####018004232#### AGUS76 KRSA 012001 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 100 PM PDT Mon Apr 1 2024 ...MAINLY DRY CONDITIONS TO START THE WEEK... ...TURNING COOLER AND WETTER FOR THE SECOND HALF OF THE WEEK... .METEOROLOGICAL CONDITIONS (SHORT TERM: MON PM - THU AM)... High pressure over the eastern Pacific is building over northern CA up toward western Canada this afternoon...while a positively tilted upr trof is slowly making its way downstream of the west coast. This will generate dry north to northeast flow over the next 24 to 36 hours...before the next polar system moves across the northern Pacific and begins to affect the region on Wednesday with cooler temperatures and increasing changes of precip. Still looking for the best amounts to fall over portions of the upper Klamath River basin and the northeast CA Plateau with 0.50- to 1.00-inch totals...where an area of upper diffluence exists and a mid-level vort max moves overhead. Freezing levels will also crash as the colder airmass filters across the area...dropping down to 2000- to 3500-feet over the upper Klamath River basin...northern CA...and portions of central CA. .METEOROLOGICAL CONDITIONS (LONG TERM: THU AM - SUN AM)... Forecast remains on track with minimal changes from the morning package. Models continue to predict a large upper low offshore of the PacNW early Thursday with troughing already engulfing nrn CA. The system will then continue to expand southward as it begins to head inland later Thursday. The low will drop scattered precipitation across the region Thursday and Friday as it moves eastward with the core of the low enveloping CA/NV Friday morning. The one thing that has changed since the previous runs is that the GFS/ECMWF have come closer together on the timing of the system's inland progression. The 12z ECMWF is now better in line with the GFS on a quicker movement, beginning to carry the low into the Four corners overnight Friday with some brief ridging building in behind it. However, the GFS is still quicker at moving the back edge of the low out of NV showing the ridge mostly covering the region mid Saturday morning while the ECMWF still has the western side of the system over most of NV. The result remains then that the GFS brings in precip from the next approaching system sooner, though by not as much as before, now closer to a 6-12 hr difference. Between the two models' ensembles, arrival times of initial showers vary between early Saturday morning and Sunday afternoon. Most of the region is expected to remain dry on Saturday with showers over nrn CA/NV, and srn OR between the exiting and approaching systems. Afternoon forecast was a combination of the previous forecast and the latest WPC guidance. This amounted to relatively small changes on the order of 0.10-0.25" over the 3 day period in the higher spots and less than a tenth of an inch for the rest of the region. QPF 12z Thurs-12z Sun: 0.50-1.50" over the Sierra and the eastern Transverse Range/San Diego mountains, 0.30-0.75" Santa Cruz/Santa Lucia mountains, 0.75-1.75" nrn NV mountains, 0.25-0.50" coastal soCal, and 0.10-0.50" for much of the rest of the region aside from se CA which is expected to see little to no precip. The approaching system will lower freezing levels from nw to se on Thursday starting at 2-5 kft north of I-80 and 3-11.5 kft to the south. Lowering freezing levels will spread across the region the rest of Thursday down to 2-3.5 kft north of Point Conception and 2.5- 5.5 kft to the south. Some fluctuation but generally predicting levels between 2.5-5 kft through Saturday morning. Some uncertainty after that as levels will depend on how quickly the system exits and the ridge moves in, but for the moment expecting levels to rise to 4- 8 kft from nw to se Saturday afternoon and 3.5-5 kft north of I- 80/4.5-9 kft south of I-80 by the end of the period. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php Kozlowski/AS $$