textproduct: Great Falls

KEY MESSAGES

- Warm and breezy Independence Day across the region.

- Afternoon showers and thunderstorms increase in coverage heading into next week.

DISCUSSION

Meteorological Overview:

Upper level flow across the region will remain zonal through the day today associated with troughing moving onshore across central BC. Daytime mixing will facilitate in translating these westerly winds aloft to the surface through the day which largely look to peak in the 30-40 mph range. While these gusts aren't particularly notable in standard circumstances for the region, it seems as if a special mention is warranted given increased recreation for the day. Temperatures above the mixed layer today look to be a touch too warm to allow for any convection, though it cannot be completely ruled out over terrain.

Flow aloft begins to trend more southwesterly as the troughing in Canada progresses eastward. Instability looks to increase quickest across Southwest Montana and vicinity by Sunday afternoon. Meanwhile the more favorable kinematic environment looks to be further north across North-central Montana closer to the troughing. Should there be a blend of both of these factors (Increased instability and modest shear) there will be the risk for a briefly organized thunderstorm or two capable of producing briefly large hail and gusty winds. This conditional risk looks to be in place across Central Montana Sunday.

Heading into Sunday evening the upper troughing continues eastward. In its wake a weak Canadian cold front looks to slide southward across the plains. The main impact from this front will be for a cooler day heading into Monday. By Monday afternoon a wave progressing northeastward, originating from central/southern CA, will overspread the region. There is still some timing uncertainty, but should this wave arrive during peak diurnal heating there would be sufficient shear to result in at least briefly organized strong to severe thunderstorms across Southwest MT and vicinity. Both large hail and gusty winds would be a concern with this conditional risk. A later to arrive wave would still result in some showers and thunderstorms, but the risk for severe weather would be lower.

Uncertainty increases further heading into Tuesday. Shortwave ridging looks to build in briefly behind a departing wave, but there are considerable discrepancies as to how quickly the departing wave actually moves out. Slower solutions result in another round of showers and thunderstorms, this time across the plains east of I-15. Quicker guidance results in a largely dry day Tuesday, save for some terrain driven showers and thunderstorms.

From a parameter space and overall pattern setup standpoint Wednesday looks to be the most intriguing. Ensemble mean dew points across the plains look to be well into the 50s, if not lower 60s. This looks to coincide with another upper level wave passing across the region during the afternoon or evening. Should this wave and associated stronger flow aloft arrive by the early afternoon, there looks to be more than enough instability around to result in a few organized strong to severe thunderstorms. Conversely, if this wave is slower to arrive, flow aloft and the overall steering pattern will be much weaker and result in an increased risk for flash flooding given ample moisture and instability that would still be in place. -AM

Uncertainty:

There are several pieces of uncertainty for the forecast, largely associated with timing of disturbances. Should these disturbances trend toward afternoon timing, the risk for stronger thunderstorms would increase and vice versa. -AM

AVIATION

04/06Z TAF Period

Expect VFR conditions to prevail over the next 24 hours. Some gusty winds are expected during the afternoon hours, but otherwise benign conditions are expected. Ludwig

PRELIMINARY POINT TEMPS/POPS

GTF 86 55 85 53 / 0 0 0 0 CTB 80 50 80 48 / 0 0 0 0 HLN 85 56 86 55 / 0 0 20 10 BZN 85 54 87 53 / 0 0 10 30 WYS 80 43 84 46 / 0 0 0 0 DLN 84 51 86 52 / 0 0 20 20 HVR 86 56 86 53 / 0 0 0 0 LWT 84 52 83 50 / 0 0 10 40

TFX WATCHES/WARNINGS/ADVISORIES

None.