Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
526 FXUS65 KTFX 280218 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 818 PM MDT Mon May 27 2024 .SYNOPSIS... Tuesday will be warmer with highs 10 to 15 degrees above normal across north central, central, and southwestern Montana. By Tuesday evening, a cold front will make its way into western Montana and bring a chance of showers and thunderstorms to the region. Some storms may produce gusty winds and small hail as well as locally heavy rain which may lead to minor flood concerns for areas that have recently received rain and snow. Behind the front, the rest of the week will remain closer to normal with drier conditions persisting through the weekend. && .UPDATE... Transient ridging will remain in place through Tuesday morning before shortwave energy within an increasing southwesterly flow aloft brings a round of scattered late afternoon and evening showers and thunderstorms. The previous forecast package is handling current trends well, so no updating will be necessary this evening. - RCG && .AVIATION... 28/00Z TAF Period Upper level ridge of high pressure shifts east across the Northern Rockies through Tuesday morning, maintaining a dry west to northwest flow aloft. VFR conditions are expected at all terminals for much if not all of the TAF period; however, higher level clouds will increase tonight into Tuesday morning and scattered shower and thunderstorm activity associated with an approaching Pacific trough will develop and cross the Continental Divide around 29/00Z and beyond. Winds become light this evening and turn more southerly by Tuesday morning. - RCG 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. && .PREV DISCUSSION... /ISSUED 218 PM MDT Mon May 27 2024/ Today through Thursday... A calm day remains for Memorial Day with generally partly cloudy skies and winds that will slowly calm down heading into the evening/overnight hours before the front arrives on Tuesday. Things heat up for Tuesday afternoon with widespread chances of 80 degrees or higher along the northern plains and lower elevations across central and southwestern Montana. Along with the heat and the arriving front comes the risk for afternoon showers and thunderstorms across the region. The severe weather threat across western and central Montana remains under a Marginal Risk (<15% chance) for Tuesday evening. The latest model guidance has storms first moving into the areas immediately east of the divide between 4 and 6 PM. After that, scattered thunderstorms will continue pushing eastward through portions of central and southwestern Montana. The biggest threat with these thunderstorms will be the wind potential with small hail remaining a secondary threat. A small wild card with the severe threat will be how far east it extends. Currently SPC cuts off the Marginal Risk at a line between Inverness and Bozeman. Some convective allowing models are proposing around a 20% chance that some stronger showers/ thunderstorms may last a little further east, possibly impacting Havre and Lewistown. More than likely these areas will see less on the side of strong convection and rather the remnants of whatever is propagating from the west but precipitation chances were increased to reflect the probability of some heavier showers and gusty winds further east. -thor 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. 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 && .PRELIMINARY POINT TEMPS/POPS... GTF 45 82 53 66 / 0 10 40 50 CTB 42 78 49 61 / 0 20 50 20 HLN 48 84 53 65 / 0 20 50 60 BZN 41 81 47 64 / 0 10 40 90 WYS 34 71 41 62 / 0 0 40 90 DLN 42 78 46 65 / 0 20 50 80 HVR 44 83 54 70 / 0 0 30 50 LWT 41 76 51 65 / 0 0 20 90 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls