Area Forecast Discussion
FXUS66 KMTR [Monterey CA] 180508 AFDMTR ∨
FXUS66 KMTR 180508 AFDMTR [HEAD]
National Weather Service San Francisco Bay Area 10:08pm PDT Wed Sep 17 2014 ∨
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA 1008 PM PDT WED SEP 17 2014 [HEAD]
Synopsis. A Pacific weather system will bring a chance of light rain to much of the region tonight and Thursday… mainly across the north and also in coastal areas. There is also a slight chance of thunderstorms on Thursday… mainly across the North and East Bay. Temperatures will cool below seasonal averages through the remainder of the work week. Dry conditions will return and temperatures will rebound during the upcoming weekend as low pressure moves off to the east. ∨
.SYNOPSIS...A PACIFIC WEATHER SYSTEM WILL BRING A CHANCE OF LIGHT RAIN TO MUCH OF THE REGION TONIGHT AND THURSDAY...MAINLY ACROSS THE NORTH AND ALSO IN COASTAL AREAS. THERE IS ALSO A SLIGHT CHANCE OF THUNDERSTORMS ON THURSDAY...MAINLY ACROSS THE NORTH AND EAST BAY. TEMPERATURES WILL COOL BELOW SEASONAL AVERAGES THROUGH THE REMAINDER OF THE WORK WEEK. DRY CONDITIONS WILL RETURN AND TEMPERATURES WILL REBOUND DURING THE UPCOMING WEEKEND AS LOW PRESSURE MOVES OFF TO THE EAST. [DISC]
Discussion. As of 9:15pm PDT Wednesday… an upper trough is located off the Northern California coast along 130°W [410 mi west of Montara] this evening. An associated cold front swept into northwest California late this afternoon and early evening… producing widespread rainfall north of Point Arena [Mendocino CA] and even scattered thunderstorms near Eureka. The front has since weakened quite a bit… but light rain has made it as far south as occidental and Santa Rosa within the past hour. The 00Z [5pm PDT] NAM [North American Meso (formerly Eta) model] is consistent with previous NAM [North American Meso (formerly Eta) model] output in spreading light rain across most of the North Bay overnight… as well as into all of our coastal areas by late tonight. Current forecast looks on track except have taken out mention of thunderstorms for tonight as it appears there is no longer any convection along the front and the models show very little instability for the remainder of tonight. ∨
.DISCUSSION...AS OF 9:15 PM PDT WEDNESDAY...AN UPPER TROUGH IS LOCATED OFF THE NORTHERN CALIFORNIA COAST ALONG 130W THIS EVENING. AN ASSOCIATED COLD FRONT SWEPT INTO NORTHWEST CALIFORNIA LATE THIS AFTERNOON AND EARLY EVENING....PRODUCING WIDESPREAD RAINFALL NORTH OF POINT ARENA AND EVEN SCATTERED THUNDERSTORMS NEAR EUREKA. THE FRONT HAS SINCE WEAKENED QUITE A BIT...BUT LIGHT RAIN HAS MADE IT AS FAR SOUTH AS OCCIDENTAL AND SANTA ROSA WITHIN THE PAST HOUR. THE 00Z NAM IS CONSISTENT WITH PREVIOUS NAM OUTPUT IN SPREADING LIGHT RAIN ACROSS MOST OF THE NORTH BAY OVERNIGHT...AS WELL AS INTO ALL OF OUR COASTAL AREAS BY LATE TONIGHT. CURRENT FORECAST LOOKS ON TRACK EXCEPT HAVE TAKEN OUT MENTION OF THUNDERSTORMS FOR TONIGHT AS IT APPEARS THERE IS NO LONGER ANY CONVECTION ALONG THE FRONT...AND THE MODELS SHOW VERY LITTLE INSTABILITY FOR THE REMAINDER OF TONIGHT. [DISC]
Shower activity will continue into Thursday as the upper trough gradually digs southeast along the coast. Instability will increase on Thursday as a colder air moves in aloft. Based on latest model guidance… the best chance of isolated thunderstorms on Thursday will be across the North and East Bay. ∨
SHOWER ACTIVITY WILL CONTINUE INTO THURSDAY AS THE UPPER TROUGH GRADUALLY DIGS SOUTHEAST ALONG THE COAST. INSTABILITY WILL INCREASE ON THURSDAY AS A COLDER AIR MOVES IN ALOFT. BASED ON LATEST MODEL GUIDANCE...THE BEST CHANCE OF ISOLATED THUNDERSTORMS ON THURSDAY WILL BE ACROSS THE NORTH AND EAST BAY. [DISC]
The trough is forecast to drop south on Thursday night and Friday and eventually form a cutoff low [A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft] near Point Conception [Santa Barbara CA]. Earlier runs of the NAM [North American Meso (formerly Eta) model] had indicated that showers and thunderstorms might still be a possibility across the far southeastern part of our forecast area into Friday… but now now the NAM [North American Meso (formerly Eta) model] is on board with the other models in keeping all precipitation out of our area after Thursday evening. ∨
THE TROUGH IS FORECAST TO DROP SOUTH ON THURSDAY NIGHT AND FRIDAY AND EVENTUALLY FORM A CUTOFF LOW NEAR POINT CONCEPTION. EARLIER RUNS OF THE NAM HAD INDICATED THAT SHOWERS AND THUNDERSTORMS MIGHT STILL BE A POSSIBILITY ACROSS THE FAR SOUTHEASTERN PART OF OUR FORECAST AREA INTO FRIDAY...BUT NOW NOW THE NAM IS ON BOARD WITH THE OTHER MODELS IN KEEPING ALL PRECIP OUT OF OUR AREA AFTER THURSDAY EVENING. [DISC]
850 mb temps are forecast to drop by about 8°C by tomorrow and so we can expect widespread cooling with temperatures falling below normal in most areas. Temps will then gradually warm from Friday through the weekend as the low first sinks south of our area and then moves off to the east. ∨
850 MB TEMPS ARE FORECAST TO DROP BY ABOUT 8 DEG C BY TOMORROW AND SO WE CAN EXPECT WIDESPREAD COOLING WITH TEMPERATURES FALLING BELOW NORMAL IN MOST AREAS. TEMPS WILL THEN GRADUALLY WARM FROM FRIDAY THROUGH THE WEEKEND AS THE LOW FIRST SINKS SOUTH OF OUR AREA AND THEN MOVES OFF TO THE EAST. [DISC]
During past few days the medium range models had been forecasting a rather robust warming trend during the first half of next week when a strong upper ridge was expected to develop off the California coast. But the latest models now show another upper trough approaching the West Coast with rain possible across at least the northern portion of our area by Tuesday night or Wednesday of next week. ∨
DURING PAST FEW DAYS THE MEDIUM RANGE MODELS HAD BEEN FORECASTING A RATHER ROBUST WARMING TREND DURING THE FIRST HALF OF NEXT WEEK WHEN A STRONG UPPER RIDGE WAS EXPECTED TO DEVELOP OFF THE CALIFORNIA COAST. BUT THE LATEST MODELS NOW SHOW ANOTHER UPPER TROUGH APPROACHING THE WEST COAST WITH RAIN POSSIBLE ACROSS AT LEAST THE NORTHERN PORTION OF OUR AREA BY TUESDAY NIGHT OR WEDNESDAY OF NEXT WEEK. [DISC]
Aviation. As of 10:00pm PDT Wednesday… rain… mostly light in intensity and on the immediate coast continues to move south this evening. Ceilings at area terminals will gradually lower to MVFR [Marginal Visual Flight Rules] levels overnight. ∨
.AVIATION...AS OF 10:00 PM PDT WEDNESDAY...RAIN...MOSTLY LIGHT IN INTENSITY AND ON THE IMMEDIATE COAST CONTINUES TO MOVE SOUTH THIS EVENING. CIGS AT AREA TERMINALS WILL GRADUALLY LOWER TO MVFR LEVELS OVERNIGHT. [AVIA]
Vicinity of KSFO [San Francisco CA]… light drizzle reported at KSFO [San Francisco CA] now. The lower to mid level clouds will move in later tonight. MVFR [Marginal Visual Flight Rules] ceilings with light rain late tonight. Wet runways. ∨
VICINITY OF KSFO...LIGHT DRIZZLE REPORTED AT KSFO NOW. THE LOWER TO MID LEVEL CLOUDS WILL MOVE IN LATER TONIGHT. MVFR CIGS WITH LIGHT RAIN LATE TONIGHT. WET RUNWAYS. [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] holding together so far but potential is there for low cloud development ahead of the incoming low. Cloud layers tonight forecast to be primarily MVFR [Marginal Visual Flight Rules]. ∨
MONTEREY BAY AREA TERMINALS...VFR HOLDING TOGETHER SO FAR BUT POTENTIAL IS THERE FOR LOW CLOUD DEVELOPMENT AHEAD OF THE INCOMING LOW. CLOUD LAYERS TONIGHT FORECAST TO BE PRIMARILY MVFR. [AVI2]
Marine. As of 6:35pm PDT Wednesday… very large long period southwesterly swell will impact the waters through Thursday. A low is centered 350 miles west of Ukiah. This low will move through the northern waters on Thursday resulting in light southerly winds with a chance of showers. There will be a slight chance of thunderstorms this evening north of Point Reyes [San Francisco CA] and in the San Francisco Bay Thursday. Winds will become light to moderate northwest Friday through Sunday after the low moves inland. ∨
.MARINE...AS OF 6:35 PM PDT WEDNESDAY...VERY LARGE LONG PERIOD SOUTHWESTERLY SWELL WILL IMPACT THE WATERS THROUGH THURSDAY. A LOW IS CENTERED 350 MILES WEST OF UKIAH. THIS LOW WILL MOVE THROUGH THE NORTHERN WATERS ON THURSDAY RESULTING IN LIGHT SOUTHERLY WINDS WITH A CHANCE OF SHOWERS. THERE WILL BE A SLIGHT CHANCE OF THUNDERSTORMS THIS EVENING NORTH OF POINT REYES AND IN THE SAN FRANCISCO BAY THURSDAY. WINDS WILL BECOME LIGHT TO MODERATE NORTHWEST FRIDAY THROUGH SUNDAY AFTER THE LOW MOVES INLAND. [AVIA]
.MTR WATCHES/WARNINGS/ADVISORIES... .TNGT...NONE. [WARN]
Public Forecast: Dykema; Aviation: Canepa; Marine: W PI/Canepa ∨
PUBLIC FORECAST: DYKEMA AVIATION: CANEPA MARINE: W PI/CANEPA [CRED]
VISIT US AT WEATHER.GOV/SANFRANCISCO [LINK]
FOLLOW US ON FACEBOOK, TWITTER, AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSBAYAREA WWW.TWITTER.COM/NWSBAYAREA WWW.YOUTUBE.COM/NWSBAYAREA [DISC]
9:05pm PDT Sep 17 2014
Overnight. A 40% chance of showers. Cloudy, with a low around 58°. SSW wind around 14 mph, with gusts as high as 18 mph.
Thursday. A 30% chance of showers. Cloudy, with a high near 68°. South wind 10 to 14 mph.
Thursday Night. Cloudy, with a low around 57°. WNW wind 5 to 13 mph.
Friday. Patchy fog before 11am. Otherwise, mostly cloudy, with a high near 67°. West wind 7 to 9 mph.
Friday Night. Patchy fog after 11pm. Otherwise, mostly cloudy, with a low around 57°. West wind 7 to 13 mph.
Saturday. Patchy fog before 11am. Otherwise, partly sunny, with a high near 66°.
Saturday Night. Patchy fog after 11pm. Otherwise, mostly cloudy, with a low around 57°.
Sunday. Patchy fog before 11am. Otherwise, partly sunny, with a high near 66°.
Sunday Night. Patchy fog after 11pm. Otherwise, mostly cloudy, with a low around 56°.
Monday. Patchy fog before 11am. Otherwise, partly sunny, with a high near 67°.
Monday Night. Patchy fog after 11pm. Otherwise, partly cloudy, with a low around 56°.
Tuesday. Patchy fog before 11am. Otherwise, mostly sunny, with a high near 67°.
Tuesday Night. Mostly cloudy, with a low around 57°.
Wednesday. Mostly sunny, with a high near 69°.