textproduct: Great Falls

KEY MESSAGES

- Much warmer and drier conditions return this weekend into early next week, with near record highs by Tuesday and Wednesday.

- The next Pacific trough approaches the Northern Rockies on Wednesday and will bring breezy to windy conditions and widely scattered showers and thunderstorms.

- Cooler temperatures, mountain snow, and lower elevation rain or mixed precipitation move in for next Thursday and Friday.

DISCUSSION

- Meteorological Overview:

The cold pool aloft quickly gives way to a building ridge this weekend through early next week for much warmer and drier conditions with seasonable breeziness. Lower elevation afternoon temperatures warm into the 50s and 60s today before 70s and even higher readings become more common Sunday through Wednesday.

Southwesterly flow aloft begins to develop heading into the mid- week period in response to a split flow trough moving onto the Pacific coast. As this system progresses eastward, moisture and forcing from associated shortwave energy may become sufficient for isolated shower and thunderstorm activity as early as Tuesday evening, though Wednesday looks to be most favorable day for more widespread activity with CAPE rising to the 300 to 800 J/kg range amid better forcing from the approaching cold front. The primary hazards look to be localized strong to severe wind gusts and occasional lightning given warm/dry surfaces and inverted V forecast soundings, though hail and downpours may be secondary hazards.

The evolution of this trough still looks rather complex with multiple transient circulations capable of producing at least localized heavier precipitation. Over half of the ensembles support now widespread precipitation Wednesday night through early Friday, although the with slightly differing synoptic setups. This system will be much warmer than the most recent one, but colder north/northwesterly on the backside flow looks to bring at least some snow to all elevations. - RCG

- Forecast Confidence & Scenarios:

Mid- week breezy to windy conditions and widely scattered showers and thunderstorms...

Ensembles continue to highlight Wednesday for the next chance for more impactful widely scattered showers and thunderstorms. The timing of the Pacific trough and its attendant cold front will have an impact on the phasing of instability and synoptic forcing and will ultimately determine storm coverage/intensity. An earlier passage will shunt the richest moisture and instability to plains locations east of I15. Localized strong to severe wind gusts look to be the primary impact, but hail and brief downpours may also come into play dependent on instability and moisture. Winds may even be strong outside of showers and thunderstorms with a 40kt southwesterly low level jet passing through. Currently, the probabilities for 55 mph wind gusts or greater are running in 20 to 40% range, highest along the Rocky Mountain Front and the narrow north to south oriented southwest valleys.

Mountain snow and lower elevation rain/snow Thursday into Friday...

sufficient and stable forcing to ensure widespread precipitation next Thursday into Friday remains in question outside of transient smaller scale circulations and the trough itself; however, ensembles continue to increase precipitation coverage and amounts. NBM probabilities for 1 inch or more of liquid equivalent precipitation have increased to around 30 to 50% over and near the central MT island ranges, with around 20 to 30% in other areas. Ensembles continue to favor warmer temperatures aloft compered to the most recent storm, but colder northerly flow on the backside of this system will ensure mountain snow accumulation and at least a brief transition to snow at lower elevations. - RCG

AVIATION

18/06Z TAF Period

The primary concern this TAF period will be for some lingering snow showers through the remainder of the night. Otherwise concerns are minimal with upper level ridging building in during the day Saturday. Surface winds increase somewhat late morning and persist through early evening. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 58 29 68 39 / 0 0 0 0 CTB 57 25 67 36 / 0 0 0 0 HLN 59 31 69 40 / 0 0 0 0 BZN 56 25 67 35 / 0 0 0 0 WYS 45 15 58 25 / 0 0 0 0 DLN 56 27 66 35 / 0 0 0 0 HVR 59 27 69 38 / 0 0 0 0 LWT 55 28 65 40 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.