textproduct: Great Falls

KEY MESSAGES

- Critical Fire Weather conditions will develop through Sunday evening with low humidity, hot temperatures, and gusty winds. - A series of cold fronts will work through the area tonight and Sunday, bringing gradually cooler temperatures. - While Monday will be refreshingly cooler, temperatures will rise through the work week.

DISCUSSION

- Meteorological Overview:

The persistent upper level ridge that has been the main driver of our weather the past two weeks or so will break down over the next 24-48 hours as an upper level trough and a series of surface cold fronts push through the Treasure State. The first in this series of fronts will begin to push through the area this afternoon, but ahead of that front temperatures will be rather warm, with many lower elevation locales seeing temperatures rise through the upper 90s to around 105 in some of the warmest spots (pushing some areas into top 10 warmest day on record territory). As the first front pushes through the area this afternoon and evening, winds will begin to increase first, then gradually shift from southwesterly to westerly as the front progresses. Winds will remain elevated through the overnight, keeping the atmosphere well mixed overnight, which will keep us relatively warm with low overnight humidity recovery. Temperatures will be noticeably cooler on Sunday behind this first front, but expect temperatures to still warm up to the 80s for most lower elevations. With winds remaining gusty and the humidity low, critical fire weather conditions will persist into the day on Sunday. It will not be until a second, stronger cold front pushes into the area Sunday evening that fire danger will begin to decrease, though the wind shift associated with this front will pose a risk to any fire suppression operations.

Behind this second front, Monday will be quite a bit cooler than we have seen recently, with most population centers struggling to reach the mid 70s for afternoon highs. While these cooler temperatures will likely feel nice to most folks, they will not last long as upper level ridging begins to build back in across the Western CONUS. While not as intense as the ridge we have had recently, temperatures will once again warm up across the area, with most lower elevation locales seeing low to mid 90s return for the second half of the work week.

- Forecast Confidence & Scenarios:

Fire weather conditions today: There remains very high (>90%) confidence in widespread critical fire weather conditions across North Central and Southwestern Montana today and Sunday as the series of fronts mentioned above roll through the area. The first in this series of cold fronts will arrive later this evening, but southwesterly winds will increase across the area through the afternoon before gradually shifting to the west this evening. With winds behind the first front remaining elevated through the overnight, overnight temperatures will not drop as much as one might expect, with lower elevations expected to generally get to around 60, though eastern Fergus County along the Missouri River Breaks may struggle to get below 70. With these warmer temperatures, relative humidity values will struggle to rise overnight, with some areas struggling to rise above 30% which will keep fires a bit more active overnight. Winds and low humidity will continue on Sunday, keeping our fire danger high ahead of another, stronger cold front that will arrive late Sunday evening. While this front will eventually mean a reduction in fire weather conditions, the initial wind shift will be rather sudden and will pose a danger to the men and women who are fighting the fires. Ludwig

AVIATION

01/18Z TAF Period

The main concern this TAF period will be for breezy winds developing through the day and for a cold front to result in a wind shift this afternoon and evening across the region. Wildfire smoke will continue to result in slant-range visibility impacts, with surface visibility largely remaining VFR. Mountain wave activity increases near the Rocky Mountain Front after 00Z.

For KGTF: Southwest winds on the order of 15 kts through mid- afternoon will result in little to no cross-winds ahead of a cold front. The cold front looks to arrive between 21Z Sat and 00Z Sun, which would shift winds northwesterly. The probability that crosswinds exceed 15 kts is less than 10% before 21Z Sat, but rapidly rises to 70% after 21Z.

The gusty west/northwest winds slowly diminish near and after sunset and are expected to stay below 15 kts through the overnight hours before increasing Sunday morning after 15Z. Mountain wave activity cannot be ruled out near KGTF beginning late evening Saturday, persisting into Sunday morning. -AM/thor

FIRE WEATHER

Critical fire weather conditions are forecast this weekend across all fire weather districts. Gusty winds developing this afternoon ahead of a dry cold front will combine with relative humidities approaching single digits to result in critical fire weather conditions in most areas. Winds shift to a more northwesterly direction this afternoon and evening behind the cold front. While gusty winds diminish somewhat out over the plains tonight, strong winds will develop along the Rocky Mountain Front this evening, slowly expanding over the plains through the night and into the day Sunday. Overnight humidity recovery is forecast to be nearly non-existent where these winds are persistent overnight near the Rocky Mountain Front, but even where winds do decrease a bit overnight RH recoveries will be very poor, with many areas struggling to achieve RH values above 30-40%.

Although cooler temperatures will be moving in behind the cold front for Sunday, very low relative humidity will persist (Approaching single digits once again). Combine this with forecast breezy winds, and critical fire weather conditions will be around once again for Sunday. A second, more defined, cold front is also expected to push through the area Sunday evening, which will bring a sharp wind shift from westerly to more northerly. -AM/Ludwig

PRELIMINARY POINT TEMPS/POPS

GTF 61 85 49 72 / 0 0 0 0 CTB 54 74 44 68 / 0 0 0 0 HLN 59 84 50 74 / 0 0 0 0 BZN 57 90 49 73 / 0 0 0 0 WYS 48 85 39 75 / 0 0 0 0 DLN 55 87 46 72 / 0 0 0 0 HVR 61 85 46 72 / 0 0 0 0 LWT 59 84 47 66 / 10 0 30 10

TFX WATCHES/WARNINGS/ADVISORIES

Red Flag Warning until 9 PM MDT Sunday for Central and Eastern Lewis and Clark National Forest Areas-Chouteau and Fergus Counties-Eastern Glacier/Toole/Central/Eastern Pondera/Liberty- Helena and Townsend Ranger Districts of the Helena National Forest-Hill and Blaine Counties-Lewis and Clark National Forest Rocky Mountain District-Rocky Mountain Front-Lincoln Ranger District of the Helena National Forest.

Heat Advisory until 3 AM MDT Sunday for Beaverhead and Western Madison below 6000ft-Canyon Ferry Area-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton- Eastern Toole and Liberty-Elkhorn and Boulder Mountains-Gates of the Mountains-Helena Valley-Hill County-Meagher County Valleys- Missouri Headwaters-Northern Blaine County-Snowy and Judith Mountains.

High Wind Warning from 6 PM this evening to noon MDT Sunday for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains.

Extreme Heat Warning until noon MDT Sunday for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Western and Central Chouteau County.

Extreme Heat Warning until 3 AM MDT Sunday for Cascade County below 5000ft-Gallatin Valley-Judith Basin County and Judith Gap- Madison River Valley.