textproduct: Great Falls

KEY MESSAGES

- Areas of fog this morning.

- Slightly cooler Sunday behind a cold front.

- Windy on Tuesday ahead of an unsettled rest of the week.

DISCUSSION

- Meteorological Overview:

Upper level ridging building in across the west is resulting in a clearing trend early this morning across the region. Given recent precipitation and snow melt, areas where clearing skies overlap with light winds will see at least patchy fog through the morning. Some high level cloudiness should limit spatial extent of fog that does form this morning, however. A weak wave traversing the building ridge will push slightly cooler air in from north to south this afternoon into the evening, and get reinforced by a building surface high well off to the northeast in SK. Slightly cooler temperatures that move in late Saturday will persist through the day Sunday, with just a low end opportunity for a light shower at times Sunday.

Some moderation occurs into Monday as upper level ridging persists. The ridge is quickly broken down heading into Tuesday by an upper level system diving southeastward from BC toward the Northern Rockies. The initial impact will be strong westerly to southwesterly winds through the day, driven by the combination of daytime mixing and at least a briefly strong pressure gradient. As the surface low in southern BC shifts eastward Tuesday night, a Canadian cold front pushes southward across the region Wednesday morning. There remains tremendous uncertainty with respect to how much precipitation would result from this frontal passage, though confidence is high that temperatures will fall to or a touch below normal behind the front.

A convoluted pattern with cutoff lows and additional waves within a broadly cyclonic flow aloft across the west will keep the region unsettled through the remainder of the week, with daily opportunities for at least light precipitation. -AM

- Forecast Confidence & Scenarios:

Winds Tuesday and cold front into Wednesday:

Ensembles are largely on board with this system moving across the region during the day Tuesday. This will result in favorable conditions to mix down stronger winds aloft through the afternoon and early evening ahead of a cold front. EFI (European Extreme Forecast Index) values are largely between 0.6 and 0.8 for wind gusts Tuesday, with a shift of tails of 0 across nearly the entire region. These values indicate increased confidence in strong winds, with potential for anomalous gusts for this time of the year.

While confidence is increasing in the aforementioned gusty period, it remains low with respect to how much precipitation follows the cold front Wednesday into late week. Timing and location (which is low confidence at this range) of several trailing upper level disturbances later in the week will play a large part in which areas receive precipitation. -AM

AVIATION

04/06Z TAF Period

A northwesterly flow aloft will maintain scattered to broken low VFR clouds through around 04/12Z. Once these clouds begin to clear out, patchy fog will begin developing over the wind protected plains and southwest valleys. The probabilities for impactful fog are generally less than 20% for all terminals; however, the meteorological set up favors localized instances of MVFR/IFR fog over KBZN, KHVR, and KLWT between 04/10 and 04/16Z. On Saturday, a weak front will move through the plains and bring a northerly wind shift. A moist easterly low level becomes more established over central/north-central MT after 05/06Z and gradually increases MVFR/IFR stratus from north to south. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 27 58 28 51 / 0 0 10 10 CTB 27 55 27 49 / 0 0 10 10 HLN 29 59 32 59 / 0 0 0 10 BZN 23 59 29 62 / 0 0 0 0 WYS 18 51 19 57 / 0 10 10 0 DLN 26 59 29 65 / 0 0 0 0 HVR 28 58 30 50 / 10 0 10 10 LWT 26 53 28 50 / 10 0 10 10

TFX WATCHES/WARNINGS/ADVISORIES

None.