A potent shortwave can be seen diving into the northwestern US this evening. This shortwave will carve out a trough over the central US and will be the catalyst of a moderate snow event from portions of the central US points eastward starting Thursday.
As the jet streak with the aforementioned shortwave rounds the base of the developing trough over the Plains Thursday morning, upper level divergence caused by the jet streak should encourage surface low pressure development over the central Plains along a relatively strong baroclinic zone. Initially snow will be caused by the mid level shortwave creating notable height falls and lift along with mid level frontogenesis Thursday as it dives into the Plains…note the potent mid level shortwave and associated lift as shown on the 18z NAM, valid Thursday afternoon:
The Euro, UKMET and Canadian (CMC) models are all a bit slower and farther south than the NAM solution (shown above) and the GFS solution…however, it should be noted that the 18z run of the NAM (shown above) is a bit slower and farther south with the mid level energy than the previous run, which is a trend towards the non-American model camp.
Eventually, the jet streak associated with the shortwave diving into the Plains will partially phase with the subtropical jet stream…and although the cut off energy that has been previously discussed will remain out of the picture and over Mexico through the duration of this event, the phasing of the northern branch jet streak with the subtropical jet stream will create a 150kt speed max in the upper level jet stream, and the large scale lift in the left-exit portion of this powerful jet streak will allow the storm to deepen.
The main difference in the model camps (GFS/NAM/SREF vs. ECM/CMC/UKMET) lies in exactly how fast/far south the northern shortwave tracks and when the streams partially phase…
-The GFS and NAM are a bit faster with the northern branch energy initially, leading to an earlier phase and a storm that hooks a bit harder to the north by Friday morning. This also results in the storm tracking east a bit faster than the foreign models show.
-The Euro, CMC, and UKMET are a bit slower with the northern branch energy and do not amplify it as fast, resulting in the partial phase occurring a bit further east, resulting in the storm strengthening a bit farther east, resulting in a more southerly track overall.
The differences in the speed/amplitude of the northern branch shortwave and phasing can be seen well in the above comparison of the 12z Euro/GFS and 18z NAM’s depiction of 300mb wind speeds valid 6z Friday…The GFS is the farthest east and along with the NAM is fairly well phased by this point, with the NAM a bit slower with the shortwave but still fairly amped up, while the Euro is the slowest with the shortwave and is just starting to phase the northern branch and southern branch, resulting in a storm that tracks farther south and a bit slower than what the American models currently show.
Given the current lack of any blocking on our side of the hemisphere, I believe the less amplified (and thus further south) storm track is more likely…in addition, current observations indicate that today’s GFS/NAM runs are a bit too fast with the low pressure.
For this forecast I will assume a track close to the Euro/UKMET will verify.
As for how much snow will fall and where…as mentioned above, strong mid level lift associated with the northern shortwave diving into the Plains through Thursday will result in a swath of snowfall. The intense lift associated with this feature will likely result in a narrow swath of 6”+ high ratio snowfall from the Dakotas south into Iowa.
The precip shied will begin to expand Thursday night as partial phasing with the subtropical jet stream will occur and as large scale lift and mid level frontogenesis increase to the north of the surface low track…
Note how by 6am (CST) Friday, the Euro shows a small area of very intense positive vorticity and height falls over the Great Lakes region. This will result in positive vorticity advection which will combine with decent height falls and large scale lift in the left-exit portion of an upper level jet streak to produce heavy precipitation rates in a relatively narrow corridor to the north of the mid level low track.
To illustrate the upper level divergence created by the jet streak associated with the storm, note the Euro model valid for Friday morning with the quadrants of the upper level jet streak drawn on…also, note how the height lines spread out in the left-exit portion of the jet streak, representing upper level divergence which encourages large scale lift under that portion of the jet streak.
To illustrate a point, also note the 18z NAM’s depiction of converging mid level winds and temp gradient just north of the mid level low:
This all indicates a likelihood for mesoscale banding and heavy snow rates just north of the mid level low track…but given the progressive pattern and speed of the system, this snow will last 12-18 hours to the north of the mid level low, likely limiting amounts somewhat.
In addition to banded deformation snows to the northwest of the mid level low, overrunning precip will increase dramatically Thursday night as the low pressure system intensifies and the low to mid level jets increase, creating decent low to mid level lift to the east of the strengthening low pressure…note the strong H85 jet shown developing by the 12z Euro Friday morning to the east of the mid level low:
This will result in light to moderate snows breaking out over Upstate NY, southern Ontario and eventually east into northern New England east of the mid level low. This may add a couple to a few inches of snow where the overrunning precip falls as snow.
For precip type, I believe boundary layer temperatures will be overcome, especially along and north of the mid level low track as dynamic cooling becomes dominant after dark due to the expected moderate to heavy precip rates north of the mid level low track…note how the ECM shows temperatures nearing or plunging below freezing at the surface by Friday morning just north of the surface low track:
The boundary layer may be more of an issue during over western IL/IA when the heaviest precip will fall during the day Thursday. Elsewhere, I believe where mid level temps are cold enough for snow, surface temps will relatively quickly cool enough for snow to reach the ground and accumulate.
For accumulations east of the Plains, given the 12 hour +/- duration of snow over IA/northern IL/WI, with a burst of moderate to heavy snow likely, will go with a 3-6” of snow where I believe precip will fall as all snow. Farther east, dynamics will improve, duration will improve somewhat especially as overrunning precip becomes more widespread ahead of the storm, and a good portion of the snow will fall as night…therefore, will slightly broaden the swath of heavier snow and increase max amounts to 8”. The bullseye will likely be over the eastern lakes east into southern Ontario and Quebec, where locally 10” will be possible.
Despite the intense dynamics and likely heavy rates in banding, the wet nature of the snow and relatively short duration will keep amounts from getting out of control.
An additional few inches of snow will likely fall in the favored NW flow Great Lake Snow Belts with a marginally cool but moist airmass and cyclonic flow behind the storm. I will not really try to show this in the forecast for the synoptic snows, shown below.
My first and likely final call: