textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Mild conditions the rest of the week with only minor disturbances.

- Shortwave Monday to bring breezy conditions.

- Potential for a cold front Friday into the weekend, but confidence is low.

- Chances are increasing for a warm, dry early October beyond the 7 day forecast.

Overall mild conditions are expected the rest of the week. The weather patterns we expect throughout the week are: weak zonal (west-to-east) flow aloft, a shortwave on Monday in split flow, and some weak ridging. The shortwave on Monday will cause some breezy conditions, but it is a result of a cutoff low splitting out of the flow and diving into the U.S. Southwest. As a result, most of the energy and moisture will go to the south. During periods of zonal flow, there will be some moisture mixed in, but it will only affect far northwest Montana and Glacier National Park around Tuesday and Wednesday. Even for that though there's only a 40% chance of up to 0.10 inches of precipitation in the mountains.

Some models, namely the GFS, are gearing up for another cold front to end the week on Friday into Saturday. Looking at ensemble clusters for rain, current forecasts only show an 11% chance of some moisture with that event again in northwest Montana. Most ensemble members don't show a cold front at all.

Active weather in the tropics right now is setting up the pattern in the long term. With 3 hurricanes in the central and east Pacific Ocean from Hawaii to Baja and a typhoon approaching the eastern periphery of Japan, it's been a difficult forecast for models. However, they're starting to coalesce around higher chances for a potent ridge over the northern Rockies into the first third of October. Ensemble mean temperature forecasts are coming in 15 degrees above seasonal averages at about 9-12 days out. Those are some large anomalies for that long of a lead time! Add in that the signal is coming from multiple ensemble systems including both dynamical (physics based) and AI modeling systems, it gives above normal levels of confidence for such a long lead time forecast. Looking back, the last time this kind of warm ridge set up over the northwestern US in early October was october 9-11 and 14, 1991. Eureka, Montana hit 86 degrees, Grangeville, Idaho 87 degrees, and Butte 81 degrees! The Center for Climate Prediction has issued forecasts for the northern Rockies region with a 60-70% chance of above average temperatures and precipitation forecasts are leaning below average for that time period as well.

AVIATION

Today an upper level low is exiting the region with a weak ridge building in behind it. Generally expect mid-level ceilings rising throughout the day with some clearing overnight. With stable conditions expected going into the evening, valley fog is likely across the region. However, with dew points in the low to mid 30s in the valleys and in the upper 20s on the ridges, we do not expect the valley fog to be widespread enough to impact regional terminals.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.