Riverton, Wyoming

• A weather system crosses the area today into tomorrow and brings periods of rain to many along with some mountain snow.
• Gusty to strong wind likely (greater than a 3 out of 4 chance) the eastern half of the area on Monday, with the strongest wind in Johnson and Big Horn Counties.
• More tranquil weather returns for the rest of the workweek except for a chance of showers Wednesday.

The weather has cooperated for the most part for evening activities for Halloween. We are now entering what I call the 151 days (or 152 if it is a leap year) of winter in western and central Wyoming. This is when we usually start expecting more wintery precipitation than rain. I say usually because the past couple of winters have been unusually warm and dry. We will have the precipitation, and there will be some snow, but not really in the lower elevations. Going to try something different tonight with the discussion, I will discuss the overall weather pattern over the next couple of days first with the players. Then, I will focus on the three main impacts, precipitation amounts, potential snowfall and amounts and finally potential for strong to high wind in portions of the area.

The low that will be bringing the precipitation today is now moving onshore in northern California and will open a trough crossing the Rockies late tonight and into tomorrow. The latest model trends have been downward somewhat though with QPF, with the bulk remaining to the south over Utah, Colorado and Arizona. We will see some here though, with the heaviest precipitation in the west through midnight tonight, tapering off after midnight after the trough axis passes through the area. For areas East of the Divide, although there will be some showers around this morning, the most numerous should be tonight through Monday morning. Lee cyclogenesis will occur over eastern Colorado late today and then the low will strengthen as it moves northeastward into the Northern Plains states. This will likely bring some wrap around moisture that will keep precipitation going somewhat longer. However, it will also introduce a dry slot that could keep amounts down somewhere. And this complicates the rain and snow forecast somewhat. And finally, with the deepening flow, the pressure gradient will tighten across the area. This will bring the likelihood of strong wind to much of the area and potentially high wind to portions of Johnson County. These main impacts should end Monday evening as the low moves away to the east. Now for the individual forecast elements.

Precipitation amounts...As was stated earlier, these have trended down somewhat with core of the moisture remaining to the south. However, all areas should see some precipitation. The wettest areas at this time look to be 1) Far southern Wyoming, which will be closer to the core of the deeper moisture; 2) The western facing slopes in the western mountains with a period of favorable orographic flow and 3) Favored northerly upslope flow areas East of the Divide like the Wind River Basin south of Riverton and the Lander Foothills, around Casper, the southern end of the Bighorn Basin and the eastern Bighorns. As a counter, the least amounts would be in 1) far northern Wyoming, which will be away from the core moisture and 2) Areas of downslope, like the eastern Bighorn Basin, the northern Wind River Basin and the Green River Basin. However, there are still model differences on the placement of the heaviest rain and the potential dry slot. Other differences are with the amount of wrap around, as some have it further west than others. As for amounts, most areas have at least a 1 in 2 chance of a quarter inch of QPF or more, with the exception of the aforementioned downslope areas. As for greatest chance of three quarters of an inch or more, this would be mainly in the aforementioned upslope areas. Some areas will likely see more than an inch of QPF through the system though. Most should end by sunset Monday, with all precipitation finished by sunrise on Tuesday. CONFIDENCE IN AMOUNTS...Moderate to low.

Snow...For the most part, this looks like only the mountains for this. Snow levels remain fairly high through this afternoon, ranging from 8000 feet in the far west to 9500 to 10000 feet in eastern portions of the area. The cooler air then works in as flow turns north, falling as low as minus 3 to minus 4 by Monday afternoon. This would drop snow levels down to 7000 to as low as 6500 feet. However, the deepest moisture would be out of the area by then. There could be some snowflakes mixed in or possibly a brief period of wet snow in a place like Dubois, Meeteetse or areas along US 20 26 like Hiland, but accumulation would be slim to none. Most mountains will see at least some accumulation though. The amount we are most concerned with is 6 inches, which is needed for most advisories. In the western mountains, this is largely restricted to the highest elevations, so we will not issue highlights here. The southern Absarokas have a 1 in 2 chance, but these are in areas with no population and no impact. The Wind River Range does but we already have an advisory for the western Wind River Range. I thought about extending it to the eastern Winds but will hold off for now. The other area of concern is the Bighorn Range, mainly the northern portion west of Buffalo and northward. There is an area of a greater than 1 in 2 chance of over 12 inches but it is in a relatively small area with few people of impacts. In addition, the worst impacts would not be until after midnight tonight. So, we will hold off of issuing highlights for now. One complicating factor is the potential for strong wind to increase impacts, we will touch on that in the next section. And finally, we have Casper Mountain. There could be a period of upslope flow here, but a lot depends on how much wrap around moisture is present. Almost all snow here would be on Monday, so we will hold off on any potential highlights for now. There is a 1 in 2 chance of 6 inches or more above 8500 feet on the highest portions of the mountain. Almost all snow should end by sunset Monday. CONFIDENCE IN AMOUNTS AND PLACEMENT...Moderate.

Wind...Wind should remain fairly light through the day today except for the favored southwest flow areas, even here is nothing unusual for Wyoming though. The main time of concern will be from later tonight after the flow turns northward and the pressure gradient tightens, especially across northern Wyoming. There will be around 12 to 14 millibar pressure gradient from the eastern border of Wyoming to Greybull. The 700 millibar winds are also rather impressive, increasing to 50 knots across Johnson County from 12Z to 21Z on Monday. The NBM ensemble guidance gives a greater than 4 out of 5 chance of wind gusts past 55 mph across northern Johnson County. With this, we will hoist a high wind watch for this area starting at 3 am and ending at 5 pm Monday. I also gave thought to one for Bighorn County, but with the chance less than 1 out of 2 and the main chance in the higher elevations I held off. The day shift will have to watch this though and a warning may be needed if the wind speeds trend up. Most areas east of a Cody to Riverton Line as well as portions of southwestern Wyoming have at least a 1 in 2 chance of gusts past 40 mph so most places will see a blustery and cool day, it will definitely feel like November in Wyoming. Wind should decrease after sunset Monday as the low moves away and the pressure gradient decreases. CONFIDENCE IN WIND SPEED AND TIMING...Moderate to high.

Tuesday sees a return to more tranquil weather with somewhat warmer temperatures. The next potential maker will be a fast moving low moving in for Tuesday night into Wednesday. There is a rather substantial model spread on this though, with some bringing some showers to the state Wednesday and others keeping the system south of the area as high pressure builds in from the north. Riding should then bring mainly dry weather for the end of next week with near to somewhat above normal temperatures.

A weather system will move across western and central Wyoming through 12Z Monday. Periods of rain will spread from southwest to northeast across the area, reaching all locations by 21Z Sunday. There could be a lull in the rain at times this afternoon if a dry slot can work it, but confidence remains low in exactly where this will occur. Most TAF locations will see several hours of MVFR/ IFR conditions. LIFR conditions will be possible at time with low ceilings and possible fog, but not enough confidence to pinpoint the times at this time. So, like the previous shift, we have kept the lowest flight category as IFR for all terminals. To state again, confidence remains low in timing of the lowest ceilings and periods of rain.

A cold front will drop through the area tonight and bring breezy to windy conditions. This will especially the case at the terminals East of the Divide, with an emphasis on KCOD, KWRL and KRIW where gusts to and over 25 knots are likely between 6 and 12Z Monday (greater than a 3 out of 4 chance). Wind should remain under 11 knots through most of the day at all terminals until that time.

High Wind Watch from late tonight through Monday morning for WYZ010.

Winter Weather Advisory from 8 AM this morning to 5 AM MST Monday for WYZ014.