594 FXUS65 KTFX 190003 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 603 PM MDT Sat May 18 2013 ...UPDATE TO AVIATION... .DISCUSSION... Tonight through Monday...Scattered showers and isolated thunderstorms are developing this afternoon as instability increases with an upper-level trough pushing through the Northern Rockies. With the cooler air aloft, some of these showers and thunderstorms may contain graupel. On Sunday, the upper-level low associated with the trough will further deepen as it moves into eastern Montana, which will shift the surface flow more northerly. As abundant moisture wraps into this system, upslope precipitation will develop and become steadier and more widespread, especially during the afternoon hours. There are still some differences in the models regarding where the heaviest precipitation will be located, however confidence is increasing that the best precip chances will be for areas along and east of I-15 as well as areas along the Rocky Mountain Front. Cooler, Canadian air will also keep snow levels lower on Sunday with snow levels between 7000 and 8000 feet. Over southwest Montana, precipitation will be widespread but off and on through the period. The system will move further east on Monday with drier air pushing in west to east. High temperatures will remain below average through Sunday, then rise to near seasonal averages on Monday. MLV Monday Night through Saturday...Extended forecast period begins with mostly quiet conditions as the upper level system that will cross Montana this weekend will be moving into the Upper Midwest states and a well-amplified high pressure ridge begins to settle in over ern Montana and the Dakotas. Meanwhile, a second upper level trof is forecast to swing through western Canada and form a closed low pressure center over the PACNW coast by Tues eve. With the large ridge remaining anchored thru the middle of the country, the Pacific low essentially becomes stationary, drifting along the coast and occasionally moving inland during the latter half of next week. Minor shortwaves and accompanying moisture rotating around the central low will bring intermittent periods of showers and isolated thunderstorms each day from Wed onward. Forecast models are in decent agreement that most parts of central/southwest MT will see rain during this time, but precip amounts still look to be generally light (0.10-0.15 inch or less) over the plains and valleys. Rainfall totals are slightly higher (0.20-0.50 inch) over the melting snowfields of the Rocky Mtn Front and Little Belt Mtns, which gives concern for increased streamflows in creeks/rivers in those areas. Persistent southerly flow aloft will allow temperatures to stay close to typical seasonal values with highs in the mid-60s to the low 70s each day, though that will be modified quite a bit by a fair amount of cloud cover and the timing of the shortwave passages. Waranauskas && .AVIATION... UPDATED 00Z. Unsettled weather continues for the next 24 hours as rain showers are expected to remain in the area for the entire TAF period. An isolated thunderstorm will be possible until sunset...however not expecting any long duration impacts so no VCTS or TSRA in the TAFs. Ceilings and visibilities should remain VFR but brief MVFR/IFR conditions are possible near any precipitation areas. Suk && .HYDROLOGY... Two upper level trofs, one moving through our region this weekend and the other becoming stationary over the PACNW coast for much of next week, will bring increased chances for rainfall over the next several days. Though cooler temperatures will reduce snowmelt slightly, the primary concern is that rain on the melting snowpack will cause enhanced rises in stream/river levels. Most area rivers are currently well below flood stage but a handful of sites are near or approaching bankfull, so rainfall totals and river stage forecasts will be closely monitored for significant changes and impacts. Waranauskas && .PRELIMINARY POINT TEMPS/POPS... GTF 44 61 44 66 / 60 70 60 40 CTB 42 62 42 67 / 60 50 40 30 HLN 44 62 45 68 / 60 50 50 30 BZN 41 60 41 66 / 80 60 50 30 WEY 36 53 35 59 / 70 60 50 40 DLN 38 59 40 66 / 60 50 40 20 HVR 47 68 46 71 / 80 80 70 70 LWT 44 57 43 62 / 80 80 70 60 && .TFX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ weather.gov/greatfalls |