National Weather Service forecasters use GPS precipitable water vapor for enhanced situational awareness during the Southern California summer monsoon

Flooding across Highway 78 in San Diego County, California, 22 Jul 2013. [Photo credit: NOAA]

Flooding across Highway 78 in San Diego County, California, 22 Jul 2013. [Photo credit: NOAA]

TitleNational Weather Service forecasters use GPS precipitable water vapor for enhanced situational awareness during the Southern California summer monsoon
Publication TypeJournal Article
Year of Publication2015
AuthorsMoore A.W, Small I.J, Gutman S.I, Bock Y, Dumas J.L, Fang P., Haase JS, Jackson M.E, Laber J.L
JournalBulletin of the American Meteorological Society
Volume96
Pagination1867-1877
Date Published2015/11
Type of ArticleArticle
ISBN Number0003-0007
Accession NumberWOS:000366993900002
Keywordsassimilation; Climatology; gulf-of-california; meteorology; mountains; north-american monsoon; rainfall; southwestern united-states; surges; system
Abstract

During the North American Monsoon, low-to-midlevel moisture is transported in surges from the Gulf of California and Eastern Pacific Ocean into Mexico and the American Southwest. As rising levels of precipitable water interact with the mountainous terrain, severe thunderstorms can develop, resulting in flash floods that threaten life and property. The rapid evolution of these storms, coupled with the relative lack of upper-air and surface weather observations in the region, make them difficult to predict and monitor, and guidance from numerical weather prediction models can vary greatly under these conditions. Precipitable water vapor (PW) estimates derived from continuously operating ground-based GPS receivers have been available for some time from NOAA's GPS-Met program, but these observations have been of limited utility to operational forecasters in part due to poor spatial resolution. Under a NASA Advanced Information Systems Technology project, 37 real-time stations were added to NOAA's GPS-Met analysis providing 30-min PW estimates, reducing station spacing from approximately 150 km to 30 km in Southern California. An 18-22 July 2013 North American Monsoon event provided an opportunity to evaluate the utility of the additional upper-air moisture observations to enhance National Weather Service (NWS) forecaster situational awareness during the rapidly developing event. NWS forecasters used these additional data to detect rapid moisture increases at intervals between the available 1-6-h model updates and approximately twice-daily radiosonde observations, and these contributed tangibly to the issuance of timely flood watches and warnings in advance of flash floods, debris flows, and related road closures.

DOI10.1175/bams-d-14-00095.1
Short TitleBull. Amer. Meteorol. Soc.
Impact: 

GPS precipitable water vapor (PW) gives forecasters confidence about whether a watch or warning is needed, and if so, how soon it will likely be needed and in which area. The GPS-PW data improves the watch/warning process for several reasons, including the high temporal resolution compared with other observations and the rough correlation between high PW and heavy rainfall. GPS PW can also be particularly useful in the case of atmospheric river events, when moisture is concentrated in a band associated with the low-level jet, rain rate is minimally impacted by evaporation, orographically forced ascent is appreciably larger than synoptic ascent, and hydrometeor generation is instantaneous and 100% efficient.

Student Publication: 
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