textproduct: Great Falls

KEY MESSAGES

- Localized flooding possible in areas with moderate rainfall across parts of central and southwest Montana today.

- Drier and warmer weather pattern develops for the upcoming week

DISCUSSION

- Meteorological Overview:

A vertically stacked mid-upper level low, centered over southern Idaho early this morning, will slowly track east into Wyoming through tonight while gradually weakening. This setup will sustain a deep, anomalously moist easterly flow across southwestern Montana today. Widespread and locally moderate to heavy rain this morning will concentrate along and south of a deformation/convergence axis extending eastward across central Montana north of the low. Conversely, areas north of this axis will experience little to no precipitation. By this afternoon, rainfall will transition to a more showery pattern and pivot back southward over southwestern Montana, following the circulation moving into Wyoming.

As the system continues eastward tonight, a few lingering showers remain possible across southwestern Montana into Sunday morning as moisture wraps around its backside. Clearing skies from the west overnight into Sunday morning may favor fog formation in southwestern Montana valleys. Upper-level ridging will then rebuild across Montana and the Northern Rockies by Sunday afternoon, returning predominantly sunny, warmer, and drier conditions to the area.

A dry weather pattern with above-normal temperatures is expected next week. Broad-scale ridging will attempt to build westward across the CONUS while troughing persists off the West Coast, maintaining upper-level heights above average under a southwesterly flow regime. High temperatures in the upper 70s to around 80 are anticipated, running roughly 10 degrees above early-autumn normals.

- Forecast Confidence & Scenarios:

An area of rainfall with embedded moderate to locally heavy rates (0.2 to 0.4 in/hr) will initially focus along the US-12 corridor early this morning. This precipitation is expected to lift northward toward the Little Belt and Snowy mountains later this morning before tapering off. The heaviest rainfall accumulations are anticipated over and adjacent to the Castle, Big Belt, and Elkhorn mountains, though confidence is somewhat lower regarding how far north higher amounts will reach into the Little Belts.

AVIATION

19/12Z TAF Period

Rain will continue to move across southwest MT this morning, becoming more showery this afternoon. MVFR to IFR ceilings are most likely through mid-day with lower cigs as far north as KLWT but most widespread from KBZN to KHLN. Lower clouds and a few showers will continue to circulate across southwest MT this evening with improvement from west to east overnight. Hoenisch

HYDROLOGY

Observed 24-hour precipitation totals have reached up to approximately 1.00 in northwest Beaverhead County, primarily from yesterday evening, with 0.500.75 measured so far across parts of Jefferson, Broadwater, and Meagher counties. Although the risk for intense rainfall rates is subsiding, the potential for runoff and debris flow near burn scars persists through this morning. Today, additional rainfall up to around 1.00 is likely (6090% chance) in portions of Jefferson, Broadwater, and Meagher counties, where localized 24-hour totals near or exceeding 2 inches remain possible (3050% probability). Amounts of this magnitude represent a 1-in-5 to 10-year recurrence interval for this time of year and could trigger minor urban and rural flooding in poor-drainage areas.

PRELIMINARY POINT TEMPS/POPS

GTF 68 47 74 47 / 20 0 0 0 CTB 68 43 73 44 / 0 0 0 0 HLN 63 46 72 46 / 100 20 0 0 BZN 59 42 66 40 / 100 60 10 0 WYS 57 31 61 29 / 70 40 10 0 DLN 59 40 67 40 / 70 30 0 0 HVR 73 46 74 46 / 0 0 0 0 LWT 61 45 65 43 / 70 70 50 0

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch through this evening for Big Belt, Bridger and Castle Mountains-Elkhorn and Boulder Mountains-Little Belt and Highwood Mountains.

Flood Watch through this afternoon for Canyon Ferry Area- Gallatin Valley-Helena Valley-Meagher County Valleys.

Flash Flood Watch through this afternoon for Northwest Beaverhead County.