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
400 FXUS65 KTFX 050959 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 359 AM MDT Sun May 5 2024 .SYNOPSIS... A slow moving Pacific weather system begins to impact the Northern Rockies today in the form of northward expansion of shower and isolated thunderstorm activity. Snow also develops over the higher terrain of Southwest Montana tonight into Monday morning. Much cooler and wetter weather is expected for much of the up coming workweek, along with breezy to windy conditions and localized heavy mountain snow. && .DISCUSSION... Southerly flow aloft and abundant mid- and higher level clouds have overspread much of North-central and Southwest MT this morning as a closed Pacific low centered over N CA/NW NV slowly moves eastward toward S ID. Scattered shower and isolated thunderstorm activity will overspread the region today in a south to north fashion. Given forecast MU CAPE levels generally in the 100 to 300 J/kg range, not expecting anything beyond general thunderstorm hazards like gusty winds, brief downpours, and isolated lightning strikes. The best chance for convective activity will be this afternoon over the southwest and the eastern portions of the plains. Colder air aloft moves in tonight and introduces falling snow levels and mountain snow, most notably over the southwestern mountains. NBM probabilities support a quick 1 to 5 inches of snow, with the highest amounts expected over NW Beaverhead County, including Chief Joseph and Big Hole Passes. Peak snowfall rates look to occur between 3 and 9 am Monday morning, so a Winter Weather Advisory was posted for the three mountain zones south of I90 to address the expected impacts during the coldest part of the day. A majority of ensembles continue to highlight the closed Pacific low lifting northeastward into eastern MT while merging with another shortwave streaming eastward out of the Pacific NW. There will be a bit a lull in activity on Monday as steadier stratiform rains remain to our east/northeast and mountain snows temporarily diminish. H700 northwesterly flow begins to increase by Monday afternoon, ushering cold air advection and increasing surface winds along the Rocky Mountain Front and much of Central/North- central MT. By Tuesday, periods of moderate, to at times heavy, rainfall begin to settle in and more or less remain in place through Thursday. NBM probabilistic guidance still highlights locations east of a Great Falls to Shelby line with a 50 to 80% chance for rainfall amounts in excess of 1 inch and a 30% to 50% chance for 2 inches of rainfall or greater for the 72 hour period ending Thursday 6 am MDT. Keep in mind that there will be some areas situated southeast of higher terrain that see reduced rainfall amounts due to downsloping winds. This widespread rainfall will generally be beneficial for most, but some minor hydrologic impacts are expected for those who see the heaviest storm totals, please view the hydrology section for more information. Strong and cold northwesterly flow aloft (H700 winds around 40 to 60 kts, temperatures near -10C) will be occurring during this timeframe as well, resulting in windy conditions for all areas and heavy wet mountain snow over the central ranges. Monday night through Tuesday looks to be the overall windiest period, with most locations in both North-central and Southwest MT seeing 60 to 90% chances for wind gusts in excess of 45 mph, while the 55 mph exceedance higher probabilities (70% +) remain confined along the Continental Divide, the Central MT highway 200 corridor, and the eastern portions of the north-central plains. After collaboration with the other MT WFOs, I decided to hold off on issuing a High wind Watch for now given continued uncertainties with positioning of the strongest winds and mismatches with timing and NBM probabilistic guidance. As mentioned earlier, H700 temperatures will be cold enough to support mountain snow, heaviest over the central ranges where a Winter Storm Watch remains in effect Tuesday through Thursday and the probability for snowfall amounts in excess of 2 feet continues to run 70% and higher, mostly for the mid- slopes and higher peaks of the Little Belt Mountains. The combination of strong winds and heavy wet snow is expected to cause dangerous conditions for outback recreation over higher terrain and there is also the expectations for at least some isolated impacts to the power infrastructure in areas that receive the heaviest snow and stronger winds. Additional winter weather highlights may be needed over the higher terrain of the southwest, the Rocky Mountain Front, and possibly for the Bear`s Paw Mountains. Lower elevations will mostly see cold, wet, and raw conditions for outdoor recreation and agriculture activities with daytime temperatures mostly in the 40s Tuesday and Wednesday. Rain and mountain snow diminish on Thursday as the responsible low pressure system weakens and generally moves away from the region. Ridging aloft becomes more dominant heading into next weekend for drier conditions with temperatures moderating to near or slightly above average. - RCG && .AVIATION... 1235 AM MDT Sun May 5 2024 (05/06Z TAF Period) Note: Amendments are not scheduled for KEKS until their observation can be fully returned to service. An estimated time for return to service is not known at this time. Thank you for your patience. VFR conditions are expected to continue through at least 06/06Z for most of North Central (KCTB KHVR), Central (KLWT KGTF KHLN) and Southwest (KBZN KEKS KWYS) Montana, unless otherwise specified. A moderate southerly flow aloft with south to east winds gusting to 35 kt at times at the surface will cause periods of low level wind shear through around 15Z. Moisture in the flow aloft will spread high- and mid-level cloudiness north over the area through the period, with the best chance for persistent mountain-obscuring MVFR ceilings occurring at KWYS with an increase in precipitation. Otherwise, scattered rain showers and high mountain snow showers are forecast to spread north over the area more so after 12Z, with snow levels lowering over Southwest Montana after 00Z. A few thunderstorms may also develop over Southwest Montana after 18Z, possibly moving into Central Montana after 22Z or so; however, confidence is low at this time that thunderstorms will directly impact most terminals (except for possibly KWYS), so have mostly left mention out of TAFs. Main threats from these storms will be isolated lightning strikes and brief heavier periods of rain possibly causing MVFR conditions, but erratic wind gusts and small hail cannot be ruled out. Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A slow moving Pacific weather system will bring an extending period of lower elevation rain and mountain snow next week, with the heaviest precipitation falling Tuesday through early Thursday. NBM probabilistic guidance highlights the eastern portions of Central/North-central MT with the heaviest rainfall amounts for this period, largely supporting 1 to 3 inches of rainfall/snow melt liquid equivalent over locations east of a Great Falls to Shelby line Tuesday through Thursday. While flooding is generally not expected, some rivers, creeks, and streams along the Canadian border in Hill and Blaine Counties are expected to approach or reach action stage Wednesday and Thursday, mainly impacting unpopulated rural locations. Anyone near waterways should remain alert for changing conditions and be prepared to seek higher ground should water rises occur. - RCG && .PRELIMINARY POINT TEMPS/POPS... GTF 64 43 54 40 / 60 80 60 20 CTB 60 41 53 38 / 40 70 50 20 HLN 67 43 54 39 / 80 90 50 10 BZN 66 36 52 33 / 80 90 90 10 WYS 50 29 42 26 / 80 90 100 70 DLN 58 35 49 30 / 60 100 40 30 HVR 69 44 57 40 / 30 60 80 60 LWT 65 39 49 35 / 40 60 70 30 && .TFX WATCHES/WARNINGS/ADVISORIES... Winter Storm Watch from Tuesday morning through Thursday morning for Big Belt, Bridger and Castle Mountains-Little Belt and Highwood Mountains-Snowy and Judith Mountains. Winter Weather Advisory from 9 PM this evening to noon MDT Monday for Gallatin and Madison County Mountains and Centennial Mountains-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains. && $$ http://www.weather.gov/greatfalls