textproduct: Great Falls

KEY MESSAGES

- Temperatures will be a roller coaster over the next several days with a mid-week warmup followed by another drop to near or slightly below seasonal averages for the weekend.

- Rain and Mountain snow will overspread the region Friday into Saturday bringing healthy liquid precipitation to lower elevations and accumulating snowfall above 4kft.

- Gusty winds develop across the area Friday afternoon into early Saturday morning. Highest gusts will be across Central MT and the Hi-Line.

DISCUSSION

- Meteorological Overview:

Weak zonal flow aloft across North-Montana and weak southerly flow over SW Montana will keep conditions relatively quiet on Tuesday, with temperatures cooling to near-seasonal averages. Isolated, light showers remain possible near the Idaho border, but dry conditions will prevail everywhere else.

On Wednesday, an upper-level low settles along the California coast, increasing our southwesterly flow aloft over SW Montana while zonal flow persists across North-Central MT. Strengthened warm air advection with the southwesterly regime will rebound high temperatures back to well above seasonal averages. Mostly dry conditions will continue across the plains, though weak moisture advection supports a low chance for a shower or two along the ID/MT border.

An active synoptic pattern arrives late in the week, as the pacific low pressure eject inland, and begins to phase with the digging trough emerging out of Canada. A weak cold front will pass through the region Thursday night into Friday morning, helping initiate widespread precipitation that persists through early Saturday. This system will be fairly heavy in moisture with high confidence in a large area seeing over 0.5" of liquid precipitation. An axis of heavier amounts is likely across Southwest and Central Montana where areas could see over 1" of QPF. Precipitation will begin with high snow level Thursday night (Above 6kft). During the heaviest period of precip on Friday morning, diabatic and dynamic cooling will help pull levels down towards 5kft, transitioning much of the mountain ran to accumulation snow. As the primary deformation axis and moisture depart Friday night into Saturday, additional cold air advects in lowering snow levels below 4kft. However, mid-level drying will quick cut off precipitation rates, limiting low- elevation accumulation potential. Simultaneously, a tightening surface pressure gradient and increasing low-level lapse rates will help tap the stronger 700mb flow on Friday bringing gusty winds across the region.

- Forecast Confidence & Scenarios: Confidence is moderate to high regarding elevated post-frontal winds on Friday. The probability for gusts reaching or exceeding 45mph is 40-60% across the Hi-Line and central Plains, across the RMF and central Island ranges that rises up to 50-70%.

For snowfall, confidence is medium for accumulation snowfall above 4kft, with a 35-55% chance of exceeding 2 inches especially across the RMF and central Island ranges. For elevations below 4kft, the probability of exceeding 1" is largely under 10%. The main question mark for the weekend storm centers on how long moisture wants to hang around as snow level drop further especially with the phasing of the low and trough. -Scenario 1: The low phases and swings through more progressively, moisture exits rapidly before snow levels fall below 4kft, confining accumulation snow to the mountains and passes while valley floors see rain ended as non accumulation "mood-flakes" with no impacts. -Scenario 2: The low/trough phasing is slower, both delaying the progression of the troughing but also allowing deformation moisture to linger across the area while cold air drives snow levels down towards 3kft. This will introduce a window for a more persistent but light snowfall across lower elevations. That being said, warm ground temperatures and hours of rain proceeding the potential change would restrict any of this to elevated surface if any at all. -JR

AVIATION

20/12Z TAF Period

VFR conditions will prevail at all terminals during this TAF Period with high-level clouds around. At all terminals but the KEKS terminal there will be light winds during this TAF Period. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 71 40 56 37 / 0 0 0 0 CTB 61 36 55 31 / 0 0 0 0 HLN 68 36 61 34 / 0 0 0 0 BZN 67 35 63 34 / 0 0 0 0 WYS 59 26 59 28 / 0 0 10 0 DLN 65 35 63 37 / 0 0 0 0 HVR 66 34 60 32 / 0 0 0 0 LWT 71 41 57 34 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.