041 FXUS65 KPSR 080500 AAB AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1000 PM MST Thu Oct 7 2021 .UPDATE... Updated Aviation .SYNOPSIS... High clouds will continue today and early tomorrow as another weather system approaches the region. By Friday, a few light showers will be possible across southeast California and portions of southwest Arizona. Breezy conditions will also develop, particularly for southeast California. The weekend should be quiet with slightly below normal temperatures. Winds and clouds will increase again Monday and Tuesday ahead of the next system which should be larger and stronger than recent disturbances. Breezy and significantly cooler conditions will arrive Tuesday but with little in the way of precipitation. && .DISCUSSION... Satellite imagery continues to show widespread high clouds stretching from SoCal to Arizona as upper level moisture streams in from the Pacific. Periods of dense high clouds will begin to clear out on Friday with afternoon highs peaking near 90 degrees. An offshore trough will begin to lift northeastward on Friday into southern California. However, the base of the trough will slow considerably and partially detach from the mean flow as a second disturbance sliding down the western edge of the trough causes it deepen somewhat. This feature will become positively tilted as it finally pushes through on Saturday. Although the trough orientation will not provide favorable dynamics for lift and precipitation, it will be associated with a robust moisture tap. Model IVT may peak as high as 500-700 kg/m/s for portions of southern California. This moisture rich environment will likely allow for a few showers to form. PoPs approach 40% for JTNP, and gradually diminish to the single digits towards La Paz and Yuma counties. Besides the rain chances, wind will also be a concern. The orientation of the trough, steep height falls, and pressure rises behind the surface front will all be favorable for breezy conditions west of the Colorado River. ECMWF EFI values are near 0.5 to 0.7, representing a slightly unusual event. Wind gusts may approach wind advisory criteria by Friday evening for typically windy portions of Imperial County. Temperatures will cool somewhat Saturday through Monday in the wake of the weak storm system. Highs will generally rise to the mid 80s. Our attention then turns to the anomalously strong trough that will crash into the desert southwest starting Tuesday. Ensemble guidance is in very good agreement that this trough will develop with relatively limited variation in its depicted timing. There is still uncertainty with respect to its amplitude. About 3/4 of the WPC clusters show a relatively more amplified trough, where the low is able to completely close off, with about 1/4 showing a less amplified open trough solution. Overall, confidence continues to improve that the more amplified solution will pan out. How anomalous will this trough be? The EPS and NAEFS both show that for 500 mb heights and below it will be in the bottom 1th percentile of values for this time of year. ECMWF EFI values for surface temperatures show values of -.90 to -.95 over the area. By any metric this storm will be unusually cold for the middle of October. In terms of surface temperature, the NBM shows highs will likely only rise to the low 70s with lows in the low 50s on Tuesday for the lower deserts. For Phoenix, NBM temperature probabilities have a 45% chance of highs remaining below 70F and a 35% chance of lows falling into the 40s. Wind will also be a major factor for this storm system. This system will induce a very strong height and pressure gradient as it moves in. Even at 5 days out, the ECMWF EFI is showing values of 0.7-0.8 with SOT values approaching 1. If the forecast remains consistent, these values could rise further in the coming days as the ensemble solutions with a weaker trough are gradually removed. It is much too early to get too specific, but ensemble guidance suggests that some of the typically windier spots could see gusts approaching wind advisory criteria. The chance for rain with this system will remain low as a continental trajectory down the interior west will limit moisture. Isolated showers will likely be limited to the higher terrain in Northern Arizona, such as Flagstaff, the Rim and White Mountains. NBM precipitation probabilities have crept up just a bit compared to yesterday along the foothills north and east of Phoenix due to a slightly more favorable trough orientation and moisture values, but rain remains unlikely in the major population centers. A progressive upper level pattern will allow the trough to quickly eject towards the east within a day or two. Rebounding heights and dry conditions will favor a return to pleasant conditions the second half of next week. && .AVIATION...Updated at 0006Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Periods of thick cirrus cigs will be common through Friday afternoon with only patchy virga otherwise. Current light westerly winds to switch back to easterly shortly before midnight. The daytime switch back to west on Friday should occur earlier and more distinctly than recent days, with a bit of gustiness likely as well, with lighter westerly winds likely to continue through most, or all of Friday night. Southeast California/Southwest Arizona including KIPL and KBLH: Thick high cirrus cigs will gradually lower through Friday morning with decks falling as low as 8K ft AGL. While virga will be expansive, a few light showers reaching the surface will be possible Friday morning at KIPL. These virga/light showers Friday could lead to some variability in wind directions/speeds at times. Some enhanced breeziness is possible Friday morning in response to an approaching weather system, but likely will not become pronounced until later in the day Friday as winds aloft increase and mix down to the sfc. && .FIRE WEATHER... Saturday through Wednesday: The next weather system will brush through the Southwest on Friday, bringing low end rain chances to southeast California and southwest Arizona, along with breezy conditions and a slight uptick in afternoon humidity to all areas. Wind gusts may reach 20-30 mph Friday afternoon. Temperatures and humidity will fluctuate a bit over the weekend before the next system approaches on Monday. Much stronger wind gusts are possible Monday across southeast California before spreading to all areas on Tuesday. Temps will cool by 10-15 degrees for Tue/Wed giving a minor boost to RH levels. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Hodges/12 AVIATION...Percha/18 FIRE WEATHER...12/Iniguez