textproduct: NWS Phoenix

KEY MESSAGES

- Dry and tranquil weather conditions will continue through the middle of the week with temperatures within a few degrees of daily normals.

- An unsettled weather pattern will return to the region late in the week and persist through this weekend resulting in much cooler temperatures and chances for widespread rainfall.

- There will also be a possibility of embedded thunderstorms with heavier rainfall which could lead to localized minor flooding Friday and Sunday.

SHORT TERM /TODAY THROUGH THURSDAY/

Continued dry west quasi-zonal flow will keep quiet weather conditions over the region for the next 2-3 days before an unsettled period of weather begins Friday. Current IR satellite imagery shows clear skies across the bulk of the Desert Southwest with the next batch of clouds to eventually move in late tonight still off the California coast. Expect daytime highs today similar to yesterday's mid 80s with overnight lows Wednesday morning a bit warmer than this morning as the higher clouds move in from the west. The incoming high clouds will blanket the entire area during the daytime hours Wednesday, but they should be thin enough for some dull sunshine throughout the day. A passing weak shortwave across the Great Basin tonight into early Wednesday should also lower heights over the western deserts leading to highs dropping into the lower 80s, while the rest of the lower deserts holds in the mid 80s for highs.

Going into Thursday, a strong upper low is expected to dive south southwestward to just off the coast of Washington while a second shortwave approaches California from the west within the same large-scale trough. With the polar jet supporting the northern system and a strengthening subtropical jet supporting the southern shortwave and eventually a much stronger one by the weekend, the Western U.S. is shaping up for a busy latter part of the week. For the Desert Southwest, the deepening trough to our west will also lead to increased southerly flow on Thursday helping to gradually advect lower level moisture into southern and central Arizona leading into Friday. Forecast temperatures for Thursday are very similar to Wednesday, but expect thicker clouds and a bit warmer overnight temperatures.

LONG TERM /FRIDAY THROUGH MONDAY/

Friday is still looking like a fairly wet day across the bulk of central Arizona as moisture levels rise to 200-250% of normal and the area falls under strong difluence aloft due to the incoming near record strength (for this time of year) subtropical jet. Guidance continues to heavily favor showers and isolated thunderstorms developing at some point early Friday morning centered over south-central Arizona and then expanding northward and eastward throughout the day on Friday. Rainfall rates are not anticipated to be all that heavy with the bulk of the rainfall falling at less than 0.1" per hour, but with embedded heavier showers or thunderstorms, rates upwards of 0.5-1.0" per hour should be possible. GEFS and EPS ensemble mean QPF for the Friday event shows an average of around 0.50"-0.75" from Phoenix and into the higher terrain north and northeast of Phoenix to 0.10-0.25" over much of the rest of southern and central Arizona. The higher end of guidance (NBM 75% percentile) still shows potential for heavy rainfall with amounts of up to around 1.5" across south-central Arizona. As a result, WPC's ERO continues to depict a Slight Risk for excessive rainfall on Friday. Given the propensity for the NBM to forecast too much QPF for our area in the mid to long range, we are leaning toward the lower end amounts. Some higher end amounts of over 1" are expected to fall, but it is likely to be fairly localized.

As the subtropical jet lifts northeastward later on Friday, the best ascent will shift out of Arizona leaving only continued chances for isolated upslope driven shower activity for Friday night and Saturday. The unsettled weather pattern will persist over the weekend with an atmospheric river approaching the region from the southwest later on Saturday and likely arriving during the day Sunday with the brunt of this expected modest atmospheric river impacting areas across southern Arizona and northern Sonora. Model uncertainty remains quite high for this second weather event with differences in the track and the strength of the shortwave trough and differences in the amount of moisture. The EPS continues to show higher moisture levels and QPF amounts compared than the GEFS, but both have trended lower with QPF over the past 24 hours. The GEFS's additional 0.25-0.50" for south- central Arizona looks to be a bit too low considering the amount of moisture potential, but the EPS's 0.75-1.50" may be a bit too high. The timing with the bulk of the rain likely falling during the first half of Sunday still looks good, but as stated above how much rain will fall is still quite uncertain. Considering this will be the second rain event within 3 days, any additional moderate to heavy rain could compound any flooding issues that could occur on Friday. Rain chances may linger through Sunday night across eastern Arizona, but by Monday much of the area should be drying out as the system is expected to quickly push northeast of the area.

Temperatures during this unsettled weather pattern are expected to fall below normal starting Friday with highs dropping to between 75-80 degrees before bottoming out closer to 70 degree highs for Sunday and/or Monday. NBM guidance shows fairly large forecast temperature spread for the first half of next week as the large scale troughing may linger over the region for a couple more days. If this happens, highs will very slowly creep upward through the low to mid 70s, whereas if the ridge takes over we could see highs closer to 80 degrees by next Tuesday or Wednesday.

AVIATION

Updated at 0547Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

No aviation weather concerns under mostly clear skies expected through Tuesday. The wind pattern will continue to exhibit typical diurnal tendencies with speeds mostly aob 6 kts. There will also be extended periods of light variability to even calm conditions. A period of light southerly crosswinds is likely early Tuesday afternoon before the westerly transition by the late afternoon hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather concerns under mostly clear skies expected through Tuesday. Winds will favor diurnal patterns with light speeds, mostly below 8 kts, at both terminals. W winds are expected through sunrise before shifting toward a S-SE direction by late morning and then back W in the evening/overnight hours. KBLH will see a NW wind component through most of Tuesday morning and then shift S by the afternoon. Both terminals will also experience periods of light variability to even calm conditions.

FIRE WEATHER

Warm and dry conditions will persist through at least the middle part of the week before a much cooler and wetter pattern sets up Friday into this weekend. Minimum humidity levels will bottom out today at 15-30% Tuesday before gradual moisture improvement arrives starting Wednesday. Overnight recoveries will remain good at 40-65%. Winds will remain light each day favoring typical diurnal patterns and minimal upslope breeziness.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.