Re: August 2016: Significant Pattern Change/Rain Chances Ret
Posted: Sat Aug 20, 2016 4:18 pm
sobering excessive rainfall from WPC:Portastorm wrote:Looks like a long night ahead for us in the I-35 corridor Austin/San Antonio. We're looking at the likelihood of 6-8" of rain with as much as 15" possible in isolated spots. This on already saturated ground. Very sobering language coming out of the EWX office as well as NOAA's WPC excessive rainfall desk. Hope folks pay attention and heed all warnings.
...TEXAS...
growing very concerned about the ingredients in place that would support a convectively enhanced mid level vortex or mcv... and the associated very strong trend toward excessive rainfall amounts totaling 10 to 15 inches in most of the high resolution guidance. hand analysis of 500 mb reveals a height depression over south texas near del rio... associated with cyclonic curvature seen in radar loops at 19z. this feature is embedded within a plume of tropical moisture that lifted into northern mexico and south texas over the past few days and is now in a region of weak steering flow. the larger scale pattern related to a healthy northern stream trough and sub-tropical ridging along the gulf coast favors persistent upper difluence in the vicinity of the mcv... and also sustained low level inflow. this situation would appear to be ripe for a significant flash flood event... especially given the tendency for this style of event to occur in the texas hill country. therefore... it was not too surprising when the canadian gem regional... the first arriving hi-res guidance... depicted a very wet... nearly stationary mcv and likely flash flood event over the san antonio / austin area. this was followed up by very strong and similar qpf signals in the ncep hi-res windows and nssl wrf. the global models also have heavy rain... especially the ecmwf. the hi-res models and ecmwf support the notion of a 10-plus inch rainfall event occurring from this evening through early sunday. the qpf signal is organized... suggesting some breadth to the extreme rain totals... perhaps affecting multiple counties... rather than being very isolated.
based on the overwhelming model signal and history of rapid hydrologic response in this area... coordinated with texas local offices to upgrade not only to moderate risk... but also to a small high risk area centered around san antonio / austin. placement of the qpf maximum is not of extremely high confidence... but wpc forecasters using independent methods on the night and day shift came up with the same answer for location... which is supported in particular by the wrf-arw... which tends to perform well in south texas. also noted the wrf-arw and wrf-nmm qpf signal indicating nearly 150 percent of the one percent annual exceedance probability or recurrence interval rainfall. with perhaps more people outdoors / on the roads on a saturday night... felt it was prudent to upgrade to high risk to send a clearly heightened message. thanks wfo ewx... fwd... crp... hgx...and southern region for coordination.