Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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886 FXUS65 KPSR 192103 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 203 PM MST Thu Sep 19 2024 .SYNOPSIS... A low pressure system will pass through southern California and Arizona tonight through Saturday. In the process, there will be cooler temperatures and breezy conditions. There will also be a chance for showers and thunderstorms in some areas including eastern Riverside County, La Paz and northern Maricopa Counties, and portions of Gila County. Better chances will be north of there. Next week, a ridge of high pressure will advance inland and result in a warmup with desert highs reaching triple digits again. && .DISCUSSION... A positively tilted upper trough lies across much of the western and north-central CONUS. Within that, a low center is located near the central CA coast. Meanwhile, high pressure is centered to the southeast over northeast Mexico. With this flow pattern, there is a distinct temperature gradient aloft with rather warm temperatures over southeast AZ and rather cool temperatures near the CA coast. Precipitable water has increased a little compared to yesterday and there are some areas of few/scattered cumulus. But, there isn`t enough moisture/instability for convection concerns today - just some pockets of few-scattered cumulus. The upper low will begin entering southeast California tonight and exit Arizona by Saturday evening. In the process, it will bring a measure of dynamical forcing and some destabilization with cold air aloft for our forecast area plus a weak cold front. The system will be positively tilted before becoming neutrally tilted late Friday night which is not ideal for moisture advection. But the other aforementioned factors will help to make up for the modest moisture availability leading to an opportunity for showers and thunderstorms for northern portions of our forecast area (generally eastern Riverside County, La Paz, northern Maricopa, and portions of Gila County) with better chances to the north of there. Best chances for our forecast area are over Joshua Tree National Park Friday afternoon. For the Greater Phoenix area, precip chances will be limited to northern portions (mainly north and northeast of the 101 Freeway in the north Valley). As an aside, PoPs have trended up a bit from the previous forecast package. The passage of the system will also lead to cooler temperatures though not a dramatic drop. The coolest day for SE CA and SW AZ will be Friday; for south- central AZ it will be Saturday. Daytime and evening wind speeds will also pick up (breeziest on Friday) but fall short of Advisory conditions. An exception may be the southwest corner of Imperial County. But HREF probabilities of exceeding 40mph there are currently less than 50%. Heights and thicknesses slowly rebound next week and in turn temperatures rebound as well. In fact, triple digits return beginning Sunday and peak on Tuesday with little change through Thursday. But the probability of reaching 110 remains less than 10% for each of those days. Though there are indications of follow-on shortwaves carving out a cutoff over/near the CWA, it would likely have little impact other than muting the warm up a little but. Of note, even the 25th percentile high temperatures for PHX and YUM are in the 100-105 range next week. && .AVIATION...Updated at 1803Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Persistence wind forecast is expected again for the next 24 hours. Southerly winds, with SW-SE variability, are expected to develop by midday and persist through 00-01Z before a more predominant west to southwest component develops. Diurnal easterlies then return tonight. Daytime wind speeds will be around 7-10 kts with afternoon gusts occasionally up to 15-20 kts. FEW to SCT cumulus will be common through the TAF period with varying bases between 6-10K ft AGL (lowest overnight and highest in the afternoon) Southeast California/Southwest Arizona including KIPL and KBLH: A period of variability this morning will be followed by SE winds at KIPL and S at KBLH. Between 00-02Z, winds will shift W at KIPL, with gusts up to 20-25 kt during the evening and into the overnight hours. Speeds will mostly be in the 6-10 kt range at KBLH. Skies will remain mostly clear. FEW cumulus are expected at KBLH this afternoon, becoming FEW to SCT by late Friday morning. && .FIRE WEATHER... A drier and cooler airmass encompassing the region will result in below normal temperatures through Saturday. Stronger southerly to southwesterly will develop this afternoon, with gusts to 20-25 mph expected today and tomorrow. MinRH`s will hover around 15-25% region-wide over the next couple of days, with some slight drying first across the western districts starting Saturday and spreading eastward on Sunday, with MinRH`s more in the 10-20% range by the beginning of next week. Overnight RH recoveries will be fair to good across the region over the next couple of mornings, before degrading to generally fair to poor in spots going into next week. Warming temperatures to above normal are expected to develop late this weekend and persist through most of next week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...AJ AVIATION...Benedict FIRE WEATHER...Young