textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Chances for scattered showers and isolated thunderstorms are increased today and Thursday with the most activity focused over foothill and high terrain areas.

- Along with the chances for rain, gusty storm driven outflow winds will be possible today and Thursday, potentially leading to patchy blowing dust.

- Near to slightly above normal temperatures will persist through the weekend before additional cooling is expected early next week.

SHORT TERM /Today through Thursday/

Broad upper level trough continues to be seen to the region's southwest, with the center of the low just off the west coast of the Baja Peninsula. To the east ridging can be seen expanding over the southern Plains, with the core centered over SE TX and W LA. The low pressure system is advecting an increase in moisture into the Desert SW. The main weather impact for later this evening and tonight will be the potential for isolated to scatted thunder storms. The best chances for thunderstorms will be in the eastern higher terrain where the latest HREF shows good agreement in between a 50-70% chance of storm development. A lesser chance for thunderstorms exist for the lower deserts, and Phoenix metro, but not impossible. HREF shows between a 30-50% chance however storm are expected be be isolated in nature.

Tonight is currently shaping up to be more active compared to Thursday night, with decent moisture tonight, PWATs near 1.25" and 1000-700 mb mixing ratios between 8-10 g/kg across south-central Arizona. Additionally, good upper level ascent exists between the trough and the jet max to our north in the higher terrain. Forecasted MUCAPE values continue to be near 1000 J/kg in south- central AZ lower deserts and between 500-700 J/kg in the eastern high terrain region. However even with lower CAPE values the lift jet max and the natural orographic lift will provide the extra lift needed for storms to develop. CAMs are starting to come into better agreement in term of a strong outflow descending out of the NE sweeping through the lower deserts, where blowing dust and reductions invisibilities will be possible. Later in the evening/night additional chances for isolated thunderstorms are possible near southern Maricopa county, were another, although weaker outflow could develop out of the south-southwest, and collide with the northeastern outflow which may spark additional thunderstorms into the early overnight hours over the southern Maricopa county and even as far into Yuma, and La Paz Counties.

Thursday's outlook continues to be questionable, largely being dependent on tonight's activity. Moisture is expected to dwindle tomorrow over south-central AZ, with forecast soundings showing PWATs dropping to near 1". interestingly though, MUCAPEs are expected to be slightly higher, between 1000-1250 J/kg Thursday afternoon as upper level vorticity streams directly over head as the low weakens and progresses over the Desert SW. CAMs show little to no convection developing across southern Arizona with the majority staying to our north. Given the higher instability and at least some upper level ascent, it would seem we'd have at least similar chances as today, but NBM PoPs remain below 10% for the lower deserts. If tonight's forecast pans out then chances for Thursday will likely continue to be very slim for the lower deserts, but if things struggle tonight then there may be high chances for development tomorrow, however with the moisture moving westwards isolated thunderstorms will be expected to mostly be confined to the higher terrain. If storms actually do get going on Thursday, gusty winds and localized heavy rainfall will again be possible given the expected decent instability.

LONG TERM /Friday through Tuesday/

By Friday, the main upper level trough is forecast to shift northeastward out of a our region leaving dry southwesterly flow in place. Guidance shows low level mixing ratios quickly dropping to 5- 7 g/kg across the lower deserts to only 6-8 g/kg over the Arizona high terrain. As a result, Friday will begin another dry period with only lingering chances for some isolated showers and maybe a few thunderstorms over the eastern Arizona high terrain. Temperatures are forecast to remain quite stable over the next few days with highs right around to slightly above normal and lows a few degrees above normal.

Westerly dry flow should persist through at least Saturday with large scale troughing remaining to our northwest while the subtropical ridge slowly shifts southwestward from east Texas into the interior of Mexico. This shift in the ridge is likely to bring higher heights temporarily for our region later Friday through Saturday, so daytime highs Saturday are likely to top out between 100-104 degrees (4-6 degrees above normal) under mostly sunny skies.

Forecast uncertainty increases dramatically going into early next week as it will likely depend upon what happens with Hurricane Polo, the positioning of the subtropical ridge, and an upper level trough that may swing through our region on or around next Monday. The latest NHC track for Polo has it slowly making its way westward over the next few days before turning more to the north this weekend, but staying west of southern Baja. This track may be enough to help advect tropical moisture northward as far as our area, but uncertainty remains quite high. There is a low end potential of tropical moisture making its way northward into our area just before the aforementioned trough swings through our region and this could lead to chances for strong to severe storms along with heavy rain potential on or around Monday. The more likely scenario would involve modest moisture making its way into our area, but still with the support of the trough. The trough may still be strong enough to support some strong to severe storms due to much higher than normal shear, but with less moisture it would lower the severe threat as well as well as the threat for heavy rainfall. At this moment, we are unsure which solution will win out, but we expect to have a better idea by the weekend.

AVIATION

Updated at 2342Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Variable wind directions with the potential for gusty evening outflows followed by the threat of SHRA/TSRA will be main weather issues through Thursday afternoon under thick, high cirrus cigs and brief periods of cigs as low as 8-10K ft AGL. Confidence is good that E/SE winds will prevail ahead of thunderstorm outflows moving into the area late this afternoon. Wind directions may briefly go S/SW late afternoon/early evening, but a 140v240 oscillation may be more accurate.

Confidence is high that thunderstorms to the north and northeast will send an outflow S/SW through the terminals leading to an abrupt wind shift early/mid evening (01-05Z timing) with gusts briefly reaching 25-35kt. A few SHRA/TSRA may impact the area, and have maintained a prob30 mention of TS, though confidence in development and coverage in the PHX metro is very low. Confidence is better that once activity clears the metro later this evening, east winds will hold over the area through much of the day Thursday.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather issues will exist through Thursday afternoon under increasing high cirrus decks. Confidence in a specific wind direction at any given time is low given the weak pressure gradient. Extended periods of light and variable winds will be common.

FIRE WEATHER

Today into tonight will bring modest chances 30-50% for showers and thunderstorms, focused over the eastern districts, while the western districts should mostly remain dry. Gusty outflow winds with abrupt wind shifts are expected during the first part of the evening, predominately out of the NE. MinRHs between 15-20% range today for the western districts and 20-25% Thursday, while the eastern districts are between 25-40% today and Thursday. Winds across eastern districts will generally maintain a east southeast direction, frequently becoming gusty while lighter, diurnally driven winds will persist in western districts.

PSR WATCHES/WARNINGS/ADVISORIES

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