006 FXUS65 KTWC 110438 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 938 PM MST Wed Oct 10 2018 .SYNOPSIS...Increasing moisture from tropical system Sergio along with influences from a Pacific storm system will bring showers and thunderstorms to the forecast for much of the region Thursday night into early next week. Temperatures will continue to be below average. && .DISCUSSION...Our main weather maker the next 48 hours is going to the moisture increase ahead of and with the remains of Sergio as it shears into the higher latitude flow east of our area. The current NHC track still puts the heaviest influence south of our area through Sonora Friday on a track toward southern New Mexico and then the southern to central plains Saturday. It has taken it's time so far but will now start to pick up speed as it engages with higher latitude support and opens up. As for our area, the best bet for significant rainfall will be south and southeast of Tucson, including portions of Cochise and Santa Cruz counties. We'll also need to watch the flow from even heavier rainfall in northern Sonora adding to any water issues as some areas flow across the border into southeast Arizona (such as Nogales and Naco). In addition, the interaction with the low off the southern California coast will become more important as it shifts slowly into the area over the weekend behind Sergio. An early look at the latest GFS now shows a more widespread area of showers ahead of Sergio forming across our area Thursday night into Friday morning. Our current precip chances may see another 20 percent bump for that period, but our overall QPF values still look good. Please see the previous discussion below for additional details. && .AVIATION...Valid through 12/06Z. FEW-SCT clouds 4-8k ft AGL. Layered clouds above 15k ft AGL in advance of Tropical Storm Sergio will spread across the area from the S-SW after 11/09z. Some -SHRA will start to develop after 11/21Z. Surface wind variable in direction and mainly less than 12 kts. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Increasing moisture associated with tropical system Sergio is expected to result in areas of showers and thunderstorms as early as Thursday afternoon and continuing through Friday night, with an emphasis on locations southeast of Tucson through Friday. A secondary weather system will keep unsettled weather continuing through the weekend and early next week with isolated to scattered showers and thunderstorms each day. 20-foot winds will be terrain driven and generally less than 15 mph through the weekend. && .PREV DISCUSSION...It's shaping up to be a beautiful October afternoon across Southeast Arizona with mostly sunny skies and temperatures running 4 to 7 degrees below average. Conditions will gradually deteriorate over the next 24-48 hours though, as Tropical Storm Sergio continues its journey toward Baja California. The latest forecast from the NHC (2 PM PDT this afternoon) has the center located 845 mi WSW of the southern tip of Baja California moving toward the northeast at 14 mph. Sergio is expected to make landfall on the Pacific coast of Baja California Sur sometime early Friday morning as a tropical storm. Here in Arizona, we should start to feel the effects of increased moisture advected into our area from Sergio by late Thursday afternoon. This will result in scattered showers and a few thunderstorms, but the best chance for measurable rainfall looks to be during the day on Friday. Based on the official NHC forecast track along with hurricane model forecasts, believe that the remnant center of circulation of Sergio will pass southeast of Arizona and into southern NM. The deterministic GFS and ECMWF track it somewhat further north though, so there remains some uncertainty with the timing and placement of the heaviest rainfall. At the moment, do believe that Santa Cruz and Cochise Counties will receive the most rainfall through Saturday morning, with amounts anywhere from 0.75" to 1.25" and isolated heavier amounts, especially in the mountains. Accumulations will taper off toward the north and west, but valleys could still see 0.25"-0.50" with upwards of 0.75" in the mountains. As the storm-formerly-known-as-Sergio moves through NM on Saturday, an upper low is progged to settle off the southwest coast of California. This system will become quasistationary into early next week. With enough atmospheric moisture hanging around in Sergio's wake, there will be a chance of showers and a few thunderstorms Saturday afternoon into at least next Monday, and potentially beyond. Confidence is increasing for this scenario though, since for the second forecast cycle in a row, the GFS and ECMWF are in much better agreement with the evolution of this system. One thing that could throw a wrench in the works late in the forecast period, is an upper trough that the ECMWF drops through Utah and into northern NM which could redirect the aforementioned system. We'll have to wait and see. In general, expect the next several days to have periods of cool, cloudy and showery weather. Temperatures over the next 7 days will remain below normal, with high temperature departures of 10 to 15 degrees Friday into early next week. && .TWC WATCHES/WARNINGS/ADVISORIES...None. && $$ Meyer/ECC/JG Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson