Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Mon Jun 25 2018
 
Day 3
Valid 12Z Wed Jun 27 2018 - 12Z Thu Jun 28 2018 

...RISK OF FLASH FLOODING SPREADS TO PARTS OF THE NORTHEAST AND 
UPPER APPALACHIANS...

...Eastern Great Lakes Into New England...

A closed mid level system with a surface low and associated cold 
front will be making its way slowly eastward out of the Eastern 
Great Lakes at the beginning of Day 3.  The dynamical forcing will 
still be decent early on with divergent/difluent flow east of the 
mid level system and low level flow ahead of the surface low will 
be drawing moisture in place from the southwest. However, the 
mid/upper level system will be filling Wednesday night into 
Thursday morning.  With precipitable water values still running at 
or above 1.75 inches ahead of the system, the atmosphere should 
still be capable of supporting areas of heavy rain into parts of 
the Northeast U.S.. The question by that point will be whether the 
upper support and low level forcing will be focused enough to 
support widespread risk of excessive rainfall.  With some of the 
global models showing rainfall amounts in the 1.5 inch to 2.0 inch 
range, opted to place a Slight Risk.  Note the Slight Risk was 
drawn a bit of the model QPF maxima where terrain gets to be a 
concern despite fairly high flash flood guidance. 

...Ohio Valley into the Appalachians...

A Slight Risk area for excessive rainfall was raised from portions 
of the Ohio Valley eastward to the west face of the Appalachians 
where Flash Flood Guidance was an inch (or less, in spots).  Deep 
moisture should be available for storms to tap from Wednesday 
afternoon into Wednesday night/early Thursday morning.  How much 
mesoscale forcing will be available is in question given the fact 
that the mid-level system over the Eastern Great Lakes Region will 
be filling and what that implies to the low level wind flow over 
this area.  However, consensus values for QPF from the models 
approaches the Flash Flood Guidance values apparently in response 
to a combination of deep moisture and low level upslope which will 
enhance lift in the low levels. 

Bann

 
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt