Area Forecast Discussion
FXUS66 KSTO [Sacramento/WFO CA] 272212 AFDSTO ∨
FXUS66 KSTO 272212 AFDSTO [HEAD]
National Weather Service Sacramento California 3:12pm PDT Sat Aug 27 2016 ∨
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SACRAMENTO CA 312 PM PDT SAT AUG 27 2016 [HEAD]
Synopsis. Mostly a dry weather pattern with average temperatures into next week. A fast moving trough could bring scattered showers and even an isolated thunderstorm to areas north of I-80 Monday night and Tuesday morning. ∨
.SYNOPSIS... MOSTLY A DRY WEATHER PATTERN WITH AVERAGE TEMPERATURES INTO NEXT WEEK. A FAST MOVING TROUGH COULD BRING SCATTERED SHOWERS AND EVEN AN ISOLATED THUNDERSTORM TO AREAS NORTH OF INTERSTATE 80 MONDAY NIGHT AND TUESDAY MORNING. [DISC]
Discussion. Inland coastal stratus retreating to the coast in typical diurnal fashion this afternoon. Temps running near to slightly cooler than yesterday… the coolest in the far northern SAC [Sacramento CA] Valley which should end up just a few degrees cooler than yesterday. Winds and pressure gradients lighter/weaker than yesterday in the southern portion of the CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map] where temps should near persistence for maxes this afternoon. ∨
.DISCUSSION... INLAND COASTAL STRATUS RETREATING TO THE COAST IN TYPICAL DIURNAL FASHION THIS AFTERNOON. TEMPS RUNNING NEAR TO SLIGHTLY COOLER THAN YESTERDAY...THE COOLEST IN THE FAR NRN SAC VLY WHICH SHOULD END UP JUST A FEW DEGREES COOLER THAN YESTERDAY. WINDS AND PRESSURE GRADIENTS LIGHTER/WEAKER THAN YESTERDAY IN THE SRN PORTION OF THE CWA WHERE TEMPS SHOULD NEAR PERSISTENCE FOR MAXES THIS AFTERNOON. [DISC]
Marine layer holding around 2600–3000 ft this afternoon and should lower Sunday/Monday as subsidence from building high pressure increases over the region. The synoptic pattern is rather benign over California as the main synoptic features are distant from our area which is generally in between 3 main weather features. The first is the large low over northern Canada which retrogrades and acts as the kicker for the Rex block [blocking pattern where there is an upper level high located directly north of a closed low] (#2) currently in the vicinity 35°N [200 mi south of Mather]/140°W [1100 mi west of Mather] to move into our area late Monday night and Tuesday. In the interim… the retrogression of the northern Canadian low allows subtropical high pressure (#3) to build northward allowing for slight warming Sunday/Monday and a slight "squashing" of the marine layer. ∨
MARINE LAYER HOLDING AROUND 2600-3000 FT THIS AFTERNOON AND SHOULD LOWER SUN/MON AS SUBSIDENCE FROM BUILDING HIGH PRESSURE INCREASES OVER THE REGION. THE SYNOPTIC PATTERN IS RATHER BENIGN OVER CA AS THE MAIN SYNOPTIC FEATURES ARE DISTANT FROM OUR AREA WHICH IS GENERALLY IN BETWEEN 3 MAIN WX FEATURES. THE FIRST IS THE LARGE LOW OVER NRN CANADA WHICH RETROGRADES AND ACTS AS THE KICKER FOR THE REX BLOCK (#2) CURRENTLY IN THE VCNTY 35N/140W TO MOVE INTO OUR AREA LATE MON NITE AND TUE. IN THE INTERIM...THE RETROGRESSION OF THE NRN CANADIAN LOW ALLOWS SUBTROPICAL HIGH PRESSURE (#3) TO BUILD NWD ALLOWING FOR SLIGHT WARMING SUN/MON AND A SLIGHT "SQUASHING" OF THE MARINE LAYER. [AVIA]
Model consensus suggests the Rex block [blocking pattern where there is an upper level high located directly north of a closed low] will move over our area early Tuesday as an open wave ahead of a deepening upper trough next week. The NAM [North American Meso (formerly Eta) model] elevated instability forecasts suggest at least a small chance of precipitation will be possible in our CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map] mainly N of the I-80 corridor. For now… the best chance of precipitation in our CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map] would be the 12Z [5am PDT]-18Z [11am PDT] Tuesday time frame… but the timing is likely to vary due to retrogression and deepening weather systems. The Rex block [blocking pattern where there is an upper level high located directly north of a closed low] does have some moisture with it on the water vapor imagery with a small area of visible cloud and some subtropical moisture from the reservoir of higher PW [precipitable water] air vicinity of 20°N [1240 mi south of Mather]/140°W [1100 mi west of Mather] is forecast to work northeastward towards and into Northern California as the moisture becomes entrained into the southwesterly flow ahead of the Rex low/trough. This makes elevated convection and terrain forcing the most likely triggers for convection in our CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map] than surface based instability… with typical inverted-V soundings. Although our POP [Probability of Precipitation] area is rather large… the POP [Probability of Precipitation] at a point will be low with little QPF [Quantitative Precipitation Forecast]. ∨
MODEL CONSENSUS SUGGESTS THE REX BLOCK WILL MOVE OVER OUR AREA EARLY TUE AS AN OPEN WAVE AHEAD OF A DEEPENING UPPER TROF NEXT WEEK. THE NAM ELEVATED INSTABILITY PROGS SUGGEST AT LEAST A SMALL CHANCE OF PRECIP WILL BE POSSIBLE IN OUR CWA MAINLY N OF THE I-80 CORRIDOR. FOR NOW...THE BEST CHANCE OF PRECIP IN OUR CWA WOULD BE THE 12Z-18Z TUE TIME FRAME...BUT THE TIMING IS LIKELY TO VARY DUE TO RETROGRESSION AND DEEPENING WX SYSTEMS. THE REX BLOCK DOES HAVE SOME MOISTURE WITH IT ON THE WATER VAPOR IMAGERY WITH A SMALL AREA OF VISIBLE CLOUD...AND SOME SUBTROPICAL MOISTURE FROM THE RESERVOIR OF HIGHER PW AIR VCNTY OF 20N/140W IS FORECAST TO WORK NEWD TOWARDS AND INTO NORCAL AS THE MOISTURE BECOMES ENTRAINED INTO THE SWLY FLOW AHEAD OF THE REX LOW/TROF. THIS MAKES ELEVATED CONVECTION AND TERRAIN FORCING THE MOST LIKELY TRIGGERS FOR CONVECTION IN OUR CWA THAN SURFACE BASED INSTABILITY...WITH TYPICAL INVERTED-V SOUNDINGS. ALTHOUGH OUR POP AREA IS RATHER LARGE...THE POP AT A POINT WILL BE LOW WITH LITTLE QPF. [AVI2]
The deeper trough developing over the eastern Pacific next week will lead to cooler and breezier weather. (JHM [Lahaina/West Maui HI]) ∨
THE DEEPER TROUGH DEVELOPING OVER THE EASTERN PACIFIC NEXT WEEK WILL LEAD TO COOLER AND BREEZIER WEATHER. JHM [AVI2]
Extended discussion (Wednesday through Saturday) ∨
.EXTENDED DISCUSSION (WEDNESDAY THROUGH SATURDAY) [DISC]
Northern California will remain under a longwave trough pattern through the extended period, bringing cooler than normal temperatures for next week. Medium-range models have shown poor consistency handling a deeper, more robust trough late next week. However, there continues to be enough confidence to mention a slight chance of showers mainly along the mountains for Labor Day weekend. ∨
NORTHERN CALIFORNIA WILL REMAIN UNDER A LONGWAVE TROUGH PATTERN THROUGH THE EXTENDED PERIOD, BRINGING COOLER THAN NORMAL TEMPERATURES FOR NEXT WEEK. MEDIUM-RANGE MODELS HAVE SHOWN POOR CONSISTENCY HANDLING A DEEPER, MORE ROBUST TROUGH LATE NEXT WEEK. HOWEVER, THERE CONTINUES TO BE ENOUGH CONFIDENCE TO MENTION A SLIGHT CHANCE OF SHOWERS MAINLY ALONG THE MOUNTAINS FOR LABOR DAY WEEKEND. [DISC]
VFR [Visual Flight Rules] conditions across TAF [Terminal Aerodrome Forecast] sites next 24 hours. Generally light onshore winds around 5–10 kts [6–12 mph] except for gusts up to 25 knots [29 mph] near the Delta. ∨
VFR CONDITIONS ACROSS TAF SITES NEXT 24 HOURS. GENERALLY LIGHT ONSHORE WINDS AROUND 5 TO 10 KTS EXCEPT FOR GUSTS UP TO 25 KNOTS NEAR THE DELTA. [AVI2]
STO [Sacramento/WFO CA] Watches/Warnings/Advisories
.STO WATCHES/WARNINGS/ADVISORIES... NONE. [WARN]
28 Aug 1:02am PDT
Overnight Mostly clear, with a low around 62°. South wind 5 to 9 mph becoming ENE.
Sunday Sunny, with a high near 87°. East wind 5 to 8 mph becoming WSW in the morning.
Sunday Night Clear, with a low around 63°. West wind 6 to 8 mph becoming ENE after midnight.
Monday Sunny, with a high near 86°. ENE wind around 7 mph becoming SW in the morning.
Monday Night Mostly clear, with a low around 62°. West wind 5 to 7 mph becoming light and variable in the evening.
Tuesday Sunny, with a high near 84°.
Tuesday Night Clear, with a low around 60°.
Wednesday Sunny, with a high near 82°.
Wednesday Night Mostly clear, with a low around 58°.
Thursday Sunny, with a high near 80°.
Thursday Night Mostly clear, with a low around 56°.
Friday Sunny, with a high near 79°.
Friday Night Mostly clear, with a low around 55°.
Saturday Sunny, with a high near 78°.
Mostly clear, with a low around 62. South wind 5 to 9 mph becoming east northeast.
Sunny, with a high near 87. East wind 5 to 8 mph becoming west southwest in the morning.
Clear, with a low around 63. West wind 6 to 8 mph becoming east northeast after midnight.
Sunny, with a high near 86. East northeast wind around 7 mph becoming southwest in the morning.
Mostly clear, with a low around 62. West wind 5 to 7 mph becoming light and variable in the evening.
Sunny, with a high near 84.
Clear, with a low around 60.
Sunny, with a high near 82.
Mostly clear, with a low around 58.
Sunny, with a high near 80.
Mostly clear, with a low around 56.
Sunny, with a high near 79.
Mostly clear, with a low around 55.
Sunny, with a high near 78.