528 FXUS65 KTFX 221609 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 1009 AM MDT Tue Apr 22 2014 .UPDATE... Models are showing sufficient shear and instability over a small portion of southwest and central Montana this afternoon for the threat of isolated severe thunderstorms. The main threat from these storms would be winds but could not rule out a a few cells producing hail to near one inch in diameter. The best dynamics will be across Gallatin...Meagher...and southern Judith basin and Fergus counties. Will be adding mention of severe thunderstorms to much of this area. There is a threat for weaker thunderstorms across much of the remainder of the CWA...especially along and east of intestate 15. The remainder of the forecast is on track, only minor change was to increase winds over southwest Montana with the passage of the cold front this evening. Langlieb && .AVIATION... UPDATED 1150Z. A cold front is expected to move to near a KCTB-KBTM line around 21z and then to near a KHVR-KLWT line near 02z. Showers and thunderstorms are likely near and behind the front. Expect scattered MVFR ceilings and visibilities in the showers and thunderstorms with local IFR ceilings and visibilties. A few of the thunderstorms will have wind gusts to 40 kts and small hail. Showers and thunderstorms will end from the west near and after midnight over the plains but showers are expected to linger over southwest Montana past midnight. Gusty westerly winds 20 to 30 kts will develop behind the front and there is a risk of wind gusts to 50 kts along the Rocky Mountain Front after midnight. Snow levels will be dropping overnight and could see brief MVFR conditions in snow for the southwest valleys. Also expect areas of mountain obscurement tonight over the Rocky Mountain Front and southwest Montana in snow. Blank && .PREV DISCUSSION... /ISSUED 550 AM MDT Tue Apr 22 2014/ Today through Thursday...A deep upper level trough along the west coast this morning will shift inland, bringing multiple weather impacts to the region through the next 24 hrs (see specific info below). Upper level jet and diffluent flow on the front side of the trough moving into northern CA/OR early this morning will move east into the northern Rockies later today while a surface trough intensifies over central MT this afternoon ahead of a Pacific cold front associated with the upper trough. Surface trough/cold front and main forcing associated with the trough move across the region tonight with showers and thunderstorms developing from SW to NE across the forecast area late this afternoon and evening. Cooler and windy conditions move in behind the front tonight through Wednesday with precipitation diminishing to scattered showers by Wednesday morning. Westerly flow will limit precipitation chances for areas east of the Mtns Wednesday and Thursday with temperatures 5-10 degrees below seasonal averages. Thunderstorms...Convective precipitation developing ahead of the front this afternoon over western and SW MT will shift east across north-central and the remainder of SW MT this evening. Upper level forcing and instability will provide ample lift for thunderstorm development, though cloud-cover and dewpoints in the 20s/30s will likely be a limiting factor in the strength of thunderstorm activity. Main concern with thunderstorms this afternoon/evening is the potential for linear organization and strong winds with locally heavy rain and small hail a secondary threat. Winds....Strong pressure rises behind the front will contribute to gusty post frontal west winds this evening with strong lower level westerly flow developing later tonight and continuing through Wednesday as low pressure deepens over southern AB beneath the closed mid-upper level low. Area most likely to see winds 30kts with gusts near 50 kts will be along and adjacent to the Rocky Mtn front where a a high wind watch remains in effect for tonight through Wednesday. Precipitation/Hydrology...Total precipitation amounts through tonight still look to range from around 1/4 to 1/2 inch for most of the area with highest amounts likely along a swath from Madison County north-northeast through Cascade/Judith Basin counties and north to western Hill county where localized amounts around 3/4" are possible. Main concern from a hydrologic standpoint is the potential for embedded heavier precipitation late this afternoon/evening to fall on mountain snowpack resulting in rapid snowmelt runoff and flooding of creeks/rivers originating in the mtns. The potential for this situation exists across most mountain rages of central and southwest MT with streams emptying into the Gallatin River most likely to see locally moderate precipitation on a widespread mountain snowpack. Winter Weather...Strong cooling associated with the upper level trough will cause snow levels to fall rapidly tonight behind the cold front. Expect a change-over to snow on most passes overnight though precipitation largely diminishes as the cooler air moves in, resulting in around an inch of snow at pass levels, though road surfaces may remain at or above freezing, limiting accumulation on roadways. Higher elevations above 7500 feet in southwest MT could see 4 to 6 inches of snow accumulation late tonight through Wednesday morning. Hoenisch Thursday night through Tuesday...In a general sense models are on the same page with trends during the period. Expect scattered showers to diminish Thursday night as an upper ridge moves through western Montana and into central Montana. Then a wet period is expected. A strong upper trough will move through the western U.S. Friday through Saturday. Several weather disturbances embedded in the southerly flow aloft ahead of the upper trough will bring periods of rain to the forecast area during this time frame. Then Sunday the main portion of the upper trough is expected to move into the central Plains. Southeast flow aloft ahead of the upper trough will continue to feed in moisture to an area possibly as far west as a Havre to Great Falls to Bozeman line. Since the models are in better agreement with this wet period have increased the chances of precipitation for Friday night through Saturday and also east of the line mentioned above for Sunday. For the period Friday afternoon through Sunday precipitation amounts ranging from a half inch to an inch from the western mountains to the eastern plains could occur. Models are also in better agreement with forecast temperatures for Saturday and Sunday as the ECMWF is not as cold as previously while the GFS is quite a bit colder. Now expect highs only in the 40s over the plains and 50 to 55 for lower elevations of southwest Montana over the weekend. In line with the colder temperatures expect snow over the mountains with lower elevations possibly seeing a little snow as well during the overnight periods. Looking ahead to early next week models are pointing to warmer temperatures with decreasing chances of precipitation as an upper ridge develops. Blank && .PRELIMINARY POINT TEMPS/POPS... GTF 70 37 51 33 / 70 100 30 10 CTB 67 36 50 32 / 60 90 20 10 HLN 67 36 52 35 / 80 100 30 20 BZN 70 32 51 30 / 60 90 40 30 WEY 60 28 43 24 / 70 80 60 40 DLN 64 30 49 30 / 80 80 20 30 HVR 76 40 57 33 / 20 100 40 30 LWT 71 34 50 29 / 30 90 30 20 && .TFX WATCHES/WARNINGS/ADVISORIES... HIGH WIND WATCH from late tonight through Wednesday afternoon Eastern Glacier...Eastern Pondera...Northern Rocky Mountain Front...Southern Rocky Mountain Front...Toole. FLOOD WATCH from 6 PM MDT this evening through Wednesday afternoon Gallatin. && $$ weather.gov/greatfalls www.twitter.com/NWSGreatFalls www.facebook.com/US.NationalWeatherService.GreatFalls.gov