FOUS30 KWBC 200802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Fri Jun 20 2025

Day 1
Valid 12Z Fri Jun 20 2025 - 12Z Sat Jun 21 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
UPPER MIDWEST...

...Northern Plains into Upper Midwest...
Model guidance indicates a well defined mid level shortwave will
eject east out of the Rockies and into the Northern Plains this 
afternoon into tonight, with a surface low moving from southern MT 
into the Dakotas. High res guidance show convection developing near
the MT/ND border this afternoon to the north of a warm front. An 
increasing low level jet by evening likely supports upscale growth 
of this activity as it moves eastward across ND. This convection 
will likely be quick moving, however 1"+ per hour rainfall appears 
probable and a localized flash flood risk is possible. A Marginal 
risk was expanded west into these areas.

A Slight risk was maintained across portions of northeast MN,
and was expanded some with this update to include more of northern
MN and far northern WI. An impressive low level jet and moisture 
transport axis will quickly evolve this evening into the overnight 
hours. Also looking at a pool of extreme instability on the order 
of 4000-5000 j/kg, and PWs increasing towards 2". While the 
progressive MCS tracks east across ND, we should see some 
downstream development over eastern ND into northern MN on the nose
of this intense low level jet and along the instability gradient. 
Some west to east training of this activity is possible ahead of 
the upstream MCS that will also eventually push across the area. 

The degree of training remains a bit unclear, but do note that both
the 00z HRRR and RRFS show a narrow swath of 3-5" of rainfall. 
Given the instability and moisture in place, totals of this 
magnitude do seem plausible. Also some uncertainty on the axis of 
heaviest rainfall, which should end up pretty narrow. The better 
instability will be south, however a strong mid level cap will be 
advecting northward likely putting a limit on how far south 
organized convection will get. The current Slight risk area 
encompasses the highest HREF probabilities, and while a bit broader
than what will probably happen, accounts for some latitudinal 
uncertainty. Overall think isolated to scattered flash flooding 
could evolve from this setup, especially if there is overlap with 
the heavy rainfall axis from a couple days ago over northeast MN.

...Iowa...
Convection is currently moving southeast across portions of MN and
IA, and some of this will likely be still ongoing at 12z this
morning. The 12z HREF and recent HRRR/RRFS runs show some brief
backbuilding potential this morning on the nose of the low level 
moisture transport axis across eastern IA. Localized amounts over 
3" appear possible, and an isolated flash flood threat could exist 
this morning.

...Northern Rockies...
A strong mid level low moving into the Pacific Northwest will 
focus impressive synoptic ascent over MT today into tonight. This 
will result in increasing shower coverage, with PWs increasing 
towards the climatological 90th percentile as well. Instability is
likely the main limiting factor for excessive rainfall, and the 
probability of exceeding 1" per hour rain is pretty low because of
this. However do think we will get some weak instability, which 
combined with the impressive dynamics, should still allow for some
embedded heavier convective cores with over 0.5" per hour rain. 
Most areas will probably see closer to 1" of rain, however 
localized amounts over 2" are supported by the 00z high res models
which could result in an isolated flash flood risk.

Chenard


Day 2
Valid 12Z Sat Jun 21 2025 - 12Z Sun Jun 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN GREAT LAKES, INTERIOR NORTHEAST, AND NORTHERN ROCKIES...

...Northern Great Lakes...
Convection will likely be ongoing at 12z Sat across portions of
northern MI into the UP. The overall convective complex should be
propagating eastward pretty quickly by this time, although it may 
still have a west to east orientation to it allowing for some 
training. The 00z HREF indicates 1" per hour exceedance 
neighborhood probabilities upwards of 60%, and 2" exceedance around
20%. HREF blended mean QPF is in the 2-3" range and these amounts 
seem plausible wherever the convective training axis is Saturday 
morning. Amounts higher than this will probable be hard to come by 
given the quick system movement, however even totals of this 
magnitude could result in a localized flash flood threat. Some 
latitudinal differences in the convective axis are noted, with both
northern MI and the UP of MI potentially impacted.

...New England...
The aforementioned convection over MI will try to move into 
portions of NY during the day Saturday. The maintenance of the 
activity is unclear as it will be moving into a more stable 
airmass. Generally think convection will weaken as it moves into 
NY...however with daytime heating some increase in instability over
central NY could support at least some additional convective 
development during the day. 

By Saturday night steeper lapse rates advect into the
Northeast, resulting in what should be a pretty rapid increase in
MUCAPE over western NY into northern New England. A well defined
mid level shortwave and increasing low level moisture transport
will probably kick of organized convective development over
southern Canada by Saturday evening. This activity should track
into portions of NY and northern New England overnight into early
Sunday. It seems likely that this activity will have a better
chance of persisting into NY and New England given what should be 
a very strong axis of low level moisture transport over top steep 
mid level lapse rates. In fact we could very well have an 
impressive MCS diving into NY and northern New England later 
Saturday night. The specifics remain a bit unclear, but at least a 
localized flash flood risk seems probable even with a forward 
propagating system. Certainly a chance this setup keeps some 
backbuilding convection across the area, and possible we will 
eventually need a Slight risk. However, given the uncertainty 
regarding the details, think a Marginal risk will suffice for now.

...Northwest Montana...
A Marginal risk was maintained across portions of northwest MT 
where rainfall will continue into Saturday. By this time this 
region will be within the comma head of the low, and looking at 
more stratiform rain with lower rates. Thus probably not really a 
flash flood risk by this point, but with storm total rainfall 
potentially into the 3-4" range, some possible flood impacts still 
justify a Marginal risk. Snow levels will drop towards 4000 feet 
by this time as well, so flood impacts will stay below that level.

Chenard


Day 3
Valid 12Z Sun Jun 22 2025 - 12Z Mon Jun 23 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN NEW MEXICO AND SOUTHWEST TEXAS...

Sunday is likely the beginning of what will be a multi day period
of excessive rainfall potential over NM. The mid/upper level 
pattern features a near record to record ridge over the eastern 
U.S. and a well defined longwave trough over the west. Southerly 
flow in between these features will supply ample moisture to NM, 
with a connection all the way to the Caribbean Sea. Thus no 
surprise that forecast PWs are well over the 90th percentile, and 
possibly approaching near record levels for late June. The better 
forcing from the western trough should hold of until Monday and 
Tuesday (see the day 4 and 5 EROs), but by Sunday we should at 
least have enough moisture and instability around for isolated to 
scattered convective development. This appears to be a solid 
Marginal risk with localized flash flooding expected...and can not 
rule out eventually needing a Slight risk. Sometimes these first 
day of the event situations can overperform as the clean start can
allow for more destabilization and greater convective coverage 
than expected. However with Monday and Tuesday currently expected 
to be the bigger days, will keep this day 3 as a Marginal for now 
and continue to monitor trends.

Chenard

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