Monsoonal flow interrupted for the weekend but returns mid next week

Thursday, July 20, 2017

Though the rainfall from the current Southwest U.S. Monsoon has only amounted to a few tenths of an inch over the past two days in the Steamboat Springs area, we will have more chances today and Friday for locally heavy rainfall, including the overnight periods.

Subtle waves of energy moving northward in the monsoonal moisture plume make local forecasting difficult, but current model forecasts have one of these waves phasing with some energy moving across the northern U.S. and traveling over our area later on Friday. This should give us our best chances for locally heavy rainfall Friday evening and overnight.

The westerly flow behind Friday’s wave will return drier air to northern Colorado for the weekend and into the early part of next week. Along with a building ridge of high pressure over the Intermountain West, there should be minimal chances for afternoon storms on Saturday, with a better chance on Sunday.

The drier air will stick around until Wednesday, keeping precipitation chances low, before a storm off the Pacific Northwest coast elongates to the south along the West Coast. This will  promote southwesterly to southerly flow that is forecast to bring another monsoonal surge of moisture over Colorado. Some energy ejecting out of the West Coast trough of low pressure may provide a focus for stronger storms on Wednesday, with continued chances of more rains through the rest of the work week as the monsoonal plume of moisture remains mostly intact.

The evolution of the West Coast trough will dictate whether we see drier air for next weekend or a continuations of the moist conditions.

Wetting rains likely around midweek ahead of drier weekend

Monday, July 17, 2017

A storm off the coast of British Columbia has mixed with some cool air from western Canada, and a piece of the storm will travel across the northern U.S. border through today. Energy associated with this wave has flattened the dominant western ridge of high pressure that was over the Steamboat Springs area this past week and may provide a focus for a round of afternoon storms today.

Some drier air will follow for Tuesday as the western ridge tries to rebuild, though there will still be a slight chance of afternoon storms.

By Wednesday, the parent storm near British Columbia begins to move eastward, with energy ejecting out ahead of the storm nudging the flattened western ridge eastward and opening the door to a robust surge of monsoonal moisture from the south.

We should have a good chance of wetting rains on Wednesday and Thursday, including the overnight periods, with locally heavy precipitation possible, as embedded subtropical waves move through the moisture plume and phase with some passing energy from the eastward-moving British Columbia storm to our north.

Drier air is advertised to encroach on our area by the weekend in the westerly flow behind the passing storm to our north. Though we will still have a good chance of storms on Friday, the drying air should lead to a downturn in precipitation chances heading into the weekend.

Another monsoonal surge next week after drier weekend

Thursday, July 13, 2017

A building western ridge of high pressure has brought drier air to the Steamboat Springs area as of late yesterday afternoon. As discussed in the previous forecast, northern Colorado was close to the boundary separating drier air to our north and west from monsoonal moisture to our south, and Tuesday the boundary moved north allowing for another day of wetting rains. By Wednesday afternoon, drier and cooler air moved over our area as the boundary moved southward precluding significant precipitation.

The western ridge looks to dominate our weather through the weekend and into Monday. Hot temperatures and the possibility of daily afternoon storms are on tap as remaining low-level moisture is cooked by the strong summer sun as it recirculates under the ridge of high pressure.

Meanwhile, a storm off the coast of British Columbia mixes with some cool western Canadian air as advertised by the European ECMWF last week. As the storm moves to our north late in the weekend, a trailing cool front may be the focus of stronger storms later Monday and Monday night.

The storm will flatten and elongate the ridge of high pressure eastward, which opens the door to another monsoonal moisture surge in the southerly flow on the west side of the ridge. This one looks to hang around longer than the last one, with good moisture over our area through most of the work week. Some cloud cover will keep the hot temperatures at bay, and storm cells that move over our area will be capable of producing locally heavy showers.

Extended-range models are in disagreement with regards to the evolution of another weak wave moving to our north near the end of the work week. The European ECMWF is more aggressive in bringing the storm eastward, interrupting the monsoonal moisture tap a day earlier than the American GFS, as we head into a weekend advertised to be drier than the preceding work week.

Today offers the best chance of wetting rains this week

Monday, July 10, 2017

Pacific energy moving eastward from a complex of storms in the Gulf of Alaska has flattened the dominant western U.S. ridge and nudged it eastward, allowing the first significant surge of the Southwestern U.S. Monsoon to reach the Steamboat Springs area this season.

The tropical storm system discussed in the last forecast did indeed turn into hurricane Eugene and contributed to the moistening of the atmosphere over the southwestern U.S. As a result, there is a good chance of wetting rains over the Steamboat Springs area later this afternoon and early this evening. Short range models show another batch of showers that may reach far enough northward to produce more rain around midnight.

As weak Pacific energy moves eastward to our north, drier air moves in behind the wave, and our weather will depend on the outcome of the battle between this dry air to our west and north and the monsoonal moisture to our south and southwest. Current models have a chance of showers for Tuesday and Wednesday as the boundary between the dry and moist air is close to northern Colorado.

By Thursday, the western U.S. ridge rebuilds, bringing drier air across the region under hot temperatures. The strong surface heating will cook any remaining low-level moisture to produce a chance of afternoon storm for the remainder of the work week and into the weekend.

Significant differences between the models arise for the second half of the weekend, with the American GFS further rebuilding the western U.S. ridge and further warming temperatures as cool Canadian air moves southward across the Eastern U.S. The European ECMWF, on the other hand, slides this cool Canadian air westward and tries to phase it with some incoming Pacific energy, flattening the western U.S. ridge and moderating temperatures for the second half of the weekend and early next week.

Some moisture for the weekend and more next week

Thursday, July 6, 2017

A ridge of high pressure centered over the Great Basin will be nudged eastward this weekend by incoming Pacific energy. The weak southerly to southwesterly flow on the west side of the ridge will allow the monsoonal moisture pooling around the Sonora Desert to move northward, increasing mid and upper level moisture over the Steamboat Springs area for Friday and Saturday. The additional moisture will create high-based afternoon and early evening storms that will produce more gusty and erratic wind than rain, and possibly some dry lightening, as the lower levels of the atmosphere remain quite dry.

By Sunday, the Pacific energy traveling over the top of the ridge will interrupt the fledgling monsoonal flow from the southwest, with drier air moving over the area in weak westerly flow. There will still be the possibility of afternoon storms though, as the meager low-level moisture is lifted by the strong surface heating of mid-summer and interacts with some of the grazing Pacific energy to our north.

On Monday, more Pacific energy again pushes the western ridge eastward into the Great Plains, and a better monsoonal tap of moisture appears around Tuesday. While mid and upper level moisture is carried northward by southerly flow on the west side of the ridge, low level moisture may also receive a boost from a tropical storm moving westward south of Baja. Northern Colorado will have a much better chance of wetting rains starting around Tuesday and extending through the work week and into next weekend, as moisture falling from any storm will not encounter the very dry lower-level air present for this weekend.

Longer-range models, as of today, want to keep the monsoonal flow around after next weekend as well, hopefully leading to a far wetter July as compared to June as tropical waves of energy move northward over the Sonora Desert and eventually across the Great Basin.

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