Weather

Link to Local Radar Data

Radar

Link to Satellite Data

Satellite

National Digital Forecast Database Maximum Temperature Forecast

Max Temps

National Digital Forecast Database Weather Element Forecast

Graphical Forecast

Area Forecast Discussion

FXUS66 KMTR [Monterey CA] 201245 AFDMTR 

FXUS66 KMTR 201245
AFDMTR [HEAD]

National Weather Service San Francisco Bay Area 4:40am PST Sat Jan 20 2018 

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
440 AM PST SAT JAN 20 2018 [HEAD]

Synopsis. Dry and cool weather is forecast for today. The next system is expected to spread rain across primarily the northern portion of the forecast area from late Sunday through Sunday night. Rain could be heavy at times in the North Bay. The weather pattern will remain active into next week, with more rain likely by midweek. 

.SYNOPSIS...DRY AND COOL WEATHER IS FORECAST FOR TODAY. THE NEXT
SYSTEM IS EXPECTED TO SPREAD RAIN ACROSS PRIMARILY THE NORTHERN
PORTION OF THE FORECAST AREA FROM LATE SUNDAY THROUGH SUNDAY NIGHT.
RAIN COULD BE HEAVY AT TIMES IN THE NORTH BAY. THE WEATHER PATTERN
WILL REMAIN ACTIVE INTO NEXT WEEK, WITH MORE RAIN LIKELY BY MIDWEEK. [DISC]

Discussion. As of 4:37am PST Saturday… a few isolated showers have passed by south of Point Sur [Big Sur CA] early this morning. Elsewhere skies have mostly cleared and temperatures are cooling back into the mid 30s to mid 40s this morning. Air temperatures are within a few degrees of dewpoint temperatures thus there are a few places with patchy low clouds, but residual, carry-over mixing from Friday and no significant return of near surface temperature inversions should help prevent fog from becoming more than patchy til a little after sunrise. Very localized frost is a possibility in the North Bay valleys this morning. With sunny skies we'll have dry and cool weather today. 

.DISCUSSION...AS OF 4:37 AM PST SATURDAY...A FEW ISOLATED SHOWERS
HAVE PASSED BY SOUTH OF POINT SUR EARLY THIS MORNING. ELSEWHERE
SKIES HAVE MOSTLY CLEARED AND TEMPERATURES ARE COOLING BACK INTO
THE MID 30S TO MID 40S THIS MORNING. AIR TEMPERATURES ARE WITHIN A
FEW DEGREES OF DEWPOINT TEMPERATURES THUS THERE ARE A FEW PLACES
WITH PATCHY LOW CLOUDS, BUT RESIDUAL, CARRY-OVER MIXING FROM
FRIDAY AND NO SIGNIFICANT RETURN OF NEAR SURFACE TEMPERATURE
INVERSIONS SHOULD HELP PREVENT FOG FROM BECOMING MORE THAN PATCHY
TIL A LITTLE AFTER SUNRISE. VERY LOCALIZED FROST IS A POSSIBILITY
IN THE NORTH BAY VALLEYS THIS MORNING. WITH SUNNY SKIES WE`LL HAVE
DRY AND COOL WEATHER TODAY. [DISC]

Mid to upper level troughing with 500 mb temps near –33°C produced decent lapse rates [The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.], convective towers and showers on Friday, a few reports came in indicating mainly small hail. A fair amount of convective overturning took place over the coastal waters and land Friday. Instability profiles were conducive to isolated echo tops up to ~ 25 thousand feet (in Friday morning's discussion I incorrectly stated tops could reach 50 thousand feet). Helping to propel our weather toward drier conditions today will be a low amplitude short wave ridge bringing a return to stability today. 

MID TO UPPER LEVEL TROUGHING WITH 500 MB TEMPS NEAR -33C PRODUCED
DECENT LAPSE RATES, CONVECTIVE TOWERS AND SHOWERS ON FRIDAY, A FEW
REPORTS CAME IN INDICATING MAINLY SMALL HAIL. A FAIR AMOUNT OF
CONVECTIVE OVERTURNING TOOK PLACE OVER THE COASTAL WATERS AND LAND
FRIDAY. INSTABILITY PROFILES WERE CONDUCIVE TO ISOLATED ECHO TOPS
UP TO ~ 25 THOUSAND FEET (IN FRIDAY MORNING`S DISCUSSION I INCORRECTLY
STATED TOPS COULD REACH 50 THOUSAND FEET). HELPING TO PROPEL OUR
WEATHER TOWARD DRIER CONDITIONS TODAY WILL BE A LOW AMPLITUDE SHORT
WAVE RIDGE BRINGING A RETURN TO STABILITY TODAY. [DISC]

We continue to keep a close watch on the next cold front and rain potential for later Sunday into Monday morning. This front is attached to a 990 mb low center in the Gulf of Alaska, and based on the 00Z [4pm PST] ECMWF [European Center for Medium-Range Weather Forecasts model] Theta-e [Theta-e: related to the amount of heat present in an air parcel, useful in diagnosing atmospheric instability] the front is initially moving SE approx 30 mph then it decelerates to half this velocity prior to reaching the Northern California coast later Sunday. As mentioned previously, the models do indicate strong warm air advection ahead of this cold front. This combination will strengthen the thermal gradient with increasing wind shear and confluence forming a well developed band of rain and wind. Offsetting some of the stronger winds from reaching the surface will be strong upward vertical motion along the frontal boundary thus for the time being it appears this frontal passage may just fall short of meeting Wind Advisory criteria. 

WE CONTINUE TO KEEP A CLOSE WATCH ON THE NEXT COLD FRONT AND RAIN
POTENTIAL FOR LATER SUNDAY INTO MONDAY MORNING. THIS FRONT IS ATTACHED
TO A 990 MB LOW CENTER IN THE GULF OF ALASKA, AND BASED ON THE 00Z
ECMWF THETA-E THE FRONT IS INITIALLY MOVING SE APPROX 30 MPH THEN
IT DECELERATES TO HALF THIS VELOCITY PRIOR TO REACHING THE
NORTHERN CALIFORNIA COAST LATER SUNDAY. AS MENTIONED PREVIOUSLY,
THE MODELS DO INDICATE STRONG WARM AIR ADVECTION AHEAD OF THIS
COLD FRONT. THIS COMBINATION WILL STRENGTHEN THE THERMAL GRADIENT
WITH INCREASING WIND SHEAR AND CONFLUENCE FORMING A WELL DEVELOPED
BAND OF RAIN AND WIND. OFFSETTING SOME OF THE STRONGER WINDS FROM
REACHING THE SURFACE WILL BE STRONG UPWARD VERTICAL MOTION ALONG
THE FRONTAL BOUNDARY THUS FOR THE TIME BEING IT APPEARS THIS FROPA
MAY JUST FALL SHORT OF MEETING WIND ADVISORY CRITERIA. [DISC]

Rain arrives in the North Bay Sunday afternoon then tapers off early Monday morning. Rain will spread north to south with hourly rain rates in the North Bay near 0.50" per hour (heavy rate) with the rain band turning into a mainly light to moderate rain band as it moves southeastward over the remaining Bay Area and North Central Coast later Sunday night into Monday morning. From a San Francisco to Livermore line and areas south of this line will likely have rain, some ponding of rain on roadways perhaps, in the Monday morning commute. Interesting to note the ECMWF [European Center for Medium-Range Weather Forecasts model] 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.] remain firmly anchored with comparatively lower rain rates and totals. After examining the WRF [Weather Research and Forecasting model], NAM [North American Meso (formerly Eta) model], and weighing in on 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.], ECMWF [European Center for Medium-Range Weather Forecasts model] output it appears for time being highest rain rates/totals will be confined to coastal Sonoma and Marin Counties with moderate to heavy rain briefly extending over to the North Bay wildfire scars late Sunday night. A Flash Flood Watch would be based primarily on the currently wet NAM [North American Meso (formerly Eta) model] output while other dynamic model output are forecasting relatively less QPF [Quantitative Precipitation Forecast]; for now it's best to wait to see if the NAM [North American Meso (formerly Eta) model] turns up or down QPF [Quantitative Precipitation Forecast] in later output. Still looks like the tail end of a weak atmospheric river will be intertwined with this frontal passage, but here too the 06Z [10pm PST] 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.] IVT [GFS Integrated Water Vapor Transport ] values have fallen lower to about 300 kg/M/s. The front may just arrive here as a purely beneficial rain producer with limited hydro (and wind) hazards which we certainly need. 

RAIN ARRIVES IN THE NORTH BAY SUNDAY AFTERNOON THEN TAPERS OFF EARLY
MONDAY MORNING. RAIN WILL SPREAD NORTH TO SOUTH WITH HOURLY RAIN
RATES IN THE NORTH BAY NEAR 0.50" PER HOUR (HEAVY RATE) WITH THE
RAIN BAND TURNING INTO A MAINLY LIGHT TO MODERATE RAIN BAND AS IT
MOVES SOUTHEASTWARD OVER THE REMAINING BAY AREA AND NORTH CENTRAL
COAST LATER SUNDAY NIGHT INTO MONDAY MORNING. FROM A SAN FRANCISCO
TO LIVERMORE LINE AND AREAS SOUTH OF THIS LINE WILL LIKELY HAVE
RAIN, SOME PONDING OF RAIN ON ROADWAYS PERHAPS, IN THE MONDAY
MORNING COMMUTE. INTERESTING TO NOTE THE ECMWF AND GFS REMAIN
FIRMLY ANCHORED WITH COMPARATIVELY LOWER RAIN RATES AND TOTALS.
AFTER EXAMINING THE WRF, NAM, AND WEIGHING IN ON THE GFS, ECMWF
OUTPUT IT APPEARS FOR TIME BEING HIGHEST RAIN RATES/TOTALS WILL BE
CONFINED TO COASTAL SONOMA AND MARIN COUNTIES WITH MODERATE TO
HEAVY RAIN BRIEFLY EXTENDING OVER TO THE NORTH BAY WILDFIRE SCARS
LATE SUNDAY NIGHT. A FLASH FLOOD WATCH WOULD BE BASED PRIMARILY ON
THE CURRENTLY WET NAM OUTPUT WHILE OTHER DYNAMIC MODEL OUTPUT ARE
FORECASTING RELATIVELY LESS QPF; FOR NOW IT`S BEST TO WAIT TO SEE
IF THE NAM TURNS UP OR DOWN QPF IN LATER OUTPUT. STILL LOOKS LIKE
THE TAIL END OF A WEAK ATMOSPHERIC RIVER WILL BE INTERTWINED WITH
THIS FROPA, BUT HERE TOO THE 06Z GFS IVT VALUES HAVE FALLEN LOWER
TO ABOUT 300 KG/M/S. THE FRONT MAY JUST ARRIVE HERE AS A PURELY
BENEFICIAL RAIN PRODUCER WITH LIMITED HYDRO (AND WIND) HAZARDS
WHICH WE CERTAINLY NEED. [DISC]

A more widespread soaking rain event arrives midday Wednesday and lasts into Thursday morning based on recent 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.] output, but here too the 00Z [4pm PST] run of the ECMWF [European Center for Medium-Range Weather Forecasts model] is comparatively less with QPF [Quantitative Precipitation Forecast]. A middle of the road approach to QPF [Quantitative Precipitation Forecast] may be good for now as there will be better dynamics, amplification within this system in a long wave trough over the West Coast. 

A MORE WIDESPREAD SOAKING RAIN EVENT ARRIVES MIDDAY WEDNESDAY AND
LASTS INTO THURSDAY MORNING BASED ON RECENT GFS OUTPUT, BUT HERE
TOO THE 00Z RUN OF THE ECMWF IS COMPARATIVELY LESS WITH QPF. A
MIDDLE OF THE ROAD APPROACH TO QPF MAY BE GOOD FOR NOW AS THERE
WILL BE BETTER DYNAMICS, AMPLIFICATION WITHIN THIS SYSTEM IN A
LONG WAVE TROUGH OVER THE WEST COAST. [DISC]

There's a possible pattern change once again by late next week if the ECMWF [European Center for Medium-Range Weather Forecasts model] verifies indicating an Omega [term used to describe vertical motion in the atmosphere] block forming over eastern Asia to Alaska. Just checking 06Z [10pm PST] 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.] 500 mb pattern during this time and it looks more disorganized than the ECMWF [European Center for Medium-Range Weather Forecasts model]'s Omega [term used to describe vertical motion in the atmosphere] block forecast, which would be good news cause an Omega [term used to describe vertical motion in the atmosphere] block would put a ridge back over California sending the storm track north for 1–2 weeks if this verifies. 

THERE`S A POSSIBLE PATTERN CHANGE ONCE AGAIN BY LATE NEXT WEEK IF
THE ECMWF VERIFIES INDICATING AN OMEGA BLOCK FORMING OVER EASTERN
ASIA TO ALASKA. JUST CHECKING 06Z GFS 500 MB PATTERN DURING THIS
TIME AND IT LOOKS MORE DISORGANIZED THAN THE ECMWF`S OMEGA BLOCK
FORECAST, WHICH WOULD BE GOOD NEWS CAUSE AN OMEGA BLOCK WOULD PUT
A RIDGE BACK OVER CA SENDING THE STORM TRACK NORTH FOR 1-2 WEEKS
IF THIS VERIFIES. [DISC]

Aviation. As of 4:40am PST Saturday for 12Z [4am PST] TAFs [Terminal Aerodrome Forecasts]. As of 12Z [4am PST], all TAF [Terminal Aerodrome Forecast] sites are reporting VFR [Visual Flight Rules]. Fog product satellite shows patchy fog/low clouds affecting parts of the Santa Clara Valley, San Benito County, and areas just south of Monterey. Difficult to say whether fog will move over KMRY [Monterey CA] or KSNS [Salinas CA] before sunrise. This will have to be monitored closely. For now, have kept VFR [Visual Flight Rules] in TAF [Terminal Aerodrome Forecast] through the morning for both sites. Otherwise and elsewhere, VFR [Visual Flight Rules] will prevail today and into tonight. Westerly winds will increase and become a tad breezy by the afternoon, between 10–15 kt [12–17 mph]. The onshore gradient will weaken by the end of the TAF [Terminal Aerodrome Forecast] period as another upper level storm system approaches from the northwest, switching winds out of the south/southwest by Sunday morning. 

.AVIATION...AS OF 4:40 AM PST SATURDAY FOR 12Z TAFS. AS OF 12Z,
ALL TAF SITES ARE REPORTING VFR. FOG PRODUCT SATELLITE SHOWS
PATCHY FOG/LOW CLOUDS AFFECTING PARTS OF THE SANTA CLARA VALLEY,
SAN BENITO COUNTY, AND AREAS JUST SOUTH OF MONTEREY. DIFFICULT TO
SAY WHETHER FOG WILL MOVE OVER KMRY OR KSNS BEFORE SUNRISE. THIS
WILL HAVE TO BE MONITORED CLOSELY. FOR NOW, HAVE KEPT VFR IN TAF
THROUGH THE MORNING FOR BOTH SITES. OTHERWISE AND ELSEWHERE, VFR
WILL PREVAIL TODAY AND INTO TONIGHT. WESTERLY WINDS WILL INCREASE
AND BECOME A TAD BREEZY BY THE AFTERNOON, BETWEEN 10-15 KT. THE
ONSHORE GRADIENT WILL WEAKEN BY THE END OF THE TAF PERIOD AS
ANOTHER UPPER LEVEL STORM SYSTEM APPROACHES FROM THE NORTHWEST,
SWITCHING WINDS OUT OF THE SOUTH/SOUTHWEST BY SUNDAY MORNING. [AVIA]

Vicinity of KSFO [San Francisco CA]VFR [Visual Flight Rules] conditions will continue today. Westerly winds increase by late morning or early afternoon, sustained around 15 kts [17 mph] through the evening. 

VICINITY OF KSFO...VFR CONDITIONS WILL CONTINUE TODAY. WESTERLY
WINDS INCREASE BY LATE MORNING OR EARLY AFTERNOON, SUSTAINED
AROUND 15 KTS THROUGH THE EVENING. [AVI2]

SFO [San Francisco CA] bridge approach… similar to KSFO [San Francisco CA]

SFO BRIDGE APPROACH...SIMILAR TO KSFO. [AVI2]

Monterey Bay terminals… VFR [Visual Flight Rules] expected through the forecast period. Generally westerly winds this afternoon around 10 kts [12 mph], turning south/southeast overnight. 

MONTEREY BAY TERMINALS...VFR EXPECTED THROUGH THE FORECAST PERIOD.
GENERALLY WESTERLY WINDS THIS AFTERNOON AROUND 10 KTS, TURNING
SOUTH/SOUTHEAST OVERNIGHT. [AVI2]

Marine. As of 2:38am PST Saturday… high pressure over the eastern Pacific will continue to produce moderate northerly winds today. As swell continues to decrease, hazardous seas will persist over the coastal waters. High pressure will drift south Sunday morning as another storm system approaches from the northwest. This will result in winds increasing and shifting westerly, then southwesterly by Sunday afternoon. Northwest swell will also increase Sunday. A cold front will pass over the waters late Sunday night or early Monday morning. 

.MARINE...AS OF 2:38 AM PST SATURDAY...HIGH PRESSURE OVER THE
EASTERN PACIFIC WILL CONTINUE TO PRODUCE MODERATE NORTHERLY WINDS
TODAY. AS SWELL CONTINUES TO DECREASE, HAZARDOUS SEAS WILL PERSIST
OVER THE COASTAL WATERS. HIGH PRESSURE WILL DRIFT SOUTH SUNDAY
MORNING AS ANOTHER STORM SYSTEM APPROACHES FROM THE NORTHWEST.
THIS WILL RESULT IN WINDS INCREASING AND SHIFTING WESTERLY, THEN
SOUTHWESTERLY BY SUNDAY AFTERNOON. NORTHWEST SWELL WILL ALSO
INCREASE SUNDAY. A COLD FRONT WILL PASS OVER THE WATERS LATE
SUNDAY NIGHT OR EARLY MONDAY MORNING. [AVIA]

MTR Watches/Warnings/Advisories

.MTR WATCHES/WARNINGS/ADVISORIES...
.TDAY...SCA...PIGEON PT TO PT PINOS 0-10 NM
SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM
SCA...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM
SCA...PT ARENA TO PT REYES 0-10 NM
SCA...PT REYES TO PIGEON PT 0-10 NM
SCA...PT ARENA TO PIGEON PT 10-60 NM
SCA...ROUGH BAR ADVISORY FOR SF BAR
SCA...MRY BAY [WARN]

Public Forecast: Canepa; Aviation: BAM; Marine: BAM 

PUBLIC FORECAST: CANEPA
AVIATION: BAM
MARINE: BAM [CRED]
VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO [LINK]

Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea 

FOLLOW US ON FACEBOOK, TWITTER, AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSBAYAREA
WWW.TWITTER.COM/NWSBAYAREA
WWW.YOUTUBE.COM/NWSBAYAREA [DISC]

Point Forecast

Half Moon Bay Airport

20 Jan 5:55am PST

Today Mostly sunny, with a high near 54°. Breezy, with a NNW wind around 24 mph, with gusts as high as 30 mph.
Tonight Mostly cloudy, with a low around 44°. Breezy, with a NW wind 15 to 23 mph, with gusts as high as 29 mph.
Sunday Rain, mainly after 4pm. The rain could be heavy at times. High near 56°. South wind 7 to 17 mph, with gusts as high as 21 mph. Chance of precipitation is 80%. New precipitation amounts between a tenth and quarter of an inch possible.
Sunday Night Rain. The rain could be heavy at times. Low around 49°. Breezy, with a south wind around 26 mph, with gusts as high as 33 mph. Chance of precipitation is 80%. New precipitation amounts between a half and three quarters of an inch possible.
Monday A 30% chance of showers, mainly before 10am. Mostly cloudy, with a high near 57°. SSW wind 15 to 18 mph becoming NW in the morning. Winds could gust as high as 23 mph. New precipitation amounts between a tenth and quarter of an inch possible.
Monday Night Mostly cloudy, with a low around 47°.
Tuesday A 30% chance of rain. Mostly cloudy, with a high near 57°.
Tuesday Night A chance of rain. Mostly cloudy, with a low around 50°.
Wednesday Rain likely. Mostly cloudy, with a high near 57°.
Wednesday Night A chance of showers. Mostly cloudy, with a low around 46°.
Thursday A slight chance of showers. Partly sunny, with a high near 54°.
Thursday Night Mostly cloudy, with a low around 44°.
Friday Partly sunny, with a high near 57°.
Today
Mostly sunny, with a high near 54. Breezy, with a north northwest wind around 24 mph, with gusts as high as 30 mph.
Tonight
Mostly cloudy, with a low around 44. Breezy, with a northwest wind 15 to 23 mph, with gusts as high as 29 mph.
Sunday
Rain, mainly after 4pm. The rain could be heavy at times. High near 56. South wind 7 to 17 mph, with gusts as high as 21 mph. Chance of precipitation is 80%. New precipitation amounts between a tenth and quarter of an inch possible.
Sunday Night
Rain. The rain could be heavy at times. Low around 49. Breezy, with a south wind around 26 mph, with gusts as high as 33 mph. Chance of precipitation is 80%. New precipitation amounts between a half and three quarters of an inch possible.
Monday
A 30 percent chance of showers, mainly before 10am. Mostly cloudy, with a high near 57. South southwest wind 15 to 18 mph becoming northwest in the morning. Winds could gust as high as 23 mph. New precipitation amounts between a tenth and quarter of an inch possible.
Monday Night
Mostly cloudy, with a low around 47.
Tuesday
A 30 percent chance of rain. Mostly cloudy, with a high near 57.
Tuesday Night
A chance of rain. Mostly cloudy, with a low around 50.
Wednesday
Rain likely. Mostly cloudy, with a high near 57.
Wednesday Night
A chance of showers. Mostly cloudy, with a low around 46.
Thursday
A slight chance of showers. Partly sunny, with a high near 54.
Thursday Night
Mostly cloudy, with a low around 44.
Friday
Partly sunny, with a high near 57.

This page is generated dynamically from the National Weather Service's NWS point forecast and Area Forecast Discussion, reformatted for readability. NWS glossary; about point forecasts. For more information, visit the Lobitos Weather Project home page. Comments and corrections are welcome: