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May 14, 2021 – Cooling trend continues today and into the weekend, with warmer temperatures Sunday and Monday. Scattered showers and thunderstorms are possible today and into the weekend, mainly over the mountains and northern Sacramento Valley as a low pressure system moves over the region. Some showers may spread into the southern Sacramento Valley Saturday night.

Discussion

About 2 to 9 degrees of cooling at 08z-09z in our CWA, except in the far Nrn Sac Vly and mtns at press time. That will change today as a Pacific NW trof drops into the area, eventually forming a closed low along the CA coast by this evening. Onshore flow will increase from 3+ mbs to 5-6 mbs today resulting in a strong Delta Breeze along with the trof’s synoptic cooling to continue the cooling trend over the CWA today. Showers and isolated storms are forecast by high resolution Model REF progs to develop over Trinity/Wrn Shasta/Wrn Tehama counties mostly likely during evening in the proximity of the upper low, although other convection will occur north and east of our CWA.

As the upper low drops over the far Srn portion of the CWA on Sat, showers/storms will become more widespread over our CWA especially over the Sierra/Srn Cascades due to the strong cyclonic circulation north of the upper low. Storm cells will be moving to the WNW due to the cyclonic steering winds. Decaying storms could rotate/migrate into the Valley Sat nite as mountain convection weakens.

Moisture profiles/PW progs forecast a band of 1+ inch precipitable water values will be drawn into the circulation of the closed low Sat/Sat night into Sun with the mean of the NCEP QPF SREF plumes for the various mtn sites indicating about a half inch of rain, a bit wetter than yesterday’s runs. This suggests the potential of some “wet” showers/storms. This is in conjunction with the moisture profiles indicating substantially deeper moisture later in the weekend which also lessens the downdraft potential. The higher PW air will also mean a reduced threat of (small) hail later in the weekend since higher PW tends to impedes updraft velocity. Thus, any marble or dime size hail and gusty winds will likely occur early Sat before the atmosphere sufficiently moistens and reduces the hail and wind threats.

Cyclonic flow lingers into Sun and relaxes on Mon over our CWA as the upper low moves towards the 4-Corners area. Showers/storms should develop over the Sierra Sun afternoon with less coverage on Mon as ridging builds in from the west during the day. As the upper low drops SEwd Sun, and ridging builds into the area on Mon, temps will begin to rebound.

Marine layer nearing 2 kft and with the winds howling at KSUU some patchy stratus may find its way into the Srn Sac Vly this morning. However, the main push will be late tonight and Sat morning as the trof will elevate the marine layer and will increase onshore gradients. Gusts to 35 kts are possible at KSUU with stratus clouds intruding into the Valley. Synoptic clouds from the upper low as well will also spread over the area. These clouds will tend to clear from north to south on Sun/Mon.

Extended discussion (Tuesday through Friday)

Pacific frontal system approaches interior NorCal Tuesday and moves through Wednesday. Models differ with depth of short wave trough and associated precipitation. For now, forecast includes a threat of showers over the Northern Sacramento Valley and adjacent foothills and mountains on Wednesday. Main impact elsewhere will be increased onshore flow and some cooling.

High temperatures midweek forecast to be below normal with mainly upper 70s in the Central Valley. Heights slowly rise Thursday into Friday as subsidence increases. High temperatures trend back up to around 2 to 5 degrees above normal by Friday.