textproduct: Great Falls

KEY MESSAGES

- Winds will gradually subside through the evening hours tonight.

- A cold front will bring colder temperatures and increasing chances for rain during the day on Friday, most notably over the plains of Central and North Central Montana.

- A large upper level disturbance will bring even cooler temperatures and better chances for widespread precipitation during the day on Sunday, with mountain snowfall along the Continental Divide.

DISCUSSION

- Meteorological Overview:

A significant pattern shift is expected to end the work week and through the weekend after a breezy and warm day today. Longwave troughing over the Pacific Northwest this afternoon will continue to dig south along the western seaboard through Saturday night before lifting northeast and over the Northern Rockies through Sunday. An embedded wave within the longwave trough will initially slide east and over the Northern Rockies through the overnight hours tonight and the day on Friday, with a cold front diving south from Canada and across Montana during this timeframe. Precipitation will accompany the passage of this embedded wave and attendant cold front, most notably along and north of the Montana Hwy 200 corridor. Additionally, high temperatures will fall into the 50s across the plains of Central and North Central Montana on Friday, with the 60s to low 70s being observed further south across Southwest Montana where the front will arrive later in the day.

By Saturday temperatures will range from the 50s over the plains to 60s in the valleys of Central and Southwest Montana, with expansive cloud cover along and north of the I-90 corridor as Pacific moisture streams northeast ahead of the approaching longwave trough. Low level north to east upslope flow over Central and North Central Montana may support light precipitation across the Island Ranges and along the Rocky Mountain Front, with otherwise dry conditions expected elsewhere during the day on Saturday. As better forcing arrives Saturday night and on Sunday precipitation will become more widespread across Central and North Central Montana, especially along west of the I-15 corridor where low to mid-level easterly flow will increase. Precipitation is expected to persist through much of the day on Sunday, with temperatures struggling to rise into the 40s to mid-50s over the plains and upper 50s to low 60s in the valleys. - Moldan

- Forecast Confidence & Scenarios:

Lower Elevation Rainfall this Weekend...

Confidence with respect to rainfall amounts, especially along and north of the US Hwy 12 corridor, remains low given a high variability within the probabilistic data set. This is especially true given that "low end", or NBM10Pct values, are zero across all of Central and North Central Montana; with even the NBM25Pct values, which represent a "reasonable low end" scenario, suggesting a scenario in which the entire region see's less than 0.1" of rainfall. Ideally for a higher confidence forecast the spread between the 25th and 75th ("reasonable high end" scenario) percentiles would be closer, but in this case over the plains they are generally 0.5" to 1".

Listed below are NBM5.0 scenarios for "Low End" (NBM10Pct), " Most Likely" (NBM50Pct), and "High End" (NBM90Pct) rainfall amounts for select locations across North Central, Central, and Southwest Montana.

12z Saturday - 12z Monday Rainfall Scenarios LOCATION Low End | Most Likely | High End Browning 0.00" | 0.96" | 2.60" Cut Bank 0.00" | 0.86" | 2.39" Havre 0.00" | 0.40" | 2.02" Great Falls 0.00" | 0.53" | 1.82" Lewistown 0.00" | 0.42" | 1.46" Helena 0.00" | 0.42" | 1.42" White Sulphur Springs 0.00" | 0.30" | 1.20" Bozeman 0.00" | 0.12" | 1.03" Dillon 0.00" | 0.00" | 0.74" Ennis 0.00" | 0.00" | 1.16" West Yellowstone 0.00" | 0.00" | 0.61"

Mountain Snowfall this Weekend...

Confidence is high that the mountains, especially above 6500 feet along the Continental Divide north of the Montana Hwy 200 corridor, will see snowfall accumulations. Here NBM5.0 10Pct values generally range between 0.5" to 1", with some ridgetops across Glacier National Park approaching 3". Where uncertainty begins to grow is with how low snow levels will ultimately fall with this fall storm, along with how much precipitation will ultimately be observed (see earlier section). Should precipitation be on the heavier side, especially with the initial onset, it is likely that snow levels will fall significantly lower to around 5000 feet. Not only will the heavier precipitation lead to higher snowfall totals for elevations above 6500 feet, with NBM90Pct supporting amounts approaching 6" for these elevations, but minor accumulations up to 1" would be possible over the high plains adjacent to the Rocky Mountain Front. Given the degree of uncertainty that still exists we will hold off on issuing any winter weather highlights, but should confidence increase over the next 24 hours then a Winter Storm Watch may be warranted given the early season impacts to recreation (i.e. hunting season). - Moldan

AVIATION

10/18Z TAF Period

VFR conditons will prevail at the KBZN, KEKS, KWYS, and KHLN terminals during this TAF Period. There will be gusty winds at all terminals during the beginning of this TAF Period, with gusts in excess of 35 kts at the KCTB and KGTF terminals. Rain showers will affect all but the KBZN, KEKS, and KWYS terminals beginning after 11/08Z. Combined with the rain showers MVFR to IFR ceilings are expected at the KGTF, KCTB, KHVR, and KLWT terminals from 11/08Z through the end of this TAF Period. There will be mountain obscuration across North-central and Central Montana beginning after 11/05Z and continuing through the end of the TAF Period. There will be isolated instances of low-level wind shear between now and 11/08Z. -IG

For KGTF: Gusty winds will be a concern through the day today, with crosswinds exceeding 15kts from 19Z through 06Z. Winds will then weaken overnight after 06Z, at which point winds (and therefore crosswinds) will drop below 10kts. Ludwig/IG

PRELIMINARY POINT TEMPS/POPS

GTF 49 57 44 56 / 90 90 20 10 CTB 43 50 40 50 / 80 80 30 30 HLN 50 62 45 59 / 40 60 10 10 BZN 47 69 44 66 / 0 20 10 0 WYS 36 68 31 68 / 0 10 10 0 DLN 45 72 38 70 / 0 10 0 0 HVR 46 61 38 57 / 70 70 10 10 LWT 47 54 39 55 / 20 90 30 20

TFX WATCHES/WARNINGS/ADVISORIES