Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
631 FXUS65 KTFX 161150 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 550 AM MDT Mon Sep 16 2024 Aviation Section Updated .SYNOPSIS... The next few days will have a few rounds of showers and thunderstorms Monday and Tuesday. Some stronger to severe storms are possible across North Central Montana Tuesday afternoon and evening. Areas of widespread rain with snow at the higher mountian peaks arrive Tuesday night through Thursday morning. Temperatures remain below averages through the forecast period. && .DISCUSSION... An upper level wave passing through this morning will continue to bring scattered showers to mainly Southwest MT. The upper level trough moving east across the Northwest U.S will bring areas of divergence aloft today and Tuesday, which will aid in the development of showers and thunderstorms. This afternoon, isolated to scattered showers and thunderstorms develop over Southwest MT and travel north to the Montana Highway 200 corridor during the evening. Another round of scattered to widespread showers and thunderstorms arrive Tuesday afternoon/evening for areas mainly across the plains east of I-15. CAPE increases to 1,500 J/kg and to 40kts of bulk shear. This increase of instability, moisture, and wind shear supports some strong to severe storms. The Storm Prediction Center has put areas east of a Havre to Lewistown line in a slight risk for severe weather. There`s a 5-15% for wind gusts >=58mph and >=1" hail with stronger storms. The corridor for stronger storms will heavily depend on the low track, so the severe risk areas may continue to slightly shift over the future forecast periods. As the sun sets, instability decreases and the precipitation type will shift into a more widespread stratiform rain event. Tuesday night moisture increases as it gets wrapped around the low pressure system. A heavy band of precipitation will form around the axis of deformation, which is currently forecasted along and east of I-15. This will bring a prolonged period through early Thursday morning of steady precipitation. The NBM 50 percentile precipitation is giving a swath of 1-2" of rain in this region as the probability of 2" or more of rain is ranging between 40-60% during the 48hr period Tuesday- Thursday morning. The upper ranges of max rain amounts from models (75th-90th NBM percentiles) is hinting at 2-3.5" of rain, with a 20-40% of 3" or more of rain. This has prompted the Weather Prediction Center to issue a slight risk (15%) for excessive rainfall for the Golden Triangle area Wednesday morning through Thursday morning. Other areas across the CWA (Rocky Mountain Front and Southwest MT) will also see wetting rain, with a 40-80% for 0.5" of rain during this time frame. There is still a slight risk for excessive rainfall over sensitive burn scar areas should heavier convective showers move over the area Tuesday/Wednesday. Snow levels look to stay above 7,500ft(may go slightly lower with heavier bursts of precipitation). This should confine snow accumulations to higher mountian peaks. Another concern with this weather system is strong winds forming along the deformation band on Wednesday. Stronger 700mb winds increase Wednesday afternoon/evening, and will mix down stronger winds to the surface. Current probabilities for 48kts(~55mph) are ranging between 40-75% for portions of Central MT and the Northern Rocky Mountain Front. If models continue to trend towards higher probabilities, some wind highlights may be needed for future forecasts. As this trough/closed low system move out of the region Thursday, northwest flow aloft moves in. Embedded upper air disturbances along this northwest flow aloft will keep low chances for precipitation through the weekend. However, there is uncertainty regarding to the timing of this trough exiting, which can affect precipitation chances for the later half of the week. However, the northwest flow aloft will keep temperatures below average through the forecast period. -Wilson && .AVIATION... 16/12Z TAF Period A weakening line of showers will lift northwards across Southwest MT early this morning. Another round of scattered showers arrive during the afternoon in Southwest MT before lifting northward across the North Central MT plains in the evening. An approaching weather system brings another round of showers overnight into Tuesday morning. Clouds generally stay mid to high level, but thunderstorms can lower cloud bases slightly but stay above VFR. -Wilson Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PRELIMINARY POINT TEMPS/POPS... GTF 72 48 70 51 / 10 20 70 90 CTB 71 45 68 50 / 10 10 70 90 HLN 76 51 71 50 / 20 40 80 90 BZN 77 46 68 45 / 30 40 80 80 WYS 70 35 56 34 / 50 70 90 90 DLN 74 46 62 43 / 20 60 70 80 HVR 78 51 77 54 / 10 20 60 90 LWT 74 47 74 47 / 10 20 70 90 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls