Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Thu Jun 22 2023
 
Day 3
Valid 12Z Sat Jun 24 2023 - 12Z Sun Jun 25 2023 

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR MUCH OF NORTH 
DAKOTA AND MINNESOTA...

...ND/MN area...

A strengthening shortwave trough over south-central MT Saturday 
morning will track east to western MN by Sunday morning. This 
feature will allow the widespread area of rain, with some embedded 
thunderstorms, to track east across the Slight Risk area on 
Saturday. A slowly strengthening surface low will remain out ahead 
of the shortwave, which will both draw moisture from the 
ever-present LLJ northward, while also dragging the LLJ eastward 
ahead of its cold front. The front will set off additional showers 
and thunderstorms across IA/MO and WI, but since the front will be 
progressive, so too will the storms along the front, so only 
isolated flash flooding is possible, namely in urbanized areas. 
With ND and MN still in the best forcing as the low wraps up, 
training storms are likely to develop along the track of the low. 
While soil conditions are dry from eastern ND and even drier 
across northern MN, it's important to note that the abnormally 
high atmospheric moisture (characterized by PWATs over 1.5 inches) 
and favorable flow for training storms across this area may allow 
for the heaviest rain that falls in this region to overcome the 
antecedent dry conditions and result in flash flooding. If the 
rain falls hard enough, then the dry soil can't absorb that much 
water all at once, so there's increased runoff. Expect this to be 
the case in the Slight Risk area Saturday.

...Northeast...

The upper level trough will continue to ever so slowly northeast 
on Saturday, starting the day in the Ohio Valley and moving to 
Central NY by Sunday morning. The trough will remain negatively 
tilted, so the southerly LLJ of tropical Atlantic moisture will 
continue pumping northward ahead of the the trough. Since the 
forcing will be shifting north ahead of the trough, a Marginal 
flash flooding threat will primarily focus from Philadelphia 
northward. From northern New England and especially into western 
ME, rainfall totals over the past 2 weeks are 200-300% of normal, 
so the wet soils in this area will promote flash flooding, 
especially on the southeast facing slopes of the White Mountains 
of NH and ME. Very few changes were needed for this morning's 
update, as the forecast remains on track.

Wegman




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