Friday, September 15th: What a difference in conditions over the last few days in the northern Rockies! A digging trough of low pressure aloft is bringing significant rainfall and higher elevation snow to Montana and neighboring areas of Idaho and Wyoming. One to two inches of rain has fallen in the last 48 hours to 7 PM this evening from Helena to Great Falls to Billings in Montana with similar amounts across parts of northern Wyoming. Lesser amounts but still significant rainfall has occurred in western Montana and Idaho. This should be sufficient to pretty much douse the wildfires in Idaho and Montana.
No rain has yet fallen in Washington, Oregon or northern California, so wildfires in those areas continue unabated. However, significant rain is forecast by Sunday in Washington and at least northern Oregon. This should put a significant dent or even put an end to some of the wildfires in those two states. As for California, it appears they will have to wait as rainfall is not forecast to extend that far south with these systems.
In the meantime, satellite imagery this evening reveals that the denser smoke in our region is now east of us in places like Illinois and Indiana, as the smoke contribution from Idaho and Montana has pretty much come to an end, reducing the influx of high altitude smoke into our region.
Our area should start getting some rain as early as Saturday evening or Saturday night, with chances of rain continuing for several days perhaps. Significant rainfall is even possible in parts of our region over the next several days. This should be a welcome change as the ground is getting pretty dry now after about three weeks with no rainfall.
Satellite image at 6:15 PM, Friday, September 15th. Good rains ongoing in the northern Rockies and even extending in many areas across North Dakota and northern Minnesota. Showers trying to get going in our area, but the lower levels are dry and convergence and instability weak, so mainly just some considerable clouds. Wildfires rage on in Washington, Oregon and northern California with many dense smoke plumes in that region. The denser smoke in our region appears to be east of us now that the supply from the northern Rockies is cut off.
This evenings upper air chart (500 MB) shows the pattern shift with a digging, and rather strong upper trough of low pressure for this time of year over the northern Rockies and Intermountain Region. A southwest flow is developing over us associated with this trough, bringing some moisture aloft and clouds but not much yet in the way of rain as lower and mid levels remain pretty dry so far.
Surface map at 8 PM CDT shows increasing southerly flow over our area ahead of surface lows and a connecting cold front. Strong east ‘upslope’ flow at lower levels in Montana and northern Wyoming bringing lots of rain to the northern high plains and east slopes of the northern Rockies, with lesser amounts west of the continental divide.
At jet stream level (250 MB, or about 35,000 ft) we see an unusually strong jet stream pattern around the developing upper trough of low pressure. Northwest winds on the back side of the trough are stronger than southwest winds on the east side, indicating that the upper trough is still deepening (intensifying). The rather strong jet and fairly stout trough will bring some chance of severe storms to parts of our region over the next few days – though the focus is expected to be mainly to our north. However, there is decent potential with this system anywhere in our region. Note also a secondary ‘subtropical jet’ extending from west of Baja across northern Mexico and also the US southeast coast. This will tend to cause any approaching tropical systems to shear apart, including Category 1 Hurricane Jose. That long-lasting ocean-bound storm is meandering slowly northwestward in the Atlantic several hundred miles east of Florida.