Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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536 FXUS65 KRIW 191110 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 510 AM MDT Sun May 19 2024 .KEY MESSAGES... - One more day of elevated to critical fire weather conditions is expected for central and southern Wyoming. Wind gusts of 30 to 40 mph will be widespread. - Wetter and cooler conditions are expected Monday and Tuesday. Areas along and east of the Continental Divide will see the most moisture. - Mountain snow is expected with this system, especially for the eastern Winds, Absarokas, and Bighorns. Winter driving conditions are expected over the passes. - Warmer but still unsettled conditions expected Wednesday through the end of the week. && .DISCUSSION... Issued at 442 AM MDT Sun May 19 2024 Midlevel flow has become more cyclonically curved over the area this morning as the leading wave from a trough approaches Wyoming. This will keep light shower chances going across the area today, most favorably in two locations. One is across the Yellowstone area, which will see increased jet-enhanced lift. The other is across far southern areas which will benefit from a little Pacific moisture pushing in there. However, the majority of the area will see one last warm, dry, and windy day ahead of an approaching cold front. By midday a surface pressure gradient will once again strengthen as a surface low develops to our east. Wind gusts of 30 to 40 mph will be fairly widespread. With humidity values again dropping below 15 percent, elevated to critical fire weather conditions are expected through the afternoon for most of central and southern Wyoming. The front will push south this evening and overnight as the trough drops south over Idaho. Shower activity will first pick up across the southern half of the CWA through the early morning hours as reinforcing moisture and favorable jet dynamics arrive. By early afternoon precipitation chances will increase across the rest of the area as the trough begins to push in to western Wyoming. Widespread showers will continue Monday night as the associated low moves eastward, centering itself over central Wyoming by midday Tuesday. This will result in a favorable period of upslope enhancement east of the Divide through Tuesday afternoon. Precipitation should begin decreasing Tuesday evening as the system quickly departs and a drier westerly flow takes over. Looking at QPF, there continues to be notable model spread on amounts for this event. Given the low placement, this will certainly favor areas east of the Divide. The greatest amounts are expected from the Wind River Basin through the Powder River Basin, as well as far southern portions of the Bighorn Basin. NBM probabilities currently give these areas a 40 to 60 percent chance of seeing at least an inch, with most of that falling in the Monday evening through Tuesday afternoon timeframe. The current forecast is on the higher end of those probabilities, with the highest amounts around 1.5 inches for the non-mountain areas. Other areas across the CWA will see less through the event, generally below half an inch. With snow levels dropping to around 7,000 feet, this will be a decent spring storm for the eastern Winds, Absarokas, and Bighorns. Forecast amounts are currently peaking well above a foot; we considered a Winter Storm Watch, but have held off in part due to the continued QPF uncertainty. This may end up being more of an Advisory event given that the heaviest snow will fall during the long daylight hours, likely mitigating impacts on mountain highways somewhat. Regardless, we expect winter driving conditions in the mountains, including on South Pass, Togwotee Pass, Powder River Pass, and Granite Pass. Further ahead, guidance continues to favor an active pattern lingering through the week. It appears there should be at least a brief period of transitory ridging resulting in drier and warmer conditions on Wednesday. Solutions then quickly move in the next system for Thursday, though there is notable uncertainty on the placement and strength of this. As it stands now, ensemble cluster analysis shows anomalously low 500mb heights through next weekend. The forecast generally reflects this with daily precipitation chances and temperatures around or below normal. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 508 AM MDT Sun May 19 2024 VFR conditions for the period at all TAF sites. Winds will increase after 15-17Z as mid level clouds thicken with a weak shortwave to the north. Gusts up to 25 to 30 knots will be common at all TAF sites through peak heating of the afternoon until around 00-02Z once again. Lighter winds less than 10-15kts with more of a northerly component to the westerly flow ahead of a stronger wave come Monday and Tuesday. Isolated showers will be possible at some TAF sites after 06Z Monday, mainly in vicinity of KRKS, KRIW, KLND and KCPR. MVFR to IFR conditions are likely on Monday but should hold off until after 12Z. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued at 442 AM MDT Sun May 19 2024 One more day of elevated to critical fire weather conditions are expected ahead of an approaching trough and associated cold front. Widespread wind gusts of 30 to 40 mph will ramp up by early afternoon in response to a strengthening pressure gradient. The lowest humidity will be across southern and central Wyoming, dropping below 15 percent with peak mixing this afternoon. Otherwise, there will be some isolated light showers in the area, with the best chances across far northern and far southern Wyoming. Fire weather concerns decrease after sunset. Cooler and wetter conditions are expected for Monday and Tuesday. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Myers AVIATION...Hattings FIRE WEATHER...Myers