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Northern CA Forecast Discussion
FXUS66 KSTO 262212 AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
312 PM PDT Wed Jun 26 2019

SYNOPSIS...
Unseasonably cool daytime temperatures through Thursday with highs 12 to 20 degrees below normal tomorrow. A warming trend begins at the end of the week with highs gradually climbing to near normal early next week. No precipitation expected except for a small chance of showers or thunderstorms over the mountains of Shasta County. Occasionally breezy winds through the Delta and over the higher terrain during the next few days.


.DISCUSSION

.... Synoptic pattern over Wrn NOAM/Ern Pac dominated by a seasonably deep (minus 2.7 anomaly) upper trof off the coast will result in a (comfortably?) cooler than normal stretch of weather over Norcal for the rest of the week. (Non-scientific survey of selected co-workers reveals the term "comfortable" applies to these temps.) The GEFS/EPS 5H anomaly charts suggest this trof will maintain its cool influence on Norcal into the weekend. By early next week, temps should rebound to near normal, although the day-to-day modeling has been slowing/ pushing ahead the warming a day during the last few days. The return interval suggests this cool pattern occurs about every 5-10 yrs in our region this time of year. Thu looks to be the coolest day with high temps some 12 to 20 degrees below normal (mostly 70s in the Valley, and 50s to 60s over the higher terrain and foothills). The Probability Tool of less than 80 degrees reveals a high probability (mostly 35-70%, wrt CONSALL) that Valley temps will not equal/exceed 80, and at 100% in the immediate Delta influenced areas on Thu. This is by no means a lowest max temp event as a climate data base search reveals temps in the 60s for this date (6/27), most recently in 2001 for RBL and the RDD area. Overnite lows will drop into the 30s/40s in the mountains, and 50s in the Valley during the mid to latter portions of the week. The upper-level low pressure center just off the ORE coast will be rotating along the coast thru Thu, and then will move inland on Fri. Numerous thunderstorms will occur over the Pac NW with isolated thunder possible in our CWA. One storm popped-up briefly yesterday afternoon in Shasta Co, and with satellite imagery showing a growing cumulus field in Western Shasta/NWrn Tehama counties, we could/are see(ing) isolated storms develop there until diurnal heating ends this evening. The HREF probability of REFL suggests the Wrn half of Shasta Co has better than a 50/50 chance of seeing a 40+ dBZ echo late this afternoon/evening with much smaller probabilities on Thu. The elevated instability charts suggest a low probability or potential for thunder in our CWA, with the chances of convection near/around the Shasta Co border and Lassen NP. The deep/deepening trof will also keep some breezy SW winds in our CWA over the ridges and in the Valley into Thu. In the short term, the marine layer has deepened to above 3000 ft but remains mixed-out along the coast n of Pt Conception. The deep/deepening trof offshore will lift or elevate the marine layer with both the local "stratus calculator" and HREF cloud ceiling charts suggesting some marine cloudiness spreading/developing inland with strong onshore flow over 4 mbs SFO-SAC. Due to onshore flow and advection of marine air into the Valley on Thu, max temps may struggle to reach the guidance highs or adiabatic descent from 850 mbs. In addition, the synoptic trof presents its own challenge as the SPC SREF temp plumes for RDD and RBL show the mean temps about 10 deg cooler than the MOS bulletins for Thu. This presents a case to hedge the forecast temps to a cooler forecast. JHM


.EXTENDED DISCUSSION

(Sunday THROUGH Wednesday) An upper low continues to sit just off the coast of the PacNW on Sunday with little eastward progression forecast until Monday night. Deterministic models vary on the exact details of this feature although ensemble guidance keeps troughing over the west coast through at least the end of the period. Dry weather is expected as precipitation will remain north of the area. Temperatures will steadily increase by a degree or two daily Sunday through Wednesday, reaching normal values by Tuesday.








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