Area Forecast Discussion

UPDATE

18Z Aviation Discussion.

KEY MESSAGES

- Breezy to windy conditions will exist across portions of south- central Arizona this morning with the strongest gusts over the foothills and high terrain east of Phoenix.

- Temperatures will remain around 5 to 10 degrees above daily normals through Tuesday.

- A passing weather system will result in periods of breezy to locally windy conditions and cooler temperatures during the latter half of this week.

SHORT TERM /TODAY THROUGH TUESDAY/

Latest mid-lvl wv imagery and 500 mb streamline analysis reveals shortwave ridging centered over the Desert Southwest while a broad trough of low pressure continues to sweep through the Central Plains. A strong sfc high located over the Southern Plains will induce E-SE flow across the forecast area this morning. Due to the tightening of the sfc pressure gradient, easterly flow will continue to increase through this morning especially across the high terrain of southcentral AZ. Gusts up to 35 mph have already been observed in the Globe/Miami and San Carlos areas where a Wind Advisory will take effect beginning at 3 AM MST. Wind gusts will likely peak just after sunrise this morning, reaching up to 20-30 mph across the lower deserts and up to 40-50 mph in the higher terrain NE of Phoenix. Blowing/lofted dust will also be possible this morning for areas along and east of the I-10 corridor including portions of Maricopa and Pinal Counties. The sfc pressure gradient will relax later this morning, however lingering gusts up to 35-40 mph may carry into the early afternoon across the higher terrain areas and thus the Wind Advisory expiration time has been extended to 1 PM MST. Another attributing factor to gusty winds today will be the possibility of virga showers associated with a batch of increasing mid-lvl clouds. HREF members continue to reflect these virga showers developing over SE AZ this afternoon and progressing nwd into the Phoenix Metro this evening. Despite increasing cloud cover, upper-lvl ridging is still expected to result in above normal temperatures across the region today with highs topping out solidly into the mid 90s across the lower deserts.

On Monday, the upper-lvl ridge axis will begin to slide ewd as a cutoff low develops off the coast of N California. The Desert Southwest will still be under the presence of the ridge with 500 mb hghts ranging from 578-580 dam. This will maintain temperatures in an above normal category with lower desert highs again reaching the mid 90s. Deterministic guidance does indicate that another weak shortwave will pass through AZ on Monday afternoon as upper lvl winds begin to turn more southwesterly. There may be just enough midlevel moisture present for a few afternoon convective shower across the AZ high terrain, mainly along the Mogollon rim and in the White Mountains, however the lower deserts are expected to remain dry. Tuesday will be a transition day where our forecast area is located between an approaching trough off the coast of CA and the upper ridge will migrate further east into the S Plains. High temperatures are again forecast to reach the mid 90s across southcentral AZ on Tuesday afternoon, however there will be some slight cooling for areas along and W of the Colorado River valley where highs will range from the upper 80s to lower 90s. Increasing SW flow aloft on Tuesday will result in breezy conditions, especially across SE California where gusts could reach 20-30 mph.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

By the middle of the workweek, troughing over the West Coast will finally push inland over the Intermountain West, albeit in a weakening phase. Ensembles members are in good agreement on the evolution of the shortwave trough which is expected to remain well north of our forecast area. However, the Desert Southwest will still be under the influence of negative hght anomalies which will result in cooler temperatures (likely closer to seasonal levels, in the 80s across the lower deserts) beginning on Wednesday. Periods of breezy to locally windy conditions can also be anticipated across the entire forecast area due to the tightening of the 500-700 mb hght gradient. The most widespread breezy conditions look to be on Wednesday and mainly across eastern AZ on Thursday as the trough axis passes north of the forecast area. Quasi-zonal flow aloft will return by Friday and into next weekend ahead of another potential shortwave trough. This will allow for a slight boost in temperatures to a few degrees above normal.

AVIATION

Updated at 1816Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Gusty morning winds will gradually subside heading into the afternoon and earlier lofted dust will continue to settle with improving slantwise visibility. As winds subside, directions will tend to veer S/SW toward mid-afternoon and some light variability may also develop as wind speeds dip to 7 kts or less. Most guidance supports a W/SW shift around 01-02Z this evening, but only lasts for a few hours before returning easterly. Confidence is good that spotty virga showers will develop this evening south of the terminals and may spread into the metro. Any shower this evening will be capable of producing locally brief gusty and erratic winds. SCT-BKN clouds will persist through tonight before clearing, with lowest bases around 12-13K ft AGL.

Southeast California/Southwest Arizona including KIPL and KBLH: No significant weather issues will exist through the next 24 hours, with SCT-BKN high cirrus decks that will clear tonight. Confidence in the wind forecast is moderate with a E/NE component favored at KIPL this afternoon followed by a W component this evening and light variable winds at KBLH turning out of the S/SW this afternoon into the evening. Extended periods of nearly calm conditions will be common.

FIRE WEATHER

Dry conditions with above normal temperatures will continue over the next few days before cooler and breezy weather arrives mid-week. Easterly winds will increase this morning with the strongest winds residing over the higher terrain of the southcentral AZ where gusts are expected to reach 35-45 mph. Gusts up to 25-35 mph will also become common across the lower elevations shortly after sunrise. Given the time of day of strongest winds, RH values will not reach critical levels. However, elevated fire weather conditions will still be present across south-central AZ through early this afternoon. Winds will weaken on Monday before breezy to locally windy conditions return on Tuesday. Afternoon humidity levels are expected to remain below 15% and at times in the single digits over the next several days, with only minor day to day variations. Overnight recoveries will also vary through the week, but remain mostly in the 30-50% range. Due to the breezy and dry conditions, periods of elevated fire weather conditions can be expected during the middle to latter half of this week.

PSR WATCHES / WARNINGS / ADVISORIES

AZ...Wind Advisory until 1 PM MST this afternoon for AZZ547-552-555- 557-558-560>563.

CA...None.