Moisture plumes from the tropical Pacific can extend into the mid-latitudes, and the CIRA Layer Precipitable Water (LPW) product based on microwave (AMSU, MHS) and infrared (AIRS) sounding instruments from NASA and NOAA polar-orbiting satelliltes provides information on the amount of moisture in each layer. Traditional total precipitable water (TPW) data only give part of the picture and Water Vapor (WV) imagery only captures the upper tropospheric moisture. Note here how the GOES WV imagery from the NASA GHCC site agrees well with the values of 2-4 mm in the 500-300 mb layer between Hawaii and the west CONUS.
500-300 mb Layer Precipitable Water by CIRA, 13 March 2013, 2100Z
GOES Water Vapor imagery from NASA GHCC site, 13 March 2013, 2130Z
In the images below, the surface to 850 mb layer shows a wide plume of 0.5 to 0.75 inches of PW extending from Hawaii to Washington and Oregon. Moving upwards, the 850 to 700 mb layer continues to show a wide swath of moisture (~0.25 to 0.5 inches) in this same area, with a sharp gradient to the east. Lastly, the 700 to 500 mb layer shows a more narrow moisture swath, but still with values ranging from ~0.25 to 0.33 inches, and extending into the northwest CONUS. The observations of vertical distribution of moisture in data void regions can be compared to NWP models as well as applied to estimating the available moisture at low levels for potential precipitation and flooding events.
Surface to 850 mb Layer Precipitable Water by CIRA, 13 March 2013, 2100Z
850 to 700 mb Layer Precipitable Water by CIRA, 13 March 2013, 2100Z
700 to 500 mb Layer Precipitable Water by CIRA, 13 March 2013, 2100Z
The blended TPW product and associated anomaly increase the forecaster’s awareness for potential flooding rains. One such event occurred from April 6-8, 2008, affecting the Miami, FL WFO. Pablo Santos and staff forecasters had been tracking an area of moisture developing in the GOMEX and Caribbean. The TPW anomaly product for early on April 6 shows abnormally high values mostly in mid-latitudes with tropical areas closer to normal for this time of year. However, a small area to the west of FL indicated TPW anomalies of 200% with values over 2 inches for TPW itself. A frontal boundary associated with a mid-latitude cyclone was pushing south into the GOMEX and FL, and the upper level analysis showed weak divergence aloft over FL. This image of abnormally high moisture in the blended TPW provided additional lead time to forecasters that the rain event about to occur in the next 48 hours had a good potential of resulting in flooding. Over the course of the next 2 days the forcing from the boundary and the large TPW resulted in 5-10 inches of total precipitation over a large area due to training of cells.
Blended TPW for April 6, 2008 at 0500Z. Note the red area of 2 inches of TPW. This area was propagating toward Southern Florida.
The Blended TPW Anomaly shows that much of the mid-lattitude front pushing southward has TPW of 200% of normal although TPW values themselves are not extremely large. However, the area over the Gulf of Mexicon with 2 inches of TPW is abnormally high, even for the more tropical location, and is of largest concern.