textproduct: Great Falls

KEY MESSAGES

- Smoky conditions from fires burning across the Pacific Northwest are likely to linger across Southwest through North Central Montana into the day on Monday.

- A cold front will bring northerly breezes, a brief cooldown, and mostly dry conditions aside from a few stray showers or storms on Monday.

- Summer heat builds again and daily rounds of storms return heading towards the middle of the week.

DISCUSSION

- Meteorological Overview:

A mid- level trough will continue to move eastward through the Canadian prairies will bring increased westerly flow aloft over the Northern Rockies through the early evening hours . Deep layer mixing will help transfer the stronger 30 to 40 kt H700/H500 flow to the surface for breezy to windy conditons today, especially along the Rocky Mountain Front and the plains. In addition to the dry westerly breezes, ridging aloft, currently centered over the Great Basin, will continue to maintain hot temperatures. Although fuels are not quite dry enough for widespread fire weather concerns, a day like today will accelerate the curing process.

The drier westerly flow aloft will also slow down shower and thunderstorm activity, but ML CAPE will generally be running between 500 and 800 J/kg over the southwest. With little forcing available, widely scattered showers and storms will mostly be confined to far southwestern areas along the MT/ID/WY borders. If outflow boundaries or other lifting mechanisms become strong enough to erode the 20 to 50 J/kg of inhibition, then widely scattered storms may develop over much of the southwest and even central areas southeast of a Helena to Lewistown line; however, the chances for this occurring are running less than 20% at this time. Any storm that does develop will be capable of strong, gusty winds, hail, and downpours in addition to cloud to ground lightning.

Another trough running along the Canadian border will send a Canadian surface front through the forecast area and will bring a northerly wind shift and a brief cooldown tonight into Monday. There will also be variable clouds and perhaps an isolated shower or storm. Ridging aloft strengthens and brings a return to hot conditions heading towards the middle of the week. Passing shortwaves interacting with rich monsoon moisture will also increase daily shower and thunderstorm activity, along with severe weather and localized flooding concerns. Southwesterly flow aloft becomes more established over the Northern Rockies by next weekend. This will do little to cool temperatures, but will bring drier air in and increase winds for elevated fire weather concerns depending on the mid-week rainfall quality. - RCG

- Forecast Confidence & Scenarios:

Hot Temperatures from Thursday through next Sunday...

ECMWF EFIs, with respect to high temperatures, indicate the potential for an unusual period of heat across most of the Northern Rockies throughout the period. While these values are not exceptionally high, generally between 0.5 to 0.7, there are some locations that will be approaching 0.8 (very unusual) along the Rocky Mountain Front on Thursday. The fact that the end of July is the peak for average high temperatures is concerning given that most locations average anywhere from the mid-80s to near 90 degrees, with the EFI values suggesting temperatures at least in the 90s for all locations. NBM5.0 probabilities for high temperatures in excess of 100 degrees range from a 10-50% chance on Thursday over the plains, 30-80% (plains) and 15-30% (valleys) on Friday, 30-70% (plains) and 10-40% (valleys) on Saturday, and 20-50% (plains) and 10-20% (valleys) on Sunday.

Gusty Winds and Elevated Fire Weather Conditions on Friday and Saturday...

Cross barrier flow will begin to increase through the end of the work week and into the weekend as upper level ridging flattens, with H700 winds increasing to between 25-35kts which is 1 to nearly 2 standard deviations above normal for the Rocky Mountain Front. NBM5.0 probabilities support a 35-70% chance for wind gusts to exceed 34kts on both days, with a 10-30% chance to exceed 43kts. These gusty winds combined with hot temperatures will lead to elevated fire weather conditions along the entire Rocky Mountain Front, most notably across Fire Weather Zones 112. - Moldan

AVIATION

19/18Z TAF Period

Initial concern this TAF period will be for gusty west winds in many areas with wildfire smoke impacting visibility across the region. While surface visibility looks to remain VFR, some slant range visibility issues may arise where smoke is most dense aloft. The next concern is for a few showers and thunderstorms near the Idaho border between Monida Pass and KWYS this afternoon and early evening, capable of gusty and erratic winds. Attention then turns to a cold front moving south over the plains tonight into Monday morning, shifting winds to a more northwesterly direction. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 56 83 60 85 / 0 0 10 0 CTB 55 79 57 81 / 0 0 20 0 HLN 57 86 61 87 / 0 10 0 0 BZN 56 91 57 88 / 0 0 0 10 WYS 50 90 50 86 / 0 10 10 50 DLN 54 87 54 85 / 0 20 20 10 HVR 56 87 58 86 / 0 0 10 0 LWT 54 82 54 81 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.