Area Forecast Discussion
FXUS66 KMTR [Monterey CA] 230533 AFDMTR ∨
FXUS66 KMTR 230533 AFDMTR [HEAD]
National Weather Service San Francisco Bay Area 10:33pm PDT Wed Oct 22 2014 ∨
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA 1033 PM PDT WED OCT 22 2014 [HEAD]
Synopsis. Dry weather conditions and partly cloudy skies are expected tonight… but then on Thursday the weak tail end of an approaching Pacific weather system will likely bring some light rainfall to northernmost portions of our district. Friday looks to be mostly sunny and mild… but then a somewhat wetter system will move in on Saturday and spread rain chances across much of our area. Dry conditions and seasonal temperatures are projected for Sunday into the first part of next week. ∨
.SYNOPSIS...DRY WEATHER CONDITIONS AND PARTLY CLOUDY SKIES ARE EXPECTED TONIGHT...BUT THEN ON THURSDAY THE WEAK TAIL END OF AN APPROACHING PACIFIC WEATHER SYSTEM WILL LIKELY BRING SOME LIGHT RAINFALL TO NORTHERNMOST PORTIONS OF OUR DISTRICT. FRIDAY LOOKS TO BE MOSTLY SUNNY AND MILD...BUT THEN A SOMEWHAT WETTER SYSTEM WILL MOVE IN ON SATURDAY AND SPREAD RAIN CHANCES ACROSS MUCH OF OUR AREA. DRY CONDITIONS AND SEASONAL TEMPERATURES ARE PROJECTED FOR SUNDAY INTO THE FIRST PART OF NEXT WEEK. [DISC]
Discussion. As of 8:45pm PDT Wednesday… temperatures were for the most part a little milder and a few degrees above normal across the area today. The only exception to this was over the South Bay where average temperatures were 1–4°F below normal today. Highs ranged from the mid 60s coast to the 70s inland. ∨
.DISCUSSION...AS OF 8:45 PM PDT WEDNESDAY...TEMPERATURES WERE FOR THE MOST PART A LITTLE MILDER AND A FEW DEGREES ABOVE NORMAL ACROSS THE AREA TODAY. THE ONLY EXCEPTION TO THIS WAS OVER THE SOUTH BAY WHERE AVERAGE TEMPERATURES WERE 1 TO 4 DEGREES FAHRENHEIT BELOW NORMAL TODAY. HIGHS RANGED FROM THE MID 60S COAST TO THE 70S INLAND. [DISC]
Mid to high clouds are drifting in over the Bay Area from the northwest. Winds are mostly light this evening and the rain is still situated well to the north of the Bay Area. Believe it or not shortwave ridging is still in place this evening over the forecast area with 500 mb heights over the South Bay still hovering up near 580 decameters. 500 mb heights will briefly subside a little around 5am Thursday morning then bump upwards again to where they are now by Thursday afternoon. The incoming frontal system will be on the weak side with only very light amounts of rain expected… mostly over the northern Sonoma County coast where up to 1/3" total of rain could fall late tonight and Thursday morning. The newest 00Z [5pm PDT] NAM [North American Meso (formerly Eta) model] model does not predict any measurable rainfall anywhere else over the area tonight or Thursday. Dewpoint temperatures are solidly higher this evening area-wide so periodic light drizzle or light rain is still possible to the south of Sonoma County as the weakening shortwave trough squeezes out a little bit of moisture as it moves over the area. A few areas may see a tiny amount of measurable rainfall overnight… up to a couple hundredths possibly. The near term fits perfectly as a low POP [Probability of Precipitation] low QPF [Quantitative Precipitation Forecast] forecast. Not planning on making any adjustments to first or second period POP [Probability of Precipitation] forecasts. ∨
MID TO HIGH CLOUDS ARE DRIFTING IN OVER THE BAY AREA FROM THE NORTHWEST. WINDS ARE MOSTLY LIGHT THIS EVENING AND THE RAIN IS STILL SITUATED WELL TO THE NORTH OF THE BAY AREA. BELIEVE IT OR NOT SHORTWAVE RIDGING IS STILL IN PLACE THIS EVENING OVER THE FORECAST AREA WITH 500 MB HEIGHTS OVER THE SOUTH BAY STILL HOVERING UP NEAR 580 DECAMETERS. 500 MB HEIGHTS WILL BRIEFLY SUBSIDE A LITTLE AROUND 5 AM THURSDAY MORNING THEN BUMP UPWARDS AGAIN TO WHERE THEY ARE NOW BY THURSDAY AFTERNOON. THE INCOMING FRONTAL SYSTEM WILL BE ON THE WEAK SIDE WITH ONLY VERY LIGHT AMOUNTS OF RAIN EXPECTED...MOSTLY OVER THE NORTHERN SONOMA COUNTY COAST WHERE UP TO 1/3" TOTAL OF RAIN COULD FALL LATE TONIGHT AND THURSDAY MORNING. THE NEWEST 00Z NAM MODEL DOES NOT PREDICT ANY MEASURABLE RAINFALL ANYWHERE ELSE OVER THE AREA TONIGHT OR THURSDAY. DEWPOINT TEMPERATURES ARE SOLIDLY HIGHER THIS EVENING AREA-WIDE SO PERIODIC LIGHT DRIZZLE OR LIGHT RAIN IS STILL POSSIBLE TO THE SOUTH OF SONOMA COUNTY AS THE WEAKENING SHORTWAVE TROUGH SQUEEZES OUT A LITTLE BIT OF MOISTURE AS IT MOVES OVER THE AREA. A FEW AREAS MAY SEE A TINY AMOUNT OF MEASURABLE RAINFALL OVERNIGHT...UP TO A COUPLE HUNDREDTHS POSSIBLY. THE NEAR TERM FITS PERFECTLY AS A LOW POP LOW QPF FORECAST. NOT PLANNING ON MAKING ANY ADJUSTMENTS TO FIRST OR SECOND PERIOD POP FORECASTS. [DISC]
Once the weak trough ripples east away from the Bay Area Thursday another (more amplified and strengthening) low pressure system near 39°N [100 mi north of Montara]/134°W [630 mi west of Montara] Friday evening moves northeast with the main mid-upper level low pressure area reaching the Pacific Northwest early Saturday evening. The low pressure system will be trailing a cold front eastward which will reach northernmost California by early Saturday morning and then the Bay Area during the day Saturday. As much as 1" rainfall is possible over Sonoma… about 1/2"-3/4" Napa and Marin Counties and 2/10" or less over the remainder of the Bay Area and North Central Coast. By the time the low pressure area gets to California the strongest jet stream dynamics will have already positioned itself forward of the trough axis suggesting an acceleration of the low as well as the heaviest rainfall to the north of the Bay Area. If one is travelling to the Pacific Northwest for the weekend expect heavier steadier rains. Mid level temperatures reach about –18°C over the North Bay by late Saturday morning… not an exceptionally cold system. The 00Z [5pm PDT] NAM [North American Meso (formerly Eta) model] predicts a small pocket… Albeit just below 35… of modified total totals mid level instability by 21Z [2pm PDT] Saturday over northern Sonoma County. Often times low pressure areas swing over the area a little faster than models predict. It is quite possible that the mid level instability arrives in the North Bay Area as early as very early Saturday morning. The NAM [North American Meso (formerly Eta) model] depicts the mid level instability quickly jumping over to Napa County then moving east to the northern Sacramento Valley Saturday. Slight chance of thunderstorms was added in earlier today and see no reason to make any changes except there's a small possibility the thunderstorm activity may arrive a little earlier than presently indicated. ∨
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`S A SMALL POSSIBILITY THE T-STORM ACTIVITY MAY ARRIVE A LITTLE EARLIER THAN PRESENTLY INDICATED. [DISC]
There's much less confidence regarding next week's weather as the models diverge on solutions. For instance the ECWMF and 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.] models want to bring rain to the area by Halloween but the GEM [Global Environmental Multiscale model] model is dry. Way too early to say with any confidence if it'll be wet or dry for trick or treaters. ∨
THERE`S MUCH LESS CONFIDENCE REGARDING NEXT WEEK`S WEATHER AS THE MODELS DIVERGE ON SOLUTIONS. FOR INSTANCE THE ECWMF AND GFS MODELS WANT TO BRING RAIN TO THE AREA BY HALLOWEEN BUT THE GEM MODEL IS DRY. WAY TOO EARLY TO SAY WITH ANY CONFIDENCE IF IT`LL BE WET OR DRY FOR TRICK OR TREATERS. [DISC]
Previous discussion… as of 3:05pm PDT Wednesday. Skies are clear to partly cloudy with mid afternoon temperatures around the bays generally running in the upper 60s to mid 70s… a couple of degrees warmer than at the same time yesterday. Tonight… the weak tail end of a Pacific weather system now moving into the Pacific Northwest will spread in increasing mid and high cloudiness over more northern portions of our area. Then on Thursday… model solutions indicate a likelihood of some rain south to near Bodega Bay… with a slight chance as far south as about San Francisco. Rainfall totals look to be quite light… except in the far northern coastal hills of Sonoma County where local amounts up to the 1/3–1/2 inch range are possible. ∨
.PREVIOUS DISCUSSION...AS OF 3:05 PM PDT WEDNESDAY...SKIES ARE CLEAR TO PARTLY CLOUDY WITH MID AFTERNOON TEMPERATURES AROUND THE BAYS GENERALLY RUNNING IN THE UPPER 60S TO MID 70S...A COUPLE OF DEGS WARMER THAN AT THE SAME TIME YESTERDAY. TONIGHT...THE WEAK TAIL END OF A PACIFIC WEATHER SYSTEM NOW MOVING INTO THE PACIFIC NORTHWEST WILL SPREAD IN INCREASING MID AND HIGH CLOUDINESS OVER MORE NORTHERN PORTIONS OF OUR AREA. THEN ON THURSDAY...MODEL SOLUTIONS INDICATE A LIKELIHOOD OF SOME RAIN SOUTH TO NEAR BODEGA BAY...WITH A SLIGHT CHANCE AS FAR SOUTH AS ABOUT SAN FRANCISCO. RAINFALL TOTALS LOOK TO BE QUITE LIGHT...EXCEPT IN THE FAR NORTHERN COASTAL HILLS OF SONOMA COUNTY WHERE LOCAL AMOUNTS UP TO THE 1/3 TO 1/2 INCH RANGE ARE POSSIBLE. [DISC]
Any residual North Bay showers should taper off and end Thursday night with dry conditions then expected district-wide on Friday as a short wave upper level ridge temporarily builds in westward from the interior. By late Friday night however… the next Pacific weather system is expected to rapidly approach from the northwest. Latest model output continues to indicate it will be both a bit stronger and wetter than the one on Thursday… with a likelihood of at least some light precipitation spreading as far south as the Monterey Bay region by Saturday afternoon. Should also note that both latest NAM [North American Meso (formerly Eta) model] deterministic model output soundings and surface based 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.] values and SREF [Short-Range Ensemble Forecasting system] MUCAPE [Most Unstable Convective Available Potential Energy, a measure of instability in the troposphere] plumes… indicate a sufficient possibility of convection Saturday afternoon over the North Bay to warrant inclusion in the forecast of mention of a slight chance of thunderstorms. ∨
ANY RESIDUAL NORTH BAY SHOWERS SHOULD TAPER OFF AND END THURSDAY NIGHT WITH DRY CONDITIONS THEN EXPECTED DISTRICT-WIDE ON FRIDAY AS A SHORT WAVE UPPER LEVEL RIDGE TEMPORARILY BUILDS IN WESTWARD FROM THE INTERIOR. BY LATE FRIDAY NIGHT HOWEVER...THE NEXT PACIFIC WEATHER SYSTEM IS EXPECTED TO RAPIDLY APPROACH FROM THE NORTHWEST. LATEST MODEL OUTPUT CONTINUES TO INDICATE IT WILL BE BOTH A BIT STRONGER AND WETTER THAN THE ONE ON THURSDAY...WITH A LIKELIHOOD OF AT LEAST SOME LIGHT PRECIP SPREADING AS FAR SOUTH AS THE MONTEREY BAY REGION BY SATURDAY AFTERNOON. SHOULD ALSO NOTE THAT BOTH LATEST NAM DETERMINISTIC MODEL OUTPUT SOUNDINGS AND SURFACE BASED CAPE VALUES...AND SREF MUCAPE PLUMES...INDICATE A SUFFICIENT POSSIBILITY OF CONVECTION SATURDAY AFTERNOON OVER THE NORTH BAY TO WARRANT INCLUSION IN THE FORECAST OF MENTION OF A SLIGHT CHANCE OF THUNDERSTORMS. [DISC]
Here again post frontal showers look to taper off and end pretty quickly… with dry weather conditions returning across all but possibly the easternmost margin of our area by Sunday morning. Dry conditions and seasonal temperatures are expected across the district through at least Monday night. Thereafter longer range model solutions begin to diverge… with for example the 12Z [5am PDT] 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.] bringing in another weak system and light precipitation to the North Bay on Tuesday while the ECMWF [European Center for Medium-Range Weather Forecasts model] confines associated rainfall to areas north of our CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map]. ∨
HERE AGAIN POST FRONTAL SHOWERS LOOK TO TAPER OFF AND END PRETTY QUICKLY...WITH DRY WEATHER CONDITIONS RETURNING ACROSS ALL BUT POSSIBLY THE EASTERNMOST MARGIN OF OUR AREA BY SUNDAY MORNING. DRY CONDITIONS AND SEASONAL TEMPERATURES ARE EXPECTED ACROSS THE DISTRICT THROUGH AT LEAST MONDAY NIGHT. THEREAFTER LONGER RANGE MODEL SOLUTIONS BEGIN TO DIVERGE...WITH FOR EXAMPLE THE 12Z GFS BRINGING IN ANOTHER WEAK SYSTEM AND LIGHT PRECIP TO THE NORTH BAY ON TUESDAY WHILE THE ECMWF CONFINES ASSOCIATED RAINFALL TO AREAS NORTH OF OUR CWA. [DISC]
Aviation. As of 10:30pm PDT Wednesday… VFR [Visual Flight Rules] conditions will persist through late tonight with high clouds continue to streaming across the region. A frontal boundary is forecast to approach the North Bay tonight and should help increase the amount of low level cloud cover… especially over the San Francisco Bay Area northward. Light rain or drizzle will be possible over KSTS [Santa Rosa CA]… yet confidence remains low and have left precipitation out of the TAF [Terminal Aerodrome Forecast] for now. Low clouds that do develop over the region will be slow to burn-off Thursday morning. Further south… confidence is lower on the return of stratus over the terminals as boundary layer moisture will be slower to return. ∨
.AVIATION...AS OF 10:30 PM PDT WEDNESDAY...VFR CONDITIONS WILL PERSIST THROUGH LATE TONIGHT WITH HIGH CLOUDS CONTINUE TO STREAMING ACROSS THE REGION. A FRONTAL BOUNDARY IS FORECAST TO APPROACH THE NORTH BAY TONIGHT AND SHOULD HELP INCREASE THE AMOUNT OF LOW LEVEL CLOUD COVER...ESPECIALLY OVER THE SAN FRANCISCO BAY AREA NORTHWARD. LIGHT RAIN OR DRIZZLE WILL BE POSSIBLE OVER KSTS...YET CONFIDENCE REMAINS LOW AND HAVE LEFT PRECIPITATION OUT OF THE TAF FOR NOW. LOW CLOUDS THAT DO DEVELOP OVER THE REGION WILL BE SLOW TO BURN-OFF THURSDAY MORNING. FURTHER SOUTH...CONFIDENCE IS LOWER ON THE RETURN OF STRATUS OVER THE TERMINALS AS BOUNDARY LAYER MOISTURE WILL BE SLOWER TO RETURN. [AVIA]
Vicinity of KSFO [San Francisco CA]… VFR [Visual Flight Rules] conditions will continue through late tonight with low clouds expected to return after 09Z [2am PDT]. Light west winds will continue through tonight. Moderate confidence. ∨
VICINITY OF KSFO...VFR CONDITIONS WILL CONTINUE THROUGH LATE TONIGHT WITH LOW CLOUDS EXPECTED TO RETURN AFTER 09Z. LIGHT WEST WINDS WILL CONTINUE THROUGH TONIGHT. MODERATE CONFIDENCE. [AVI2]
KSFO [San Francisco CA] bridge approach… similar to KSFO [San Francisco CA]. ∨
KSFO BRIDGE APPROACH...SIMILAR TO KSFO. [AVI2]
Monterey Bay Area terminals… VFR [Visual Flight Rules] conditions will continue through late tonight with low clouds expected to return after 09Z [2am PDT]. Light west winds will persist becoming light and variable overnight. Moderate confidence. ∨
MONTEREY BAY AREA TERMINALS...VFR CONDITIONS WILL CONTINUE THROUGH LATE TONIGHT WITH LOW CLOUDS EXPECTED TO RETURN AFTER 09Z. LIGHT WEST WINDS WILL PERSIST BECOMING LIGHT AND VARIABLE OVERNIGHT. MODERATE CONFIDENCE. [AVI2]
Marine. As of 8:51pm PDT Wednesday… generally light northwest winds will persist across the coastal waters through Friday… with exception to the coastal jet south of Point Sur [Big Sur CA] where moderate winds will persist through Thursday night. Wind speeds will then increase and become southerly Friday night into Saturday ahead of an approaching cold front that will also bring chances of rain to the region. Northwest swell will slowly diminish through Friday then increase over the weekend. ∨
.MARINE...AS OF 8:51 PM PDT WEDNESDAY...GENERALLY LIGHT NORTHWEST WINDS WILL PERSIST ACROSS THE COASTAL WATERS THROUGH FRIDAY...WITH EXCEPTION TO THE COASTAL JET SOUTH OF POINT SUR WHERE MODERATE WINDS WILL PERSIST THROUGH THURSDAY NIGHT. WIND SPEEDS WILL THEN INCREASE AND BECOME SOUTHERLY FRIDAY NIGHT INTO SATURDAY AHEAD OF AN APPROACHING COLD FRONT THAT WILL ALSO BRING CHANCES OF RAIN TO THE REGION. NORTHWEST SWELL WILL SLOWLY DIMINISH THROUGH FRIDAY THEN INCREASE OVER THE WEEKEND. [AVIA]
.MTR WATCHES/WARNINGS/ADVISORIES... .TNGT...SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM [WARN]
Public Forecast: CW; Aviation/Marine: CW ∨
PUBLIC FORECAST: CW AVIATION/MARINE: CW [CRED]
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8:05pm PDT Oct 22 2014
Overnight. Mostly cloudy, with a low around 58°. Calm wind.
Thursday. A 20% chance of rain after 11am. Partly sunny, with a high near 66°. Calm wind.
Thursday Night. Mostly cloudy, with a low around 58°. Calm wind becoming ENE 5 to 8 mph after midnight.
Friday. Mostly sunny, with a high near 69°. ENE wind 6 to 11 mph becoming SSE in the morning.
Friday Night. A 20% chance of rain after 11pm. Partly cloudy, with a low around 57°. South wind 8 to 16 mph, with gusts as high as 20 mph.
Saturday. Showers likely, mainly after 11am. Partly sunny, with a high near 62°. Chance of precipitation is 60%.
Saturday Night. A 40% chance of showers. Partly cloudy, with a low around 55°.
Sunday. Mostly sunny, with a high near 61°.
Sunday Night. Mostly clear, with a low around 55°.
Monday. Sunny, with a high near 62°.
Monday Night. Partly cloudy, with a low around 56°.
Tuesday. Mostly sunny, with a high near 63°.
Tuesday Night. A slight chance of rain. Mostly cloudy, with a low around 56°.
Wednesday. Mostly sunny, with a high near 64°.