396 FXUS65 KRIW 072005 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 205 PM MDT Sun Apr 7 2019 .SHORT TERM...Tonight through Wednesday night Snow and rain showers will continue out west this evening although intensity should decrease through the evening and overnight hours as the best energy shifts further northward into Montana. Meanwhile, dry weather should continue East of the Divide although breezy to windy conditions will continue into the evening before subsiding around midnight. The Rangeland Fire Danger statement looks good for now and will be continued as is. Monday look a bit quieter. We will continue to have some showers continuing in the west, snow in the mountains and rain in the valleys. The best energy looks to remain largely north and west of the area through the day, so amounts should remain light. In areas East of the Divide, expect another mainly dry day. Wind looks to be not quite as strong and relative humidity somewhat higher. So for now, we will hold off on extending the fire statements since it looks borderline. We will let the night crew make the final call for any extension. Otherwise, it looks like a decent start to the workweek. Things go downhill from here. With the contrast in the weather starting Monday night, we will split the forecast into west and east of the divide. West of the Divide...Precipitation will begin in earnest Monday night, especially Monday night as the initial surge of moisture moves in and some jet dynamics catch up to the system. The dilemma is precipitation type. Through most of Tuesday, 700 millibar temperatures will remain fairly warm, anywhere from minus 2 in the north to plus 1 in the south, which would put snow levels anywhere from 7500 to 9000 feet. With a decent snowpack this could increase flooding. Amounts in the valleys are not that great though, generally anywhere from half an inch to an inch, so of which will fall as snow Tuesday night as snow levels fall back to the valley floors as colder air moves in behind a cold frontal passage. The Hydrologic Outlook looks good for now. As for snowfall amounts, at this point in the valleys amounts look to remain sub advisory. Advisory level amounts look to be a good possibility in the most of the mountains though. There is a very good chance there could be warning level amounts but these would be in the highest elevations above pass level. There will be some strong winds at times though and this could make for rather difficult travel conditions, especially Tuesday night into Wednesday. We are leaning toward advisories for most areas at this time, but still time to decide. Snow should end fairly quickly later Wednesday into Wednesday night as the storm moves away to the east with only showers by Thursday morning. East of the Divide...We do not have a lot of concerns through Tuesday. Most areas should be dry Monday night and even Tuesday most areas should be dry most of the time. It will also be another mild day so other than the higher mountains any precipitation would fall in the form of rain rather than snow. Rain then moves in Tuesday night and changes over to all snow by daybreak Wednesday. The models continue to have some differences in the details of where the heaviest QPF will fall. And since a lot of the precipitation would be banded, very difficult to nail down at this point. The models do show one areas of heavier precipitation moving from Sweetwater into Natrona County Tuesday evening but much of this may fall as rain. All areas will see snow Wednesday but exact amounts are difficult to nail down. For one, we will be fighting the April sun and temperatures will be close to freezing. This may be a situation similar to the late March system where there are several inches of snow on the grass and roads may be just wet to slushy. The main story may end up being the wind. Lee Cyclogenesis will occur and tighten the pressure gradient across the area. At the same time, 700 millibar winds will increase to 40 to 50 knots across much of the area with good mixing. Some areas will likely see northerly wind gusts over 40 mph that will blow the snow around and sharply reduce visibility. The north wind will also enhance snowfall amounts over the usual upslope locations, like Lander, Casper and even Buffalo and less over areas that see downsloping flow like Shoshoni and Greybull. Amounts from continuity looked OK so we only made some tweaks. Still enough time to hone in on any details. Snow should then end from north to south Wednesday night as system moves away to the east. Si if we could sum up Wednesday's weather in three words, it would be: 1) Cloudy 2) Cold and 3) Cruddy. .LONG TERM...12Z Issuance Models in fairly good agreement to begin the extended period with the stacked low along the NE/IA at 12Z/Thu. Wrap-around moisture will gradually decrease from north-to-south during the day east of the Continental Divide. Enough low-level moisture present to potentially generate afternoon snow showers with 500mb cold pool of minus 26C overhead. Expect the best chance will be off the south end of area mountain ranges, most likely the Bighorns. Moisture will linger longest west of the Divide where scattered snow chances remain. A brisk north wind will remain along the I-25 corridor early, but expect the wind to diminish as Thursday wears on. High temperatures will be 10-20F below normal as cold northerly flow aloft prevails. Models diverge Friday and Saturday with regard to the handling of the kicker system that will dive into the Desert Southwest. GFS not nearly as deep and slow as the solutions offered by the ECMWF and Canadian models. PMDEPD discussion shared that ensemble solutions also trended toward the slower ECMWF solution. Therefore, Friday finds enough moisture in northwest flow aloft to generate light precipitation along and west of the Divide. Have trended toward the chance of precipitation offered by the NBM which blends well with neighboring office forecasts. Temperatures will be slow to moderate with highs still generally 10-15F below normal. The Desert Southwest low will begin to eject across the southern Rockies Saturday as the next upstream Storm nears the Pacific Northwest Coast. This may give us a quick shortwave ridge Saturday before warmer west-southwest flow arrives for Sunday. Again nudged toward NBM solution with precipitation chances Saturday and Sunday. Temperatures will be more seasonal by the end of the weekend. In fact, the combination of warming mid- level temperatures and a breezy southwest wind may allow highs to be about 5F warmer than what is currently forecast.&& .AVIATION...12Z Issuance West of the Divide...KBPI/KJAC/KPNA/KRKS Routes Low-end VFR cloud decks persist across western Wyoming as of mid- afternoon Sunday. Embedded showers and associated MVFR conditions in the vicinity of KJAC and will make a last-minute decision as to VFR vs. MVFR for the early evening period. Mountain tops across northwest Wyoming will be obscured through 00Z/Tue. Other terminals will remain VFR through Monday. Diurnally driven surface wind speeds will decrease between 01Z-03Z/Mon. The wind will pick-up again between 16Z-20Z/Mon with frequent afternoon gusts around 25kts. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes VFR conditions will prevail with the primary aviation impacts related to gusty surface wind and LLWS. Gusty westerly surface wind will decrease early this evening, but several terminals will keep 12kts+ wind much of the night. Foothill terminals of KCOD and KLND could see LLWS tonight with forecast soundings showing 35-45kts in the lowest 2K feet MSL. The southwest surface wind will again increase at KCPR around 15Z/Mon and other terminals between 18Z- 21Z/Mon. Speeds will be similar to or just a touch less than those seen Sunday afternoon. && .FIRE WEATHER... Showers in the west will gradually taper off this evening. More showers are possible in the west Monday but any steady snow would likely hold off until Monday evening. Mainly dry conditions will continue in the west. Fire conditions should ease Monday with somewhat higher humidity and less windy conditions. Mixing and smoke dispersal will range from fair in the west to very good in portions southern and eastern Wyoming. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...Hattings LONG TERM...Jones AVIATION...Jones FIRE WEATHER...Hattings