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Rain and Wet Heavy Snow Tonight & Saturday


What is Happening?


Early this Friday morning, the area of focus is in northern Texas. The subtropical Jetstream and Polar Jet Stream are combining for a couple days despite an otherwise dominant split-flow pattern. Split-flow is generally unfavorable for storms because the moisture from the south is separated from the cold air to the north. When both Jetstreams merge, the moisture and cold air meet each other, which is what you need in combination with a developing storm system to see significant snow. We will see this in northern lower MI tonight and Saturday.


This morning, we see a 120kt Jet feeding cold air into the base of a trough located in northern New Mexico. With time, the trough will shift east and then northeast towards the Great Lakes. As it does so, a similar 120 knot Jet Streak will set up from Texas to the Ohio Valley on Saturday. A combination of the left exit region of this Jet and the downstream upper-level divergence associated with the 500mb trough will result in surface cyclogenesis. The mid-level ridge downstream of this trough that brought highs in the 50s to southern lower Michigan yesterday will force the surface Low north and east towards the Great Lakes. As it advances northeast through MO and IL, moisture and warm air will be transported towards the Great Lakes and lower Michigan. The strengthening Low Pressure system, deepening as low as 995mb to 998mb by Saturday afternoon, is anticipated to track directly through Mid-MI, approximately from Benton Harbor to the Thumb. This brings the warm air and moisture north, resulting in widespread moderate rain to areas along and south of M-46. To see snow, you need to be properly positioned on the northwest side of the Low, so widespread heavy wet snow is not expected until you get at least northwest of a line from Muskegon to Mount Pleasant. Highs on Saturday associated with the WAA (warm air advection) will climb into the lower-50s south of I-96. Not only are temperatures this warm uncommon for early to mid-December, but the amount of moisture being transported from the Gulf of Mexico is also highly unusual for this time of year. Maximum PWAT (Precipitable Water) values will be pushing 0.75-1.00" during the day on Saturday, which is more typical of summer-time in Michigan. This is also a long duration rainfall event that lasts for 18 to 24 hours, so widespread rainfall amounts in excess of 1 inch are anticipated, with amounts up around 2" possible. As for the wet snow, 1" of liquid will likely translate to approximately 7 or 8 inches of snow. With 1 to 1.5" of liquid forecast, we can see why 6 to 9 inches of snow is likely with higher amounts certainly possible. Many locations will receive 50% to 75% the normal precipitation for the entire month of December, and some areas may even see the entire month's worth of precipitation occur out of this one storm.


The trickiest part about this system is the rain/snow line. It is clear that rain will occur near and south of the low, while heavy snow will occur to the northwest. Slight shifts, even just 20 to 30 miles, can make a big difference for one county with a tight gradient in the rain/snow line. In this case the air is uniform with height and simply warmer south and cooler north. The 850mb LLJ (Low-level Jet) strengthening briefly up to 50 knots on Saturday afternoon, will transport the Gulf moisture and warm air into the region, roughly 0.6 miles aloft, which creates potential havoc near the rain/snow line. It looks like this line will setup somewhere in Isabella county so transitions from snow to rain and back again may occur several times in this area. The exact temperatures at not only the surface, but also aloft, as well as daytime vs. nighttime and other small factors, can all play a role to shift the rain/snow line ever so slightly. In the end, a very short distance will mean the difference between a cold rain and wet heavy snow and you may even transition between the two a couple times if you're lucky enough to be exactly in the right spot (likely near Mount Pleasant/in Isabella County). Where it does snow, it will be a wet, heavy, snow; the kind of snow that's difficult to shovel. This wet nature of the snow should keep snow totals in check (but still 6-9") where all snow occurs closest to the rain/snow line, with higher amounts of snow occurring further north where the air is colder and the snow therefore not as wet. Heavier banding of snow could easily send some areas, particularly north of US-10, over 10" of snow by Saturday evening.


Forecast Overview:


The rain and snow line should generally extend from near Muskegon to Mount Pleasant. This region is the most susceptible to seeing a fight between rain and snow on Saturday and a slight shift will make a notable difference between rain and snow accumulations in this area. Otherwise, Big Rapids to Clare northward will see all snow. Alma southward will see all rain.


Timing for Mount Pleasant and Clare:


Rain Showers will arrive mainly after sunset this evening. Rain will mix with and quickly change over to snow late in the evening. 1-4" is anticipated by Saturday morning. Snow will continue throughout the day Saturday, heavy at times. Rain may mix in near Mount Pleasant, cutting back snowfall totals. Clare should remain all snow. This rain/snow line will need to be watched closely. This will be a wet, heavy snow that is difficult to shovel, but should be good for snowball and snowman building. Travel will become slippery and hazardous, and a few power outages may occur given the wet, heavy snow. Snow will end Saturday evening, with more than 6 inches likely. Mount Pleasant may see less than 6 inches with the rain mixing in. Near and north of US-10, localized numbers could be as high as the 10-14 inch range if heavy banding sets up.


Snowfall Map:



IMPACTS:




Snow Forecast by City:


Clare: 6-10" (locally higher)

Mount Pleasant: 3-6"

Alma: <1"


This system will also bring alot of rainfall from Alma southward. In the liquid form, we're looking at 1-2 inches likely.


Rainfall Map:



Timing for Alma southward:


Rain will arrive mainly after sunset and continue through the day on Saturday. Temperatures will be in the 40s, and may even warm above 50 south of I-96. The rain may result in areas of ponding of water on roadways. The good news is, it has been dry and the ground is not frozen, so we shouldn't have any major flooding concerns. Cold air on the back side Saturday evening may allow for flurries or very light snow showers before the precipitation ends. Accumulations will be very little if any.

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