shift over the AZ high terrain on Wednesday. Winds will diminish later in the week, but overall very dry conditions will prevail.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Tuesday for AZZ537-540- 542>544-546-548-550-551.

CA...None.