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Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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546 FXUS65 KTFX 171128 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 525 AM MDT Sat Aug 17 2024 UPDATED AVIATION SECTION .SYNOPSIS... Afternoon temperatures warm back above average this weekend through mid- next week with lower elevation highs generally running in the upper 80s and 90s. There will be a brief pause in scattered shower and thunderstorm activity today before the daily chances return Sunday and Monday. A few storms may be on the stronger side with erratic gusty winds, hail, and brief heavy rains in addition to lightning. && .DISCUSSION... Overnight shower/thunder activity continues to weaken as it moves eastward and exits the CWA. Partial clearing to the west is combining with a decoupling boundary layer and lingering moisture for some patchy fog development, especially over river valleys and other fog susceptible areas of North-central and Southwest MT. This patchy fog may persist through mid- morning, but most locations have a 70% + chance for visibility remaining above 1 mile. Transient ridging moves in today and temporarily pauses shower and thunderstorm activity while afternoon temperatures warm back above the climatological average. The southwesterly flow aloft becomes more organized again tonight and will send the first of a series of embedded shortwaves into the Northern Rockies late tonight into early Sunday. This initial wave doesn`t appear to be very impactful, but may bring a few weak showers/virga and perhaps a rumble of thunder or two for locations along the Continental Divide and far Southwest MT. Stronger shortwaves and moisture arrive later Sunday into Monday and will bring multiple rounds of scattered showers and thunderstorms, mostly during the afternoon thru late evening period. Forecast soundings highlight ML CAPE values around 500 to 1,000 J/kg, bulk shear around 25 kts, and a sufficiently hot/dry sub cloud layer, all of which are favorable for the stronger cells to contain localized strong wind gusts, hail, and cloud to ground lightning. PWATS around the 0.75 to 1 inch mark will also maintain the potential for localized heavy downpours. Slightly cooler and drier air moves in and reduces overall shower and thunderstorm activity for the Tuesday through Thursday period. West to southwesterly flow aloft amid deep layer mixing will make breezy to windy and dry conditions the primary concern for this timeframe, particularly for Southwest MT and locations along the Rocky Mountain Front where minimum RHs look to be in the teens and 20s with probabilities for sustained winds above 20 mph running above 60%. Southwestern locations will be most susceptible for stronger wind gusts and currently have the highest probabilities (~40 to 60%) for gusts above 40 mph. Forecast uncertainty increases heading toward next weekend with a bout half of the ensembles moving the Pacific NW trough, responsible for the southwesterly flow aloft, ejecting eastward into the Northern Rockies while the other half favors the said trough retreating to the west or remaining stationary. The NBM was left as is for now, which depicts seasonably warm temperatures and breezy conditions with multiple opportunities for isolated to scattered showers and thunderstorms. - RCG && .AVIATION... 17/12Z TAF Period VFR conditions are mostly expected to continue through at least 18/12Z across North Central (KCTB, KHVR), Central (KLWT, KGTF, KHLN), and Southwest (KBZN, KEKS, KWYS) Montana, unless otherwise mentioned. Weak high pressure aloft is forecast to bring clearing and light winds to the area through the period. However, this has allowed patchy fog to form in some low-lying areas and areas that received precipitation yesterday, including KCTB. Some other areas may still see some fog development, but it will generally burn off by 15Z. Have mentioned the potential for MVFR/IFR conditions at terminals that received measurable precipitation. A weak disturbance in the southwesterly flow aloft may bring some scattered to broken mid level cloudiness to the area after 00Z. -Coulston Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PRELIMINARY POINT TEMPS/POPS... GTF 87 56 92 59 / 0 10 10 30 CTB 82 52 86 54 / 0 20 10 30 HLN 89 58 96 59 / 0 0 20 20 BZN 88 55 93 54 / 0 10 10 20 WYS 80 45 80 45 / 0 10 20 40 DLN 83 52 87 52 / 0 10 10 10 HVR 88 58 96 61 / 0 0 20 20 LWT 85 56 92 58 / 0 10 40 20 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls