AWUS01 KWNH 122148
FFGMPD
MTZ000-130245-

Mesoscale Precipitation Discussion 1077
NWS Weather Prediction Center College Park MD
547 PM EDT Fri Sep 12 2025

Areas affected...North central Montana...

Concerning...Heavy rainfall...Flash flooding possible

Valid 122145Z - 130245Z

SUMMARY...Stationary to very slow moving thunderstorms capable of
1-"/hr and localized 1.5-2.5" totals pose localized flash flooding
conditions through the evening.

DISCUSSION...GOES-E WV suite depicts area of concern is located
along the eastern edge of the northern portion of a large global
scale meridional trough that extends from southern Alberta to the
Lower Colorado River Valley in CA/AZ.  Embedded within, are
smaller shortwave centers, one lifting northwest to reinforce the
apex of the larger trough and the center core over the W MT
borders.  This placement provides solid height-falls to support
low level easterly flow out of the northern Plains where
seasonally abnormal deep layer moisture resides, with nearly 2 to
3 standard deviations from the Sept mean over the Dakotas.  Though
the anomalies are a bit less across the area of concern, the
strength of the eastern flux over the last few hours along with
solid insolation supported enhanced conditionally unstable air,
generally centered through the Little Belt Mountains and Judith
Basin of central MT.  Total Pwats are nosing up from .75 toward 1"
though heating and modest lapse rates support 500-1000 J/kg of MLCAPE.

Convergence along the terrain supported convective initiation a
few hours ago and rates of .75"/hr have been estimated with some
higher values likely contaminated with hail, though that adds to
moisture on the ground.  The concern here, though resides in
duration as mid-level steering flow is very weak at 15-30kts
(increasing to the east), but in proximity to left exit of the
larger scale anticyclonically curved jet over SE MT, providing
divergence aloft.  As such, effective cell motions are mainly
driven on propagation/regeneration of updrafts along the outflow
boundaries.  This allows for some 1-2 hours of near stationary
rates supporting some localized totals up to 2".  Current trends
in Visible/IR and GLM/NLDN lightning suggest further expansion
northward across Chouteau and Blaine counties with trends that
suggest mergers may result and increase rainfall efficiency toward
1-1.25"/hr.   Given FFG values of 1-1.25"/hr and generally less
than 2"/3hrs, widely scattered incidence of exceedance and
therefore potential localized flash flooding is considered
possible through evening. 

Gallina

ATTN...WFO...BYZ...GGW...TFX...

ATTN...RFC...KRF...NWC...

LAT...LON   49031013 48990897 48930779 48390761 47370792 
            46530846 46290904 46420964 46671004 47031069 
            47311129 47811149 48831126 

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