Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Wed Jul 15 2020
 
Day 3
Valid 12Z Fri Jul 17 2020 - 12Z Sat Jul 18 2020 

...THERE IS A SLIGHT RISK OVER THE NORTHERN RED RIVER OF THE NORTH 
VALLEY...

...2030 UTC Excessive Rainfall Discussion...
Only change was to expand the Slight Risk area in eastern North 
Dakota and northwestern Minnesota given the persistent wet 
conditions with the most recent bout of moderate to locally heavy 
rainfall occurring as recently as Tuesday.  The signals from the 
models have been consistently showing a signal for some 1 to 1.5 
inch rainfall amounts in the Day 3 period...with the better chance 
for heavier rain north of the international border.  The Margial 
Risk area surrounding the Slight Risk area was largely untouched. 
Only some minor adjustments were made to the Marginal Risk area 
extending from the Mid-South to New England...and those changes 
were largely made to fit the latest guidance and not the result of 
a change in forecast reasoning.

Bann

...0830 UTC Excessive Rainfall Discussion...

...Northern Great Plains to Lake Superior...
A shortwave trough that reaches the WA coast Thursday will shift 
east across MT Friday. Return flow behind the leading ridge axis 
and ahead of the trough brings elevated moisture across northern 
Plains with PWs reaching 2 standard deviations above normal by 
18/00Z over the eastern Dakotas which spreads east to Lake 
Superior through Friday night. The combination of increasing 
moisture at the diurnal peak puts the heavy rain focus over 
eastern ND into northern MN where many areas have had well above 
normal rainfall recently. Of particular note is the northern Red 
River Valley of the north (north of Grand Forks) as well as 
central MN southeast from Fargo. As of now guidance indicates the 
heavy rain axis may be closer to the northern area than the 
southern, so in coordination with WFO FGF raised a Slight Risk for 
only areas north of Grand Forks. Strong upper level westerly flow 
should keep activity generally progressive with large organized 
thunderstorm clusters likely. Given the elevated moisture, Slight 
Risk may be needed wherever the heaviest axis is forecast.


...Mid-South...Tennessee Valley...Mid-Atlantic to New England...
A slow moving/stalled frontal boundary near or south of the Ohio 
River should provide a focus for ongoing activity Friday with 2 
inch PWs (1.5 to 2 standard deviations above normal) and ample 
instability continuing a heavy rain threat. Farther northeast the 
frontal wave that lifts across the eastern Great Lakes Thursday 
night drags a cold front into New England during the day Friday. 
There is still sensitivity from heavy rain from Fay over northern 
NJ/southern NY, but confidence is not there yet for significant 
rainfall over that particular area. Otherwise the upper Hudson 
Valley has been rather dry recently before pockets of heavy recent 
rainfall in interior New England. Should the focus of heavy rain 
become apparent over any particularly sensitive areas a Slight 
Risk would be warranted.

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