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September 19, 2016 – Hot and dry today with high temperatures approaching record levels, then cooler heading into mid-week with a slight chance of Sierra showers and storms. A quick, but brief pattern change is expected to occur just in time for the Autumnal Equinox which occurs Thursday, the 22nd, at 721 am PDT. It will feel like autumn, right on cue on Thursday, with high temperatures some 25 degrees cooler than today. A weather is forecast to drop southward over Norcal by midweek, bringing not only the cooling trend, but also breezy winds and chance of mountain showers and thunderstorms, mainly over the Sierra Nevada and Shasta County mountains and foothills.

Discussion

One more hot day over Norcal with a virtually non-existent marine layer and a strong ridge centered over TX dominating the wx pattern into CA. 850 mbs temps in the 20-24 degree range over Norcal suggest max temps in the mid to high 90s over most of the Valley today with some locales reaching triple digits. The probability of max temps over 100 today ranges from 20% at MYV to 25-30% in the SAC area and 40-50% in the SCK/MOD areas…and over 101 ranging from 12% at MYV…to 10-20% in the SAC area and 25-40% in the SCK/MOD areas. This gives a “proxy probability” of records at these sites which are 102 at SCK (2000) and MOD (1995)…and 101 (2000) at DTS/SAC for today. Looking at VIS satellite imagery, elevated smoke layer from I-80 corridor Swd may reduce solar insolation enough to keep max temps from reaching records.

However, changes are around the corner as the cold low, presently centered over the northern portion of Vancouver Island, moves into the PacNW on Tuesday, then digs south into NorCal by later Wednesday. The result will be a dramatic cool down for the region beginning Tuesday along with gusty southwest winds. By Thu…max temps will be some 20-30 degrees cooler than today, just in time for the start of fall or the autumnal equinox which is Sept 22 at 721 AM PDT. The digging trof will suppress the weak cut-off low (which is several hundred miles off the Baja Coast) well to our S as it opens-up and moves across Socal.

The trough will be mainly dry for the Valley with showers mainly concentrated near the low center over far northern California Wednesday night and Thursday. Deeper moisture moving up from the south ahead of the system bears watching for shower potential along and south of I-80 if the system trends slower or further west. NAM layered 700-500 mbs moisture shows moisture advecting/moving up from the south Tue/Wed ahead of the digging upper low. Although the bulk of moisture from Hurricane Payne will move across Baja, Socal and into the Desert SW, the digging upper trof may entrain some of the moisture on the Nrn edge of this area moisture into the Sierra for possible scattered convection.

Extended discussion (Friday through Monday)

On Friday, interior NorCal will be between an upper low to our east, over the Great Basin, and high pressure to our west over the Pacific Ocean. Temperatures on Friday will still be influenced by the cooler airmass from the upper low so daytime highs will measure 2-7 degrees below normal for our CWA. Valley highs in the 80s with higher terrain in the 60s and 70s.

From Saturday into Monday, the high pressure over the Pacific will strengthen and build into the West Coast. This will produce a warming and drying trend for our region. Highs on Saturday will be about 1-5 degrees above normal with valley highs in the upper 80s to low 90s. By Sunday, valley highs will be in the low to mid 90s with higher terrain mid 70s (Sierra crest) to low 90s (foothills), measuring 4-8 degrees above normal. North winds along the western side of the Sacramento valley could be breezy at times.

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