Mesoscale Precipitation Discussion 1218
NWS Weather Prediction Center College Park MD
335 AM EDT Mon Sep 07 2026

Areas affected...East Central Montana...

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

Valid 070735Z - 071300Z

SUMMARY...Training/repeating cells along border with upstream
redevelopment due to approaching deep layer cyclone to support
strong cells capable of 1.5-2.5" in a few hours.  Localized flash
flooding will be possible where training/repeating occurs.

DISCUSSION...GOES-W WV suite denotes core of closed upper-level
low is shifting out of WA/OR into ID with modest negative tilt
orientation across the Northern Intermountain region of SW MT/W WY
and is slightly out of phase with southwesterly flow shortwave
though in a favorable manner for large scale ascent/cyclogenesis
across the Northern High Plains. This shortwave exists in the
split of the inner polar jet over SE ID and stronger 90kt jet
streaming across WY, resulting in strong downstream dPVA over SE
MT and increased divergence/diffluence noted by lower layer
baroclinic shield over central MT (along/west of 110W).   As such,
surface to 850mb cyclogenesis is well on it way across SE MT/NE WY
and a sharp inverted trough exists northward through NE MT; west
of which stronger northerly/north-northeasterly flow is
strengthening providing solid moisture flux with 1.25" TPW
increasingly toward 1.4" Rosebud/Yellowstone county along the
northwest side of the surface low. 

Above average theta-E air still retains heating from this
afternoon and with some steepening lapse rates results in an axis
of 1500-2000 J/kg of capped MLCAPE.  Weaker capping near the
Canadian border with initial moisture flux convergence in the
850-700mb layer has maintained scattered supercells that have
advanced eastward out of Blaine into Phillips county. 
Intersection with the divergence aloft and increasingly low level
moisture flux has seen an broadening of updrafts and upscale
growth of the complex.  Steering flow has also reduced the
eastward propagation enough for increasing residency of modest
1.25-1.5"/hr rates.  Though isolated personal weather stations
suggest stronger instantaneous/sub-hourly totals, near 1"/30
minutes.  Slowing steering will also allow for some repeating in
the near term, so localized 1.5-2" with an isolated 2.5" becoming
possible.  Naturally low FFG values <1.5"/hr and <2"/3hrs have
good possibility of locally being exceeded with potential for
localized incident(s) of flash flooding possible in the next few
hours.

As the negative tilt and shortwave lift northward, stronger
convection is expected to develop (as noted with a few cells
breaking out in Sweet Grass/Stillwater counties).  While faster
moving, deeper layer moisture convergence along the north-south
oriented boundary between the convective line along 850-700mb
effective frontal zone may promote some repeating from south to
north and while likely less effective than storms north, rates of
1-1.25" are possible and streaks of 1-2" may also be possible.  As
such, have expanded the area of concern within the
instability/moisture axis west of the surface low and along the
axis of lower FFG values as points of isolated flooding concerns
as well through 12z.

Gallina

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

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

LAT...LON   49090858 49080748 49070525 48750537 47880609
            46870676 45850801 45800955 46790892 47740867
            48280873 49010881