Summary...Onset of a long-duration heavy rain hazard is underway
as an atmospheric river makes landfall across central and northern
California. Upwind regions of high terrain could experience
periods of 0.5-1.0 inch/hr rain rates. Flash flooding is likely
through the morning hours.
Discussion...Recent radar/MRMS data depicts an uptick in heavy
rainfall/rain rates across northern and central California
currently. The greatest rates were occurring across coastal
ranges north/northwest of San Francisco, where strengthening, west-southwesterly 850mb flow (around 40 knots) was oriented
perpendicular to ridgelines/terrain in that area. The combination
of orographic ascent and 1.5 inch PW values located just upstream
of the area were supporting areas of 0.25-0.5 inch measured rain
rates over the past hour or so.
Models/observations suggest that the ongoing areas of heavy rain
will only expand and increase in intensity with time. 850mb
should increase into the 40-50 knot range over the next few hours
while impinging upon upwind slopes of the Sierra and northern
California coastal ranges. This will allow for areas of 0.5 to 1
inch/hr rain rates to materialize in terrain-favored areas and
persist for 6+ hours.
USGS Streamflow and NASA SPoRT-LiS Soil Moistures suggest ground
conditions can handle initial rainfall with somewhat dry initial
soil conditions and modest flow in local streams. Multiple hours
of heavier rainfall will likely make ground conditions more
favorable for excessive runoff and flood/flash flood impacts over
time. The greatest risk for flood/flash flood instances will
occur from 09Z/1a Pacific Time, and multiple instances of
excessive runoff are expected through 18Z/10a.