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Last Updated: 2022-09-26 07:26:24.717038 MST
Idaho
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Montana
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Meteorology/Dispersion Discussion

Last Updated: 2022-09-25 13:03:57.587428 MST

Monday 26 September Discussion: Poor dispersion conditions under strong upper ridging with slow to break valley inversions will occur for most western airsheds today and tomorrow. Latest forecast modeling is slowing down the progression of an upper trough, now scheduled in for Thursday over ID and Western MT. So Wednesday will not see as much improvement as had been expected earlier. On the other hand, the trough will also more slowly move out, so a return to widespread poor dispersion for Idaho and Western MT airsheds will not occur until next Sunday, and may be brief. As long range modeling is indicating a more active fall pattern developing next week, with more upper trough passages possible.

Today: POOR-MARGINAL dispersion will occur over North ID and Western MT with the upper level ridging and strong valley inversions. Light/variable surface winds. POOR dispersion will occur over Central ID with light/variable surface winds. POOR-MARGINAL dispersion will occur South ID and SW MT with light/variable winds. MARGINAL-GOOD dispersion will occur in Central/Eastern MT with light E-SE surface winds Central and NW surface winds Eastern.

Tuesday: POOR-MARGINAL dispersion will occur over North ID and Western MT under strong upper ridging. Light/variable surface winds. POOR-MARGINAL dispersion will occur over Central and South ID with light/variable surface winds. MARGINAL dispersion will SW MT with light SW winds. MARGINAL-GOOD dispersion will occur in Central/Eastern MT with light SE-S surface winds.

Wednesday: POOR-MARGINAL dispersion will occur over North ID and Western MT under strong upper ridging. Light S-SW surface winds. POOR-MARGINAL dispersion will occur over Central and South ID with light S-SW surface winds. MARGINAL dispersion will SW MT with light SW winds. POOR-MARGINAL dispersion will occur in Central/Eastern MT with light/variable winds Central and light SE-S surface winds Eastern.                                  

Thursday:  MARGINAL to GOOD dispersion North ID and Western MT as upper trough passes. Westerly surface winds. Scattered showers. MARGINAL-GOOD dispersion Central and Southern ID with W-NW surface winds. GOOD-EXCELLENT dispersion Central and Eastern Montana with W-NW surface winds, except SE-S far eastern. 

Friday: MARGINAL-GOOD dispersion will occur over ID and Western MT under circulation around upper low over Eastern MT. NE-N surface winds. MARGINAL-GOOD dispersion will occur over Central and Southern ID and SW MT with W-NW surface winds.  GOOD dispersion will occur in Central/Eastern MT with W-NW surface winds. Scattered showers.

Saturday: MARGINAL-GOOD dispersion will occur over ID and Western and SW MT under northerly flow aloft. NE-E surface winds. POOR-MARGINAL dispersion will occur over Central and Southern ID with light/variable surface winds.  GOOD dispersion will occur in Central/Eastern MT with NW-N surface winds, scattered showers.

Sunday: POOR-MARGINAL dispersion will occur over North ID and Western MT under upper ridging. Light/variable surface winds. POOR-MARGINAL dispersion will occur over Central and South ID with light S-SW surface winds. POOR dispersion will SW MT with light/variable winds. MARGINAL dispersion will occur in Central/Eastern MT with light SE winds Central and light NW-N surface winds Eastern.                                  

Tuesday 27 September Dispersion Forecast: 

MONTANA

Northwest (AS 1, 2): POOR.

West Central (AS 3A, 3B, 4, 5, 6): POOR except MARGINAL As 6.

Southwest (AS 7, 8A, 8B): POOR.

Central/Eastern (AS 9, 10): MARGINAL Central. GOOD Eastern.


IDAHO:

North (AS 11): MARGINAL.

North Central (AS 12A, 12B, 13): MARGINAL.

West Central (AS 14, 15): POOR.

East Central (AS 16, 17): POOR.

Southeast (AS 18, 19, 20): POOR.

Southwest (AS 21, 22, 23, 24): MARGINAL.

Prepared by Dan Borsum/Michael Richmond NRCC Predictive Services Meteorologists