NOAA’s (National Oceanic & Atmospheric Administration) Bay Area weather discussion bulletin (aka, the area forecast discussion or AFD) has been rather lively lately as we are entering into a weather pattern change. We’re going from last week’s high-pressure dominated clear skies and fair weather to a pattern of wind and rain. Here’s some excerpts from some recent AFDs talking about the upcoming systems.

First, regarding windy storms there is this from the Friday PM report…

GENERALLY IF A STORM HAS A LOW PRESSURE CENTER DEEPER THAN 1000 MB AND TRACKS OFF THE CA COAST E OF LONG 130 WE GET A LOT OF WIND FROM IT…

It looks like the first system will be too far to the west of the Bay Area to generate strong local winds per the latest bulletin on Saturday afternoon…

THE FIRST SYSTEM IS PROJECTED TO PASS WELL TO THE W OF US. AT 18Z SUN THE NEW 18Z ETA HAS A DEEP 986 MB LOW NEAR 40/133 MOVING TOWARD THE NE WHILE THE NEW 18Z GFS HAS THE SYSTEM AT 989 MB NEAR 40/130. IN BOTH CASES…ESPECIALLY WITH THE ETA…THE LOW IS TOO FAR OFFSHORE TO REALLY HIT OUR DISTRICT WITH MAJOR WIND AND RAIN. IN A MAJOR WIND EVENT THE LOW WILL USUALLY BE WELL TO THE E OF LONG 130 WHEN THE CENTER IS S OF LAT 40. THUS WE HAVE MOVED THE HIGH WIND WATCH BACK AND IT IS NOW EFFECTIVE ONLY SUN NIGHT AND MON. EVEN THE SUN NIGHT TIMING MAY BE TOO FAST.

Some translation may be helpful. 40/133 is 40 degrees north lattitude/133 west longitude. 18Z is 1800 zulu time or Coordinated Universal Time (UTC) also know as Greenwich Mean Time (GMT) and is eight hours ahead of Pacific Standard Time. ETA is the old name for the North American Mesoscale weather model, and GFS refers to the Global Forecast System weather model. MB is the abbreviation for millibar a measure of pressure. As you can see, meteorologists have their own language.

The second system on Monday looks like more of a direct hit…

THE MAIN EVENT IS NOW DEFINITIVELY SCHEDULED FOR MON/MON EVE. DURING THIS PERIOD A NEW SLUG OF SUBTROPICAL MOISTURE WILL SURGE INTO THE DISTRICT FROM THE SW. A SECOND SURFACE LOW WILL DEVELOP OFFSHORE AND THIS ONE IS FORECAST TO TRACK WELL TO THE E OF THE FIRST ONE…THE NEW ETA HAS IT AT 996 MB CENTERED NEAR 38/126 AT 00Z TUE.

And from this morning’s report there is this…

AMSU TPW IMAGERY IS SHOWING A PLUME OF TROPICAL MOISTURE STREAMING TOWARDS THE COAST. PW`S GREATER THAN 0.75 INCHES HAS MOVED TO NEAR 39/131 AS OF 230 AM. WITH THE STRONG SYSTEM NEAR 26/135 PW`S HAVE INCREASED TO 2.42 INCHES IN THE CONVECTION.

The AMSU stands for the advanced microwave sounding unit which ride on three NOAA weather satillites. The AMSU sensors are primarily used to study water vapor in the atmosphere. TPW stands for total precipitable water.. PW for precipitable water. Colorado State University has a great site for viewing these AMSU data. From the precipitable water data, I put together a short animated image showing the tropical moisture plume being tapped by the upcoming storm systems.

And again from the Saturday afternoon report all of this tropical moisture is going to result in some rather heavy rain for the district.

THE EARLIER FORECAST OF 2-5 INCHES OF QPF FROM SUN AFTN THRU TUE MRNG STILL LOOKS GOOD BUT THE TREND IS NOW TO WEIGHT THAT PRECIP MORE HEAVILY ON THE LAST 24 HOURS OF THE PERIOD…MON/MON NIGHT. STILL…IF THE PRECIP FORECAST VERIFIES OUR MAINSTEM RIVERS SHOULD NOT FLOOD. IT USUALLY TAKES AT LEAST 6 INCHES OF RAIN IN 24 HOURS TO FLOOD THE RUSSIAN AND NAPA AND WITH THE DRY WX OF THE PAST MONTH IT MIGHT TAKE A LITTLE MORE THAN THAT.

Browsing to NOAA’s Quantitative Precipitation Forecasts site you can find image maps with the predicted precipitation for the next five days. Here’s the image for Sunday through late Tuesday.

Zoinger says, batten down the hatches.