Area Forecast Discussion
FXUS66 KMTR [Monterey CA] 261235 AFDMTR ∨
FXUS66 KMTR 261235 AFDMTR [HEAD]
National Weather Service San Francisco Bay Area 4:35am PST Fri Dec 26 2014 ∨
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA 435 AM PST FRI DEC 26 2014 [HEAD]
Much cooler temperatures expected tonight with patchy frost in the valleys. ∨
...MUCH COOLER TEMPERATURES EXPECTED TONIGHT WITH PATCHY FROST IN THE VALLEYS... [DISC]
Synopsis. Winds are preventing temperatures at many locations from falling as low as previously anticipated for early this morning. However… much calmer conditions continue to be projected for tonight and Friday morning. Therefore still expect most places will then experience their coldest temperatures so far this season. Overnight lows in the inland valleys will fall to near or locally even a bit below freezing… along with some late night and morning frost. ∨
.SYNOPSIS...WINDS ARE PREVENTING TEMPERATURES AT MANY LOCATIONS FROM FALLING AS LOW AS PREVIOUSLY ANTICIPATED FOR EARLY THIS MORNING. HOWEVER...MUCH CALMER CONDITIONS CONTINUE TO BE PROJECTED FOR TONIGHT AND FRIDAY MORNING. THEREFORE STILL EXPECT MOST PLACES WILL THEN EXPERIENCE THEIR COLDEST TEMPERATURES SO FAR THIS SEASON. OVERNIGHT LOWS IN THE INLAND VALLEYS WILL FALL TO NEAR OR LOCALLY EVEN A BIT BELOW FREEZING...ALONG WITH SOME LATE NIGHT AND MORNING FROST. [DISC]
Otherwise… dry weather is expected to persist into the beginning of next week… but with latest model output now indicating possible cool and unsettled conditions in the Tuesday through Wednesday time frame. ∨
OTHERWISE...DRY WEATHER IS EXPECTED TO PERSIST INTO THE BEGINNING OF NEXT WEEK...BUT WITH LATEST MODEL OUTPUT NOW INDICATING POSSIBLE COOL AND UNSETTLED CONDITIONS IN THE TUESDAY THROUGH WEDNESDAY TIME FRAME. [DISC]
Discussion. As of 3:30am PST Friday… temperatures at many locations remain milder than previously anticipated as model projections of widespread calming of boundary layer winds during the overnight hours has thus far failed to materialize. In fact… 2 am temperatures at KSFO [San Francisco CA] and KAPC [Napa CA] are running 2 and 4° warmer than at the same time Christmas morning… at 53° and 43° respectively (and with a wind speed of 15 kt [17 mph] at KSFO [San Francisco CA] and 10 kt [12 mph] at KAPC [Napa CA]). On the other hand… there are some places where the winds have died down and temperatures at these locations have gotten quite a bit chillier. The Bradley RAWS [Remote Automated Weather Station] site in the southern Salinas Valley for example reported a 2:12am temperature of 31°… 12° cooler than 24 hours earlier. In the just-issued forecast package have updated temperature grids to bring them more in-line with latest observations. ∨
.DISCUSSION...AS OF 3:30 AM PST FRIDAY...TEMPERATURES AT MANY LOCATIONS REMAIN MILDER THAN PREVIOUSLY ANTICIPATED AS MODEL PROJECTIONS OF WIDESPREAD CALMING OF BOUNDARY LAYER WINDS DURING THE OVERNIGHT HOURS HAS THUS FAR FAILED TO MATERIALIZE. IN FACT...2 AM TEMPERATURES AT KSFO AND KAPC ARE RUNNING 2 AND 4 DEGS WARMER THAN AT THE SAME TIME CHRISTMAS MORNING...AT 53 DEG AND 43 DEG RESPECTIVELY (AND WITH A WIND SPEED OF 15 KT AT KSFO AND 10 KT AT KAPC). ON THE OTHER HAND...THERE ARE SOME PLACES WHERE THE WINDS HAVE DIED DOWN...AND TEMPERATURES AT THESE LOCATIONS HAVE GOTTEN QUITE A BIT CHILLIER. THE BRADLEY RAWS SITE IN THE SOUTHERN SALINAS VALLEY FOR EXAMPLE REPORTED A 2:12 AM TEMPERATURE OF 31 DEG...12 DEG COOLER THAN 24 HOURS EARLIER. IN THE JUST-ISSUED FORECAST PACKAGE HAVE UPDATED TEMPERATURE GRIDS TO BRING THEM MORE IN-LINE WITH LATEST OBSERVATIONS. [DISC]
Today continues to look to be much like Thursday… with sunny conditions prevailing districtwide and afternoon highs generally in the mid to upper 50s… but less wind. And with these significantly calmer conditions… in conjunction with the continuing cool and dry nature of the air mass and one of the longest nights of the year… the setup should then be ideal for strong radiational cooling tonight. This is expected to be the coldest night of the season thus far at many locations… with the chillier inland valleys seeing temperatures dip down to near 30° and locally even into the upper 20s. Patchy frost is likely in these interior areas late tonight and early Saturday morning and there may even be some patchy frost in low lying areas near the coast and bays. These cold temperatures are not expected to persist long enough… or cover a large enough area… to warrant Freeze Warnings. However… the hazardous weather outlook will be continued in order to highlight this cold weather situation. ∨
TODAY CONTINUES TO LOOK TO BE MUCH LIKE THURSDAY...WITH SUNNY CONDITIONS PREVAILING DISTRICTWIDE AND AFTERNOON HIGHS GENERALLY IN THE MID TO UPPER 50S...BUT LESS WIND. AND WITH THESE SIGNIFICANTLY CALMER CONDITIONS...IN CONJUNCTION WITH THE CONTINUING COOL AND DRY NATURE OF THE AIR MASS AND ONE OF THE LONGEST NIGHTS OF THE YEAR...THE SETUP SHOULD THEN BE IDEAL FOR STRONG RADIATIONAL COOLING TONIGHT. THIS IS EXPECTED TO BE THE COLDEST NIGHT OF THE SEASON THUS FAR AT MANY LOCATIONS...WITH THE CHILLIER INLAND VALLEYS SEEING TEMPERATURES DIP DOWN TO NEAR 30 DEG...AND LOCALLY EVEN INTO THE UPPER 20S. PATCHY FROST IS LIKELY IN THESE INTERIOR AREAS LATE TONIGHT AND EARLY SATURDAY MORNING...AND THERE MAY EVEN BE SOME PATCHY FROST IN LOW LYING AREAS NEAR THE COAST AND BAYS. THESE COLD TEMPERATURES ARE NOT EXPECTED TO PERSIST LONG ENOUGH...OR COVER A LARGE ENOUGH AREA...TO WARRANT FREEZE WARNINGS. HOWEVER...THE HAZARDOUS WEATHER OUTLOOK WILL BE CONTINUED IN ORDER TO HIGHLIGHT THIS COLD WEATHER SITUATION. [DISC]
The airmass over our area will gradually moderate a bit as the weekend progresses and result in not quite as chilly nighttime temperatures Saturday night and Sunday night. Latest model output also indicates the tail end of a weak upper level disturbance projected to propagate through northeastern California in the Saturday night to Sunday morning time frame could result in a bit of cloudiness spilling into our area… potentially further mitigating overnight cooling. Nonetheless… some patchy frost can again be expected in the coldest inland valley locations. ∨
THE AIRMASS OVER OUR AREA WILL GRADUALLY MODERATE A BIT AS THE WEEKEND PROGRESSES...AND RESULT IN NOT QUITE AS CHILLY NIGHTTIME TEMPERATURES SATURDAY NIGHT AND SUNDAY NIGHT. LATEST MODEL OUTPUT ALSO INDICATES THE TAIL END OF A WEAK UPPER LEVEL DISTURBANCE PROJECTED TO PROPAGATE THROUGH NORTHEASTERN CALIFORNIA IN THE SATURDAY NIGHT TO SUNDAY MORNING TIME FRAME COULD RESULT IN A BIT OF CLOUDINESS SPILLING INTO OUR AREA...POTENTIALLY FURTHER MITIGATING OVERNIGHT COOLING. NONETHELESS...SOME PATCHY FROST CAN AGAIN BE EXPECTED IN THE COLDEST INLAND VALLEY LOCATIONS. [DISC]
In the longer range. The picture is now getting a bit more complicated. Basic model agreement continues in maintaining a high amplitude longwave upper level ridge off the West Coast. But especially the latest ECMWF [European Center for Medium-Range Weather Forecasts model] deterministic output continues to trend in the direction of a more westward displaced trajectory for a much stronger shortwave trough propagating southward from British Columbia early in the week and ultimately resulting in a cold-core upper level low drifting southward down the central California coast in the Tuesday to Wednesday time frame. Should this prove out… the net result for us would be cold and potentially showery conditions with snow levels falling down into the higher hills. Given both corresponding ECMWF [European Center for Medium-Range Weather Forecasts model] MOS [Model Output Statistics] POPs [Probability of Precipitation] for sites in our district now broadly up into the 20–40% range and that the latest UKMET [A medium-range (3-7 day) numerical weather prediction model operated by the UK Meteorological Agency] model deterministic solution is basically similar… have gone ahead and introduced shower chances and snow shower chances above 2000–3000 feet in the hills. Have refrained from going with higher POPs [Probability of Precipitation] both because the 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.] continues to project a much more inland path for this upper level disturbance and thus dry conditions across our district throughout and to try to maintain consistency with neighboring offices which are generally going with lower POPs [Probability of Precipitation]. But do want to lean a bit in the direction of the ECMWF [European Center for Medium-Range Weather Forecasts model] (and UKMET [A medium-range (3-7 day) numerical weather prediction model operated by the UK Meteorological Agency]) solution noting that this is exactly the sort of situation in which higher model resolution can really pay-off. ∨
IN THE LONGER RANGE...THE PICTURE IS NOW GETTING A BIT MORE COMPLICATED. BASIC MODEL AGREEMENT CONTINUES IN MAINTAINING A HIGH AMPLITUDE LONGWAVE UPPER LEVEL RIDGE OFF THE WEST COAST. BUT ESPECIALLY THE LATEST ECMWF DETERMINISTIC OUTPUT CONTINUES TO TREND IN THE DIRECTION OF A MORE WESTWARD DISPLACED TRAJECTORY FOR A MUCH STRONGER SHORTWAVE TROUGH PROPAGATING SOUTHWARD FROM BRITISH COLUMBIA EARLY IN THE WEEK...AND ULTIMATELY RESULTING IN A COLD-CORE UPPER LEVEL LOW DRIFTING SOUTHWARD DOWN THE CENTRAL CALIFORNIA COAST IN THE TUESDAY TO WEDNESDAY TIME FRAME. SHOULD THIS PROVE OUT...THE NET RESULT FOR US WOULD BE COLD AND POTENTIALLY SHOWERY CONDITIONS WITH SNOW LEVELS FALLING DOWN INTO THE HIGHER HILLS. GIVEN BOTH CORRESPONDING ECMWF MOS POPS FOR SITES IN OUR DISTRICT NOW BROADLY UP INTO THE 20 TO 40 PERCENT RANGE...AND THAT THE LATEST UKMET MODEL DETERMINISTIC SOLUTION IS BASICALLY SIMILAR...HAVE GONE AHEAD AND INTRODUCED SHOWER CHANCES...AND SNOW SHOWER CHANCES ABOVE 2000 TO 3000 FEET IN THE HILLS. HAVE REFRAINED FROM GOING WITH HIGHER POPS BOTH BECAUSE THE GFS CONTINUES TO PROJECT A MUCH MORE INLAND PATH FOR THIS UPPER LEVEL DISTURBANCE AND THUS DRY CONDITIONS ACROSS OUR DISTRICT THROUGHOUT...AND TO TRY TO MAINTAIN CONSISTENCY WITH NEIGHBORING OFFICES WHICH ARE GENERALLY GOING WITH LOWER POPS. BUT DO WANT TO LEAN A BIT IN THE DIRECTION OF THE ECMWF (AND UKMET) SOLUTION NOTING THAT THIS IS EXACTLY THE SORT OF SITUATION IN WHICH HIGHER MODEL RESOLUTION CAN REALLY PAY-OFF. [DISC]
Further into the extended… it gets murkier as model solutions increasingly diverge. But there does continue to be general consistency with the basic idea of maintaining a high-amplitude upper level ridge axis not far off the West Coast. ∨
FURTHER INTO THE EXTENDED...IT GETS MURKIER AS MODEL SOLUTIONS INCREASINGLY DIVERGE. BUT THERE DOES CONTINUE TO BE GENERAL CONSISTENCY WITH THE BASIC IDEA OF MAINTAINING A HIGH-AMPLITUDE UPPER LEVEL RIDGE AXIS NOT FAR OFF THE WEST COAST. [DISC]
Aviation. As of 4:30am PST Friday… a dry northerly wind flow will continue for the period leading to VFR [Visual Flight Rules] conditions. The ACV [Arcata/Eureka CA]-SFO [San Francisco CA] pressure gradient is almost 6 mb… in-line with the NAM [North American Meso (formerly Eta) model] model forecasts for today. Nocturnal de-coupling of the surface winds is mostly found in the normally wind protected Bay Area Valleys while hill top/mountain top winds are still fairly breezy with some instances of surface wind to wind aloft linkage continuing in and around the Bay Area. For example the NW-NNW winds at KSFO [San Francisco CA] have been slowly increasing since late last evening to a presently sustained 20 knots [23 mph]. Vertical wind profiles not quite supportive of non-convective low level wind shear but will monitor through day break. ∨
.AVIATION...AS OF 4:30 AM PST FRIDAY...A DRY NORTHERLY WIND FLOW WILL CONTINUE FOR THE PERIOD LEADING TO VFR CONDITIONS. THE ACV- SFO PRESSURE GRADIENT IS ALMOST 6 MB...IN-LINE WITH THE NAM MODEL FORECASTS FOR TODAY. NOCTURNAL DE-COUPLING OF THE SURFACE WINDS IS MOSTLY FOUND IN THE NORMALLY WIND PROTECTED BAY AREA VALLEYS WHILE HILL TOP/MOUNTAIN TOP WINDS ARE STILL FAIRLY BREEZY WITH SOME INSTANCES OF SURFACE WIND TO WIND ALOFT LINKAGE CONTINUING IN AND AROUND THE BAY AREA. FOR EXAMPLE THE NW-NNW WINDS AT KSFO HAVE BEEN SLOWLY INCREASING SINCE LATE LAST EVENING TO A PRESENTLY SUSTAINED 20 KNOTS. VERTICAL WIND PROFILES NOT QUITE SUPPORTIVE OF NON-CONVECTIVE LOW LEVEL WIND SHEAR BUT WILL MONITOR THROUGH DAY BREAK. [AVIA]
Vicinity of KSFO [San Francisco CA]… VFR [Visual Flight Rules]. ∨
VICINITY OF KSFO...VFR. [AVI2]
KSFO [San Francisco CA] bridge approach… VFR [Visual Flight Rules]. ∨
KSFO BRIDGE APPROACH...VFR. [AVI2]
Monterey Bay Area terminals… VFR [Visual Flight Rules]. ∨
MONTEREY BAY AREA TERMINALS...VFR. [AVI2]
Marine. As of 4:13am PST Friday… gusty north to northwest winds will continue across the northern outer coastal waters. Elsewhere winds will gradually diminish as the gradient relaxes through the day. Moderate northwest swell will gradually diminish through the weekend and into early next week. The weather will remain dry through the weekend followed by a chance for showers by Monday and Tuesday of next week. ∨
.MARINE...AS OF 4:13 AM PST FRIDAY...GUSTY NORTH TO NORTHWEST WINDS WILL CONTINUE ACROSS THE NORTHERN OUTER COASTAL WATERS. ELSEWHERE WINDS WILL GRADUALLY DIMINISH AS THE GRADIENT RELAXES THROUGH THE DAY. MODERATE NORTHWEST SWELL WILL GRADUALLY DIMINISH THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK. THE WEATHER WILL REMAIN DRY THROUGH THE WEEKEND FOLLOWED BY A CHANCE FOR SHOWERS BY MONDAY AND TUESDAY OF NEXT WEEK. [AVIA]
.MTR WATCHES/WARNINGS/ADVISORIES... .TDAY...SCA...PT ARENA TO PT REYES 10-60 NM SCA...PT REYES TO PIGEON PT 10-60 NM [WARN]
Public Forecast: Blier; Aviation/Marine: Canepa ∨
PUBLIC FORECAST: BLIER AVIATION/MARINE: CANEPA [CRED]
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3:02am PST Dec 26 2014
Today. Sunny, with a temperature rising to near 54 by noon, then falling to around 45 during the remainder of the day. North wind 13 to 16 mph, with gusts as high as 21 mph.
Tonight. Clear, with a low around 41°. NE wind 11 to 14 mph.
Saturday. Sunny, with a high near 54°. ENE wind 5 to 14 mph.
Saturday Night. Partly cloudy, with a low around 44°. Light NE wind.
Sunday. Mostly sunny, with a high near 54°. NE wind 6 to 9 mph.
Sunday Night. Mostly clear, with a low around 45°.
Monday. Sunny, with a high near 52°.
Monday Night. A slight chance of showers. Partly cloudy, with a low around 42°.
Tuesday. A slight chance of showers. Mostly sunny, with a high near 51°. Breezy.
Tuesday Night. A slight chance of showers. Partly cloudy, with a low around 41°. Breezy.
Wednesday. Mostly sunny, with a high near 52°.
Wednesday Night. Mostly clear, with a low around 43°.
New Year's Day. Sunny, with a high near 55°.