textproduct: NWS Phoenix

UPDATE

18Z Aviation Discussion.

KEY MESSAGES

- A broad ridge of high pressure encompassing the Desert Southwest will allow temperatures to hover around 5 to 10 degrees above daily normals through Tuesday.

- A weather system passing north of the region 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 a broad ridge encompassing much of the Intermountain West and 4-Corners region. Meanwhile, a deep cutoff low is approaching the West Coast, brining abundant moisture and cloud cover to the Pac NW and NorCal. Our region will remain under the influence of the ridge today and to a lesser extent on Tuesday which will keep temperatures in an above average category. A weak disturbance on the western periphery of the ridge did bring extensive cloud cover and some virga through the area yesterday, but this initial shortwave is departing to the north and clouds are expected to clear out this morning. Under presences of the ridge today, H8 temperatures will again reach 18-20C this afternoon. This will translate to another round of highs in the mid 90s across the lower deserts and low to mid 80s in the high terrain which is around 8-10 degrees above normal for late April. Another weak shortwave in southwesterly flow will translate through AZ late today and will provide enough mid-lvl moisture and lift to generate showers and even a few isold storms over N Arizona. There is around a 10% chance of an isolated shower or storm over S Gila County this afternoon, however the main focus will be farther N along the Mogollon Rim. Elsewhere, tranquil weather conditions will exist with much lighter winds across southcentral AZ.

On Tuesday, the aforementioned cutoff low off the coast of NorCal will begin to move onshore. Ahead of this feature, southwesterly flow will continue to strengthen over the western half of our forecast area. The tightening of the H7-H5 gradient will lead to breezy conditions, especially across SE California and SW Arizona where gusts could reach 30-35 mph at times. Winds will be strongest over the higher terrain and at ridgetop lvl, especially in SW Imperial County where low end advisory winds will be possible. As the upper-lvl trough continues to approach from the west, 500 mb hghts will lower to around 573-575 dam for areas along and west of the CO River Valley where highs will be slightly cooler in the upper 80s and low 90s Tuesday afternoon. Areas farther east including the lower deserts of southcentral AZ will still be under the influence of the departing upper ridge and thus highs will be warmer in the low to mid 90s.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Deterministic and ensemble guidance are in good agreement that the cutoff low over the West Coast will transition to an open wave and progress through the Intermountain West from Wednesday into Thursday. Negative hght anomalies from the passing system will result in cooler temperatures (likely closer to seasonal levels, in the mid 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 H7-H5 gradient remaining compact. The most widespread breezy conditions look to be on Wednesday and then stronger winds will transition to the high terrain of eastern AZ on Thursday as the trough axis passes through the region. Quasi-zonal flow aloft will return by Friday and into next weekend ahead of another potential shortwave trough. This will allow for a temporary boost in temperatures Friday and Saturday before the next weather system and subsequent cooler temperatures arrive on Sunday.

AVIATION

Updated at 1725Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected during the TAF period. Winds will follow familiar diurnal trends, though it will take longer than usual to achieve the afternoon/evening W'rly shift, especially at KPHX where a S'rly component may be observed for several hours. A brief window of marginal breeziness is shown between 04-06Z this evening with gusts around 15-20kt, with those higher readings favored at Sky Harbor. FEW mid-level clouds over the high terrain will be present over the next several hours before SCT-BKN high clouds move in overnight.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. KIPL will follow familiar spring time diurnal trends while KBLH will see generally S'rly winds with some afternoon breezes near 20-25kt. Clear skies this afternoon will give way to SCT-BKN high clouds tonight.

FIRE WEATHER

A very progressive pattern with multiple dry weather systems passing north of the region will result in breezy conditions and periods of elevated fire weather throughout this week. During the first half of the week, above normal temperatures and breezy conditions combined with min RH values of 8-15% will result in widespread elevated fire weather conditions, especially across the western districts on Tuesday and the higher terrain of southcentral AZ on Wednesday where gusts up 20-30 mph will be common. Breezy and dry conditions will carry through the rest of this week and into this weekend with relative humidity remaining at or below 15% each afternoon. Overnight recoveries will also vary through the week, but remain mostly in the 30-50% range.

PSR WATCHES/WARNINGS/ADVISORIES

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