Trowal Winter Storm Event April 19-21, 2009
Overview
An upper level low pressure system moving from western Wisconsin to near Saginaw Bay in eastern Lower Michigan, dumped heavy precipitation across much of western and central Upper Michigan from April 19th through April 21st. Storm total precipitation ranged from an inch or less over eastern Upper Michigan to more than three inches at a few favored higher terrain locations over western and north central Upper Michigan. Much of the precipitation which occurred over the western half of Upper Michigan fell as wet heavy snow with more than 20 inches of total snow reported over upslope higher terrain locations along Lake Superior.

This storm system illustrated a classic example of a "trowal" winter storm event. A trowal, or "TROugh of Warm air ALoft", is formed when a low pressure system occludes (the northern portion of the cold front overtakes the warm front) causing a trough or wedge of warm air to be forced aloft by strong southerly winds on the eastern periphery of the low. As seen in the graphic below, this warm conveyor belt continues to wrap the trowal to the backside, or northwest quadrant, of the storm system forming the "comma head". As is commonly the case, this trowal area or comma head of the storm strengthened the mid-level frontal circulations and became the focus for the heaviest precipitation and snowfall during the event.
Origin of the Towal Concept
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Satellite & Model Data
IR Satellite LoopMosaic Radar Loop
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The infrared satellite (left) and radar (right) loops above show the evolution of the low pressure system. Notice at the end of the radar loop how the higher terrain was still receiving snow while the rest of Upper Michigan was dry. This was mainly due to the upslope processes which is explained in the Storm Reports Section under the Snowfall Totals Map. The low pressure system was located near the Arkansas/Missouri border in the morning hours of the 19th, with cold cloud tops (noted in the yellow, red, and pink shadings) moving into the Upper Peninsula. Most areas across the Upper Peninsula received between 0.2 and 0.75 inches of precipitation for the day, with some areas across the west half receiving up to an inch and areas across the east half receiving no rain at all. Areas across far western Upper Michigan received between 1 and 4 inches of snow.

At 8Z (4 AM EDT) on the 20th, the trowal feature is located across the entire Upper Peninsula, with colder cloud tops wrapping all the way back to southwestern Wisconsin. By the afternoon on the 20th, high pressure can be seen moving into North Dakota, with the low pressure center rotating over northern Lake Michigan. The colder cloud tops are not as evident at this time, but snow and rain were still falling across Upper Michigan. Snowfall amounts from 8 to 18 inches were reported across the higher terrain of western and central Upper Michigan on the 20th. Areas across central and eastern Upper Michigan were warmer and received from 0.50 inch to over 1 inch of rain.

By early morning on the 21st, dry air wrapped up into the system through western Upper Michigan, which can be seen as a sharp line west of Munising near the end of the infrared satellite loop. High pressure began to move out of the Northern Plains and into Minnesota on the 21st, causing the low pressure system to push northeast into Canada. This created a fairly quick end to the snow from west to east during the late afternoon and evening on the 21st.

The warm conveyor belt and trowal/comma head region of the storm can be clearly observed on the water vapor imagery below. Notice how the trowal and warm conveyor belt wrap back into the western half of Upper Michigan during the peak of the storm when the heaviest snow was being reported.
Put Water Vapor Image Here
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Much of the snow fell while the temperature was between 30 and 34 degrees across the Upper Peninsula. In the mid and upper levels, the air was not very cold either. Therefore, the snow that fell was very wet and heavy, with a snow to water ratio averaging around 10 to 1 (10 inches of snow for every 1 inch of water). The warm air that the low pressure wrapped around to its northwest side kept the eastern half of Upper Michigan just warm enough to where they received mostly rain. This can be seen in the thickness map below (left). Thickness is the vertical distance between two pressure surfaces or how "thick" the atmosphere is between those surfaces. In this case, the pressure surfaces are 1000mb (near the ground) and 850mb. Over Western Upper Michigan, the 1000-850mb thickness was less than 1300 meters (around 4200ft). As a general rule, a thickness less than 1300m indicates snow, while a thickness greater than 1300m indicates rain. The grey shading is the lower thicknesses (less than 1300m) across Upper Michigan. Notice the sharp cutoff, which corresponds very nicely to where the rain/snow line was located for most of the event.
1000-850MB Thickness Chart on April 21, 2009 Topographic Image of Upper Michigan
On the topographic image above (right), the yellow and red areas are the higher elevations of the Huron Mountains in Marquette and Baraga Counties, and the Porcupine Mountains located across the west. The purple areas correspond to water areas of Lake Superior and Lake Michigan. Areas located along the mountains received the heaviest amounts of snow, anywhere from 15 to 29 inches. The western part of Marquette County probably received a lot of snow as well, but there were no reports from the area. 
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Storm Reports
Upper Michigan Snowfall Reports from April 19-21, 2009
The image above shows the 3-day snowfall totals from April 19th-21st across the Upper Peninsula. The heaviest snow fell in the higher terrain of Upper Michigan due to upslope effects from northerly winds. There is a sharp elevation change from the city of Marquette to Negaunee, as well as other areas in west and central Upper Michigan from the Lake Superior shoreline to a few miles inland. In this case, northerly winds blew off Lake Superior and up the slopes to higher terrain.  As the air move upslope, the moisture near the surface is lifted and cooled, which produces condensation. This condensation reinforces the clouds and precipitation over the higher terrain, increasing the snowfall intensity and amounts. For example, the city of Marquette (700ft)and nearby locations along the Lake Superior shoreline (602ft) only received 3-6 inches of snow, while the National Weather Service (NWS) office in Negaunee Township (1412ft) accumulated more than 20 inches. The lower snowfall amounts around Marquette were due to the lower elevation and moderated temperatures from Lake Superior. Therefore, the city received mainly rain, except during the overnight hours from the 20th-21st, when the temperature dropped enough for heavy wet snow to fall. From the snowfall map, it is easy to see that Lake Michigan had similar impacts on snowfall amounts as most areas near Lake Michigan received no snow from the event.
Upper Michigan Precipitation Reports from April 19-21, 2009
Most places across the Upper Peninsula received anywhere from 1 inch to over 3 inches of liquid precipitation from the trowal event. The NWS office had a 3-day precipitation total of 2.31 inches. The eastern parts of Upper Michigan received only 0.5 inch to 1 inch of rain. Dry air wrapped up into the system overnight from the 20th into the 21st ending precipitation earlier than across the west and central parts of the Upper Peninsula. So, while other areas of the Upper Peninsula received 0.5 to 1.5 inches of water on the 21st, western Upper Michigan received little to nothing.

The late season snow event across western and central Upper Michigan was much needed, since very dry conditions were experienced during March and the first half of April. The NWS office recorded its driest March through April 17th on record, only receiving 0.98 inches of precipitation during that time. During the first 2 weeks of April leading up to the event, the National Weather Service Office in Negaunee Township recorded only 0.02 inches of precipitation. This was about 1.5 inches below normal for the month of April. Numerous grass fires broke out across south central Upper Michigan in the week leading up to the event in areas that had lost their snow pack. The precipitation from the event essentially wiped out the deficit for the month of April in many locations. In addition, the fire weather concerns before spring green-up were greatly reduced across west and central Upper Michigan.

Pictures during the storm
Mouse over the images below for a larger view.

Snow covered trees in Negaunee, MI (Photo by Andrew Kennedy)

Snow covered trees in Negaunee, MI (Photo by Andrew Kennedy)

Snow covered US 41 east of Negaunee, MI (Photo by Andrew Kennedy)

Snowman in Negaunee, MI (Photo by Andrew Kennedy)


Damage Reports
There were sporadic reports of trees and tree limbs down as well as power outages across the Upper Peninsula. The Rockland area reported bushes bent over, trees weighed down, and that roads were in very bad shape. The Negaunee area lost power for a couple of hours during the day on the 21st. Areas in the Keweenaw and Northern Houghton reported power outages for multiple hours, tree limbs down, and that many roads were impassable.
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Summary
This late season trowal event was one for the record books, with new daily record snowfalls tied or set at the NWS on April 20 and 21. On April 20, the NWS office received 10.6 inches of snow, which tied the previous record snowfall that was set in 1982. On the 21st, 9.9 inches of snow fell, which broke the previous snowfall record of 2.3 inches in 1986. In addition, it was the 3rd largest late season 2-day snowfall at the NWS office (late season is defined as after April 15th). This storm created the largest late season snowfall since 1996 and was the first time over 20 inches fell this late in the season since 1993. In addition to the snowfall records, the NWS office set a new precipitation record on April 20, with 1.08 inches of precipitation. This broke the previous record of 0.78 inches, which was set in 1970.

Here is a list of all of the locations that reported storm total snowfall greater than or equal to a foot and a half.
Snowfall (inches) Location
29.0 Alberta - Ford Forestry
26.0 Atlantic Mine 5NW
24.9 Herman
24.0 Painesdale
23.1 Rockland
22.0 Twin Lakes
20.8 Watersmeet 1NNE
20.5 NWS Marquette
20.3 Sidnaw 2E
18.0 Bergland Dam
18.0 Calumet
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