textproduct: Great Falls

KEY MESSAGES

- Showers and thunderstorms will push through Montana on Sunday, some of which may bring some gusty winds. - Widespread soaking rainfall expected Sunday through Tuesday, with some areas on the Hi-Line having a 40-60% chance of seeing 2 inches of rainfall. - Temperatures start next week below average, before returning to near average for the second half of the week.

DISCUSSION

A deep and slow moving upper level trough over the Eastern Pacific/Western US will slowly move eastward over the coming days. As it moves eastward, expect rain along the Continental Divide today to slowly shift eastward and expand, with more widespread showers and thunderstorms Sunday, Monday, and early Tuesday before this system departs late Tuesday morning and Wednesday. Behind this system, drier weather is expected Wednesday and Thursday as weak upper level ridging transits the area, which will also bring temperatures back closer to average after a cool start to the week. However, more unsettled weather looks likely to close out the week as yet another upper level trough pushes into the Western US.

- Forecast Confidence & Scenarios:

Thunderstorms Sunday: Recent Hi Res model guidance has begun to hint at the possibility of a round of thunderstorms on Sunday capable of producing some strong to severe wind gusts. While it does not currently look quite as damaging as Wednesdays event was, there are still a few things to note about the severe threat. Model soundings and hodographs (temperature/humidity profiles and wind profiles, respectively) show quite a bit of instability in the mid/upper levels, along with ample shear to help storms form and then organize and strengthen as they race northeastward thanks to relatively strong winds (in excess of 50 knots/60 mph). Given the wet environment, it will be difficult for any larger hail to reach the surface, but some smaller hail may still reach the surface. Winds are a bit tricky as well, as while there is both sufficient dry air near the surface and winds aloft to result in some strong wind gusts, there are also hints of some capping which may limit transport of the winds to the surface. Given the uncertainty, SPC has placed much of North Central and Central Montana on Sunday, which seems reasonable at the moment given the uncertainty.

Rainfall through Tuesday: Models remain fairly consistent on a period of widespread soaking rainfall across North Central and Southwestern Montana through Tuesday. Most models also remain consistent in placing the heaviest precipitation north of a Helena to Great Falls to Lewistown Line, where amounts of 1-2 inches appear likely. However, there remains some possibility that some areas could see more significant rainfall totals (in excess of 3 or 4 inches), which could result in some increases to rivers and streams. Widespread flooding remains unlikely, though some areas could see some ponding of poor drainage areas. Ludwig

AVIATION

05/06Z TAF Period

Scattered rain showers will continue through the rest of the night with minor impacts possible for KGTF and KLWT and more significant impacts expected around KCTB where periods of MVFR/IFR conditions are possible. For areas along and west of the I-15 corridor, low stratus is expected to linger through the rest of the night into Saturday morning before another round of moisture is expected Saturday afternoon. Mountain obscuration will be an ongoing concern along the Continental Divide through the period.

Otherwise there are some concerns that fog could develop at both KBZN and KWYS right around sunrise on Saturday but confidence in the exact impact and timing was too low to include it in the TAF.

For KGTF: VFR conditions will prevail through the period with a low stratus deck expected to linger through at least the mid- morning hours on Saturday. Winds in the early afternoon will start to pick up out of the east/northeast with gusts up to 20kts possible. Overall the risk for exceeding the crosswind threshold is rather low at around 15%. The time frames to watch for will be right as the winds start to increase out of the east towards the early afternoon and then as showers and thunderstorms move through which may produce gusty/erratic winds at times. The main threat for thunderstorms at KGTF will be between 06/00Z and 06/05Z. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 73 48 72 50 / 10 30 20 90 CTB 65 45 62 45 / 80 80 80 90 HLN 76 49 76 52 / 10 30 10 70 BZN 79 45 81 48 / 0 0 0 60 WYS 72 37 73 39 / 0 0 0 80 DLN 78 43 79 46 / 0 10 0 70 HVR 79 50 77 51 / 10 20 50 90 LWT 76 48 77 49 / 0 10 20 90

TFX WATCHES/WARNINGS/ADVISORIES

None.