Area Forecast Discussion
Issued by NWS Glasgow, MT
Issued by NWS Glasgow, MT
657 FXUS65 KGGW 042136 AFDGGW Area Forecast Discussion National Weather Service Glasgow MT 336 PM MDT Sat May 4 2024 .DISCUSSION... KEY MESSAGES: - Prolonged rainfall event in store for northeast Montana from Monday through Wednesday night. The WPC has issued a Slight Risk for Excessive Rainfall for the 24 hr period ending on 6am Tuesday. - The general probability of exceeding 1 inch, 2 inches, and 3 inches of total rainfall during this aforementioned period ranges from 60-90%, 30-60%, and 10-20%, respectively. Locations in the Little Rockies have a 40-50% chance of exceeding 3 inches. - Strong wind gusts in excess of 65 mph are expected for locations south of the Missouri River Breaks and west of Jordan (70-90% chance) on Tuesday, leading to extreme crosswind concerns for roadways in this area. WEATHER PATTERN OVERVIEW: An upper level trough pattern is in store for Montana, leading to sufficient synoptic-scale ascent to support a broad area of rainfall from Monday. Ensemble models,the National Blend of Models, and the WPC have consistently kept QPF amounts in excess above 1 inch for much of the forecast area. This has been further highlighted by machine learning tools from CSU that highlights much of the western half of the forecast area with up to a 40% probability of excessive rainfall, which is further reinforced by the upgrade to a Slight Risk for excessive rainfall per the WPC. As mentioned in the highlights, the probability of 2 inch accumulations still remains as high as a medium chance. In addition to the rainfall concerns, the low pressure system in the low levels of the atmosphere will strengthen on Tuesday, causing a tightening of the pressure gradient over much of the southeastern half of the forecast area. The National Blend of Models have a mean max gust value approaching 70 mph. Should this trend continue tomorrow, the next forecast package may consider a High Wind Watch for that area, which may influence the validity of the Lake Wind Advisory in the stead of a High Wind Watch issuance. Given this, impacts for motorists would be in areas where small streams and creeks approach roadways and areas where construction zones are located. Additionally on Tuesday, extreme crosswinds may affect high-profile vehicles south of the Missouri River Breaks. Confidence or Potential Deviation from the Base Forecast: The main deviation from the NBM was on Monday morning, where some dry slotting may begin to affect eastern Montana, so I lowered values in our southern zones. This area then moves off to the east into the Dakotas the remainder of the day, leading to no further adjustments. -Enriquez && .AVIATION... LAST UPDATED: 2130Z FLIGHT CAT RANGE: VFR DISCUSSION: Shallow cumulus and light showers (mostly virga) north and east of KGGW and KOLF. These diminish tonight, with partly cloudy skies overnight and into Sunday morning. WIND: Light and variable this evening. Overnight, easterly winds increase from midnight, increasing to 15-25 kt gusting to 35 kts by Sunday afternoon. -Enriquez && .GGW WATCHES/WARNINGS/ADVISORIES... Lake Wind Advisory from midnight tonight to noon MDT Monday for Central and Southeast Phillips-Central and Southern Valley- Garfield-McCone-Petroleum. && $$ weather.gov/glasgow