Area Forecast Discussion
FXUS66 KMTR [Monterey CA] 242118 AFDMTR ∨
FXUS66 KMTR 242118 AFDMTR [HEAD]
National Weather Service San Francisco Bay Area 2:18pm PDT Fri Oct 24 2014 ∨
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA 218 PM PDT FRI OCT 24 2014 [HEAD]
Synopsis. A cold front will approach the coast later tonight increasing south winds and rain chances. The front will pass through quickly in the morning… but cooler unstable air will bring showers to much of the area Saturday afternoon and evening. A few thunderstorms may develop especially Santa Cruz Mountains northward. Drier and cooler weather is expected Sunday with a warming trend beginning Monday as high pressure rebuilds along the coast. ∨
.SYNOPSIS...A COLD FRONT WILL APPROACH THE COAST LATER TONIGHT INCREASING SOUTH WINDS AND RAIN CHANCES. THE FRONT WILL PASS THROUGH QUICKLY IN THE MORNING...BUT COOLER UNSTABLE AIR WILL BRING SHOWERS TO MUCH OF THE AREA SATURDAY AFTERNOON AND EVENING. A FEW THUNDERSTORMS MAY DEVELOP ESPECIALLY SANTA CRUZ MOUNTAINS NORTHWARD. DRIER AND COOLER WEATHER IS EXPECTED SUNDAY WITH A WARMING TREND BEGINNING MONDAY AS HIGH PRESSURE REBUILDS ALONG THE COAST. [DISC]
Discussion. As of 01:29pm PDT Friday… offshore winds have turned to southerly as a forerunner to the approaching cold front and marine layer deepening and starting to mix out. Cold core trough located around 135°W [690 mi west of Montara] with vort center around 35°N [180 mi south of Montara] and has bottomed out. Feature will progress eastward and shear to the northeast as it moves onshore Saturday. Given extent of cold front to the south… believe this system will produce rainfall across most of the area with the heaviest precipitation in the North Bay and Santa Cruz Mountains northward. Amounts will vary considerably with less than a tenth of an inch most of Monterey County to as much as three quarters inch North Bay mountains. ∨
.DISCUSSION...AS OF 01:29 PM PDT FRIDAY...OFFSHORE WINDS HAVE TURNED TO SOUTHERLY AS A FORERUNNER TO THE APPROACHING COLD FRONT AND MARINE LAYER DEEPENING AND STARTING TO MIX OUT. COLD CORE TROF LOCATED AROUND 135W WITH VORT CENTER AROUND 35N AND HAS BOTTOMED OUT. FEATURE WILL PROGRESS EASTWARD AND SHEAR TO THE NORTHEAST AS IT MOVES ONSHORE SATURDAY. GIVEN EXTENT OF COLD FRONT TO THE SOUTH...BELIEVE THIS SYSTEM WILL PRODUCE RAINFALL ACROSS MOST OF THE AREA WITH THE HEAVIEST PRECIPITATION IN THE NORTH BAY AND SANTA CRUZ MOUNTAINS NORTHWARD. AMOUNTS WILL VARY CONSIDERABLY WITH LESS THAN A TENTH OF AN INCH MOST OF MONTEREY COUNTY TO AS MUCH AS THREE QUARTERS INCH NORTH BAY MOUNTAINS. [DISC]
Main meteorological concerns will be significant pre frontal south winds along the coast and higher elevations of coastal mountains. Planning on issuing Wind Advisory for these areas late tonight and Saturday morning. Higher resolution NAM [North American Meso (formerly Eta) model] and WRF [Weather Research and Forecasting model] models indicate quick frontal passage into the Bay Area early in the morning and through the rest of the district by early afternoon. Cool unstable airmass in the wake of the cold front will trigger convection with possible thunderstorms Santa Cruz Mountains northward. Should see a brief lull in precipitation after the cold front… then Saturday afternoon and evening should be active with convection. ∨
MAIN METEOROLOGICAL CONCERNS WILL BE SIGNIFICANT PRE FRONTAL SOUTH WINDS ALONG THE COAST AND HIGHER ELEVATIONS OF COASTAL MOUNTAINS. PLANNING ON ISSUING WIND ADVISORY FOR THESE AREAS LATE TONIGHT AND SATURDAY MORNING. HIGHER RESOLUTION NAM AND WRF MODELS INDICATE QUICK FRONTAL PASSAGE INTO THE BAY AREA EARLY IN THE MORNING AND THROUGH THE REST OF THE DISTRICT BY EARLY AFTERNOON. COOL UNSTABLE AIRMASS IN THE WAKE OF THE COLD FRONT WILL TRIGGER CONVECTION WITH POSSIBLE THUNDERSTORMS SANTA CRUZ MOUNTAINS NORTHWARD. SHOULD SEE A BRIEF LULL IN PRECIP AFTER THE COLD FRONT...THEN SATURDAY AFTERNOON AND EVENING SHOULD BE ACTIVE WITH CONVECTION. [DISC]
Believe the NAM [North American Meso (formerly Eta) model] is a little overdone with high CAPE [Convective Available Potential Energy. A measure of the amount of energy available for convection. Related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.] and helicity values… but GFS [Global Forecast System: global spectral model used primarily for aviation weather forecasts; provides guidance out to 384 hours at 00, 06, 12 and 18UTC.] is likely underdone with lower model resolution issues. Interesting to note that unusually warm sea surface temperatures could assist convective development Saturday afternoon as the cooler airmass moves overhead. Confidence in convective strength is low… but confidence is moderate for a thunderstorm or two at least North Bay. ∨
BELIEVE THE NAM IS A LITTLE OVERDONE WITH HIGH CAPE AND HELICITY VALUES...BUT GFS IS LIKELY UNDERDONE WITH LOWER MODEL RESOLUTION ISSUES. INTERESTING TO NOTE THAT UNUSUALLY WARM SEA SURFACE TEMPERATURES COULD ASSIST CONVECTIVE DEVELOPMENT SATURDAY AFTERNOON AS THE COOLER AIRMASS MOVES OVERHEAD. CONFIDENCE IN CONVECTIVE STRENGTH IS LOW...BUT CONFIDENCE IS MODERATE FOR A THUNDERSTORM OR TWO AT LEAST NORTH BAY. [DISC]
Dynamics with this system are not particularly impressive with an apparent stable layer above 700 mb that should cap any significant convection. However… if dew points stay high enough… would be able to break through this stable layer aloft. ∨
DYNAMICS WITH THIS SYSTEM ARE NOT PARTICULARLY IMPRESSIVE WITH AN APPARENT STABLE LAYER ABOVE 700 MB THAT SHOULD CAP ANY SIGNIFICANT CONVECTION. HOWEVER...IF DEW POINTS STAY HIGH ENOUGH...WOULD BE ABLE TO BREAK THROUGH THIS STABLE LAYER ALOFT. [DISC]
The coolest day should be Sunday with northwest flow and the airmass will begin to dry out. Model trends are leaning toward stronger high pressure next week keeping the jet stream well to our north. Look for drier and warmer conditions through midweek. Could be a few showers extreme North Bay as weak systems push through the Pacific Northwest. Lots of differences in the medium range guidance late next week with a trend toward slower approach of next trough and delaying next significant threat for rain until at least next weekend. ∨
THE COOLEST DAY SHOULD BE SUNDAY WITH NORTHWEST FLOW AND THE AIRMASS WILL BEGIN TO DRY OUT. MODEL TRENDS ARE LEANING TOWARD STRONGER HIGH PRESSURE NEXT WEEK KEEPING THE JET STREAM WELL TO OUR NORTH. LOOK FOR DRIER AND WARMER CONDITIONS THROUGH MIDWEEK. COULD BE A FEW SHOWERS EXTREME NORTH BAY AS WEAK SYSTEMS PUSH THRU THE PACIFIC NW. LOTS OF DIFFERENCES IN THE MEDIUM RANGE GUIDANCE LATE NEXT WEEK WITH A TREND TOWARD SLOWER APPROACH OF NEXT TROF AND DELAYING NEXT SIGNIFICANT THREAT FOR RAIN UNTIL AT LEAST NEXT WEEKEND. [DISC]
Aviation. As of 10:52am PDT Friday… for 18Z [11am PDT] TAFs [Terminal Aerodrome Forecasts]. No concerns for this afternoon with mainly clear skies and some light southerly winds developing but they should trend southwest this afternoon. Fairly impressive low will deepen off the coast later this afternoon and evening with increasing southeast flow developing later this evening and overnight. Pretty high confidence for strong southeast winds and wet runways for Saturday morning. Thunderstorms even possible Saturday afternoon but won't show that in TAF [Terminal Aerodrome Forecast] at this time. ∨
.AVIATION...AS OF 10:52 AM PDT FRIDAY...FOR 18Z TAFS. NO CONCERNS FOR THIS AFTERNOON WITH MAINLY CLEAR SKIES AND SOME LIGHT SOUTHERLY WINDS DEVELOPING BUT THEY SHOULD TREND SOUTHWEST THIS AFTERNOON. FAIRLY IMPRESSIVE LOW WILL DEEPEN OFF THE COAST LATER THIS AFTERNOON AND EVENING WITH INCREASING SOUTHEAST FLOW DEVELOPING LATER THIS EVENING AND OVERNIGHT. PRETTY HIGH CONFIDENCE FOR STRONG SOUTHEAST WINDS AND WET RUNWAYS FOR SATURDAY MORNING. T-STORMS EVEN POSSIBLE SATURDAY AFTERNOON BUT WONT SHOW THAT IN TAF AT THIS TIME. [AVIA]
Vicinity of KSFO [San Francisco CA]… VFR [Visual Flight Rules] skies this afternoon and evening. Light southeast winds through early afternoon should trend southwest and not impact operations. Increasing southeast winds overnight will likely cause issues for Saturday morning planning. ∨
VICINITY OF KSFO...VFR SKIES THIS AFTERNOON AND EVENING. LIGHT SOUTHEAST WINDS THROUGH EARLY AFTERNOON SHOULD TREND SOUTHWEST AND NOT IMPACT OPERATIONS. INCREASING SOUTHEAST WINDS OVERNIGHT WILL LIKELY CAUSE ISSUES FOR SATURDAY MORNING PLANNING. [AVI2]
KSFO [San Francisco CA] bridge approach… similar to KSFO [San Francisco CA]. ∨
KSFO BRIDGE APPROACH...SIMILAR TO KSFO. [AVI2]
Monterey Bay Area terminals… clear for KSNS [Salinas CA] this afternoon with cloud bank currently hovering over KMRY [Monterey CA]. Expect partial clearing at the coast. Southerly winds should develop overnight with some light rain showers by around 12–15Z [5–8am PDT] Saturday. ∨
MONTEREY BAY AREA TERMINALS...CLEAR FOR KSNS THIS AFTERNOON WITH CLOUD BANK CURRENTLY HOVERING OVER KMRY. EXPECT PARTIAL CLEARING AT THE COAST. SOUTHERLY WINDS SHOULD DEVELOP OVERNIGHT WITH SOME -SHRA BY AROUND 12-15Z SATURDAY. [AVI2]
Marine. As of 10:52am PDT Friday… high pressure off the Southern California coast will weaken as a deepening low pressure system approaches from the west. The low will lift northeast on Saturday moving inland over the Pacific Northwest Saturday night. South winds will increase today through Saturday then switch to northwest Saturday night and Sunday. The low will bring a slight chance of thunderstorms to the northern waters Saturday. ∨
.MARINE...AS OF 10:52 AM PDT FRIDAY...HIGH PRESSURE OFF THE SOUTHERN CALIFORNIA COAST WILL WEAKEN AS A DEEPENING LOW PRESSURE SYSTEM APPROACHES FROM THE WEST. THE LOW WILL LIFT NORTHEAST ON SATURDAY MOVING INLAND OVER THE PACIFIC NORTHWEST SATURDAY NIGHT. SOUTH WINDS WILL INCREASE TODAY THROUGH SATURDAY THEN SWITCH TO NORTHWEST SATURDAY NIGHT AND SUNDAY. THE LOW WILL BRING A SLIGHT CHANCE OF THUNDERSTORMS TO THE NORTHERN WATERS SATURDAY. [AVIA]
.MTR WATCHES/WARNINGS/ADVISORIES... WIND ADVISORY...CAZ505-507-509-512-529 FROM 3 AM UNTIL NOON SATURDAY SCA...SF BAY NORTH OF THE BAY BRIDGE FROM 3 AM SCA...SF BAY SOUTH OF THE BAY BRIDGE FROM 3 AM SCA...PIGEON PT TO PT PINOS 0-10 NM FROM 3 AM SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM FROM 3 AM SCA...PT PINOS TO PT PIEDRAS BLANCAS 10-60 NM FROM 3 AM SCA...PT ARENA TO PT REYES 0-10 NM FROM 9 PM SCA...PT REYES TO PIGEON PT 0-10 NM FROM 9 PM SCA...PT ARENA TO PT REYES 10-60 NM FROM 9 PM SCA...PT REYES TO PIGEON PT 10-60 NM FROM 9 PM SCA...PIGEON PT TO PT PINOS 10-60 NM FROM 9 PM SCA...ROUGH BAR ADVISORY FOR SF BAR FROM 5 AM [WARN]
Public Forecast: KBB; Aviation/Marine: RWW ∨
PUBLIC FORECAST: KBB AVIATION/MARINE: RWW [CRED]
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3:10pm PDT Oct 24 2014
Tonight. A 40% chance of rain after 11pm. Increasing clouds, with a low around 58°. Breezy, with a south wind 15 to 20 mph increasing to 23 to 28 mph after midnight. Winds could gust as high as 38 mph.
Saturday. Showers, with thunderstorms also possible after 11am. High near 66°. Windy, with a SSW wind 23 to 31 mph, with gusts as high as 41 mph. Chance of precipitation is 80%.
Saturday Night. A 40% chance of rain. Mostly cloudy, with a low around 56°. SSW wind 7 to 14 mph becoming NW after midnight.
Sunday. Mostly sunny, with a high near 62°. NNW wind around 16 mph, with gusts as high as 20 mph.
Sunday Night. Mostly clear, with a low around 54°. Breezy, with a NNW wind 20 to 22 mph, with gusts as high as 28 mph.
Monday. Sunny, with a high near 61°.
Monday Night. Mostly clear, with a low around 55°.
Tuesday. Sunny, with a high near 64°.
Tuesday Night. Partly cloudy, with a low around 57°.
Wednesday. Mostly sunny, with a high near 66°.
Wednesday Night. Partly cloudy, with a low around 58°.
Thursday. Partly sunny, with a high near 66°.
Thursday Night. Mostly cloudy, with a low around 56°.
Friday. A slight chance of rain. Partly sunny, with a high near 65°.