After a rather messy opening to the winter season, it's been rather quiet in terms of winter weather, especially across the Midwest and lower Great Lakes. Apart from the traditional lake-effect snow zones, most areas haven't seen a major winter storm yet (apart from central Michigan and Wisconsin last week). However, this may all be about to change over the next week or so as the pattern begins to shift once again.
Last week, a major cold front pushed through the area, leaving behind temperatures well below freezing for most of the Great Lakes region and periods of lake-effect snow for a large area. However, last Friday into Saturday, a warmer Pacific air began to push back through the Ohio Valley and Great Lakes as the upper level flow began to flatten out. The zonal pattern has been rather conducive for weak waves to push through, bringing clouds and slight chances for rain and snow. In the next few days, a strong upper level trough is expected to drop southward out of Canada, and as it does, the upper level ridge across the eastern Pacific will build, cutting off the low in the Desert Southwest. As this happens, another shortwave pivoting around the large upper-level trough centered over the Hudson Bay will drop southward, allowing the closed upper level wave to open up, and causing weak cyclogenesis in the southern Plains and eventually the lower Tennessee Valley. There are still model differences on this, but the position and orientation of this shortwave as it pushes eastward will drastically influence whether there is backend wintry precipitation for the Ohio Valley or not on Saturday into Sunday.
The latest 12z ECMWF has come more into line with the 12z GFS on the issue of precipitation placement on Saturday into Sunday as it has shifted northward with the precipitation shield. Ultimately, I think this trend may continue, which would continue to put some parts of the Ohio Valley at risk for wintry weather and/or heavy precipitation. If the shortwave continues to exhibit more amplification on model runs in the next few days, the chance for wintry precipitation shifts westward and we may see even heavier precipitation. This is more likely if the shortwave embedded in the northern stream (from Canada) is faster. The resultant forward speed makes it more likely that the opening wave and the upstream wave will constructively interfere to create a more amplified wave. As a result, there is more downstream amplification. There are a few physical reasons for this. One, the stronger resultant circulation due to the PV anomaly (potential vorticity (PV) is a physical quantity defined by the product of the absolute vorticity vector and the static stability (proportional to d(theta)/dz)-- it is conserved by the flow [meaning, dPV/dt = 0, or that local changes in PV are entirely due to advection] in an adiabatic, frictionless regime) induces warm air advection on the downstream periphery of the wave. Warm-air advection in the low and mid-levels translates to height rises at the 500 MB level (by the quasi-geostrophic chi equation) which will lead to an amplification of the wave due to the reduction in wavelength (that was a result of the downstream amplification of 500 MB heights). This creates a feedback process because the resultant amplification increases the relative vorticity at the base of the trough. But due to the requirement that PV is conserved (assuming frictionless, adiabatic flow), an increase in the relative vorticity must result in a decrease of the static stability of the system. As the static stability decreases, the depth through which the PV anomaly may influence circulation increases. This positive feedback loop allows the system to continuously intensify. In reality, this is the Sutcliffe self-development theorem in the potential vorticity framework. This process of amplification is more likely to occur for the Sunday night-Monday system that is forecasted to track through the Great Lakes, Ohio Valley and Northeast.
However idealistic that explanation was, it somewhat mirrors reality when it comes to the phasing of shortwaves of different forward velocities. This is why it is important to track the individual waves in each model run because they influence the outcome of each solution for the larger systems they forecast. On the contrary, if we don't see a faster shortwave coming out of southern Canada, there will be no phasing and the main primary shortwave will simply shear out and weaken, which will ultimately limit cyclogenesis. This is what the ECMWF was showing on the 00z run. In that solution, precipitation will develop along a lingering frontal boundary from the Gulf into the Mid-Atlantic, and weak cyclogenesis may result for Sunday into Monday for the Northeast, bringing additional chances for wintry precipitation.
One thing to note is that on the ECMWF 00z ensemble, the majority of the members had the surface low placement farther west (for the Saturday afternoon system expected to track through the Ohio Valley and Appalachians) than the ensemble mean and the deterministic 00z solution. Simultaneously, the 12z GEFS is showing a weaker, farther south and east surface low, which doesn't surprise me given the GFS's blatant progressive bias. The 12z ECMWF ensemble mean was similar to the 00z but slightly farther east by 18z Saturday. Many of the individual members have low placements that are west of the mean, which could be significant. However, given trends in the deterministic models, it appears that the overall 24 hour trend is for most of Illinois and Indiana to stay dry after Friday's system while a rain/snow mix may fall for portions of southern Indiana, southern and central Ohio and northeastward into western New York. There are still too many issues regarding the thermal profiles and placement of precipitation to address snowfall and icing. More details will be coming tomorrow and Friday on this.
An even bigger story than the weekend will be what's coming for portions of the Great Lakes, Ohio Valley and Northeast for Christmas Eve night into Christmas Day. At this time, it is anticipated that a shortwave ejecting southward out of Canada and into the Rockies will move eastward into the central Plains by Sunday. How quickly this happens and how deep this system will be is very much dependent on how the mean upper level flow evolves from the Saturday-Sunday system. Given the weak nature of the first wave, it is likely that heights upstream will be higher than what was originally anticipated. This will help amplify the main wave ejecting out of the Rockies and may aid in primary cyclogenesis across Indiana and Ohio early Monday morning. Unfortunately, the GFS has a very different scenario and this is because it has a much flatter, sheared out wave ejecting from the Rockies Sunday afternoon. The differences in intensity of the wave is crucial in where the resulting low pressure system develops. The ECMWF has a primary low developing in the Great Lakes and then secondary development off the coast of New England, bringing a swath of moderate snow from Indiana and Ohio into New England. If the GFS is correct, a weak clipper system will strengthen slightly bringing periods of light to moderate snow for Michigan, New York and parts of New England. The GFS solution is likely due to its progressive bias, while the ECMWF solution appears more likely if energy out of Canada digs farther south and west than what the GFS is indicating. Either way, this is an event to watch as it could allow for a white Christmas anywhere from the Ohio Valley into New England.
Apart from potential snow chances, some of the coldest weather of the season is expected to arrive by early Christmas Day. A massive Arctic high is expected to sink southward into the U.S. from western Canada. Surface high pressure may be as strong as 1044 or 1048 millibars across southern Canada and northern Montana. This strong high will allow Arctic air to anchor itself across most of the U.S. through most of next week.
As the Arctic air mass begins to relax, it is possible that yet another winter storm may develop for portions of the Ohio Valley and Northeast. Models, particularly the GFS, have been indicating the potential for a winter storm for a few runs now. While it is too early to hammer down details, I do believe that the flow pattern will favor shortwaves ejecting east northeastward and amplifying in the central and southern Plains, leading to cyclogenesis and the development of widespread precipitation from the Plains into the Northeast and Mid-Atlantic. At this time, it appears that the Thursday-Friday time frame has the best chance for such a system.
Last week, a major cold front pushed through the area, leaving behind temperatures well below freezing for most of the Great Lakes region and periods of lake-effect snow for a large area. However, last Friday into Saturday, a warmer Pacific air began to push back through the Ohio Valley and Great Lakes as the upper level flow began to flatten out. The zonal pattern has been rather conducive for weak waves to push through, bringing clouds and slight chances for rain and snow. In the next few days, a strong upper level trough is expected to drop southward out of Canada, and as it does, the upper level ridge across the eastern Pacific will build, cutting off the low in the Desert Southwest. As this happens, another shortwave pivoting around the large upper-level trough centered over the Hudson Bay will drop southward, allowing the closed upper level wave to open up, and causing weak cyclogenesis in the southern Plains and eventually the lower Tennessee Valley. There are still model differences on this, but the position and orientation of this shortwave as it pushes eastward will drastically influence whether there is backend wintry precipitation for the Ohio Valley or not on Saturday into Sunday.
The latest 12z ECMWF has come more into line with the 12z GFS on the issue of precipitation placement on Saturday into Sunday as it has shifted northward with the precipitation shield. Ultimately, I think this trend may continue, which would continue to put some parts of the Ohio Valley at risk for wintry weather and/or heavy precipitation. If the shortwave continues to exhibit more amplification on model runs in the next few days, the chance for wintry precipitation shifts westward and we may see even heavier precipitation. This is more likely if the shortwave embedded in the northern stream (from Canada) is faster. The resultant forward speed makes it more likely that the opening wave and the upstream wave will constructively interfere to create a more amplified wave. As a result, there is more downstream amplification. There are a few physical reasons for this. One, the stronger resultant circulation due to the PV anomaly (potential vorticity (PV) is a physical quantity defined by the product of the absolute vorticity vector and the static stability (proportional to d(theta)/dz)-- it is conserved by the flow [meaning, dPV/dt = 0, or that local changes in PV are entirely due to advection] in an adiabatic, frictionless regime) induces warm air advection on the downstream periphery of the wave. Warm-air advection in the low and mid-levels translates to height rises at the 500 MB level (by the quasi-geostrophic chi equation) which will lead to an amplification of the wave due to the reduction in wavelength (that was a result of the downstream amplification of 500 MB heights). This creates a feedback process because the resultant amplification increases the relative vorticity at the base of the trough. But due to the requirement that PV is conserved (assuming frictionless, adiabatic flow), an increase in the relative vorticity must result in a decrease of the static stability of the system. As the static stability decreases, the depth through which the PV anomaly may influence circulation increases. This positive feedback loop allows the system to continuously intensify. In reality, this is the Sutcliffe self-development theorem in the potential vorticity framework. This process of amplification is more likely to occur for the Sunday night-Monday system that is forecasted to track through the Great Lakes, Ohio Valley and Northeast.
However idealistic that explanation was, it somewhat mirrors reality when it comes to the phasing of shortwaves of different forward velocities. This is why it is important to track the individual waves in each model run because they influence the outcome of each solution for the larger systems they forecast. On the contrary, if we don't see a faster shortwave coming out of southern Canada, there will be no phasing and the main primary shortwave will simply shear out and weaken, which will ultimately limit cyclogenesis. This is what the ECMWF was showing on the 00z run. In that solution, precipitation will develop along a lingering frontal boundary from the Gulf into the Mid-Atlantic, and weak cyclogenesis may result for Sunday into Monday for the Northeast, bringing additional chances for wintry precipitation.
One thing to note is that on the ECMWF 00z ensemble, the majority of the members had the surface low placement farther west (for the Saturday afternoon system expected to track through the Ohio Valley and Appalachians) than the ensemble mean and the deterministic 00z solution. Simultaneously, the 12z GEFS is showing a weaker, farther south and east surface low, which doesn't surprise me given the GFS's blatant progressive bias. The 12z ECMWF ensemble mean was similar to the 00z but slightly farther east by 18z Saturday. Many of the individual members have low placements that are west of the mean, which could be significant. However, given trends in the deterministic models, it appears that the overall 24 hour trend is for most of Illinois and Indiana to stay dry after Friday's system while a rain/snow mix may fall for portions of southern Indiana, southern and central Ohio and northeastward into western New York. There are still too many issues regarding the thermal profiles and placement of precipitation to address snowfall and icing. More details will be coming tomorrow and Friday on this.
An even bigger story than the weekend will be what's coming for portions of the Great Lakes, Ohio Valley and Northeast for Christmas Eve night into Christmas Day. At this time, it is anticipated that a shortwave ejecting southward out of Canada and into the Rockies will move eastward into the central Plains by Sunday. How quickly this happens and how deep this system will be is very much dependent on how the mean upper level flow evolves from the Saturday-Sunday system. Given the weak nature of the first wave, it is likely that heights upstream will be higher than what was originally anticipated. This will help amplify the main wave ejecting out of the Rockies and may aid in primary cyclogenesis across Indiana and Ohio early Monday morning. Unfortunately, the GFS has a very different scenario and this is because it has a much flatter, sheared out wave ejecting from the Rockies Sunday afternoon. The differences in intensity of the wave is crucial in where the resulting low pressure system develops. The ECMWF has a primary low developing in the Great Lakes and then secondary development off the coast of New England, bringing a swath of moderate snow from Indiana and Ohio into New England. If the GFS is correct, a weak clipper system will strengthen slightly bringing periods of light to moderate snow for Michigan, New York and parts of New England. The GFS solution is likely due to its progressive bias, while the ECMWF solution appears more likely if energy out of Canada digs farther south and west than what the GFS is indicating. Either way, this is an event to watch as it could allow for a white Christmas anywhere from the Ohio Valley into New England.
Apart from potential snow chances, some of the coldest weather of the season is expected to arrive by early Christmas Day. A massive Arctic high is expected to sink southward into the U.S. from western Canada. Surface high pressure may be as strong as 1044 or 1048 millibars across southern Canada and northern Montana. This strong high will allow Arctic air to anchor itself across most of the U.S. through most of next week.
As the Arctic air mass begins to relax, it is possible that yet another winter storm may develop for portions of the Ohio Valley and Northeast. Models, particularly the GFS, have been indicating the potential for a winter storm for a few runs now. While it is too early to hammer down details, I do believe that the flow pattern will favor shortwaves ejecting east northeastward and amplifying in the central and southern Plains, leading to cyclogenesis and the development of widespread precipitation from the Plains into the Northeast and Mid-Atlantic. At this time, it appears that the Thursday-Friday time frame has the best chance for such a system.
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