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Mesoscale Discussion 139
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   Mesoscale Discussion 0139
   NWS Storm Prediction Center Norman OK
   0909 PM CST Sun Feb 05 2017

   Areas affected...Portions of northwest MT and far northern ID

   Concerning...Heavy snow 

   Valid 060309Z - 060915Z

   SUMMARY...Embedded within a long-duration snow event particularly
   impacting the higher elevations, a prominent increase in snowfall
   rates is expected through 06Z. Snowfall rates in excess of 2 inches
   per hour will be possible, especially for locations above 5500 feet,
   with similar snowfall rates forecast to persist through the
   overnight hours.

   DISCUSSION...Surface observations suggest surface pressure falls
   around 2 mb per 2 hours becoming increasingly widespread across WA
   and northern ID. This is where midlevel height falls/large-scale
   ascent is becoming increasingly focused downstream of cyclonic
   perturbations approaching the Pacific Northwest Coast per water
   vapor loops.

   The isallobaric response to these pressure falls is becoming
   manifest in an increase in upslope flow along the front range of the
   northern Rockies west of the Cut Bank area, where radar mosaic loops
   imply more prominent consolidation of modest reflectivity. Through
   06Z, further strengthening in upslope flow will contribute to an
   increase in snowfall rates -- firstly across the Lewis Range. This
   will be the area of most consistent upslope flow along the western
   flank of an Arctic air mass supporting large snow:liquid ratios
   owing to appreciable ascent through a saturated dendritic growth
   zone in the lower levels. Snowfall rates of 2-3 inches per hour will
   become likely, continuing through the overnight hours.

   To the west of the Lewis Range, for areas in proximity to the
   international border -- i.e., parts of extreme northwest MT into
   extreme northern ID -- areas of snow rates locally reaching 2 inches
   per hour will become likely -- especially more toward 06Z. The more
   spatially consistent low-level upslope-flow pattern will be somewhat
   more interrupted by a more variable terrain-slope pattern.
   Regardless, the southern fringes of strengthening low/mid-level
   frontogenetic circulations downstream of the aforementioned cyclonic
   perturbations will support an increase in snow rates later this
   evening continuing into the overnight hours, just south of the
   international border. Windward slopes of mountains amidst a
   spatially-variable low-level flow pattern will be the predominant
   locations of higher snow rates.

   The overall trend in observational and high-resolution model
   guidance has been a slightly earlier initiation of snowfall rates
   exceeding 2 inches per hour. This activity could continue even into
   the morning hours of Monday. Areas of strong winds -- enhanced in
   terrain-enhanced flow-channeled regimes -- may support greatly
   reduced visibility in blowing snow, with localized areas of blizzard
   conditions possible. Areas around the Lewis Range could be most
   susceptible to such conditions, owing to a tighter pressure gradient
   along the flank of the High Plains centered Arctic air mass.

   ..Cohen.. 02/06/2017


   ATTN...WFO...TFX...MSO...OTX...

   LAT...LON   48581403 48711465 48741545 48731602 48751672 48881697
               48991681 49021630 49011522 49001437 49011385 48961338
               48731307 48601294 48101277 47971295 48041317 48261331
               48431364 48581403 

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Page last modified: February 06, 2017
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