Yesterday afternoon (Saturday March 19th 2011) a surface low began developing over the eastern Pacific to the south of the upper cut-off. This low intensified dramatically west of San Francisco and moved rapidly northeastward during the night. The top water vapor image shows the feature at 0115 UTC west of San Francisco. The lower image is a 20 March,10 UTC surface plot from NCAR RAP and indicates surface pressures near 987 mb north of San Francisco - indicating falls of 20 mb or more since the 00 UTC observations. The NAM model forecasts at 12 UTC yesterday morning hinted at this development, the 18 UTC NAM had a deeper low forecast along the coast (but weaker than what actually occurred), and the 00 UTC NAM more or less caught up with the intense cyclogenesis, but in what was essentally a diagnostic time frame. Thus, a challenging situation for forecasters in the Bay area, who were following rapidly changing, short-term observations due to the not so stellar model forecasts. The Forecast Discussion from the San Francisco NWS office this morning provides a brief summary of the events during the night:
--------------------------------------------------------------------
.DISCUSSION...AS OF 4:30 AM SUNDAY...A VERY DEEP SURFACE LOW
DEVELOPED RIGHT OFF OUR COAST LAST EVENING. THE LOW IS CURRENTLY
CENTERED NEAR POINT ARENA AND ITS CENTRAL PRESSURE IS DOWN AROUND
984 MB...AND PROBABLY A BIT LOWER. SUCH A DEEP LOW IN CLOSE
PROXIMITY TO OUR COAST GENERATED VERY STRONG WINDS OVERNIGHT...
BEGINNING IN THE SOUTHERN COASTAL WATERS EARLY IN THE EVENING AND
SPREADING EAST AND NORTH AS THE NIGHT WORE ON. STRONGEST WIND GUSTS
OCCURRED LATE LAST EVENING THROUGH ABOUT MIDNIGHT. WIND GUSTS IN
EXCESS OF 60 MPH WERE REPORTED IN THE NORTH BAY AND THE SAN FRANCISCO
AIRPORT HAD A PEAK WIND GUST OF 53 MPH. WINDS HAVE BEEN GRADUALLY
SUBSIDING SINCE ABOUT 1 AM BUT ARE STILL QUITE STRONG.
No comments:
Post a Comment