Tag Archives: Mountains

Welcome to Alaska, Baroclinic Style.

First, the weather in Alaska is different, very different. Everything you thought you had a decent grasp on is out the window; you are starting from scratch as a forecaster. Synoptic/marine storms take on a completely different structure than their land based relatives, and you are truly forecasting in a relatively data void region. Entire surface lows can form and exist in regions of the Bering Sea completely in-between all available marine observations (buoy, ship observations, C-MAN stations, etc.). Old school forecasting techniques such as advanced satellite analysis, advanced synoptic analysis, and pattern recognition are stressed to the maximum. The Bering Sea/North Pacific is the perfect breeding ground for spectacular synoptic scale cyclones amplifying on both extreme baroclinic energy (polar air mass proximity) and latent energy originating from the subtropics. 940 lows are common; 930 lows are not rare.

971 hpa low forming west of the Aleutian chain. The aftermath, for Anchorage, was the first substantial windstorm of the fall, resulting in substantial damage and widespread power outages across the area:

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Second, Alaska terrain is incredibly complex. Glacier carved mountain terrain ranging from 4000 feet to 16,000 feet surround Anchorage and much of the other populated regions of Alaska.

Google Earth view looking into Valdez from Prince William Sound. In the background, on the left, is the 16,000 shield volcano, Mt. Sanford, which dominates the local terrain:

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Complex terrain, defined:

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Local pressure gradients, complex terrain flows, barrier jets, gap accelerations/winds, mountain wave dynamics, etc. dominate the local weather patterns, and they are innately intertwined with the track, position, intensity, and deepening/weakening rate of the synoptic scale cyclone. Small errors in the synoptic forecast can result in the difference between a major downslope windstorm or a minor wind event.

Instead of writing excessively about synoptic storm dynamics (or trying to, atleast), for once, I am going to sit back and enjoy the first (actually, second, see the above images) true rapid cyclone event since I arrived in Anchorage. Like all significant cyclones, it is a multi-faceted event ranging from 100+ MPH winds across the mountains and Turnagain Arm, storm force winds across the marine forecast zone in the Bering Sea and North Pacific, and heavy flooding rains across the coastal ranges..

NWS Anchorage weather headline for this weekend event:

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15 Sep 00z NAM 4 km Hi Res 500 hpa:

15 Sep 00z ECMWF 39 hr simulation depicting a powerful 972 hpa bent-back occlusion.

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The Froude Number and Stable Flow: Mountain Blocking

A powerful low amplitude shortwave ejected into Montana this morning in association with a 160 kt Pacific Jet.

The 0Z NAM from yesterday clearly depicts this feature:

Large scale and mesoscale ascent developed rapidly as the jet core amplifed over the region.  Note the large increase of high level moisture associated with a region of strong vertical ascent:

0545Z:

Three hours later at 0845Z:

Low amplitude intense shortwaves such as these have a tendency to develop significant upward vertical velocity/downward vertical velocity couplets which support rapid cyclogenesis and regions of strong pressure gradients over small areas (i.e. rapid intensification, or the second partial of p with respect to x, gradient of the gradient).

Note the rapid pressure rises, on the order of 8+ mb’s / 3 hours over northern MT as extreme cold air advection set in behind the front.

The surface analysis depicts the strong surface ridging associated with the extreme subsidence mainly owing to strong cold air advection behind the cold front.  Also note how surface ridging amplifies as the high pressure region interacts with the Rockies.  The Rockies “block” the subsident air from progressing westward, therefore air builds at a faster rate east of the Continental Divide resulting in stronger surface ridges:

The Great Falls sounding at 0Z shows the flow was mainly out of the N in the low levels and NW in the mid levels.

Great Falls is around 3700 feet, so in this sounding, stable N flow extended to nearly 10,000 feet, or over 6000 feet AGL.

The Belt Range south of Great Falls extends to around 6000-8000 feet and reaching top elevations greater than 9000 feet.  Also note they form a “bowl” type shape around the region.  This makes it very difficult for air to flow around the mountains.

The Froude number,

relates the inertial forces to the gravitational force.  Think of it as a relation of kinetic energy to potential energy where V is velocity, N is the brunt vaisala frequency, and L is the height of the mountain.  Therefore, think of it as relating KE= 1/2mv^2 to PE = mgh.  The brunt vaisala frequency is: 

Note the gravity term (remember mgh) and the static stability d-theta/d-z (the more stable the air mass is, the greater the kinetic energy will need to be for air to ascend the range).

A series of radar images shows how stable N-NW flow “bunches up” into the valley as stable flow is blocked by the mountains south of the valley.  Low level stable air builds into the valley and it acts to “uplift” air above it, much like Cold Air Damming:

Note in the surface obs the heaviest snow develops coincident with rapidly rising pressure as stable air builds into the valley while V simultaneously weakens (weak V, which means lower kinetic energy, therefore the flow can not ascend the mountain).  Note also that downslope flow into the valley was not able to kill of the qpf.  Also note the powerful cold front (green) with G into the 60s.

High res models were trying to show a large weather hole over Great Falls associated with downsloping into the valley.  A good example showing high res models can struggle mightily in compex terrain:


Weak Elevated Convective Instability Associated With Dynamic Height Falls

There is always something interesting going on in weather.  What initially may seem run-of-the-mill can become more interesting upon closer inspection.  Sunday featured a migratory baroclinic wave, cutoff from the primary westerlies, passing through the intermountain west and “ejecting” into the central plains during the afternoon.  What was particularly interesting was the interaction of a deep frontal circulation within a region of “dynamic” height falls aloft which supported the release of elevated instability.  The co-location of a mesoscale divergent jet stream aided in the rapidly increasing cloud field and elevated “gusty” virga showers.

The wave in question is quite clear in WV with the mid-level trough axis over Colorado at 12Z Sunday:

The 300 hpa upper air analysis clearly shows the upper wave and jet stream level winds as well as the large amounts of divergence (plotted yellow):

The 500 hpa analysis at 12Z.  Note the lower amplitude of the wave at 500 hpa as compared to 300 hpa (this is important!), suggesting this was largely an upper tropospheric wave.  Also note the large values of cyclonic vorticity near the base of the trough owing largely to horizontal shear (of course curvature vorticity is also present, but it does not play as large a role), simply expressed by :

in the natural coordinate system.

This is typical of low amplitude baroclinic waves owing to the amount of shear.  Why do low amplitude waves propagate at a greater speed than longwave troughs?  The rapid forward propagation of the wave is explained (compared to a longwave trough) by the dominance of cyclonic vorticity advection and subsequent height falls ahead of the shortwave trough.  Planetary vorticity advection on the backside of the wave is minimal due to the short wavelength, therefore height falls on the upstream portion of the wave are much smaller than height falls downstream.

The advection of cyclonic relative vorticity by the geostrophic wind dominates over f in the QG Chi equation regarding shortwave troughs:

And the 12Z GFS, once again, note the high values of cyclonic vorticity near the base of the shortwave:

Note the thermal pattern at 500 mb.  There is little to no cold air advection at 12Z:

Note that, by 18Z, as progged by the GFS, the cold air overspreads much of Colorado with little to no advection.  Why?  “Dynamic” height falls:

A combination of QG Chi and the hypsometric equation can help explain this.  As mentioned earlier, large values of cyclonic vorticity are being advected by the geostrophic wind near the base of the shortwave.  The more cyclonic vorticity and/or the stronger the wind, the greater the height falls.

The winds at 300 hpa are on the order of 70-90 kts:

And 40-60 kts at 500 hpa:

This suggests the geostrophic wind at 300 hpa is advecting more cyclonic vorticity than at 500 hpa due to the strengthened flow aloft (I don’t have the map, but the amount of cyclonic vorticity at 300 hpa is similar to 500 hpa).  Heights are falling faster aloft than they are at lower levels (this makes more sense now…remember the higher amplitude 300 hpa shortwave compared to 500 hpa?  This implies heights must fall at a greater rate aloft than regions below with a forward propagating wave).  As a result, because the atmospheric column is shrinking from some level below 500 hpa to the upper troposphere, temperatures must cool in response, implying forced ascent.

The effects of these dynamic height falls results in cooling of the upper levels and steepened lapse rates, which can be enhanced by diurnal insolation in the lower levels.

By 18Z, note the large convective cloud field which has developed post-front over the Colorado Rockies.  Also note the plume of high cirrus associated with the divergent jet stream (green) with a crudely drawn streamline:

By 20Z, the front has progressed into the plains, but note the still cloud free region in western NE:

The 12Z GFS @ 18Z over western NE shows the influence of dynamic induced height falls in the model Skew-T.  Note the mid-level lapse rates:

BY 21Z, note the steep mid-level lapse rates and the deep cold front (red).  Also note the GFS upper level winds are mainly S-SW.

Air parcels lifted to the top of the frontal circulation will be able to ascend freely somewhere around (after calculating a crude elevated LFC) 500 hpa before reaching the Equilibrium Level near 400 hpa.  Also note the very narrow and shallow zone of CAPE–likely around 50 j/kg or less.  Note the rapid expanse of the cloud shield over western NE along with the small pockets of weak convection from 20z to 23z:

Also note the continued SE flow aloft.  The rapid expanse of the cloud field was likely enhanced by the mesoscale jet circulation.  Also note how cold the cloud temperatures are in western NE:

The 12Z GFS @ 0Z is forecasting winds at 300 mb to be nearly due SSW (180-200 degrees).  Also note the ridge axis upstream.

Air parcels exiting the jet streak would become supergeostrophic and flow to lower heights aloft, in this case towards the NW.  This enhances the ageostrophic wind field/divergence and mesoscale forced ascent.  Remember that, in the case of jet stream circulations, this must be considered in addition to the large scale synoptic vertical motion field.  Jet streaks and their associated circulations are mesoscale and are not in any way related to synoptic scale ascent and the QG equations of vertical motion (which can be shown through a scale analysis).   Note that, in the 0Z analysis, which employs upper air soundings, the flow is indeed S-SE (also indicated in the cloud flow pattern and in satellite derived winds).  In this case, the 12Z model runs were likely underestimating the strength of the jet circulation and associated vertical motion field.  While the impacts were relatively minimal here, in winter, that could result in models significantly underestimating the potential for heavy snow banding (just one of many potential high impact events), for instance.

Also worth noting here is the 18Z 300 hpa wind forecast at the same time (compare this to the 12Z forecast above).  Satellite derived winds ingested by the numerical model data assimilation systems were able to adjust the upper level wind field to the satellite observations.  This is a good example showing “off” hour runs are not worthless and can have operational significance if used intelligently by the forecaster.

A large area of showers formed over the region including heavy convective showers.

 

Surface observations suggest most of the shower activity over western NE never hit the ground and cloud bases (AGL) remained high at around 10-12000 feet.  The main effect of the showers was to enhance horizontal momentum transport downwards and increase wind gusts along the frontal circulation.  Most surface observations showed peak wind gusts with the arrival of the showers, some in excess of 40 mph, hence the name “gusty” showers.

This case is a good example of the importance of “dynamic” height falls in meteorology, especially in terms of summer convective potential when deep, moist convection is often initiated/and or enhanced by very low amplitude waves/upper level “impulses” due to cap erosion, steepening lapse rates (increased CAPE), and regions of low level mass convergence (surface based and/or elevated).  Also important is the co-location of differing meteorological circulations (e.g. mesoscale jet circulations, frontal circulations, regions of synoptic ascent, etc.), especially during winter storm events.  Being able to diagnose and forecast these regions is of utmost importance, even more so in the short-term forecast and NOWcast.


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:



Mountain Valley Fog in Western Montana

Widespread valley fog developed this morning in the complex terrain of western Montana and Idaho.

The visible satellite clearly shows how widespread this was:

Let’s take a look at Helena, MT, circled here:

Helena is on the southern end of a bowl shaped valley around 3900 feet.  The valley low point is around 3600 feet and cool katabatic flows off the higher terrain after sunset flow in from all directions.  The only location for air to “drain” out of the valley is along the NE end at the opening of Gates of the Mountains Wilderness along the Missouri River–in essence, the entire valley is enclosed.

A Google Earth view of the only outlet on the NE end of the Valley.

In the winter (mainly under the influence of high pressure), this region is notorious for long-lasting inversions as the snow-pack reflects the weak insolation, supporting long lasting inversions and decreased air quality as stagnant air is essentially “trapped” until a synoptic scale system can mix the air mass out.

A few things worth mentioning in the surface observations.  Note, that by the 8:53 observation, the temperatures are on the increase with morning insolation.  Pressure slowly decreases (seen on the right) as rising thermals slowly develop.  Dew point slowly increases as well, but nearly saturated conditions and low stratus hang on until noon.  The depth of the fog is obvious here.   A common misnomer in the world of meteorology is that fog “burns off”.  It is too bad meteorologists insist on using this term as it can be confusing to the public.  Of course what is occuring is the development of a convective boundary layer which promotes mixing and drying of the saturated layer as insolation (even through the deep fog) will develop thermals with time.  Note the rapid decrease in surface pressure (boxed on the right) after the stratus layer mixes out–a good example of the “mass” of air that can subside into valleys at night.


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