textproduct: NWS Phoenix

UPDATE...Updated Aviation Discussion

KEY MESSAGES

- Unseasonably hot conditions will continue to challenge daily temperatures records through the rest of the week and into the weekend.

- These hot conditions may be dangerous, especially for any strenuous outdoor activities without proper hydration and frequent breaks in the shade, or air conditioning.

- Temperatures should finally begin to back away from record territory by the end of the weekend as the high shifts east and cloud cover and shower chances increase.

SHORT TERM /TODAY THROUGH SUNDAY/

High pressure continues to dominate our region. Although the center of the high pressure has shifted southeast of us our region is still under the influence of the western flank of the high. Additionally, the shortwave that moved through the region has shifted east and is now moving into the TX panhandle. This has allowed for H5 heights to increase to around 585-587 dm, which in turn will allow for a slight increase in temperatures today. Afternoon high temperatures are once again forecasted to be in the 98-103 degree range across the lower deserts and in the 90s across the higher terrain. Temperatures of this magnitude will once again break daily record high temperatures. Morning low temperatures will also be well above normal, in the 60s, but will make for a rather pleasant morning.

By tomorrow the weather in AZ and southeast CA will finally become more lively, breaking out of the multi-week long dry, tranquil, and very hot pattern. A very strong surface high (1037-1040mb) moving through the Plains Friday and Saturday will push a backdoor cold front (minus the cold) into southern AZ tonight into Saturday morning. A strong pressure gradient will develop and lead to strong easterly winds across southern AZ Saturday morning, especially across many high terrain areas east of Phoenix. Latest ECMWF EFI is highlighting parts of southeast AZ for a highly anomalous wind event. The 12Z HREF is showing high probabilities (>80%) of sustained wind speeds above 25 mph beginning around midnight in the mountains east of Phoenix, including southern Gila County. There is always uncertainty in the peak wind forecast with these easterly wind events, as models often overdo the strength of the winds. So, I would not anticipate winds hitting 60-65 mph like some of the models are suggesting for some mountain peaks, but this is still expected to be one of the strongest gradient winds over the past few years and some mountainous areas may still see peak winds in excess of 50 mph. Easterly breezes may increase as early as midnight in the lower deserts as well, including Phoenix, but the strongest breezes - gusts up to 25-30 mph - will become more common and widespread after sunrise Saturday and then the gradient should slowly relax through the afternoon.

In addition to the gusty winds Saturday, there will be an initial push of moisture into the region from the east and south, leading to more mid and high level clouds. After what will likely be a warmer morning due to the easterly breezes Saturday, afternoon high temperatures will end up several degrees cooler (3-5 degrees) than Friday, mainly across south-central AZ. Temperatures across SE CA and SW AZ will be similar or only a degree or two cooler than on Friday. Despite the slightly cooler temperatures, highs are still forecast to break daily records in Phoenix, Yuma, and El Centro.

LONG TERM /SUNDAY THROUGH THURSDAY/

Heading into Sunday a better push of moisture into the region is expected as high pressure shifts further east to the Gulf States and southerly to southwesterly flow taps into some subtropical moisture from the south. Ensemble members continue to show PWATs increasing to around 200-250% of normal across AZ by late Sunday into Monday. The increased cloud cover in combination with the high shifted further east will result in further cooling temperatures. The caveat of coming out of a wave of historical heat is that even a little bit of cooling will still result in near record temperatures. Afternoon high temperatures are forecast to be in the mid to upper 90s across the lower deserts both on Sunday and Monday and may still tie or break daily record highs. The influx of moisture late this weekend and early next week will also result in some low-end shower and thunderstorm chances, with best chances across the AZ high terrain to the north and east of the Phoenix Metro. NBM PoPs remain quite low for the lower deserts of central and southwest AZ, generally under 10%, despite the abundance of moisture. With the unseasonably hot temperatures, the lower troposphere and PBL will still be pretty dry. So, showers that do develop may mostly fall as virga. Additionally, the forcing will be very limited Sunday through Monday. There is at least some guidance support for some mid-level confluence and convergence that could better the chances for scattered showers or a band of light showers. Most global ensemble guidance has the best instability Monday (few hundred J/kg of CAPE), but with the minimal forcing it may be difficult to realize any instability to get thunderstorms. With the dry sub-cloud air, any convective or stronger shower or storm that develops will be capable of producing gusty outflow boundaries, but the hope of outflow boundaries generating new activity is pretty low given LCL heights likely remaining fairly high for this time of year.

Beyond Monday, ensemble guidance continues to show an East Pacific trough moving through the region during the middle of next week. There are still uncertainties regarding the exact track and strength of this system. However, it currently looks to drop H5 heights into a 570-575dam range during the middle of the week. As a result afternoon high temperatures are forecast to fall back into the 80s by Wednesday, which is near to slightly above normal for this time of year. The last time our region saw afternoon high temperatures in the 80s was on March 11, which will have been 3 weeks by next Wednesday. Additionally, this trough will bring additional shower and storm chances to the region, but as of now primarily favoring northern AZ. The NBM currently has the best rain chances (20-30%) across the higher terrain to the north and east of Phoenix. The latitudinal progression of the trough through the region will influence whether the rain chances increase across the southern deserts (a deeper southerly track) for the beginning of April. Stay tuned for future updates.

AVIATION

Updated at 1115Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns expected through the periods with winds mostly following diurnal tendencies. There will likely be a period of southerly winds late morning/early afternoon before the westerly shift mid/late afternoon. Clear skies will persist through most of the afternoon before some higher clouds move in by this evening. Late in the period through Saturday morning, easterly winds will be elevated and at times gusting to around 20kts.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns expected through the period. Winds will remain light and variable at times with directions settling out of the SE at KIPL and the S at KBLH during the afternoon. Winds will eventually turn out of the W at KIPL during the evening and out of the NNW at KBLH late Friday night. Clear skies will persist through this afternoon before high cirrus cloud decks move in by the evening.

FIRE WEATHER

Unseasonably hot temperatures with very dry conditions will continue through at least the start of the weekend. MinRH values will be around 5-15% today before increasing to 10-20% starting tomorrow. MaxRHs will follow a similar uptrend with readings close to 20-40% the next few mornings before rising up closer to a 40-60% range by the beginning of next week. Moisture levels will be highest Sunday into Monday and Wednesday with some moisture fluxes into the region. Winds will be light for the majority of today and follow their typical diurnal tendencies. Stronger winds (gusts 25-35 mph) enter the picture late tonight into Saturday, mainly for areas east of the Colorado River. There are good chances for higher gusts upwards of 40+ mph (~80% chance) in the mountainous areas east of Phoenix. Additionally, winds will be strongest Saturday morning and then should gradually subside midday through the afternoon. Although minimum RH values are forecast to stay above the critical threshold (15%) most of the day, there may still be a period of elevated, to near critical, fire weather conditions Saturday due primarily to the widespread breezy easterly winds and unseasonable temperatures. There will be at least slight chances for rain (10-30%) Monday and Wednesday, but the CWR will be under 10%. Any showers that do develop will be capable of producing gusty and erratic outflow winds, which could impact any existing fires.

CLIMATE

Daily record highs through this week:

Date Phoenix Yuma El Centro ---- ------- ---- --------- 3/27 98 in 1986 100 in 1986 99 in 1988 3/28 95 in 2015 98 in 2015 98 in 2015 3/29 97 in 2015 100 in 1897 97 in 1969

PSR WATCHES/WARNINGS/ADVISORIES

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