Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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243 FXUS65 KCYS 182326 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 526 PM MDT Sat May 18 2024 .KEY MESSAGES... - Warm & breezy conditions are expected again for this afternoon. Overall chances for showers are low, but an isolated storm cluster may develop during the late afternoon and/or early evening along and southeast of a line from Cheyenne to Alliance. - The risk for showers and thunderstorms is expected to increase from Sunday afternoon through Monday as significant upper- level troughing deepens and digs into the 4 corners region. A Marginal risk for severe weather exists Sunday afternoon. - More widespread rain showers with high elevation snowfall expected for Monday PM through Tuesday night. Exact snow levels remain uncertain at this time. - Cooler air behind Tuesday`s storm system may bring near freezing temperatures to portions of the area Wednesday morning. && .SHORT TERM /THROUGH MONDAY NIGHT/... Issued at 300 PM MDT Sat May 18 2024 The quiet weather pattern continues early this afternoon as breezy winds and low relative humidity readings remain present across portions of southeast WY. A weak boundary is evident on the radar as of 20Z this afternoon, and is likely to continue lifting north ahead of an approaching weak weather disturbance from the south. This will bring chances for isolated rain showers and even a couple rumbles of thunder along the I-80 corridor by early evening, and the NE Panhandle. A longwave trough will dive south on Sunday towards the Four Corners area, brining an increased amount of moisture, instability, and forcing to our cwa by Sunday afternoon. The best locations for this to result in organized convection is along and east of the Laramie Range. SPC Day 2 has a Marginal Risk for isolated severe weather to occur from the I-25 corridor toward western Nebraska. There is a Slight Risk from the SPC just to our east. We will have to monitor CAMs in the coming 24 hours as current model guidance shows the strongest convection initiating in western Nebraska and pushing east from Sunday afternoon in the evening time frame. Main hazards would be strong, damaging winds and hail up to 1" in diameter of in areas of the Marginal Risk. At this time of inspection, model support doesn`t show widespread chances of strong convection. However, model soundings have the infamous "inverted-V" feature, so rain showers that develop will have the potential to gust to severe thresholds easily. The aforementioned longwave trough will continue to dig toward the Four Corners on Monday, with an attendant surface cold FROPA heading towards our cwa. Model guidance has an aggressive approach towards 700mb and 850mb temperatures decreasing rapidly behind the surface cold front, well below 0C. Cold fronts typically translate faster than model guidance across our cwa, so have gone with a decrease in daytime highs for our northern forecast zones slightly, and nudged temperatures down slightly along the I-80 corridor also. Looking at scattered to numerous rain showers as this FROPA advects southward, with isolated to scattered thunderstorms as well. Will begin to see a rain and snow mix in the higher terrain by Monday evening, after dark. Colder temperatures will increase in coverage overnight, with sub-freezing temperatures likely in the higher terrain. Have nudged down the overnight lows for Tuesday morning across the entire cwa to reflect this. Overall, expect a period of active weather to ramp back upward beginning Sunday afternoon into early next week. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Issued at 300 PM MDT Sat May 18 2024 A cooler and more unsettled weather pattern will continue through much of next week. A broad longwave over the western CONUS will dominate the weather pattern, with numerous shortwaves rotating around the trough through the area over the course of the week. The first shortwave ejects to the east Monday night, but a secondary shortwave just behind will likely bring the coldest air of this cycle. Due to the two shortwaves in close proximity and the second vorticity maximum tracking more or less directly over the area, the lifting mechanism is somewhat muddled. This reduces confidence that widespread rainfall will continue through the day Tuesday, but several ensemble members still show a more southerly track of the second wave which allows heavier precipitation to continue longer. Precipitation rate will also have an impact on precipitation type. There`s pretty good confidence this will be cold enough to support some snow flakes mixing in above 7000 ft and accumulation above 8500 feet. However, the atmosphere is expected to be cool enough that heavier precipitation rates could locally drop snow levels to around 6000 ft, which would include Cheyenne. The current most likely scenario of lighter precipitation rates may lead to a few flakes mixing in, but we`d need the heavier scenario to lead to more noticeable snow event. This scenario is shown by approximately 30% of ensemble members at this time. Precipitation should come to an end sometime Tuesday night as drier air works in behind the second shortwave. We`ll be watching temperatures into Wednesday morning, as the potential for near freezing temperatures exists. Many locations are a week or more past their climatological last freeze date, so would likely need frost/freeze products in this scenario. Currently, a freeze looks likely for areas along and west of the Laramie range (but this isn`t very unusual for those areas), with about a 30-50% chance of a freeze for the higher elevations east of the Laramie range, including the corridors from Cheyenne to Kimball near I-80 and Douglas to Harrison near US-20. The cool snap should be pretty short-lived, with a weak ridge pushing back in by Wednesday. Highs will probably remain below normal Wednesday, but climb back above normal for Thursday. Models are starting to come into better agreement for the end of the week, leaning towards a sharp trough passing through Thursday night or Friday. However, most guidance keeps the upper level low north of our area, and thus the precipitation potential is not very impressive. Nonetheless, looking at another shot of cooler air (but probably not as significant as the early week front), with continued chances for afternoon showers and thunderstorms each day through the end of the week. && .AVIATION /00Z TAFS THROUGH 00Z MONDAY/... Issued at 521 PM MDT Sat May 18 2024 Southwest flow aloft will develop overnight and continue on Sunday. Scattered to broken clouds from 8000 to 15000 feet will prevail. Winds will gust to 33 knots at Rawlins, Laramie and Scottsbluff until 02Z, and to 33 knots at Cheyenne until 06Z, then winds will gust to 30 knots at Sidney, Chadron and Alliance after 05Z, and to 35 knots at all terminals after 14Z Sunday. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...BW LONG TERM...MN AVIATION...RUBIN