000 FXUS65 KPSR 281720 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1020 AM MST Thu Mar 28 2024 .UPDATE...Updated 18z Aviation Discussion. && .SYNOPSIS... Dry conditions and slightly above normal temperatures are expected through at least Friday but with increasing winds each day through Saturday. A strong low pressure system moving through the region over the weekend is expected to bring windy conditions on Saturday followed by a strong cold front moving through Saturday night and Sunday morning. Much of the area can expect light to moderate rainfall late Saturday and Sunday with some isolated thunderstorms and brief heavy rainfall possible. Temperatures will drop to well below normal for Sunday and Monday with rain chances lingering through at least Monday across much of Arizona. && .DISCUSSION... The main forecast concerns over the next several days are the strong gradient winds expected for Saturday across Arizona and the potential for a squall line with embedded thunderstorms tracking eastward across southern and central Arizona during the first half of Sunday. In the meantime, the region will continue to fall under weak ridging aloft through Friday resulting in overall quiet conditions and temperatures at or slightly above seasonal normals. The weather system expected for our region over the weekend is currently spinning off the coast of the Pacific Northwest and it's already quite strong even for that region. Little movement of the upper level trough is expected today, but it is forecast to expand in size with the base of the trough reaching southern California. As a result, this will lead to an increasing pressure gradient over the western deserts and gusty winds upwards of 35 mph in some spots this afternoon and evening. By tonight, a trailing shortwave trough is expected to very quickly dive southward across the western periphery of the trough reaching the base of the trough early on Friday as a new low center develops. At the same time the trough is expected to link up with a strengthening southern jet branch situated from west west of Baja to Arizona. As this phasing occurs, the trough is forecast to undergo a period of strengthening off the coast of central California and then from Friday night through early Sunday maintaining 500mb heights near climatological records for the period in that area. For our area on Friday, the gradient will continue to strengthen likely bringing near advisory level winds across portions of southeast California into far southwest Arizona. For Saturday, as the unseasonably strong low pressure system slowly tracks south southeastward just off the California coast, very windy conditions are likely to develop across Arizona. Latest guidance is now suggesting widespread advisory level winds across the majority of Arizona Saturday afternoon/early evening, including south-central Arizona. Forecast 850mb winds for Saturday afternoon and evening shows a swath of 35-45 kts centered across south-central Arizona, while skies are likely to be just partly cloudy. The combination of the strong winds within the boundary layer and some insolation should result in mixing much of those winds down the the surface. Wind gusts of 35-45 mph are likely to occur over much of southern and central Arizona on Saturday with potential of reaching 50mph in some spots. By Saturday evening, the advancing strong cold front with some showers developing along and ahead of the front is expected to move into southeast California. By the time the front reaches southwest and then south-central Arizona during the overnight hours Saturday night, we should see the rain band fill in with the leading edge of the band potentially evolving into a broken squall line with embedded thunderstorms due to strong frontal forcing and favorable boundary-relative flow. Any available instability is expected to be very minimal, but conceptually based on the strong front, it should be possible for a few strong thunderstorms to develop capable of producing heavy rainfall and near to severe wind gusts. For the rest of Sunday, the cold core portion of the upper low will move east across the region with additional showers expected later in the afternoon and through the evening. There also very well could be a few isolated thunderstorms later on Sunday as lapse rates improve and some weak instability develops mainly from Phoenix and areas west of Phoenix. There is also model evidence showing good shower potential lasting through the overnight hours Sunday night across south-central Arizona. As the cold core low remains in place over much of the region through Monday, shower chances should persist. Depending on how much insolation can be realized during the daytime hours Monday, its very possible some areas could see some low-topped convective thunderstorms Monday afternoon with small hail and brief heavy downpours the most likely impacts. As far as expected rainfall amounts, ample moisture will be present this weekend, especially along and just ahead of the front when there will be the most widespread rainfall. The additional shower and potential thunderstorm activity later on Sunday and Monday should also add a decent amount to the rainfall totals from Maricopa County and areas to the east. The latest storm total forecast rainfall amounts are calling for between 0.10-0.25" for areas west of the Colorado River to 0.50-1.00" in the Phoenix area. Higher terrain areas northeast of Phoenix could receive as much as 1.5" in a few spots. This rainfall is likely to be spread out across a 2-day period, so at this point we are not anticipating much if any of a localized flood threat. Temperatures this weekend will also be something to take note of as high temperatures Saturday will drop roughly 20 degrees to Sunday's daytime highs. Given the timing of the cold front during the early morning hours Sunday, much of south-central Arizona is likely to see highs reached just after midnight in the mid to upper 60s with temperatures falling well into the 50s by sunrise Sunday morning. Daytime temperatures will recover somewhat by the afternoon, but depending on the cloud cover some areas Sunday afternoon may struggle to get into the lower 60s. For early next week as the weakening upper low is trying to linger over eastern portions of the region, temperatures will be slow to moderate. Forecast highs for Monday are 10-13 degrees below normals with highs still in the 60s in Phoenix before likely improving into the mid 70s on Tuesday. Ensemble guidance heavy favors a ridge to begin building into the region by next Wednesday and this should briefly push highs back into the 80s for 2-3 days later next week. && .AVIATION...Updated at 1720Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation concerns are expected during the TAF period. Winds will follow typical diurnal tendencies with speeds generally aob 10 kts. High clouds will continue to stream over the region before gradually clearing by Friday morning. Southeast California/Southwest Arizona including KIPL and KBLH: Breezy to windy conditions will be the main aviation concern during the period. W'rly gusts 25-35 kts will be common this evening at IPL with elevated gusts potentially lasting through Friday morning. These enhanced winds will be capable of producing areas of of blowing dust leading to periods of reduced visibilities. At BLH, S/SW'rly gusts 20-25 kts will commence this evening before speeds calm through the early tonight. SCT high clouds can be expected through tonight before clearing out by tomorrow morning. && .FIRE WEATHER... Seasonably warm and dry conditions will persist through Friday, but with increasing winds each day. Winds today will be light across the eastern districts, but breezy to locally windy across portions of southeast CA this afternoon and evening. Winds Friday will increase further across the entire area with afternoon wind gusts between 30-40 mph across the western districts to around 25 mph over the eastern districts. MinRHs both days are likely to reach into the mid teens across the lower deserts to 20-25% over higher terrain areas. Friday afternoon may bring a brief period of elevated fire weather conditions within the Lower CO River Valley. By the weekend, a strong low pressure system will be moving in from the west bringing widespread windy conditions during the daytime Saturday, fairly widespread wetting rains Saturday night into Sunday, and cooler temperatures starting Sunday. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...Wind Advisory from 5 PM this afternoon to 11 PM PDT Friday for CAZ562. Wind Advisory from 5 PM this afternoon to 2 AM PDT Friday for CAZ563-566-567. && $$ DISCUSSION...Kuhlman AVIATION...RW FIRE WEATHER...Kuhlman