Mesoscale Precipitation Discussion 1083
NWS Weather Prediction Center College Park MD
517 PM EDT Sat Sep 13 2025
Areas affected...Northwest NDak...East-central & Northeast MT...
Concerning...Heavy rainfall...Flash flooding possible
Valid 132115Z - 140230Z
SUMMARY...Highly anomalous moisture, increasing instability and
slow cell motions with some back-building training possible.
Rates of 1.5"/hr and localized 2-3" totals may result in possible
scattered incidents of flash flooding through early overnight
period.
DISCUSSION...GOES-E WV suite shows a conducive dynamic environment
aloft to maintain and support new convective development within a
seasonally, highly anomalous moisture regime. The northern,
smaller closed low/short-wave to the parent large scale meridional
trough is well defined and compact over central MT. It is
connected via a boundary layer convergence/confluence axis that
waves eastward before turning north along/just west of the MT/ND
border where it eventually connects back up to the exiting
vorticity center over southwest Canada. Along and east of the
confluence boundary, deep layer moisture with highly anomalous
(2-4 standard anomaly units) above average exists across the
Northern Plains. Upper 60 to low 70s Tds with southeasterly flow
strengthening to 20kts at 850 provide deep layer convergence to
the boundary.
GOES-E Visible imagery shows developing overshooting tops through
the mid-level cloud deck along the boundary, some denoting some
Wedge/'V' shaped signatures. This is indicative of the exiting
upper level 70kt speed max and the cells moving into better right
entrance ascent. Combine this with sharply arched cirrus canopy
over central SD denoting the nose of the next speed max suggests
this favorable divergence aloft will likely maintain for quite a
few hours into the early overnight period. Limited motion of the
convergence boundary is expected given the surface to low level
flow, providing a solid opportunity for convective cells to
train/repeat over NW ND through the evening. The limiting factor
continues to be the lack of surface heating due to the mid-level
canopy, so vertical vigor may limit overall rainfall
production/intensity in the range of 1-1.5" (given total PWat
values of 1.5-1.7") and CAPE 500-750 J/kg. As such spots of 2-4"
are possible resulting in possible flash flooding.
Further west,
Clearer skies, has supported some low level heating near the
stationary mid-level feature and eastward along the low to
mid-level shear axis into eastern MT. A few stronger cells have
developed, utilizing the enhanced surface heating and weak to
moderate convergence. Limiting factor with these cells mainly in
in the deeper moisture profile. While still above normal Tds in
the low 50s and total Pwats range from 1-1.25". This should
reduce overall efficiency and rates to around 1-1.25"/hr but slow
cell motions perhaps even stationary near the vorticity center
itself could result in localized 2-3" totals in 3-6hrs. Recent
rainfall and FFG values in the 1-1.5"/hr & 1.5-2"/3hrs suggest
these rates/totals may result in an incident or two of flash
flooding through the evening.
