Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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452 FXUS65 KTFX 181540 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 940 AM MDT Wed Sep 18 2024 .SYNOPSIS... Widespread rain continues through Thursday morning across Central and North Central MT. Drier conditions settle in for the weekend and beginning of next week, with a few disturbances moving through causing low chances for precipitation throughout the forecast period. && .UPDATE... The flood advisory for Teton County has been cancelled due to heavy rain having ended and there is no longer a risk for flooding. For other Hydrology impacts see the HYDROLOGY section below. Rain will continue along and east of the I-15 corridor in North-central and Central Montana through the day today. The heaviest rainfall is currently along the Cascade and and Judith Basin County lines. This will shift eastward this morning. Today it will also be windy across the plains of North-central and Central Montana. For the update winds and wind gust were increased. Hourly temperature and sky grids were adjusted. Pops this afternoon and evening in Blaine and Fergus Counties was adjusted. QPF this morning west of the I-15 corridor was decreased. The rest of the forecast is on track. -IG && .AVIATION... 18/12Z TAF Period Areas of widespread rain continue through the evening tonight, with the main band of rain slowly moving east throughout the day today. There`s a 30% for emebedded thunderstorms with in precipitation. However, it is hard to pinpoint the timing and coverage at the terminals, so it was left out of the TAFs at this time. MVFR/IFR visibilities with low ceilings continue through the day today with the main rain band. Gusty west to northwest winds increase today across North Central/Central MT, with some gusts in excessive of 35kts possible at times. Low ceilings will cause mountain obscuration through the TAF period, and mountain turbulence will be a concern at times as well. -Wilson Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A Flash Flood Watch remains in effect for the Horse Gulch fire burn scar area through the afternoon. Saturated soils from rain yesterday with the concerns for periods of steady moderate precipitation today will maintain a risk for flash flooding and debris flow in the burn scar area. Local minor flooding concerns remain across eastern portions of North-Central MT and Central MT with steady moderate rainfall. This is especially true in the Central MT mountain ranges where the highest rainfall totals are forecasted. Current flash flood guidance is 1-2.5"(higher amounts in the mountains) over 3 hours, with some areas as low as 1" over 1 hour in the plains. The main concern will be moderate steady rain can cause concern for excessive runoff in low lying areas and poor drainage. Some standing water built up is a concern as a result, so the Flood Watch remains in effect through the evening. -Wilson && .PREV DISCUSSION... /ISSUED 610 AM MDT Wed Sep 18 2024/ Current radar shows areas of widespread rain with a few embedded thunderstorms being wrapped around the low station in Eastern MT. This low hangs around the region through out the day, keeping the TROWEL region centered along and east of the Great Falls to Chester line. This will keep the steady rain with pockets of moderate to heavy rainfall through Wednesday night. Snow levels look to continue to stay above 8,500ft this morning, keeping snow accumulations to the high mountain peaks. Convective showers that occurred yesterday evening along with additional lighter rainfall through the night has posed a concern for minor flooding/road ponding in flood prone areas in portions of eastern/central Teton County. Therefore a flash flood advisory was issued through 9 a.m this morning. Models have continued to uptrend in additional rainfall amounts through Thursday morning. The HREF model guidance along with other deterministic models suggests there`s a 50-80% for an additional 2" of rain across the plains east of a Great Falls to Hingham line. The main concern today will be areas of minor flooding in low lying areas/poor drainage and road ponding with moderate to heavy rain bands. Some moderate rain bands through the night have produced local areas of 1-2" of rain over the span of 3 hours across the North-Central MT plains. Additional flood advisories may need to be monitored throughout the day as a result of these moderate rain bands continuing throughout the day. See the HYDROLOGY section for more details. Another area of concern is the Central/North Central mountain areas and lower elevations across Judith Basin and Fergus Counties in receiving additional moderate to heavy rainfall today. The HREF and deterministic model guidance suggests that some areas may receive and additional 3-4" of rain. Isolated higher amounts are possible, with model guidance suggesting higher end amounts of up to 6" (20-40% possible) across the Bear`s Paw, Juidth`s/Snowy`s, and the Little Belts/Highwoods. The Flood Watch for those regions remain in effect through Wednesday evening. See the HYDROLOGY section for more details. Besides the concern for flooding, a strong mid/low level jet sets up along the backside of the low pressure. This will cause gusty northwest/west winds across the North-Central/Central MT plains today. The High Wind Warning in effect for Judith Basin and Fergus counties remain in effect through Wednesday night, where there`s a 70% and greater of 55 mph wind gusts. Other areas across North- Central MT have less than a 40% of 55mph wind gusts, but still cannot rule out the possibility of an isolated 55mph gust. The low pressure system pushes off to the east Wednesday night, and rain begins to wind down, before fully exiting Thursday morning. Northwest flow aloft moves in behind. A weak upper level trough moves into the region over the weekend, producing low end chances for precipitation. Ensemble clusters suggests some upper level ridging moves in beginning of next week, keeping the forecast drier. However, some guidance suggests a few disturbances bring low end precipitation chances to the area. -Wilson && .PRELIMINARY POINT TEMPS/POPS... GTF 64 47 73 47 / 90 50 0 0 CTB 67 45 71 45 / 40 10 0 0 HLN 65 47 74 47 / 80 20 0 0 BZN 61 42 71 42 / 100 40 0 0 WYS 51 32 61 31 / 90 40 10 0 DLN 60 39 68 40 / 60 10 0 0 HVR 65 49 73 48 / 100 80 0 0 LWT 56 45 68 45 / 100 80 0 0 && .TFX WATCHES/WARNINGS/ADVISORIES... Flood Watch through late tonight for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Snowy and Judith Mountains. High Wind Warning until midnight MDT tonight for Fergus County below 4500ft-Judith Basin County and Judith Gap-Snowy and Judith Mountains. Flash Flood Watch until 6 PM MDT this evening for Big Belt, Bridger and Castle Mountains. && $$ http://www.weather.gov/greatfalls