Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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891 FXUS65 KRIW 041111 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 511 AM MDT Sat May 4 2024 .KEY MESSAGES... - A nice start to the weekend today with mild temperatures and light to moderate wind. - Rain and snow returns to the west Sunday, spreading east at night. Strong to high wind is likely East of the Divide Sunday afternoon. - Much of next week looks cool, damp and blustery with rounds of showers and mountain snow. Details on the placement and timing of heaviest precipitation remain rather uncertain this morning. && .DISCUSSION... Issued at 332 AM MDT Sat May 4 2024 This morning I have some good news and bad news. The good news at least one half of the weekend, mainly today, looks like a nice Spring day with mild temperatures and light to moderate wind. With the rest of the period rather active, this is all I will say about this. Because, the rest of the forecast, if you like warm and dry weather, will not be to your liking. And with the contrast in conditions, I will split the discussion into West and East of the Divide starting on Sunday. West of the Divide...The main concern here will be snow, mainly in the higher elevations. The approaching Pacific system continues to slow it`s progress somewhat though. So, we have removed the POPs for this evening and some models even have late night dry. The precipitation will get underway on Sunday. During the day though, snow levels will remain rather high, remaining above 8500 feet for the most part. Levels will then fall to the valley floors during the evening after the Pacific cold front passes through. Most models are keying in on Sunday night with the heaviest snow as this will associated with the trough axis passage. An assist from the left front quadrant of an 100 knot jet streak should enhance precipitation rates during this time, with over an inch an hour snowfall rates possible. Most models are keying in on the Tetons and southern Yellowstone with the heaviest snow, and as a result we have issued Winter Storm Watches starting at noon Sunday, although the main impacts would not be until at night. Decent snowfall rates will continue through Monday as mid level flow turns to a favorable westerly direction, bring orographic enhancement to the snow. The one question this morning was how long to run it, as guidance is split on decent snow extending into Tuesday. For now, we decided to run it through Tuesday night, since it is only a watch. Future shifts can decide on duration when they are upgraded. These areas were selected since there is greater than a 1 in 2 chance of exceeding 12 inches of snow. As for the other areas, the chance of warning criteria is less than 2 in 5 right now. As a result, advisories would be more appropriate in places like Jackson, the Star Valley and the Salt and Wyoming Range. In addition, with borderline temperatures and the strong May sun, accumulation would be limited on roads during the daylight hours. More uncertainty creeps in starting Wednesday. It will still be unsettled and cool with a longwave trough hanging around, but details are very hard to figure out with slow moving upper level lows. For now, we made few changes to continuity. To sum up the extended, we have high confidence (greater than a 3 in 4 chance) of cool and damp weather continuing, but confidence remains very low in specific timing and precipitation amounts. East of the Divide and Sweetwater County...After a nice day today, things will turn more active Sunday as the aforementioned Pacific low approaches the area. Most of Sunday still looks to be rain free and warm. We have left some small chances of convection late as a hedge, but the chance of an location seeing a shower or thunderstorm before 6 pm is 1 in 4 or less. The main concern here continues to be wind. There will be a good set up for potential high wind with a tightening pressure gradient, southwest flow, 700 millibar wind approaching 55 knots at times and enhanced downward momentum from the right front quadrant of an 100 knot jet. These areas have at least a 3 in 5 chance of wind gusts past 55 mph. As a result, we have issued a high wind watch for the favored areas from Sweetwater County through Natrona County starting Sunday. A tight pressure gradient may keep high wind going into Monday as well. So, we ran the watch through Monday with a mention that wind will decrease somewhat Sunday night as the wind decouples after sunset. Strong wind may continue into Tuesday as well, especially across Sweetwater County where the best jet energy will be. We do not have enough confidence to extend the watch this far though. Temperatures will be quite warm on Sunday as well with the southwest flow, rising well into the 70s in some locations. Now for precipitation, showers will overspread the area Sunday night as the cold front moves through. Exact coverage is still in question though as the position of the low will be key. Some models take the low further north which could dry slot the area somewhat, keeping coverage down somewhat. So, QPF amounts were kept on the low side through Sunday night with POPs remaining higher. There continues to be uncertainty for coverage of showers for much of the week as well. It, again, depends on what happens with the low. With blocking ridging to the east and the low getting cut off from steering flow; it will only move slowly over much of next week. The big question is how much moisture can wrap around the low. Most models favor northern Wyoming, especially the Bighorns and Johnson County through Wednesday night, which makes sense given the low will be somewhere east or northeast of the area. There are still large differences in how far south the moisture can get though and a lot depends of the exact position of the low. There is the potential for a decent amount of snow in the Bighorns with this though, given the favorable upslope flow. It is too early for highlights, since the decent snow would not begin until Monday at the earliest. The greatest chance may be Wednesday or Wednesday night when the low, emphasis on may, get pushed further south. But consistency with the guidance has been very low. With 700 millibar temperatures remaining at minus 5 or warmer, snow levels should remain at 6000 feet or higher. The result will be the lower elevations having mainly rain. This unsettled weather will continue through the workweek. So to sum things up, we have high confidence (greater than 9 in 10) of a cool, blustery and unsettled pattern through much of next week. Confidence in details of timing and placement of the heavier precipitation remains very low, especially starting on Monday. && .AVIATION /12Z TAFS THROUGH 12Z SUNDAY/... Issued at 509 AM MDT Sat May 4 2024 Skies have cleared overnight, with mostly clear skies expected through the remainder of the morning. Patchy fog is looking less likely, with KCPR and KRIW only now having VCFG through 15Z. The prevailing wind will remain light and variable at most terminals before gradually becoming southerly by Saturday afternoon. High clouds (SCT200) will pass overhead Saturday afternoon, with increasing cloud cover heading into Saturday evening. All terminals will have prevailing VFR conditions through the TAF period. Winds begin to increase at the end of the TAF period, with KRKS currently being the only terminal to see winds increase before 12Z Sunday. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Storm Watch from Sunday evening through late Tuesday night for WYZ001-012. High Wind Watch from Sunday morning through Sunday evening for WYZ018. High Wind Watch from Sunday morning through Monday evening for WYZ019-020-028>030. && $$ DISCUSSION...Hattings AVIATION...Dziewaltowski