textproduct: Great Falls

KEY MESSAGES

- Another round of valley fog or low stratus is expected within the valleys of Southwest and Central Montana tonight through Friday morning.

- Fast moving weather systems will bring windy conditions and spotty mountain precipitation through the weekend into next week.

- High Winds are possible along the Rocky Mountain Front Friday into Saturday morning and again on Sunday.

- Temperatures will generally run above average for the next seven days.

DISCUSSION

- Meteorological Overview:

An approaching Pacific trough will displace the ridge aloft to the east and bring increased winds and even some spotty mostly mountain rain and snow showers later today. This system will generally be moisture starved and will offer little to no precipitation for most locations. H700 winds dont look overly impressive, generally in the 30 to 40 kt range, but this will bring a definitive shift towards breezy to windy conditions, especially following the light wind regime of the last several days.

Ridging aloft then builds back in for the first half of the weekend, but a trough passing to our north will tighten the pressure gradient along the Canadian border. Forecast soundings have been steadily becoming more bullish in regards to mountain wave activity bringing bouts of strong winds along the Northern Rocky Mountain Front and adjacent plains Friday into Saturday, with localized wind gusts between 60 and 80 mph. Winds dont look quite as strong for the plains east of the highway 89 corridor, although gusts between 30 and 50 mph will be common.

A slightly deeper trough moves in Sunday into Monday and brings another round of winds and some more light mountain precipitation, mostly along the northern divide. This system will have a broader wind shield with H700 peaking at well over 60 kts; however, there may be more reliance on diurnal mixing for surface transfer momentum due to less ensemble confidence in more robust mountain waves. The winds certainly look to cover more real estate, but the top winds may not be as strong as the previous system if mountain waves are weaker and less frequent. The progressive westerly flow pattern continues well into next week with another peak in winds heading towards the mid- week period. - RCG

- Forecast Confidence & Scenarios:

High Winds from Friday afternoon through Saturday morning and Sunday

Confidence in impactful winds occurring during the day on Sunday has risen to the point where a High Wind Watch has been issued for the East Glacier Park Region, Northern and Southern High Plains, and Eastern Glacier/Western Toole/Central Pondera counties from the late morning through early evening hours. BUFKIT analysis throughout this period indicates increasing, unidirectional cross barrier flow, which is likely to support momentum transfer to the surface solely through diurnal mixing processes. This alone supports the need for a High Wind Watch given the anomalous (2-3 standard deviations above normal) H700 flow of 50-80kts moving over the region. Additionally within BUFKIT over this timeframe exists ideal thermal profiles to support mountain wave activity, which combined with a moderately strong surface pressure gradient will further enhance winds speeds during the day on Sunday.

For the timeframe from Friday afternoon through Saturday morning uncertainty in high winds materializing still exists given a veering wind profile through the critical layer (i.e. ridgetop stability) from the surface. This veering profile during past events has limited how widespread the most intense winds are from the more traditional locations across the East Glacier Park Region, Northern and Southern High Plains, and Eastern Glacier/Western Toole/Central Pondera counties; however, some locations do still see impactful winds. For this reason the inherited High Wind Watch was left as, but the Southern High Plains zone was added given the susceptibility of areas like Dellwo and Bynum hitting on criteria. If it was nott for this veering profile BUFKIT soundings support another ideal thermal profile for mountain wave activity on Friday night, with a strong surface pressure gradient of 0.15 to 0.2 mb/km existing from the afternoon hours on Friday through Saturday morning along the immediate eastern slopes of the Rocky Mountain Front.

The table below lists the reasonable low-end (25th Percentile), most likely (50th Percentile), and high-end (75th Percentile) wind gust scenarios within the NBM5.2 for select locations across the plains of Central and North Central Montana over the aforementioned timeframe. The low-end scenario has a 75% chance that wind gusts will exceed the listed value or a 25% chance that gusts will be lower than what is listed while the high-end scenario has a 75% chance that wind gusts will be lower than what is listed or a 25% chance that gusts will be higher than what is listed.

Reasonable Wind Gust Scenarios on Friday LOCATION 25 Pct | 50 Pct | 75 Pct ------------------------------------------------------------- East Glacier 60 mph | 65 mph | 70 mph Browning 60 mph | 70 mph | 75 mph Cut Bank 45 mph | 50 mph | 60 mph Havre 35 mph | 40 mph | 45 mph Choteau 30 mph | 30 mph | 35 mph Augusta 30 mph | 35 mph | 45 mph Great Falls 45 mph | 45 mph | 50 mph Lewistown 25 mph | 30 mph | 30 mph

Reasonable Wind Gust Scenarios on Saturday LOCATION 25 Pct | 50 Pct | 75 Pct ------------------------------------------------------------- East Glacier 50 mph | 60 mph | 70 mph Browning 50 mph | 60 mph | 70 mph Cut Bank 45 mph | 50 mph | 60 mph Havre 35 mph | 40 mph | 45 mph Choteau 25 mph | 30 mph | 35 mph Augusta 25 mph | 30 mph | 40 mph Great Falls 40 mph | 45 mph | 50 mph Lewistown 25 mph | 30 mph | 30 mph

Reasonable Wind Gust Scenarios on Sunday LOCATION 25 Pct | 50 Pct | 75 Pct ------------------------------------------------------------- East Glacier 60 mph | 65 mph | 77 mph Browning 70 mph | 70 mph | 80 mph Cut Bank 65 mph | 75 mph | 80 mph Havre 45 mph | 55 mph | 60 mph Choteau 40 mph | 50 mph | 55 mph Augusta 50 mph | 55 mph | 60 mph Great Falls 45 mph | 50 mph | 55 mph Lewistown 35 mph | 40 mph | 45 mph

- Moldan

AVIATION

05/18Z TAF Period

Main upcoming concern will be breezy winds in north-central and north MT. Breezy winds in these areas will remain consistent through most of the TAF period with peak gusts between 25-35KT. Additionally, multiple periods of mountain waves are expected Friday through Sunday along the Rocky Mountain Front, north of the MT 200. As for flight categories, widespread VFR conditions are expected through the TAF period.-ESH

PRELIMINARY POINT TEMPS/POPS

GTF 35 55 46 63 / 0 0 0 0 CTB 32 51 40 59 / 0 0 0 0 HLN 27 51 35 57 / 0 0 0 0 BZN 27 48 30 54 / 0 0 0 0 WYS 21 39 19 47 / 0 0 0 0 DLN 25 47 28 52 / 0 0 0 0 HVR 32 55 39 63 / 0 0 0 0 LWT 31 54 38 61 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Watch from Friday afternoon through Saturday morning for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains.

High Wind Watch from Sunday morning through Sunday evening for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains.