Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 730 AM PDT Wed May 7 2025 ...LINGERING SHOWERS THIS MORNING OVER SE CA AS A LOW EXITS, A FRONT MOVING THROUGH TODAY MAY GENERATE A SHOWER OR TWO OVER NRN CA... ...DRY CONDITIONS THE REST OF THE WEEK WITH WELL ABOVE NORMAL TEMPS... ...LARGER SYSTEM MAY RETURN PRECIP CHANCES TO NRN CA SUN/MON... .METEOROLOGICAL CONDITIONS (WED AM - TUE AM)... An upper low is exiting SE CA into AZ this morning leaving about 0.75" PW of moisture over far se CA and generating showers over AZ near the CA border. Some showers possible as the system continues to pull out of the region with chances diminishing throughout the morning. Behind the trough, a ridge is building in overhead while a trough approaches the PacNW along with a cold front. Can see the front on satellite imagery making its way inland over the WA/OR coast with the southern end of the front just offshore of the north coast. TPW imagery shows around 1" PW of moisture surrounding the front while water vapor imagery shows a stream of dry air just behind it. The ridge over CA/NV will help steer the upper shortwave to the north moving inland around WA later today while the surface front undercuts the ridge and moves through CA. The moisture it carries is shown quickly weakening when the front moves inland in the model forecasts. There may be enough lingering moisture to generate a shower or two later today over northern CA, but accumulations would be minimal (maybe a few hundredths). None of the GFS ensemble members show a drop of rain at Arcata while only 3 of the 50 ECMWF ensemble members show more than a trace. This front is not expected to be strong enough to prevent the start of a warming trend across interior northern CA which is forecast to see temperature anomalies today of +5 to +10 deg F. Coastal areas should remain relatively cool along with se CA as the low still exerts some influence. For the rest of the week, high pressure will strengthen overhead keeping conditions dry and continuing to raise temperatures. Some lingering moisture from today's cold front may allow for a few light showers to form over the Sierra. Most of the region can expect to see temperature anomalies of +5 to +15 deg F for Thursday and +10 to +20 deg F for Friday and Saturday. Models still have a larger system approaching over the weekend beginning to kick the ridge off to the east on Saturday, but not soon enough to prevent another relatively warm day. This larger low pressure system is progged to send showers to nrn CA Sunday and Monday. There is still a good amount of uncertainty as models disagree on the timing and movement of this system. Expecting showers to reach the north coast Sunday morning spreading inland overnight into Monday as the system pushes onshore. The GFS/ECMWF show the system digging deeper into CA throughout Monday as it gradually swings eastward moving into NV at the end of the current period. Again, there are disagreements on the exact timing as well as the general overall pattern. The GFS/CMC are wetter than the ECMWF at the moment while also spreading lighter precip further to the south (across the Bay Area vs mostly north of Point Arena). There is a decent spread amongst the ensembles as well with 24 hr QPF at Arcata ranging from 0 to nearly 2". The official forecast was a combination of the latest NBM and morning WPC guidance. Less than a tenth of an inch is expected today from stray showers over se CA and maybe a few hundredths over parts of the Sierra Thursday/Friday. The large bulk of the 6 day QPF will fall Sunday and Monday. Highest amounts over the Smith Basin with little to no precip expected to make it south of the Golden Gate. QPF 12z Sun-12z Tues: 0.50-1.50" north coast, 0.50-1" Shasta, 0.10- 0.75" northern Sierra, and a few hundredths to 0.25" central Sierra and the rest of nrn CA. In addition to precip, this next system will begin to lower temperatures from nw to se on Sunday with much of nrn/cnt CA expected to see near to below normal temperatures. For Monday, anomalies are forecast at -10 to -25 deg F. Freezing levels will drop from 10-13 kft or so this week to 5.5-11 kft north of I-80 Sunday afternoon. Lower freezing levels will spread across the region early next week down to 4.5-9.5 kft north of I-80 mid Monday morning and down to 4-6.5 kft in the evening. By the end of the period, these lower levels will push as far south as Monterey County. Please Note: This product may NOT be routinely updated this summer. Please refer to the following product issued by the CNRFC www.cnrfc.noaa.gov/Daily-Briefing for a graphical summary of weather and hydrologic conditions. |