textproduct: Great Falls

KEY MESSAGES

- Showers and a few thunderstorms this evening along and behind a cold front.

- After a slightly cooler Saturday, temperatures trend warmer Sunday through Tuesday.

- Breezy Tuesday and Wednesday.

DISCUSSION

- Meteorological Overview:

Upper level troughing dropping south from Canada is helping propel a cold front southward across the region this afternoon. Showers and a few thunderstorms will continue along and behind the front through the afternoon and early evening. Small hail and gusty winds are the primary concern, but a rogue stronger storm cannot be ruled out. See more on this in the forecast confidence section. Most precipitation ends this evening and tonight ahead of the coldest air aloft. A few flakes of snow near the higher peaks is a reasonable expectation at this point for the overnight.

Lingering cooler air aloft Saturday will combine with sufficient daytime heating to result in additional showers and perhaps isolated thunderstorms, mainly across Central and Southwest Montana from late morning through early evening. Upper level ridging then begins to build in, promoting largely clear skies and a warming trend. Before the warming occurs there appears to be a risk for some frost Sunday morning.

The building ridge looks to get flattened Tuesday into Wednesday, which will promote breezy conditions, particularly over the plains. The nature of the flow aloft looks to be a bit more cyclonic Tuesday night into Wednesday, which would promote a return of precipitation wherever the best forcing ends up developing.

Thereafter a northwesterly flow aloft slowly trends southwesterly, which will gradually reintroduce afternoon/evening showers and thunderstorms to the region toward next weekend. -AM

- Forecast Confidence & Scenarios:

Stronger thunderstorms this afternoon:

Shear looks to gradually increase across Central Montana through the afternoon. Instability increasing toward 500 J/Kg or so combined with the locally higher effective shear will result in a narrow corridor for a stronger thunderstorm or two across Central Montana, capable of producing some larger hail (Perhaps up to an inch in diameter) and gusty winds. Both shear and instability look to be maximized across Central Montana this afternoon, with a less favorable environment for stronger thunderstorms further north across North-central Montana as well as further south across Southwest Montana. -AM

Warm, windy, and, dry Tuesday...

Westerly winds begin to increase on Tuesday in advance of an approaching through and cold front. The NAEFS anomaly index already has H700/H500 winds approaching two to three standard deviations above climatology with NBM probabilities for 50 mph + gusts already running above 50% for portions of the forecast area, including the Rocky Mountain Front/plains west of I15 and areas over and near the higher terrain of central/southwest MT. The biggest uncertainty will be when the strongest winds aloft move through and whether it will be during peak diurnal mixing or later in the evening with less momentum transfer.

With this event being four to five days out, there's plenty of time to iron out any changes in trough/jet streak timing and positioning. At this time, the primary impact looks be inclement outdoor recreation and difficult travel for those operating high profile vehicles. The combination of temperatures warming well into the 80s and RHs falling to near critically low levels will also be monitored for fire weather concerns, although fuels will be less receptive given recent rainfall and greenup. - RCG

AVIATION

13/00Z TAF Period

Showers and thunderstorms will continue to affect the CWA through the overnight hours and into Saturday morning. Overall the rainfall will become lighter and less widespread by Saturday afternoon. Mountains/passes will be obscured into Saturday morning, with some clearing on Saturday afternoon. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 43 62 39 70 / 80 20 0 0 CTB 38 61 36 71 / 40 10 0 0 HLN 44 63 40 73 / 60 20 0 0 BZN 41 62 35 70 / 60 40 10 0 WYS 34 62 30 70 / 10 10 10 0 DLN 41 63 36 70 / 20 10 0 0 HVR 40 66 39 73 / 20 10 0 0 LWT 39 57 36 66 / 50 20 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.