FXUS65 KTFX 191757 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1157 AM MDT Sun May 19 2024 ...Updated Aviation Section... .SYNOPSIS... Seasonably cool temperatures will be around the next week or so, with opportunities for precipitation most days. There is still some uncertainty in amounts, but Wednesday into Thursday looks to be the most impactful from a precipitation perspective. && .UPDATE... Updated forecast is out. Main adjustment was to increase pops to 100 this morning in areas where it is raining/snowing over Central MT. Rainfall amounts around 0.10 to 0.20 are likely, with 1 to 3 inches of snowfall in the mountains. Accumulations on roadways will generally be less than one half inch, thus just going to continue to highlight the impact with an SPS. Otherwise, it will be a cooler day today for most. The trend for much of this week will be below normal temperatures and periods of rain and mountain snow. Brusda && .AVIATION... 19/18Z TAF Period For all but the KBZN and KLWT terminals VFR conditions will prevail during this TAF period. At the KBZN and KLWT terminals rain is reducing ceilings to MVFR levels and will continue to do so for the first few hours of this TAF period. At the KCTB, KGTF, and KHLN terminals this afternoon and early evening there will be isolated thunderstorms. At the KBZN, KEKS, KLWT, KHVR, and KWYS terminals this afternoon and evening there is a slight chance (%20) for a thunderstorm or two but there wasn't enough probabilistic support to include it in those TAFs. For all terminals light rain showers will continue through this afternoon into the early evening with minimal impacts to aviation operations. For the KBZN, KEKS, and KHLN there is a 40% chance of rain/snow Monday morning which could reduce visibility and ceilings to MVFR levels and so a PROB30 group was included for those terminals. Tonight there is a 50% chance of rain/snow reducing visibility and ceilings to MVFR levels at the KWYS terminal and so a PROB30 group was included for it. For the KLWT and KGTF terminals there is a 20% chance of rain on Monday morning but there wasn't enough probabilistic support to include a PROB30 group for it. This afternoon and Monday morning there will be mountain obscuration across Southwestern Montana. This afternoon and early evening there will be instances of mountain wave turbulence. - IG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 520 AM MDT Sun May 19 2024/ Through Tuesday... To start the period, a series of upper level disturbances are in places from the Pacific NW northeastward across much of southern Canada, forming a congealed mess of a broad upper level trough. The next disturbance in the parade will shift eastward from the Pacific NW toward the Northern Rockies this morning, shifting eastward away from the region late today. A brief period of frontogenesis in the 700-600 mb layer will result in a band of precipitation early this morning, generally in the Helena area east-northeastward toward Lewistown. This band will slowly shift southeastward through the morning, dissipating as mid-level support diminishes and shifts eastward late morning. High resolution ensembles give high confidence in snow as the precipitation type in terrain. The Kings Hill Pass area has the best chance for a couple inches of snow with this band. A mix of rain and snow looks to be a good bet at lower elevations of Central Montana south toward the I-90 corridor through late morning. Given warm roadways I am not expecting too many impacts outside of reduced visibility at lower elevations where snow does mix in. Given the cool temperatures aloft working in with this system, numerous showers and a few thunderstorms look to develop late morning into the afternoon, especially in areas that see less cloud cover early in the day. Most showers will diminish by sunset. The next in the parade of upper level disturbances will largely miss the region to the south cool Monday into Tuesday. Although this is the case, cool temperatures aloft stick around into Monday and Tuesday, allowing additional showers to develop late morning through early evening across much of the region each day. Overall, not expecting much impact from these showers. Cool temperatures at the surface trend a touch warmer Monday, and get close to average for Tuesday. Wednesday into next weekend... Ensembles are converging on a solution where a closed upper low dives southward from northern BC late Tuesday into the Pacific NW Wednesday, eastward across Southwest Montana Wednesday night into Thursday, finally exiting eastward late Thursday night or early Friday. This solution would bring a round of accumulating precipitation to the region, largely rain at lower elevations, with high mountain snow. The chance for a half an inch of total precipitation Wednesday through Thursday night is largely greater than 50% across much of the plains and across terrain. Ensembles get a bit murky on the Friday through Sunday period, though temperatures near to slightly below normal with chances for lingering precipitation look reasonable at this point. -AM && .PRELIMINARY POINT TEMPS/POPS... GTF 55 35 59 37 / 80 30 60 20 CTB 55 33 57 35 / 70 30 50 10 HLN 57 37 60 39 / 100 30 60 20 BZN 55 32 56 34 / 60 20 70 40 WYS 52 27 51 28 / 40 30 70 50 DLN 53 31 55 33 / 30 20 60 30 HVR 59 37 60 38 / 60 20 40 10 LWT 52 32 54 33 / 70 30 70 20 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls