Monthly Archives: October 2010

Subtle Terrain Effects Across Eastern South Dakota

The morning satellite imagery shows the subtle effects of terrain on sensible weather.  The Coteau des Prairies in Eastern SD (sometimes referred to as the Sisseton Hills) rises above the local terrain by 600-800 feet.  A good colored relief map clearly shows this feature:

Strong nearly unidirectional NW flow was present over the entire region, and the synoptic flow pattern was not conducive to any significant regions of forced ascent.  Upper level heights were rising with strong subsidence owing to the effects of differential anticyclonic vorticity advection aloft.  However, a weak mid-level trough axis was present owing to weak thermal advection as the powerful low continued to “wrap” warm and moist air around the large upper trough.

700 hpa trough axis noted on both the 12 GFS and NAM.  Also note the mass convergence at this level–a response to the forced ascent, not the other way around.

Note the region of weak warm thermal advection.

And weak resulting synoptic ascent:

The 0Z sounding from Aberdeen depicts the moist inversion sandwiched in between two dry layers, one aloft and one near the surface.

The subtle effects of terrain across the region had very visible and profound weather effects.  Stable low level NW flow and subsequent downsloping resulted in drying of the low level air mass over Aberdeen with subtle upslope effects clearly visible over the higher terrain.  Let’s look a little closer.

Radar depicts the showers over the area at 21Z.  Note the radar “hole” around Aberdeen (red) and the regions of enhanced returns over the higher terrain (green).

The observations at Watertown (KATY) and Aberdeen (KABR) also show the effects of terrain enhancement.  Note the dewpoint depression at Aberdeen hovers around 4-5 degrees C while Watertown hovers around saturation to 1-2 degrees C.  Ignore the -SN reports from Aberdeen–seems to be a sensor error (also note it reports a constant 9SM visibility, a good indicator something is not right).

Aberdeen:

Watertown:

The visible satellite image the day after clearly shows the terrain enhanced snowfall over the higher terrain in Eastern SD (red).  Also note the west slope favored snowfall across the Bighorns, Black Hills, and Laramies (green).  Also worth noting here is the lack of snow cover over the whole of the Coteau des Prairies region.  This suggests the air mass precipitated out before before reaching farther south and east.

This shows up in even more spectacular fashion on the MODIS multi-spectral image.

This is a clear example  the significant effects even “subtle” terrain can have on weather.  In this case, strong and stable low level flow won out over weak mid level ascent.  If precipitation were heavier, it is unlikely terrain would have played nearly as significant of a role.

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Satellite Loop of the Powerful Northern Plains Storm

First storm animation (more are coming) of the record breaking low.  This animation is more than 3.5 days long from the 24th-27th of October (make sure to watch in 720):

I suggest clicking the link to see it in a slightly larger view (use the expand) from the youtube website.


The Second Lowest U.S. Atmospheric Surface Pressure Ever

Not only has the state of Minnesota record surface low pressure been shattered, now the second lowest non-tropical (extratropical) surface pressure has been set for the Lower 48.

KFOZ recorded 955.2 hpa earlier in the afternoon.  The old record for MN was 962.7 hpa, set November 10, 1998.

KFOZ 262213Z AUTO 06005KT 5SM -RA OVC005 11/10 A2820 RMK AO2 P0004

Analysis at a later date, but a few initial thoughts.

First, the deepness of the surface low (yes, deepness, intensity is related to the pressure gradient, not the actual central pressure) caught everyone off-guard, including the numerical guidance.  It is rare when the models consistently underestimate the surface pressure as the system is underway.  Usually the model data assimilation system and objective analysis, in the presence of sufficient observations (as is the case over northern MN), can “nudge” the model analysis towards reality.  This did not happen with this storm.  Looking at this system from start to this point, the models consistently underestimated the strength of the jet stream.  Observed satellite winds over the Pacific were as high as 205 knots (well above the numerical guidance) and 190 knots over the mainland, also above guidance.  Even as the system ejected into the plains, the observed satellite wind speeds exceeded the numerical guidance, sometimes by quite a bit (see previous post).  Preliminary evidence seems to suggest this was a likely contributor to the underestimation of this storm by the numerical guidance.

The big question to answer is why, with the presence of satellite derived winds and RAOB data suggesting otherwise, were the data assimilations systems of the various models unable to incorporate these features better in their analysis?

Final thought.  This storm has been an epic example of how powerful baroclinic waves act to enhance and develop their own baroclinic zones by increasing the thermal gradients over large regions as opposed to simply developing over regions of existing baroclinity.


Incredible Jet Stream Divergence

No amount of superlatives can describe the storm taking shape over the central and northern plains.  My projections of surface intensity in the previous post were completely wrong (I believed 966 was too low).  With any sort of extreme weather system, any particular dynamic and/or kinematic field is expected to be impressive.  This storm, however, is displaying an incredible amount of jet stream divergence which shows up in spectacular fashion on satellite imagery.    Let’s take a look.

IR satellite image at 15Z with the center of the jet stream noted at 250 hpa with the red line.  The green circle denotes the upstream jet streak in excess of 160-180 knots.

12Z 300 hpa analysis with winds and divergence plotted (thanks to http://www.patricktmarsh.com/, I never knew jet stream level winds with divergence plotted existed!):

As analyzed by the 12Z GFS @ 18Z (250 hpa):

Note the increasing jet level winds on the eastern side of the upper level trough from 12Z to 18Z.  Let’s investigate further.

The 12Z HPC surface analysis has the cold front analyzed in northern CO:

Note the well defined lee cyclone in the Front Range of CO extending into New Mexico, an atmospheric response due to the cross-barrier flow blocking effect of the Rockies.  Large and long mountain ranges block the otherwise orderly flow of cold air advection, resulting in a geostrophic adjustment process.  Lee cyclogenesis acts to enhance the low-level south flow and, in the case of the US, the flow of warm and moist Gulf air northward.  The “blocking” of cold air into the plains acts to “displace” the cold air aloft from the low level warm air in the plains in the vertical.  Mentioned in the previous post as well, the thermal wind equation comes into play here.  http://amsglossary.allenpress.com/glossary/search?id=thermal-wind-equation1

The change in the geostrophic wind with height (vertical shear) is related to the thermal gradient.  The jet stream, therefore, is a manifestation of intense baroclinic zones and upper level fronts, not the other way around.

Let us put it together a little more.  Take a look at the GFS 12Z analyzed 1000-500 mb thickness fields:

The location of the surface trough is noted with the green line with strong surface ridging behind the front (as expected).

Note that, at 12Z, the upper level cold air isotherm packing is lagging behind the low level cold air:

The effects of large scale flow blocking become much more apparent here as we put things together.  The effect of the broad and high Colorado Rockies is to block or retard the low level progression of otherwise orderly cold air advection.

BY 18Z, the region of cold air aloft has now become superimposed over the region of lower level cold air associated with the low level front, currently being blocked by the high terrain of the Rockies.

Oh, but wait.  What did the thermal wind equation state?  The picture is becoming slightly more clear now.  The juxtaposition of cold air aloft and at low levels along with the continued effect of lee cyclogenesis due to cross barrier flow results in southerly warm air advection in the low levels of the high plains.  These processes work to enhance the baroclinic zone along the mountain barrier.

18Z GFS forecast shows how much tighter the 1000-500 mb thickness field has become due to the aforementioned processes.  Also note the high-low pressure couplet that has developed across CO with the decrease in the surface pressure of the lee cyclone, now to 984 mb.  Cold bora winds downslope into the plains as the cold air “pours” over the Front Range.

As expected from the thermal wind equation, our jet stream has now become stronger on the eastward side of the curved jet stream over our now enhanced baroclinic zone across the high plains (circled).:

Also worth noting here are some of the terrain flows that can develop under such circumstances.  In the case of the Front Range, mesoscale terrain flows can develop around or over regions of decreased height in the Rockies.  Extreme pressure gradient forces are relaxed through relatively narrow regions of the terrain, resulting in terrain enhanced gradient forces.

Both the Ferris Mountains and the Laramies reach elevations above 10,000 feet with the Snowy Range (named the Medicine Bows in Colorado) extending to over 12,000 feet.  Gaps in the terrain extend down to 7500 feet in Laramie, WY before reaching approximately 4600 feet in Akron, CO.  With I-80 along southern WY being the only large scale “outlet” for subsident air over the Great Basin, winds can become rather extreme.

The obs from Akron, CO clearly show frontal passage (boxed red) with the typical pressure falls preceding the front followed by rapid pressure rises.  Of course, peak winds occur during the time period of rapid pressure rises (boxed green) and strong descent due to efficient mixing in the convective boundary layer acting in conjunction with descent on the backside of the frontal circulation (circled red).

Let’s move on.

Curved jet dynamics result in regions of strong ascent/descent (ascent on the exit region, descent on the entrance region) on the poleward (cold) side of the jet stream.

Also note the increasing amplitude of the trough and the “digging” nature of the jet.  Is this a result of QG Chi interepreted height falls associated with abnormal thermal advection patterns noted earlier?  Think about that.  Do jets “dig” or do heights fall?  I will let the readers decide.

Goes satellite derived WV winds at 18Z suggest both the NAM and GFS are under observing the jet streak winds on the downstream portion of the trough which would result in even greater values of jet divergence.  Circled isotach at 120 kts (18Z GFS peaked at 90 kts from 300-200 hpa).

This jet stream divergence was manifested in spectacular fashion on satellite imagery:

And on multi-spectral satellite imagery:

Here is an animation of the cloud patterns associated with this divergence over Colorado.  This is the best way to see the divergence pattern and associated cloud field:

Also note the “folds” oriented perpendicular to the flow (easily seen in the visible sat images).  Personally, I have no explanation for these features.  It seems plausible the N-S oriented CO Rockies have an influence, but I personally have no reasoning.  Anyone with ideas or explanations please let me know.

Update:  The expected smooth nature of the jet cloud pattern over WY is typical earlier in the day.  As the system interacts with the Front Range of Colorado, the folds seem to originate in the region where enhanced vertically propagating mountain waves often develop.  This seems like a plausible explanation, but I will have to do more of an analysis before coming to such a conclusion.

This analysis ends here, but note this is just one explanation (also the more simplistic and less mathematical approach and reasoning) of lee cyclogenesis and further baroclinic development associated with an intense jet stream (lee troughing is possible with little to no jet stream/weak baroclinity).  Other authors have proposed a QG explanation (Bluestein uses this approach in his Synoptics in Midlatitudes) as well as potential vorticity reasoning.  In general, differing “theories” and interpretations seem to come to relatively similar conclusions (in the last 20 years at least).

Meteorology is a beautiful thing when it makes sense.

Additional reading for those interested.

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(1989)117%3C0154:NAOTIO%3E2.0.CO;2

http://www.atm.helsinki.fi/~dschultz/pubs/19-SchultzDoswell00.pdf


Big Pattern Change Next Week

The first large fall storm across the western and central U.S. is looking more likely with the breakdown of the mean ridge across the western U.S. and an increasingly active Pacific Polar Jet.

What we do know.

An Aleutian Low will deepen near 175 W and will act to strengthen the baroclinic zone across the central Pacific as cold air streams in along the backside of the stationary low from Russia and the Barents Sea.

Current IR imagery and the western tip of the Aleutian Islands:

The 6 hour forecast from the 0Z 20th Oct GFS:

CIMMS analyzed precipitable water over the Pacific.  Increasing cold air streaming in from the north associated with the stationary Aleutian Low will act to enhance the baroclinic zone (boxed).  Also note Typhoon Megi east of China.

A low amplitude upper tropospheric wave disturbance, partially visible in this WV satellite loop off the coast of China and south of Japan, will set in motion the amplification of the Polar Jet over the Pacific, the development of a rapidly intensifying surface low off the west coast, and the subsequent intrusion of cold air over the northwestern U.S.  Also note the tropical systems east of Megi in the WV image (this is important!).

84 hr GFS forecast of 1000-500 mb thickness fields and SLP forecasting the rapid intensification of the surface low over the Pacific (circled) and the cold air reinforcing Aleutian Low (boxed).

Note the significantly high precipitable water values associated with the system, forecast to be around 2 inches:

What we don’t know.

A fair amount of variability exists even in the first 100 hours of the forecast period (as expected).  The large stacked upper low over the Gulf of Alaska is projected to slowly translate eastward, deamplifying with time.  An embedded shortwave in the base of the stacked upper low is projected to amplify over the existing baroclinic zone, developing a compact surface low ahead of the larger incoming Polar Jet.

The Large trough south of Alaska and the already developing embedded shortwave at the base of the trough (circled):

The 24 hour 0Z GFS forecast projecting the development of a surface low over the leftover baroclinic zone associated with the mean trough (circled) and a surface wave and developing triple point low (boxed) associated with the occlusion from the aforementioned Aleutian Low.

The development of these two systems will have profound impacts on the development of the mainland U.S. storm system next week, especially the latter system.

The 60 hour GFS 500 hpa vorticity fields clearly show the forecasted development of these systems.  Circled is the former stacked upper level low with embedded shortwave deamplifying into a compact shortwave trough as noted above in the previous WV image (circled), the development of a surface low along the Aleutian Low occlusion as noted in the previous WV image (boxed), and the incoming Pacific Polar Jet as mentioned earlier (pointed line).

By the 72nd forecast hour, note the rapid disintegration of the upper low clearly visible above in the 60 hour forecast of 500 mb vorticity fields (circled).  Why?  First, the presence of the long Coastal Range along the British Columbia coast disrupts the otherwise orderly flow of air (will go far more in-depth on this topic at a later date) across regions of even terrain (the ocean, in this case), and second, the lack of reinforcing cold air associated with this rather compact low (essentially an occlusion), and the total lack of baroclinity along the mainland of the U.S. results in rapid disintegration of the system by 72 hours.

The second system is also of significant interest.

Note that, by 84 hours, the GFS forecasts the closed upper low (earlier associated with the Aleutian Low occlusion noted above) interacting with the coastal ranges of the United States.  Once again, rapid weakening ensues as the mountains “perturb” the orderly flow of air, resulting in a region of disorganized vorticity.

This weak upper level low will have profound impacts on the development and amplification of the Polar Jet over the Northern and Southern Plains as the main storm system crashes on shore.

As forecasted by the 0Z GFS at 126 hours, the above mentioned system has now progressed over the intermountain west as an open wave, and the intense cyclone (as mentioned earlier in the post) is now quite evident over the northern B.C. coast (a track which is still highly uncertain at this point, boxed in this image).  The secondary jet on the backside of the occlusion is pointed to with the green line.

Why am I keying in on the open wave across the intermountain west?  Baroclinity.  Without a reinforcing snow pack across the Canadian Prairies and the mountain west, the lack of a significant baroclinic zone will not support the development of a significant surface cyclone even with the presence of an intense Pacific Polar Jet (which, of course, will weaken due to the lack of significant baroclinity as defined by the thermal wind equation… http://amsglossary.allenpress.com/glossary/search?id=thermal-wind-equation1).  With no reinforcing snow pack, we need to look elsewhere.  Where?  The Gulf of Mexico!

Note, by forecast hour 132, the GFS has now positioned the open wave over Texas (circled) while the powerful Polar Jet above now plows across the intermountain west.

Note that, under this flow, the low level flow off the Gulf of Mexico is limited to the SE U.S. (surface theta-e for simplicity):

Looking back at the 12Z GFS run, note here at the same forecast hour (144 here) as the 0Z run, the position of the open wave is projected to be over the SE U.S. instead of Texas:

This vastly different solution supports a prolonged period of lee troughing and subsequent low level S-SE flow off the Gulf of Mexico, and the establishment of a much more pronounced baroclinic zone over the plains.  The 12Z GFS goes on to blow up a 966 mb surface low over the northern plains by forecast hour 180 while the 0Z run develops a still strong but much tamer 978 mb surface low slightly farther east.  I am going out on a limb here, but it is highly unlikely the 12Z GFS solution verifies across the plains due to no reinforcing snow pack across the intermountain west and the Canadian Prairies (some air mass modification is likely) and what is looking to be a closed Gulf of Mexico due to the slower progression of the upper low across the intermountain west.

The 12Z’s rather generous surface low:

Why does all this discussion matter?  It goes to show just how complex weather can be even 5-6 days out.  Small deviations in the simulations of rather “insignificant” features (as we have shown here) can have far reaching effects with time as errors rapidly amplify.  Don’t forget models are ingesting hundreds of different data observations at differing times and all with varying errors associated with them–these errors will also grow with time (hence ensemble modeling and the perturbation method).  Think of everything going on in this scenario: a low amplitude wave over China, three tropical systems over the Pacific, the development of two compact surface lows over the Pacific, the interaction of those systems with the coastal range, etc. etc. etc.  For this post, we won’t even talk about the models themselves, all the parameterizations and assumptions they are making, the complete lack of a turbulence solution in the Navier-Stokes Equations, lack of infinite and continuous observations, model filtering, etc etc etc.  Maybe a post on numerical models is in the making…

I hope this post illustrates why it is not a good idea to rely on one operational model run for longer range weather forecasting.


First “bomb” Cyclone of Fall 2010

I have been so transfixed with the large cyclone transitioning into a vigorous shortwave trough across the western U.S. lately I had paid little attention to weather events along the east coast.  My special love for vertically propagating mountain waves, downslope windstorms, and intermountain/mountain west weather in general may have blinded me (albeit very briefly) slightly to the events along the other coast of America.  I apologize, and I ask for forgiveness from any east coasters I know.

WV imagery during the initial stages of rapid deepening:

12 hours later.  Note the rapid increase in mid-upper tropospheric moisture as the system interacts with the Gulf Stream.  Also note the rapid development of a significant “dry-slot” off the east coast–quite common in rapidly intensifying cyclones (will also go more in-depth during later posts…more complex dynamically and thermodynamically than one may think!) :

No analysis needed here (I will do a more thorough analysis of the dynamics and thermodynamics sometime this winter).  The interaction of Canadian cold air advection and the semipermanent zone of baroclinity along the Gulf Stream results in some of the most spectacular weather in the U.S. during the fall/winter.

Surface pressure falls at Portsmouth, NH.  27 mb/20 hours, and the very impressive nearly 15 mb in the last 5 hours:

The late renowned MIT professor Dr. Fred Sanders, a synoptician for whom I have the utmost respect for, was the first to “coin” the term bomb in the case of rapid marine cyclogenesis.  http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(1980)108%3C1589:SDCOT%3E2.0.CO;2

Update:

Portsmouth, NH finally reached a low pressure of 982 mb and nearly 35 mb/24 hrs.

Atmospheric Bombogenesis.  Fred Sanders would be proud.  Enjoy the spectacular satellite signature:



Instability Vortices over the NW United States

On the 6th of October, a series of vortices developed over the northwestern portion of the United states over a region high horizontal wind shear aloft.  This was preceded on the 5th by a large cutoff low over the US SW and a sharp shortwave trough along the US-CAN border.  A region of deformation develops later on the 5th seen here in WV:

This deformation zone transitions into a region of strong cyclonic shear on the 6th, streamlines on WV :

The NAM also captures this region of enhanced cyclonic vorticity (250 hpa):

From there, a vortex street develops, seen here in WV:

Seems to be a case of barotropic instability, but there are other potential (or simply other mechanisms) theories on the development of these vortices including stratospheric intrusions.  Unfortunately I do not have cross section data, so this simple analysis will have to suffice for this post.

http://journals.ametsoc.org/doi/full/10.1175/1520-0493(1997)125%3C2504:SOIVAM%3E2.0.CO;2

Update:  It is very well possible the unusually sharp shortwave (PV anomaly) trough penetrated the stratosphere resulting in the very dry air behind the system, resulting in a combination of the two effects discussed above.


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