textproduct: Great Falls

KEY MESSAGES

- Warming peaks on Tuesday with temperatures near daily records.

- Transitional day on Wednesday with showers and few thunderstorms.

- Potential for an impactful spring storm system with widespread precipitation, locally heavy snow and gusty north winds Wednesday night into Thursday.

- Much colder temperatures by Thursday with below average temperatures through the weekend.

DISCUSSION

- Meteorological Overview:

Upper level ridging remains in place across the Rockies ahead of a large cut-off upper level low offshore the west coast, which begins moving onshore Tuesday. This maintains mild and dry conditions early this week with afternoon temperatures on track to peak Tuesday as southerly flow increases ahead of the upper level system. Thermal/surface troughing also begins to deepen across the area Tuesday, supporting deeper mixing and breezy/gusty south to southwest winds Tuesday afternoon, particularly across southwest MT.

A fairly complex situation evolves late Wednesday through Thursday as the initially closed upper level low moving out of the Great Basin interacts with shortwave energy dropping south through western Canada with a fairly potent mid-level wave forming and ejecting out across MT/WY into the Northern Plains by late Thursday. This initially leads to the development of showers and thunderstorms late Wednesday as moisture increases along with instability from cooling aloft beneath the approaching upper low from the SW. Meanwhile, cooler air also begins to move south from Canada into north-central MT late Wednesday with much colder air spreading south across the area Wednesday night through Thursday as surface low pressure and the mid-level circulation deepens across eastern MT and ND. An area of widespread precipitation is likely to pivot around the mid-lvel circulation Wednesday night through Thursday morning affecting much of north-central and southwest MT with the highest amounts focused across the central MT mountains and adjacent areas to the north. A change to snow at lower elevations is likely to occur from north to south Wednesday night with gusty north winds also developing by Thursday morning and increasing through Thursday afternoon as precipitation diminishes behind the departing system.

Longer range model ensembles are in general agreement to maintain a cyclonic northwesterly flow across the area through the upcoming weekend with differing versions of troughing or a broad upper low centered to the northeast across central Canada. This will maintain below average temperatures through the upcoming weekend with additional chances for precipitation as weaker disturbances rotate across the Northern Rockies and MT. Hoenisch

- Forecast Confidence & Scenarios:

Warm, dry and windy conditions will elevate fire danger on Tuesday, especially across southwest MT where gusts in excess of 35 mph are likely (>70% probability) as far north as southern Jefferson and Broadwater counties with a similar probability for gusts in excess of 40 mph for areas south of I90. The risk for enhanced fire spread will be greatest in areas where greenup may be more limited.

The extent and strength of convection/thunderstorms Wednesday is still fairly uncertain with latest guidance hinting at a somewhat narrow corridor from central MT through eastern portions of north- central MT late Wednesday afternoon.

There is still considerable uncertainty with with the evolution of smaller scale features with the the upper level trough/low moving across the region Wednesday night/Thursday. However, latest model guidance trends show a favorable track of the mid level circulation to bring significant precipitation, including heavy wet snow, to portions of central MT. Probabilities for precipitation amounts in excess 1.00" are as high as 50-60% across the central MT mountains and 10-30% across adjacent lower elevations, though there is still a spread of 0.50" or more between the 25th and 75th percentile amounts. Even higher uncertainty exists for snow amounts, especially at lower elevation, however this bears watching as there is also increasing support for gusty north winds in excess of 35-40 mph (>70% probability across north-central MT) on Thursday, which combined with heavy wet snow could lead to power impacts in addition to winter driving conditions in areas that see the wet snowfall. Monitor the forecast for additional details this week. Hoenisch

AVIATION

20/06Z TAF Period

VFR conditions prevail under some high-level cloudiness. Winds will largely be light, peaking in the mid-afternoon Monday in most areas. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 40 75 43 80 / 0 0 0 0 CTB 36 73 40 78 / 0 0 0 0 HLN 41 75 43 81 / 0 0 0 0 BZN 35 75 39 78 / 0 0 0 0 WYS 24 65 28 67 / 0 0 0 0 DLN 37 74 40 74 / 0 0 0 0 HVR 38 75 42 84 / 0 0 0 0 LWT 40 71 44 79 / 0 0 0 10

TFX WATCHES/WARNINGS/ADVISORIES

None.