Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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595 FXUS65 KTFX 271123 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 523 AM MDT Mon May 27 2024 ...Updated Aviation Section... .SYNOPSIS... A high pressure ridge will help warm temperatures to between 10 and 15 degrees above normal by Tuesday across North Central, Central, and Southwest Montana, while keeping the area mostly dry. However, a Pacific weather system with an associated cold front will bring scattered showers and thunderstorms to the area beginning Tuesday afternoon through Wednesday. Some storms could become strong with erratic wind gusts and heavy downpours. Southerly winds ahead of the front will also shift more westerly on Wednesday, as temperatures cool back to near normal. Cool and unsettled conditions will then continue through the work week. && .DISCUSSION... Today through Thursday... A high pressure ridge will continue to strengthen over the Northern Rockies through Tuesday, which will keep North Central, Central, and Southwest Montana mostly dry and warm to end the Memorial Day holiday weekend. This warming will be aided by a shift in the breezy winds to a more southerly direction ahead of an approaching shortwave trough from the Pacific Ocean and its associated cold front. Many lower elevation locations have an 80 percent or greater probability of reaching 80 degrees for the first time this year on Tuesday. However, the increasingly moist and unstable southwest flow aloft ahead of the system will get some added lift as it becomes more difluent, which should generate some added lift and cause scattered showers and thunderstorms to form west of the Continental Divide by late afternoon. These storms are forecast to move east of the Divide into the evening hours. The warm and dry air at the surface and increased wind shear due to the southerly surface winds will make strong erratic wind gusts the initial threat from these storms, so the Storm Prediction Center (SPC) has this area under a Marginal Risk (5 to 15% chance) for severe thunderstorms, primarily due to wind. However, the slightly higher than normal precipitable water with them will transition the threat more so to heavy downpours, possibly with some small hail, during the late evening and overnight. As the cold front moves east across the area Tuesday night into Wednesday, so will the focus for additional showers and thunderstorms, as gusty winds shift more westerly behind the front. The increased moisture and cooler temperatures will be firmly in place by Wednesday, continuing the main threat for heavy downpours with the storms. This could cause increased runoff out of the mountains of Southwest and Central Montana, so the Weather Prediction Center (WPC) has that area under a Marginal Risk (5 to 15% chance) for flash flooding (for more details on this potential, please see the HYDROLOGY section). As the cold front exits the area Wednesday night into Thursday, the main portion of the shortwave trough will move into the area. This will further cool temperatures below normal, but keep a chance of showers and isolated thunderstorms over the area. In fact, snow levels are forecast to lower down as low as 5000 feet in some areas, which may result in snow accumulations of between 1 and 3 inches mainly to areas above 7000 feet. Friday through next Monday... Model ensemble clusters seem to agree with keeping a broad upper level trough over the area on Friday, likely continuing the near to slightly below normal temperatures with a chance of showers and thunderstorms. Slightly more than half of the clusters (56%) then start to bring weak high pressure ridging in for Saturday, while the other 44% want to keep the weak trough in place. There is then better agreement in building in the weak ridge for Sunday into Monday, but 25% of the clusters forecast the ridge to start shifting east with the approach of another trough. The National Blend of Models (NBM) seems to resolve this uncertainty well by warming temperatures through the weekend into early next week, but bring in a slight chance of showers and thunderstorms Sunday into Monday. -Coulston && .AVIATION... 27/12Z TAF Period VFR conditions will prevail during this TAF period for all terminals. At all terminals excluding the KCTB and KLWT terminals there will be light and variable winds due to high pressure moving into North-central, Central, and Southwestern Montana. At the KCTB terminal there will be winds gusting to 19kts until 02Z this evening. During the middle of this TAF period at the KLWT terminal there will be winds gusting to 17kts. At all terminals there will be few high-level clouds around due to high pressure moving into the area. During the beginning of this TAF period there will be instances of mountain wave turbulence. -IG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A Pacific weather system will bring a threat of heavy downpours with associated showers and thunderstorms, first to the Continental Divide and adjacent areas Tuesday afternoon and evening, then to the remainder of North Central, Central, and Southwest Montana Tuesday night through Wednesday. The concern is that this precipitation will fall on warming high mountain snowpack, which could increase the amount and speed of runoff out of the mountains. While most rivers should be able to handle this runoff with a low risk of minor flooding, smaller creeks and streams may run out of their banks. This situation will continue to be monitored for possible flood highlights. && .PRELIMINARY POINT TEMPS/POPS... GTF 71 45 82 53 / 0 0 10 40 CTB 69 43 79 49 / 0 0 10 50 HLN 75 48 82 54 / 0 0 20 50 BZN 71 41 80 48 / 0 0 10 30 WYS 63 33 71 41 / 0 0 0 30 DLN 71 42 78 47 / 0 0 10 40 HVR 73 44 82 54 / 0 0 0 30 LWT 66 43 77 52 / 0 0 0 20 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls