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FXUS65 KTFX 200520 AFDTFX

Area Forecast Discussion National Weather Service Great Falls MT 1120 PM MDT Sun May 19 2024

Aviation Section Updated

SYNOPSIS

Showers, thunderstorms, small hail and snow will all decrease over the area tonight, as drier air moves in for a few hours. However, expect showers and thunderstorms to redevelop over Southwest MT on Monday, with the showers becoming more widespread on Tuesday. A spring storm will affect much of the area Wed and Thu, with periods of heavy rain and mountain snow expected. Generally, below normal temperatures are expected for most areas this week.

UPDATE

Two main changes with this update:

1. Increased precipitation chances in central Montana using the convective allowing models to fit the band of showers currently moving through. At this point in the evening, the main thunderstorm threat is over. However, a couple weak thunderstorms are still popping up across the area which may produce locally heavier rain and gusty winds. But no major concerns are present at this time.

2. The chance for fog was expanded along the hi-line overnight.
The best chances for fog will be along the Milk River Valley north to the Canadian border east of Havre. In those locations temperature and dewpoint will be within a couple degrees of each other, skies will be mostly clear, and winds will be generally light. Outside the main fog area, there is a broad chance for patchy fog across north-central Montana with the probability decreasing rapidly towards the south. Higher fog chances (up to 40%) were brought down along the Highway 87 corridor as far south as Loma while lower fog chances (20-40%) were brought down to just north of Great Falls. At this point, conditions are too marginal for fog in Great Falls proper but should skies clear a bit more after the band of showers move through then that will be something to watch heading into tomorrow morning.

Otherwise, no major changes were made to the going forecast.

-thor



AVIATION
20/06Z TAF Period

The concerns this TAF period will be for primarily mountain snow across Southwest Montana through mid-morning or so Monday. Attention then turns to afternoon showers and isolated thunderstorms across most of the region. Confidence in thunder was too low to warrant mention in TAFs at this time. A brief period of fog will be possible in the vicinity of KHVR around sunrise Monday. Mountains will likely be obscured around precipitation. -AM

Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information.

PREV DISCUSSION
/ISSUED 544 PM MDT Sun May 19 2024/ Tonight through Tuesday...Expect a variety of weather conditions over the CWA tonight, with rain, snow, thunderstorms and small hail all diminishing this evening, as the main upper level trof moves off to our east. No significant impacts are expected from the weather this evening. Most snow or small hail accumulations will only occur on grassy areas. Much of the area starts out dry tomorrow morning, but a slow moving upper level disturbance moves into Southwest MT by mid day, producing scattered showers/thunderstorms by late morning and then continuing into Tuesday. Again, this thunderstorm activity is expected to be on the light side for most storms. Afternoon temperatures will warm up a bit for Monday and Tuesday, with near normal afternoon temperatures expected on Tuesday.

Wednesday into Friday...A spring storm is expected to affect much of the CWA for the middle portion of the week. There is a 50 percent chance of 0.25 to 0.75 inches of liquid at lower elevations and 1 to 1.50 inches in the mountains. There is less than a 10 percent chance of getting over 2 inches of liquid in the mountains at this time. The heavy rain could cause some impacts in the Monarch and Neihart areas. Additionally, snow levels look to be around 7000 feet. There is a 50 percent chance of over 8 inches of snow in the mountains of Northwest Beaverhead County and the Tobacco Root Mountains, and about a 30 percent chance over the Rocky Mountain Front and the Kings Hill area. Thus winter statements are likely for portions of the CWA for this event. This system is slow, and it will take almost 2 days to move through the CWA Below normal temperatures are expected during this time.
There is also a small concern that snow levels could lower to near valley floors during the morning hours on Thurs/Fri, which could impact some bigger lower elevation cities.

Saturday and Sunday...Some forecast models show the potential for a 2nd spring storm to move through the CWA, while other models have lighter precipitation. Never the less, below normal temperatures and the chance for precipitation is looking likely for most areas at this time next weekend. Brusda

PRELIMINARY POINT TEMPS/POPS
GTF 33 59 36 65 / 40 50 20 30 CTB 33 56 34 63 / 30 60 20 20 HLN 36 60 38 66 / 20 60 20 30 BZN 31 54 32 62 / 20 70 40 40 WYS 26 50 28 52 / 30 70 60 60 DLN 34 54 31 59 / 40 60 30 20 HVR 35 60 37 66 / 30 40 10 20 LWT 31 54 33 59 / 20 60 20 40

TFX WATCHES/WARNINGS/ADVISORIES
Flood Watch from Wednesday evening through Friday morning for Little Belt and Highwood Mountains.

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