Friday, October 2, 2015

Extraordinary Flooding Possible In South Carolina

Although I generally keep my view on Pacific Northwest weather, the forecasts for South Carolina and vicinity are so unusual and threatening that I wanted to mention it. And because of Boeing, there are concerns about what happens around Charleston, SC.

Huge rainfall totals are forecast to hit that region, with 10-20 inches being expected in some locations.  Flooding should be expected.

This flooding is NOT directly associated with Hurricane Joaquin, but with the confluence of tropical moisture streams.

Let me begin by showing you the cumulative rainfall totals from the National Weather Service GFS model for the next 66 hours. Over Washington State, some very light precipitation (mainly tonight), but over the eastern U.S. it is the end of the world.   You can see one area of heavy precipitation offshore, with values between 5 and 10 inches....that is the precipitation accompanying Joaquin.   But there is another, even HEAVIER precipitation region with values getting to 15-20 inches stretching from South Carolina and into western North Carolina.  That is what I am talking about.   And this is from a relatively coarse global model--local rainfall could be much heavier.


The NWS Storm Prediction Center three-day forecast totals parallel the GFS runs, giving a conservative total of 13.2 inches.

What I showed you  above was the precipitation totals over several days.   The area of heavy rainfall is associated with a narrow band of precipitation that is forecast to be  extraordinarily persistent and nearly stationary, as illustrated by the precipitation forecast for a 3-h period ending 5 PM PDT tomorrow (Saturday).  You can see the hurricane offshore.


So why does this narrow feature exist?   Substantial insights can be derived from looking at the winds near 10,000 ft (700 hPa), relative humidity, and upward motion at the same time (see map).  There is an area of high pressure to the north (H shown in red) and a low to the south (L in red).  Between them there is a region of strong easterly winds that are bringing moisture into the region.  Moisture that is forced to rise by the nearby Appalachians and their eastern foothills.   But there is more.


Some flow is circling around the low and converging into the easterly flow--right over south Carolina.  Converging air flows cause upward motion that produces precipitation.  But there is still more.  This converging flow is forced to rise by the mountains. The red lines shows upward motion (very subtle but look close and you will see it over S. Carolina).  That upward motion in the presence of tropical moisture is producing intense precipitation.

The latest (9 PM PDT) NWS radar image shows heavy precipitation moving into the area.


The National Weather Service has flood watches, flood warnings, flash flood warnings, and more posted right now (see graphic), and some areas are being evacuated. This is a serious, unusual, and life-threatening event, one that our models have been warning about for several days.  I hope the Charleston Boeing facility is ready for intense rainfall...they are going to get it.


2 comments:

typingtalker said...

Early morning update from the NWS ...

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
736 AM EDT SAT OCT 3 2015

...RECORD RAINFALL AND LIFE-THREATENING FLASH FLOODING LIKELY
THROUGH SUNDAY...
...COASTAL FLOOD WARNING ISSUED FOR PORTIONS OF THE LOWER SOUTH
CAROLINA COAST...

THE MUCH ANTICIPATED HEAVY RAIN EVENT IS NOW WELL UNDERWAY. THE
AXIS OF TROPICAL MOISTURE STREAMING NORTHWEST FROM HURRICANE
JOAQUIN AHEAD OF UPPER LEVEL LOW PRESSURE MEANDERING OVER
SOUTHWEST GEORGIA WILL REMAIN ANCHORED ACROSS SOUTH CAROLINA FOR
THE 48 HOURS. MODELS ARE COMING INTO BETTER AGREEMENT THAT HEAVY
RAINS WILL SETUP ACROSS MUCH OF THE SOUTH CAROLINA LOWCOUNTRY WITH
THE AREA BOUNDED BY BEAUFORT-WALTERBORO-SUMMERVILLE AND DOWNTOWN
CHARLESTON LIKELY SEEING THE HIGHEST STORM TOTALS THROUGH THE
EVENT. RAIN TOTALS OF 10-15 INCHES ARE LIKELY IN THIS AREA WITH
LOCALIZED HIGHER AMOUNTS POSSIBLE. SEE THE HYDROLOGY SECTION BELOW
FOR A MORE DETAILED ASSESSMENT ON THE HEAVY RAIN AND FLASH FLOOD
POTENTIAL.

[ ... ]

.HYDROLOGY...
THE METEOROLOGICAL SETUP REMAINS ON TARGET FOR A RECORD RAINFALL
EVENT THROUGH SUNDAY MORNING WITH A POTENTIAL FOR WIDESPREAD
FLASH FLOODING. A FLASH FLOOD WATCH REMAINS IN EFFECT FOR ALL OF
SOUTHEAST SOUTH CAROLINA UNTIL SUNDAY AFTERNOON. FOR THE GEORGIA
COUNTIES BORDERING THE SAVANNAH RIVER...THE ENDING TIME OF THE
WATCH HAS BEEN SHORTED UNTIL EARLY SUNDAY MORNING AS THE CORE OF
HEAVIEST RAINS SHOULD EXIT TO THE NORTH BY THEN...IF NOT SOONER.

http://forecast.weather.gov/product.php?site=NWS&issuedby=CHS&product=AFD&format=CI&version=1&glossary=1&highlight=off

richard583 said...

It looks as if quite a bit of the moisture feeding into the subregion was Nino generated. http://climate.cod.edu/flanis/satellite/polar/index.php?type=polar-wv-48