Area Forecast Discussion
Issued by NWS Sacramento, CA

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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.


&&

.AVIATION...

VFR conditions next 24 hours except slight chance of thunderstorms
northern Sacramento valley/Shasta Co 22z to 03z this afternoon
and evening. Surface winds mostly south 10 to 20 knots Sacramento
valley and northwest 10 to 20 knots San Joaquin valley. Occasional
gusts to 25 knots afternoon and early evening hours, with stronger
gusts over higher terrain and Carquinez Strait. Elevated marine
layer increases the chance of local MVFR CIGS developing in the
Sac Vly Thu morning.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$



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