Issued Wednesday May 1, 2024 8:00 P.M. MDT
Tonight…Mostly cloudy. Isolated rain and snow showers in the evening then rain and snow showers becoming likely along, north and east of Lake Koocanusa and the Fisher River overnight. Snow level lowering to near valley floors. Snow accumulations of up to 1 inch possible below 4000 feet with 1 to 3 inches possible above 4000 feet. Snow accumulations mainly along and east of Highway 93. Lows in the 30s with upper 20s around 5000 feet. Light wind except ridge top winds west 5 to 15 mph.
Thursday…Rain and snow showers likely in the early morning along, north and east of Lake Koocanusa and the Fisher River otherwise variable clouds with scattered afternoon rain and snow showers with isolated thunderstorms. Snow level rising to near 4500 feet in the afternoon. Local snow accumulations of up to 1 inch possible above 4000 feet in the early morning east of Highway 93. Highs in the 50s to near 60 except locally upper 40s higher valleys east of Lake Koocanusa with lower 40s around 5000 feet. Light winds except gusty winds possible near heavier showers and thunderstorms.
Thursday night…Partly cloudy in the evening with isolated rain and snow showers then slow clearing overnight with patchy fog and low clouds developing in the valleys. Snow level near 4500 feet in the evening. Lows in the mid 20s to mid 30s with lower 30s around 5000 feet. Light wind.
Friday…Patchy early morning fog and low clouds in the valleys otherwise mostly sunny in the morning. Partly cloudy and a little warmer in the afternoon with isolated showers, mainly over the higher terrain. Highs in the mid 50s to mid 60s with upper 40s around 5000 feet. Light wind.
EXTENDED OUTLOOK (SATURDAY THROUGH WEDNESDAY):
Saturday…Increasing clouds and warmer. Lows in the lower 30s to lower 40s with upper 30s around 5000 feet. Highs in the mid 60s to lower 70s with lower 50s around 5000 feet.
Sunday and Monday…Cooler with a chance of valley rain and mountain snow. Lows in the upper 30s to mid 40s with mid 30s around 5000 feet. Highs in the mid 40s to mid 50s with upper 30s around 5000 feet.
Tuesday and Wednesday…A little warmer with a chance of valley rain showers and high mountain snow showers. Lows in the 30s with lower 30s around 5000 feet. Highs in the 50s with near 40 around 5000 feet.
In-Depth Weather Discussion for Northwest Montana/North Idaho/Northern Rockies
ISSUED 05/01/2024 8:25 P.M. MDT/7:25 P.M. PDT
SHORT TERM (WEDNESDAY NIGHT THROUGH FRIDAY):
Cold upper level low continues to spin around the Northern Rockies and Northwest US keeping our weather chilly and conditionally unstable with several vort maxes rotating around within the upper level low. I will add that this cold upper level trough combined with a very warm upper level ridge over the Central and Eastern US is ejecting out various shortwave disturbances into the Midwest and combining with a strong upper level jet stream which creates tremendous upper level diffluence along with a good low level jet feeding into each passing shortwave is what is triggering these rounds of strong to severe thunderstorms and tornadoes the Plains states are seeing each evening and overnight. It’s also interesting that similar to our region that area sees its greatest thunderstorm coverage during the evening and overnight hours and not during peak daytime heating which shows once again that thunderstorm development is not dependent on moisture so much as it is on instability, lift and dynamics as the models already have moisture factored into them. Back to our weather, the cold upper level trough will not move very much over the next 24 hours or so with another shortwave rotating back around the north side of the upper level low in Alberta then dropping southward overnight bringing good surface convergence and lift into Northwest Montana overnight. This will likely produce a band of snow across northeastern Lincoln County mainly along and east of Lake Koocanusa so if you live in the Tobacco Valley south towards Trego and Olney then don’t be surprised to see some snowfall overnight into the morning hours on Thursday as that shortwave pushes through. Elsewhere across Northwest Montana and North Idaho some clearing will occur again overnight and allow near to below freezing temperatures once again across most locations away from the immediate shorelines of the larger lakes of Priest and Pend Oreille. Another shortwave will drop southward during peak heating on Thursday afternoon so another day of sunshine and showers with the odd thunderstorm will be possible so keep the umbrella handy if traveling out. Temperatures will warm up a bit on Thursday from Wednesday’s chilly readings but will remain below early May seasonal averages. By Friday shortwave upper level ridging will begin to nose into the region and while we are still likely going to see a fair amount of fair weather cloud bubble up with daytime heating my gut feeling is that the upper levels of the atmosphere will be warming up faster than daytime heating will allow and thus will likely see capping develop to prevent more than perhaps one or two mainly mountain showers on Friday afternoon. Temperatures will rise further on Friday and begin to approach or reach seasonal averages.
LONG TERM (FRIDAY NIGHT THROUGH WEDNESDAY):
A very cold for early May upper level low will be approaching the West Coast at the start of this forecast period with the center of the cold core tracking southeast towards southwest Oregon and northwest California. As this occurs it will force upper level ridging to amplify across the Northern Rockies allowing for a mild, if not warm day on Saturday under increasing atmospheric heights and thicknesses but also increasing cloudiness as well. Saturday is looking rather warm with above average temperatures and likely many of our warmer valleys in Northwest Montana especially likely to hit 70 degrees or a little warmer. All other valleys should easily warm into the 60s. Will also see a tightening northeasterly gradient as a deepening surface low tracks east along the Oregon/California border with surface high pressure east of the Continental Divide in the morning before daytime heating along with the approaching upper level low cause surface pressures to reverse and drop east of the divide in the afternoon and the gradient relaxes. Some uncertainty still on the exact track of the upper level low which really only affects how much rain and mountain snow our region will receive. A further south track would result in less rain and snow along with slightly warmer temperatures while a further north track would result in a washout most likely for both Sunday and Monday along with chilly temperatures and very beneficial rains and snows. If the further north solution pans out high temperatures on Sunday and possibly Monday may not rise out of the 40s in valley locations and possibly even remain in the upper 30s. If you’re heading over to Spokane on Sunday for Bloomsday it does look wet and cold over there so be prepared. The main upper level low will scoot east by Monday night but will leave in its wake a broad upper level trough along with much lower atmospheric heights and thickness values which will result in continued cool temperatures. The atmosphere will also remain rather cold aloft and with conditional instability a daily dose of sunshine and showers will also likely persist beyond Monday with the sunniest periods of the day likely to be in the morning hours before cloud quickly bubbles up and showers develop. Overall a rather typical spring pattern here in the Northern Rockies. Some signs in the long range if you want warm and dry conditions of this occurring by the second week of May as CPC’s 8-14 day outlook is hinting at this change but we will see.