textproduct: Great Falls

KEY MESSAGES

- It will be mostly dry and warm through Monday.

- It will be windy on Tuesday across North-central, Central, and Southwestern Montana.

- Mountain snowfall will return for Wednesday and Thursday with cooler temperatures.

DISCUSSION

- Meteorological Overview:

This afternoon there is an upper-level ridge over North-central, Central, and Southwestern Montana. This afternoon a weak upper-level shortwave associated with a surface cold front will move south from Canada through North-central Montana. This will bring isolated light rain showers to portions of the Hi-Line this afternoon and evening. Due to the upper-level ridge it will be warm across North-central, Central, and Southwestern Montana this afternoon. Tonight in areas that clear and have light winds across North-central Montana fog has the potential to form. See the Forecast Confidence & Scenarios section for details. On Sunday the upper-level ridge remains over North-central, Central, and Southwestern Montana. Due to the surface cold front that moved across North-central Montana on Saturday, on Sunday it will be a few degrees cooler than today with the coldest temperatures along the Hi-Line where below seasonal average temperatures are expected. On Sunday there will be isolated light rain showers across the Hi-Line. On Monday the upper-level ridge remains over North-central, Central, and Southwestern Montana. This will bring warm temperatures with dry weather to the area on Monday.

On Tuesday an upper-level low associated with a surface cold front tracks along the Canadian/Montana Border. This will briefly bring a strong surface pressure gradient to North-central Montana. Low-level and mid-level winds increase during the day Tuesday. This will lead to windy condtions across North-central, Central, and Southwestern Montana on Tuesday. See the Forecast Confidence & Scenarios section for more details on the winds. The cold front/upper-level low will bring primarily mountain precipitation Tuesday afternoon and evening. On Wednesday there will be an upper-level trough over North- central, Central, and Southwestern Montana. On Wednesday due to the cold frontal passage and upper-level trough it will colder across the area with light mountain snow. On Thursday there will be upper- level zonal flow with a brief upper-level shortwave over North- central, Central, and Southwestern Montana. This will bring another day with cool temperatures and light mountain snowfall. On Friday there will be upper-level zonal flow over North-central, Central, and Southwestern Montana. This will allow temperatures to warm up slightly with mostly dry weather. -IG

- Forecast Confidence & Scenarios:

From Great Falls up to Cut Bank there is a 10 - 40% chance for fog to form tonight. Along the Rocky Mountain Front there is greater than a 70% chance for fog to form tonight. The biggest uncertainty is that winds in some of those areas might not decrease enough to allow fog to from.

On Tuesday there will be a strong pressure gradient for a few hours across North-central Montana and winds aloft will increase. On Tuesday the strongest winds will occur, in the areas where winds at low and mid levels peak in the mid to late afternoon. Another area of uncertainty is that cold air from Canada could move in faster than currently the models are projecting and undercut the strong winds along the Hi-Line. There is also a lower potential for a brief period of strong winds in Southwestern Montana Tuesday afternoon. On Tuesday there is greater than a 50% chance for wind gusts of 55 mph or greater across the lower-elevations of Cascade and Judith Basin Counties. Across the Rocky Mountain Front down to Macdonald Pass and the plains west of I-15 there is greater than an 85% chance for wind gusts in excess of 55 mph. Along the Rocky Mountain Front there is greater than a 80% chance for wind gusts in excess of 75 mph on Tuesday. As a result in these areas on Tuesday a High Wind Watch was issued. Across portions of Central and Southwestern Montana there is greater than a 50% chance for wind gusts of 55 mph or greater. There was low confidence in there being strong wind gusts in these areas for more than an hour or two. Future shifts will continue to monitor the need to expand the current watch to other areas. The areas that are currently in the watch are the areas where there is the highest confidence in receiving impactful wind gusts and or sustained winds on Tuesday. -IG

AVIATION

04/18Z TAF Period

A weak cold front moving south and east over the plains of Central and North Central Montana will bring a wind shift from the southwest/west to northwest/northeast this afternoon. While a shower can't be ruled out along and then behind the front through Sunday morning at the KCTB and KHVR terminals the probability for any one shower impacting these two terminals is too low to mention at this time. Main impact from the passing front will be lowering CIGS at the KCTB, KHVR, and KGTF terminals; with IFR/MVFR CIGS expected beyond 09z Sunday. Further south VFR conditions will prevail at the KHLN, KBZN, and KEKS terminals.

PRELIMINARY POINT TEMPS/POPS

GTF 29 51 30 57 / 0 10 0 0 CTB 27 47 27 54 / 20 20 0 0 HLN 32 57 34 62 / 0 0 0 10 BZN 30 61 31 60 / 0 0 0 10 WYS 20 55 24 56 / 0 0 0 0 DLN 29 63 34 65 / 0 0 0 0 HVR 31 48 29 51 / 20 20 0 0 LWT 29 50 28 50 / 0 10 10 10

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Watch from Tuesday morning through Tuesday evening for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains- Southern Rocky Mountain Front.

High Wind Watch from Tuesday morning through Tuesday evening for Cascade County below 5000ft-Judith Basin County and Judith Gap- Upper Blackfoot and MacDonald Pass.