FXUS66 KMTR 201731
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
931 AM PST TUE JAN 20 2009
.DISCUSSION...AS OF 9:05 AM PST TUESDAY...MAIN FORECAST FOCUS WILL
BE ON UPCOMING LIGHT RAIN CHANCES BEGINNING WEDNESDAY.
MOSTLY CLEAR SKIES PREVAIL THIS MORNING WITH ONE MORE DAY OF WELL
ABOVE NORMAL TEMPERATURES. THE LARGE SCALE OFFSHORE GRADIENTS ARE
STILL IMPRESSIVE WITH 11 MB FROM WMC TO SFO RESULTING IN SOME
LOCALLY BREEZY CONDITIONS ACROSS THE EAST BAY HILLS THIS MORNING.
HIGH CLOUDS SHOULD START TO INCREASE FROM SOUTH TO NORTH BY LATER
THIS AFTERNOON AND OVERNIGHT. THE INCOMING STORM SYSTEM IS ACTUALLY
LOCATED WELL SOUTHWEST OF POINT CONCEPTION AT THIS HOUR WITH
PRECIPITABLE WATER VALUES OF NEARLY 1.5 INCHES. AN INCOMING ZONAL
TROUGH WILL TAP INTO THIS PIECE OF MOISTURE AND SPREAD IT NORTHWARD
INTO SOUTHERN CALIFORNIA AND AT LEAST PORTIONS OF OUR CWA WEDS INTO
THURSDAY. OFFSHORE LIGHTNING DATA INDICATED A STRIKE JUST WEST OF
130 WEST AT 1315Z THIS MORNING SO THERE IS SOME INSTABILITY AND
PLENTY OF MOISTURE AVAILABLE. SYSTEMS COMING IN FROM THE SOUTHWEST
ARE OFTEN NOT MODELED AS CLEANLY AS OUR NW FLOW STORMS BUT STILL
FEEL THAT RFC QPF NUMBERS APPROACHING 0.50 FOR THE MONTEREY COUNTY
HILLS IS REASONABLE GIVEN THE HIGH TPW VALUES. THIS SYSTEM WILL BE
LIKE OTHERS WE HAD BACK IN DECEMBER WHERE THE "HEAVIEST" PRECIP
OCCURS ACROSS SOUTHERN PORTIONS OF THE CWA WITH ONLY TRACE OR LIGHT
AMOUNTS FROM THE BAY AREA NORTHWARD.
WILL LIKELY INCREASE POPS FOR THE AFTERNOON PACKAGES FOR SOUTHERN
PORTIONS OF THE CWA WITH THE IDEA OF TRYING TO PAINT A HIGHER POP
BUT LOW QPF TYPE SCENARIO AND WILL CONTINUE TO WORD IT AS SHOWERS.
THE GFS AND ECMWF SOLUTIONS KEEP SHOWERS GOING ON AND OFF WEDS NIGHT
INTO THURSDAY WHILE THE NAM IS MORE PROGRESSIVE AND DRIES THINGS
OUT QUICKER ON WEDS. WILL LEAN MORE TOWARDS THE GFS/ECMWF SOLUTIONS.
THE ECMWF IS THEN THE ONLY MODEL TO KEEP AT LEAST SLIGHT RAIN
CHANCES GOING FRIDAY AND INTO THE WEEKEND WHILE THE OTHER MODELS
BRING IN DRIER NW FLOW. ITS HARD TO GO AGAINST THE ECMWF BUT DONT
SEE ANY SIGNIFICANT RAINS DURING THE FORECAST PERIOD.
&&
.AVIATION...AS OF 9:31 AM PST TUESDAY...CHANGE IN THE PATTERN OVER
THE NEXT 24 HOURS AS A TROUGH APPROACHES FROM THE WEST. WEAK
OFFSHORE FLOW WILL LINGER EARLY TODAY BEFORE ONSHORE FLOW DEVELOPS.
MID LEVEL CLOUDS WILL GRADUALLY INCREASE LATER THIS AFTERNOON AS
THE TROUGH SLIDES EASTWARD TOWARDS THE COAST. WINDS WILL BE
HARD TO NAIL DOWN AS EASTERLY FLOW WILL GRADUALLY MOVE TOWARD THE
WEST TO NORTHWEST.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...NONE
&&
$$
PUBLIC FORECAST: RWW
AVIATION/MARINE: SSA
NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO