Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Fri Sep 10 2021
 
Day 1
Valid 01Z Sat Sep 11 2021 - 12Z Sat Sep 11 2021 

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PARTS OF 
THE PACIFIC NORTHWEST TO THE NORTHERN ROCKIES...

...01Z Update...
Periods of moderate to heavy rain continue to impact portions of 
the Northwest/Northern Rockies as a an upper level system and 
associated surface feature advances to the north and east this 
evening.  As this feature and mid-level wave aloft move into 
northern ID and western portions of MT through the next several 
hours, expect heavier rain rates to help realize the 1+ PWATs 
across the region. While precipitation may be somewhat beneficial 
to the drought conditions and wildfires, if rates are high enough 
and over burn scars this may still result in localized debris 
flow.  Therefore, maintained the Marginal Risk area with only 
minor adjustments across central OR.  

Along the east coast of FL, convection that developed on the 
Atlantic sea breeze continues with coverage and intensity 
diminishing. All indications suggest that this activity should 
decrease through the next couple of hours with loss of daytime 
heating and surface convergence. A bulk of the activity across 
east central FL is creating torrential downpours but is shifting 
to the east in response to the increased westerlies. Therefore, 
given this activity is fairly progressive, do not anticipate 
significant flash flooding, though ponding on roadways can not be 
ruled out over the next hour or two.  Thus, the Marginal Risk area 
was removed at this update.  

Pagano

...Previous Discussion... 

...Pacific Northwest/Northern Great Basin/Northern Rockies...
A shortwave trough continues to amplify along the coast of the 
Pacific Northwest and will continue to advance inland into the 
Northern Great Basin and Northern Rockies through tonight. The 
moisture flux anomalies in the 850/700 mb layer are forecast by 
the global models to be quite impressive as a belt of strong 
southwest mid-level flow wraps around the eastern flank of the 
shortwave trough. This coupled with anomalously high PWs, as much 
as 3 to 4+ standard deviations above normal, is expected to yield 
an efficient environment for heavier rates. There will be strong 
dynamic forcing associated with the height falls and the 
expectation is for broken areas of heavy showers and a few 
thunderstorms with locally enhanced rainfall rates. This will be 
further aided by orographics and diurnally driven instability.

The 00Z HREF model consensus supports locally as much as 1 to 2+ 
inches of rain from central Oregon to northern Idaho and northwest 
Montana. Given the sloped terrain, lower FFGs overall, and 
sensitivities involving area burn scars, there will be concerns 
for at least some flash flooding. Forecast rainfall amounts 
remained steady across these regions, and an upgrade to a Slight 
Risk during the day remains possible depending on convective 
evolution and if convection repeats across the same areas. A minor 
extension to the Marginal Risk was made to add more of 
north-central Oregon into the area based on some of the 00Z hi-res 
guidance.

...Florida Atlantic Coast...
A surface front is expected to meander over the Florida Peninsula 
today with possibly a wave of low pressure along it, while a weak 
500mb vort max passes over the northern part of the state as well. 
These features could provide additional lift and focus for 
thunderstorms that develop today/tonight compared to "normal" 
daily convection. PWs could be slightly above average (1 to 1.5 
standard deviations above normal) with PW values themselves right 
around 2 inches. While much of the peninsula (especially the 
northern half) could see scattered showers and thunderstorms, it 
appears the best chance for the heaviest amounts and flash 
flooding could be in eastern Florida near the Atlantic Coast per 
00Z hi-res CAMs and the HREF. HREF probabilities of exceeding FFG 
are the highest in this area, and urban areas such as Jacksonville 
could be especially susceptible to flash flooding.

Tate/Orrison
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt