wxgeek's weather-Big Change in Weather this wk-Update Oct 5

Update Wed Oct 5

So models still have some differences, but better overall concensus so that forecast has more confidence, but still some uncertainties especially when it comes to foothill snowfall.

Large upper low has moved onshore along the West Coast today, and brought heavy rain and snow to the Pacific States, and is now moving into the Great Basin and SoCal. Up to 3 inches of rain was reported in CA coastal hills, with up to 18 inches of snow at higher elevations in the Sierras. Forecast is solid in bringing the upper low into eastern NV on Thursday. The upper trough elongates and splits with a northern low moving into WY, and a southern low forming near the 4 Corners area on Friday. The initial upper trough will bring heavy rain and snow to western CO beginning late tonight into Thursday, but will remain mostly west of the Divide, with some spilling over to a line from Longs Peak to Kenosha Pass by Thursday afternoon. It is the southern upper low that may bring us our first snow of the season in the foothills. new forecats looks like this:

Precip will move into western CO tonight and become heavy during the day on Thursday. Associated cold front will move across CO on Thursday. Snow level ahead of the front will be 10,000-11,000 ft, while on the back side of the front snow level will be 6000-7000 ft, so much colder air behind the front. Winter Storm Warning in effect for the southwest mountains, and Winter Storm Watch in effect for central and northern mountains from late Wednesday night into Thursday night. Travel across western CO will become difficult on Thursday from the Divide west. Cold front will pass over the foothills by Thursday late afternoon/evening and may bring a brief shower, but very limited precip due to strong southwest flow which is downsloping and dry. Speaking of winds, south to southwest winds will increase along the foothills and Plains on Thursday, with peak winds in the afternoon of 30-55 mph which may cause downed trees and power outages. After the cold front passes winds should die down some, and temps will begin to drop. Air behind the front looks very dry, so do not expect any precip post frontal passage Thursday night and Friday. Friday will be partly cloudy and cool and breezy.

Saturday is now the focus of any precip we may get from this system. Models are in fair agreement that the southern branch of the system will form over the 4 Corners region late on Friday, and meander along the south central CO area through Sunday. If this forecast verifies, some decent upslope flow will develop late Friday night into Saturday and provide some good rain and snow for the foothills and adjacent Plains. Snow level would be 7000-8000 ft on Saturday, with some flakes down to 6500 ft. Accumulation should be confined to areas above 7500-8000 ft. So for possible amounts, if we get the good upslope that models are currently advertising, we could see 3-6 inches of snow above 8000 ft from Friday night to Sunday morning. If upper low and upslope are weaker, then maybe just a dusting to a few inches. If the upper low intensifies beyond current forecast, then even more snow. It now appears likely that most foothill areas will see our first snow of the season this weekend. Unsetttled weather with showers possible on Sunday as well, with snow level around 7500-8500 ft. System should exit late on Sunday, so warmer and drier conditions on tap for next week.

Another update Thursday.
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Update Tue Oct 4

Large and powerful Pacific storm on track to affect the west coast from tonight into Wednesday. Winter Storm Warnings are posted for the Sierras above 6000 ft for a foot of snow, and up to 20 inches above 8000 ft, so travel across the Sierras could be difficult on Wednesday.

Models still not in very good agreement on the track of this system across CO, but general trend is starting to appear. Large upper low will move across the west coast Wednesday and into the Great Basin on Thursday. The main part of the upper low moves into WY on Friday. GFS model takes a southern branch and creates a small upper low over south central CO on Saturday, while the NAM and ECMWF models just move an open trough across CO on Saturday.

So, here is how the forecats looks today:

Isolated showers and thunderstorms increase west of the Divide today and Wednesday, with a chance for a stray shower or thunderstorm across the foothills. Main storm precip enters western CO Wednesday night with snow level initially around 10,000 ft. Cold front will move across CO on Thursday and bring heavy precip to areas west of the Divide during the day. Snow levels will lower to 8000-9000 ft west of the Divide Thursday, where Winter Weather Advisores will likely be issued for the southwest and central mountains. Winter Storm Watch already issued for the southwest mountains above 9000 ft for Wednesday night til Thursday evening for up to a foot of new snow. Ahead of the system, very strong southwest winds will occur along the foothills and Plains, with winds in the 25-50 mph range during the day on Thursday, so winds may cause downed trees and power outages on Thursday. Fire danger will aslo be high due to very strong winds. Cold front will pass over the foothills Thursday evening, so an isolated shower is possibe with frontal passage. Cooler temps will transpire behind the cold front Thursday night into Friday.

Currently looks like Friday will be cooler and breezy, but no precip as main upper and surface lows move into eastern MT. If the GFS solution verifies, then we have a chance for rain and snow above 8000 ft on Saturday. If the other models verify, likely just cooler and very isolated showers possible on Saturday with no snow accumulation. Right now it is hard to determine which model has the correct solution, so will have to wait and see additional model runs. Overall this looks like the first big winter storm of the season for western CO, and will be mostly a high wind event for eastern CO. If the secondary upper low does form over south central CO, then we would see our first snow with some upslope flow on Saturday, but that is far from certain at this time.

Next week continues to look mostly dry with seasonal temps.


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Our very warm early Fall weather should continue the early part of this week. Isolated afternoon and evening thunderstorms are possible Monday through Thursday, with the majority of the activity remaining on the west side of the Divide.

Some big changes will take place later this week. Our upper ridge that has been parked over the inter-mountain region most of September has begun moving east into the Plains, and will continue to move eastward during the week. This will allow a powerful upper trough currently in the Gulf of Alaska to move across the west coast and into the Great Basin later this week. Forecast models are having a hard time coming to concensus on the track of this system as it moves across CO, so low confidence in the current forecast as it will likely change between now and later this week. If the forecast were to verify, here is what it looks like:

Upper trough will move into the Pacific Northwest late in the day on Tuesday and bring precip across WA and OR Tuesday night, spreading south to CA on Wednesday. Winter Advisories are already posted for the Cascades and Sierras, with 12-18 inches of snow expected above 7000 ft in the Sierras, so chain restrictions across Sierra highways likely by late Wednesday and Wednesday night. The system will move into the Great Basin on Thursday with snow expected across the Wasatch Range above 7000 ft. Precip is expected to make it's way into western CO late on Thursday with snow level around 8000-9000 ft. Precip should remain west of Vail Pass on Thursday. Now for the uncertainty, models currently split the energy in the upper trough on Friday, with part going into an upper low in WY, and part going into an upper low to form over western CO. The southern upper low then moves slowly northeast across CO and into southeast WY on Saturday. The end result of this forecast is for only light precip across our foothills. Best chances for precip currently looks like during the day on Friday, with showers continuing into Friday night and during the day on Saturday. Snow level looks to begin around 8500-9500 ft on Friday, lowering to 7000-8000 ft by Saturday morning, and remaining around 8000 ft during the day on Saturday. Best chance for accumulating snow would be Friday night. At this time, amounts look to be from a dusting to a few inches for most foothill locations. Heavier snow is likely west of the Divide. Model ensemble forecast, which is a spread of all the possible model solutions, is all over the board, hence the low confidence at this time. If the track of the upper trough stays farther north, we could see little to no precip. If the track moves farther south and we get an upslope component, we could see considerably more precip. So will have to wait til later this week when models hopefully come into better agreement. The certain part of the forecast is that much cooler and more unsettled weather in on the way for late this week.

The other impact will be for us to see an increase in southwest winds aloft during the week, which will translate to strong south to southwest surface winds across the foothills. Conditions could prompt a Red Flag Watch or Warning for Tuesday into Thursday if relative humidity values drop. Thus, increased fire danger this week through Thursday night. Surface winds could get quite strong on Thursday into Thursday night in advance of our storm, so downed trees and power lines are possible on Thursday and Thursday night. With temps so warm this week, I doubt roads will be much of a problem even with snow below 9000 ft, as most will melt or just be slushy Friday night into Saturday morning.

Stay tuned for further updates during the week.
_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

Update Oct 5