Scientific Forecaster Discussion
000 FXUS65 KTFX 201124 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 520 AM MDT Thu Jul 20 2017 ...UPDATED AVIATION SECTION... .SYNOPSIS... A few showers or thunderstorms may linger along the Hi-Line today, but the area will otherwise remain dry through the weekend. A dry cold front will bring gusty west winds to the area this afternoon through the day on Friday, with some decrease in winds tonight. Ongoing very dry conditions will combine with this wind for critical fire weather conditions. High pressure aloft will then help decrease winds for Saturday into Sunday. The cold front will cool temperatures from 10 degrees above normal today closer to normal for Friday into Saturday, before temperatures warm back up for Sunday. && .DISCUSSION... Today through Saturday...A weak disturbance moving through the high pressure ridge will keep a chance of weak showers and thunderstorms along the Hi-Line today. Otherwise, a shortwave trough will move east along the Canadian border, breaking down the ridge temporarily this afternoon through Friday. An associated relatively dry cold front will also move through the area this afternoon into this evening, and the strong pressure gradient in the wake of it will bring gusty west winds. Winds will decrease somewhat overnight as the pressure gradient weakens, but mid level winds will increase to around 30 kt as the shortwave passes. These winds will be brought down to the surface during the day on Friday due its associated cold air advection. Despite temperatures falling to near seasonal averages on Friday, the airmass will remain quite dry. (Please see the Fire Weather section for details on fire weather highlights in effect.) The high pressure ridge will then rebuild into the area Friday night into Saturday, which will help decrease the winds, keep the area dry, and warm temperatures a few degrees in the process. Coulston Saturday Night Through Thursday...Dry weather prevails through the weekend with temperatures climbing back into the 90s. Broad ridging aloft amplifies through Sunday, but moves east of the forecast area Sunday night. Increasing southwest flow aloft associated with a closed low over western Canada will allow shortwave energy to move into the region, raising chances for showers and thunderstorms. Models are arriving at a better consensus for the large scale pattern, but timing and coverage of thunderstorm activity has yet to be resolved. Temperatures will remain near to above seasonal averages into next week. PN && .AVIATION... Updated 1120Z. VFR conditions will continue at least through Thursday night. Isolated showers/thunderstorms are possible along the Hi-Line today due to a passing disturbance, but am not anticipating them to impact the KCTB and KHVR terminals directly. Light east winds are also moving smoke from fires in Canada into the KCTB area and from fires in Petroleum County into the KLWT area early this morning. Am expecting high MVFR visibilities at worst before westerly winds move the smoke out of those areas later this morning. These winds will develop across the forecast area as a dry cold front associated with an approaching shortwave trough moves east through the area between 20Z and 00Z. West winds gusting between 20 and 30 kt will follow the frontal passage, but they will become less gusty after 04Z. Coulston && .FIRE WEATHER... The forecast models continue to move a relatively dry and weak cold front through the area this afternoon. As a result, sustained west winds of 10 to 20 mph will spread east across the area with gusts as high as 35 mph at times. Relative humidity will fall well into the 10 to 20 percent range once again which, when combined with the gusty winds and high to very high fire danger class, will make for critical fire weather conditions this afternoon and evening. Breezy but less gusty west winds will continue overnight with only moderate to poor humidity recovery. Winds will then increase again to 15 to 25 mph with gusts to 40 mph on Friday due to mixing down of stronger mid level winds, as humidity falls into the 10 to 20 percent range. Have therefore upgraded the previous Fire Weather Watch to a Red Flag Warning, expanded that Warning to include all of TFX's fire weather zones, and extended the valid time from this afternoon and evening to this afternoon through Friday evening. NWS Missoula also has a Red Flag Warning in effect for much of Southwest Montana this afternoon and evening due to the dry conditions and passage of the cold front. Coulston && .PRELIMINARY POINT TEMPS/POPS... GTF 94 54 85 52 / 0 10 0 0 CTB 88 51 79 49 / 10 0 0 0 HLN 93 56 86 55 / 0 0 0 0 BZN 92 50 86 48 / 0 0 0 0 WEY 81 40 78 38 / 10 10 0 0 DLN 90 48 84 48 / 0 0 0 0 HVR 96 55 87 53 / 20 10 0 0 LWT 91 53 84 52 / 0 10 0 0 && .TFX WATCHES/WARNINGS/ADVISORIES... Red Flag Warning from noon today to 9 PM MDT Friday Central and Eastern Lewis and Clark National Forest Areas...Chouteau and Fergus Counties...Eastern Glacier/Toole/Central/Eastern Pondera/Liberty...Helena and Townsend Ranger Districts of the Helena National Forest...Hill and Blaine Counties...Lewis and Clark National Forest Rocky Mountain District-Rocky Mountain Front...Lincoln Ranger District of the Helena National Forest. Red Flag Warning from noon today to 9 PM MDT this evening Deerlodge/Western Beaverhead National Forest...Eastern Beaverhead National Forest.