May 17, 2018 – Afternoon and evening thunderstorms possible over higher terrain for much of this week. Temperatures will gradually warm through Saturday, and then cool slightly on Sunday.

Discussion

Less convective activity expected over NorCal today as the upper level low moves Ewd across NV, farther away from NorCal which will weaken the cyclonic flow over our CWA. Less activity, however, does not mean nil as models forecast a relatively large amount of instability over the Sierra, and then over the Tehama/Shasta Co Wrn borders this afternoon. The NAM 3km limits the coverage to the Sierra Crest with isolated cells on the coastal range, while the HRRR shows a trend towards some cells in the Sierra foothills.

Weak ridging builds over the CWA on Fri with WNWly flow limiting the convection to the Sierra Crest. Then more areal coverage of storms expected as the weekend wears on as a weak trof is progged to move through on Sat, followed by a digging trof over the Wrn States on Sun leading to the development of a closed low over the Srn half of CA on Mon.

Generally, the SREF MUCAPE plumes show higher CAPE values over the Coastal mtns this afternoon, very little on Fri, and modest values on Sat. This is the opposite of the Sierra plumes (e.g. BLU and TVL) which increase their values through Sat. This shows the weaker synoptic forcing today, the ridging on Fri, and then the weak trof approaching on Sat. The next forecast period will include Sun which should show a marked increase in instability over the Shasta County and Coastal Range mtns with the digging trof. Except for the N end of the Sac Vly, the Vly is expected to remain dry.

Modest warming expected through Saturday, before a cooling trend on Sunday. Max temps today could be up to 8-10 degrees warmer than yesterday over some locations which may be predicated on the amount of sunshine today. A broad deck of SC and AC is rotating around the Wrn NV upper low with its Srn edge near SAC. Some of this cloud cover should erode during the day as the cyclonic flow weakens and turns NNWly. Temps are trending mild tonight under the cloud deck and only have to warm some 20 degrees to reach their normals while the normal diurnal range is about 27-28 degrees for example. The LAMP guidance is still trending towards upper 70s to low 80s in the Valley for today.

Extended discussion (Monday through Thursday)

A deepening upper trough will form a closed low over California on Monday, then very slowly move eastward toward the Great Basin Tuesday into Wednesday. There are still some model differences with the exact track this upper low will take, which will affect coverage of showers and thunderstorms. Precipitation will be most likely over the mountains during this period, but a few lower elevation showers/storms cannot be ruled out. Look for high temperatures in the upper 70s or 80s across the Valley, with 60s to 70s over the mountains.