####018004244#### AGUS76 KRSA 262043 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 1245 PM PST Mon Feb 26 2024 ...MAINLY LIGHT PRECIP SOME LOCALLY MODERATE TODAY... ...LONGER PERSISTENT PRECIP EVENT FOR LATTER HALF OF WEEK... .METEOROLOGICAL CONDITIONS (SHORT TERM: MON PM - THU AM)... The s/wv trof along the Pacific Northwest coast has shifted inland and is tracking toward the east-southeast this afternoon...while the upr low off the southern CA coast is continuing to slowly meander eastward generally along 30N. Scattered precip associated with both of these features will bring mainly light with locally moderate totals across the region...focused primarily on the areas near the CA/OR border...northeast NV...and the length of the Sierra with 0.25- to 0.75-inch (local to 1.00-inch near Crater Lake) totals. This thinking remains the pretty much the same as this morning. Brief anti-cyclonic flow slides across the area tomorrow into early Wednesday for mainly dry conditions...ahead of a deepening system over the Gulf of Alaska that will make its way toward the northeast Pacific and elongate off the west coast. This will bring the next round of precip that looks to affect the region for the latter half of the week. On Wednesday...precip will slowly spread across northwest CA and the crest of the southern OR Cascades. Best totals will be over the Smith River basin with 1.00- to 2.00-inches possible...tapering off southward and inland. .METEOROLOGICAL CONDITIONS (LONG TERM: THU AM - SUN AM)... Forecast overall remains on track for a large surface/upper low to impact the region late this week and into the coming weekend. The low is progged to approach from the Gulf of Alaska sending a cold front towards the PacNW and northern CA into early Thursday spreading moderate to heavy precip into the north coast. The front will gradually traverse the region the rest of Thursday into Friday as the low heads south towards the area. The low and overall trough will begin to push inland on Friday spreading across the region into Saturday allowing for widespread precipitation through the extended. There are differences between the GFS and the ECMWF on some of the details of the system as it approaches with the GFS now showing three embedded circulations within the larger core of the low while the ECMWF does not. The GFS is also heavier with the precip across the Sierra throughout the duration of the event. The 12z ensembles are particularly divided on Sierra precip for Saturday as the QPF clusters are split into 5 groups (31%, 27%, 27%, 8%, 7%) with 24 hr amounts across the Sierra ranging from 1" to over 3-4". Models since the 00z/06z runs have trended a bit drier resulting in a 0.10-0.50" decrease in the QPF for most areas and as much as 0.50- 1" over the mountains. What happens will depend on the positioning of the low and how quickly it moves through as favorable flow into the Sierra could result in additional accumulations. For the afternoon update, went with a blend of WPC guidance and the latest NBM. Highest precip amounts are still forecast over the north coast, Shasta, and the Sierra. QPF for 12z Thurs-12z Sun: 1.50-4.50" CA coast Sonoma northward (locally 4-5.5+" parts of the north coast), 3- 5.50 " Shasta, 3.5-6.5" Sierra (5-8" parts of northern Sierra), 1.50- 2.75" central coast mountains, 0.75-1.50" rest of Bay Area, 0.25-1" down the valleys, 0.30-1" coastal soCal, and 0.75-1.50+" Transverse/mountains east of San Diego. Freezing levels 5-7 kft north of I-80 6.5-10 kft to the south Thursday morning lowering to 2.5-5.5 kft north of I-80 and 5.5-10 kft to the south in the evening. Levels will lower the rest of the week and into the weekend as the low approaches all the way down to 1.5-2.5 kft north of I-80 and 2-5.5 kft from I-80 to Point Conception Saturday morning. By the end of the period most of the region will be down to 1.5-3.5 kft. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php Kozlowski/AS $$