Category Archives: Forecasting

Forecasts, forecasting techniques, computer models

Forecasting, Tropical

Emily Organizes; Gulf Threat Decreases

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Tropical Storm Emily struggled through most of last night and today with disorganization, an aftereffect of its multivortex structure as an unnamed disturbance.  However, it has become better stacked today, with convection blowing up over its center.  It still has a long way to go, despite its more pleasing appearance in satellite photos.

Steering in the short term is straightforward.  Emily has been generally on the left side of the forecast track for most of the day, and it is now expected to make landfall in the Dominican Republic as a tropical storm.  Weaker systems generally weather the mountains better than stronger ones, provided that they do not linger in the area; therefore dissipation of the system seems comparatively unlikely.

 

Models conclude that trough will miss Emily

The models have largely converged on a scenario in which the trough that is to weaken the Bermuda High will be gone before it can force the full recurvature of Emily.  The ridge is expected to build back in, and the GFS shows the hurricane being trapped off the coast of Florida, unable to move ashore because of another ridge, stalling until a shortwave trough lifts it away.  The GFDL and HWRF models, which take their input from the GFS, both show a very close approach to the east coast of Florida, with the HWRF showing near-hurricane-force winds onshore.  The NOGAPS shows this same scenario without the stall.  In this scenario, a landfall on the Outer Banks of North Carolina occurs, followed by a pull up the Atlantic seaboard (offshore) and out to sea.

The Canadian model shows a very weak system, probably no more than a mild tropical storm, making landfall on the east coast of Florida and then being merged into the shortwave.  I should observe that the Canadian model now shows the first low strengthening to 988 mb at sea and reducing the Bermuda High to its winter stage (the Azores High), which does not seem remotely reasonable to me for an August system.  I am not putting a lot of faith in this aspect of the Canadian solution.

The European model also shows a “screwy” solution, amplifying the shortwave trough to 992 mb at sea, while completely dissipating Emily over Hispaniola.  Although unlikely in my opinion, dissipation of the tropical storm is certainly possible, as yesterday’s blog entry said, but I am having great difficulty believing that either of the baroclinic low pressure systems involved in this will reach 990 mb levels.  The first trough is currently located in New England producing a severe weather risk, and it is analyzed at 1002 mb.

Bottom line, I am giving a highly skeptical eye to anything that destroys the Bermuda High at the beginning of August and amplifies low pressure cores to winter levels, especially when they have not been doing this consistently.  Any land-free recurvature of Emily depends entirely on such “bombs,” and the approach to the East Coast will be so close that a weaker trough or shortwave will make all the difference in the world in what wind speeds are felt onshore and whether landfall occurs.

 

Gulf Coast threat decreases… for now

As should be apparent, the threat to the Gulf Coast states has decreased over the course of the day (with a caveat).  The current thinking is that the trough will lift Emily northward enough to miss an entrance into the Gulf of Mexico.  However, this could change if the storm stays south and west enough, or the trough is weaker than expected at sea.

Forecasting, Tropical

Emily Is a Threat To the U.S.

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After days of teasing weather watchers (and the National Hurricane Center), a tropical wave in the Atlantic has formed into Tropical Storm Emily.  The storm is rather disorganized and not at all “attractive” in the tropical cyclone sense, an artifact of its having had multiple competing vortices for several days that prevented its consolidation into a single system.

Because of its delay in getting organized, Emily is a threat to the United States.  I am going to blog regularly about this system as long as that remains the case.

 

What’s the synoptic setup?

Emily is going to move mostly west, slightly WNW, along the southern end of the Bermuda High.  Its strength will depend primarily on possible land interaction during this time.  The National Hurricane Center forecasts an impact on the island of Hispaniola, which would weaken the system.  How much remains to be seen; many a major hurricane has been reduced to a tropical storm by this island, but some systems that are much weaker have survived passage.  It is terribly difficult to forecast how much effect the mountains will have on any particular storm.  A lot depends on how well-organized the system is when it reaches the island (I do not mean its intensity; intensity and cyclonic organization are not the same thing), how long it stays there, and whether there are any additional destructive factors such as dry air intrusion and wind shear that are hitting the storm at the same time.  It is arguable that there’s not a lot of point in making a forecast for Emily after its interaction with Hispaniola at all, because what happens to it after that will be heavily influenced by its strength at that point.  I’ll discuss the various possibilities, however.

The high is going to be weakened on its left flank by a trough coming off the Atlantic coast in a couple of days.  This should pull the storm to the north.  How much depends on how weak Emily is and how strong the trough has managed to become.  The stronger Emily is, the more northward it is expected to move, all things being equal with respect to the trough.  I think that the trough will be the most important player here, though, and should be watched at least as closely as the tropical system.  It is very uncommon to have a strong trough coming off the East Coast at the beginning of August, and there has been a pattern this year of the GFS (the U.S.’s long-range weather forecast model) overdoing the strength of lows in the days before they arrive.  I am not inclined to buy into a strong trough unless I see it materialize, but it’s always best not to count anything out, either.

Emily is probably too far south and west to have a land-free recurvature (“fish storm”) path.  It’s not impossible, but it is unlikely.  It simply took too long to develop for that to be the most likely track.

 

What’s the model spread?

Models generally have clustered around the state of Florida as of Monday evening, with the NOGAPS (U.S. Navy model) farthest west and the GFDL farthest east.  The NOGAPS is showing an implied strike on the Florida panhandle (it has Emily stalling in the Gulf and not making final landfall within a 7-day period), and the GFDL shows a “fish storm” recurvature.  It is important to observe that this trend for the GFDL is relatively new; until the past 24 hours or so, that model was showing a strike on the East Coast of Florida and the HWRF model was showing a recurvature.  Now that has reversed itself.  In the meantime, the NOGAPS has been consistent in its Gulf track.  Consistency alone is not a reason to support a model’s output, but it is generally indicative of a model’s having a better grip on the environment than one that is prone to the “windshield wiper effect.”

 

Is the Gulf Coast at risk?

Short answer:  yes, but it’s not set in stone.  As of now, I still would say that the Florida peninsula is most likely to get hit, but the Gulf is a definite possibility, especially if Emily is weakened by interaction with land and/or the trough is weak.

Several forecasts indicate that the storm will remain weak for long enough to stay south and get into the Gulf of Mexico before making the recurvature.  This is not a fluke, or a one-off from some model; it has been a solution for the NOGAPS, Canadian, and UK several times over the past two days.  Furthermore, models such as the HWRF have been hinting at a Florida East Coast strike at a perpendicular angle, indicating a strengthening ridge that would force Emily westward again.  While these models do not go out far enough yet to indicate what would become of Emily after the Florida strike, entry into the Gulf (in a weakened state) is certainly possible in this scenario.

 

Forecasting, Tropical

90L: Weak and Into the Gulf

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The area of interest in the Atlantic, 90L, has become more likely to enter the Gulf of Mexico.  After a time yesterday when it was trying to spin up, the system has stayed weak and is now beginning to encounter land.  This land interaction will keep 90L weak as it passes through the Caribbean, making it even more likely to avoid the weakness in the Bermuda High that will be created by a trough.  90L currently has an area of moderate 700 mb to 850 mb vorticity associated with its convection.  This area of vorticity is what currently passes for a circulation.

It is important to note that, even though the current state of the system is less organized than yesterday and the National Hurricane Center has lowered its percentage of becoming a tropical cyclone in the next 48 hours (which I would completely agree with), 90L has gained additional model support for its long-term development prospects.  The cyclone-specific HWRF model was on board with 90L yesterday, taking it just south of Cuba and bringing it to 60 mph by the time it passes by.  Today the HWRF keeps the system even farther south, intensifies 90L to a Category 1 hurricane, and sends it into the Yucatan.  Additionally, the GFDL cyclone model, which was not doing anything at all with 90L yesterday, is today showing a Category 2 hurricane striking the Yucatan.  I think that is overdoing it, personally, but this system is showing indications of going into the Gulf of Mexico and intensifying then.

In recent hours, it has become possible that 90L is experiencing a center reformation.  The center has been located in the part of the system that is now south of Puerto Rico.  However, increased convection just south of Hispaniola (Fig. 1) is changing the polarity of the system, as is evident in upper-level divergence charts (Fig. 2).  This convection is likely associated with the mountains and therefore does not indicate improvement in the tropical structure of 90L.  However, if the center reforms to the northwest, this will throw a great deal of uncertainty into even the survival of 90L, as it will come much closer to the destructive mountains of Hispaniola and Cuba.  If the reformation does not occur, we are looking at a track like that of the GFDL and HWRF.  For my part, I am finding it hard to get on board with a center reformation over a more destructive environment that will make it hard for existing centers to stay together, let alone new ones to form, but time will tell.

One more important point to note for the GFDL model run is the strong ridge that would, in that scenario, serve to block 90L from moving north after it enters the Gulf.  The blocking ridge does not extend that far west in the HWRF run, making a Central Gulf landfall possible.

In summary:  90L is in a state of transition at present, and the outcome of a number of possibilities will determine its fate.  If the center reforms to the northwest, the GFDL and HWRF tracks should not be considered because they assume the present center.  The result of a reformation would be more land interaction, which means a weak system, delays in development, and the possibility of complete dissipation.  If the center does not reform, the GFDL and HWRF scenarios are in play, opening the doors for a significantly stronger system (and it should be noted that those models only go out to 126 hours, and have the system as an organized hurricane or near-hurricane in the middle of 90°F waters and low shear).  The ultimate landfalling location of 90L will then depend on the strength and extent of the ridge.


Figure 1: Rainbow-enhanced infrared image of 90L, Saturday evening.


Figure 2: Upper-level divergence over 90L.

Forecasting, Tropical

A Tropical System For the Gulf To Watch

A tropical wave, designated 90L by the National Hurricane Center, is worthy of being watched by the Gulf Coast states. This system is arguably the first tropical system of real interest to the Gulf states in the U.S., as Tropical Storm Arlene was regarded as a Mexican storm (correctly so) almost from its inception, and Tropical Storms Bret and Cindy were never a threat to any land areas.  However, 90L is in a situation that will strongly favor its reaching the Gulf of Mexico, where conditions are favorable for development.

The system has been steadily increasing its convection over the course of the day, and with this increase has come an improvement in its cyclonic structure.  Cyclonic curvature is evident on satellite (Fig. 1), and upper-level divergence (Fig. 2) indicates good ventilation for the system.  Lower-level convergence (not shown) is not so impressive, indicating that the system needs to form a strong low-level circulation to be considered a tropical cyclone.  This is usually the last step that developing tropical cyclones take.

90L is in a simple steering regime, being located south of the Bermuda High.  In about 3 days, a trough associated with a cyclone is expected to be located off the East Coast of the U.S., eroding the high somewhat.  It was previously assumed that this temporary weakening of the ridge would result in 90L being drawn north for a recurvature.  However, recently, it has become likely that the trough will be weaker than previously believed.  90L is also expected to take longer to develop owing to shear and likely land interaction.  The net result will be a stronger ridge and a weaker tropical system, and the consensus is that 90L will be forced into the Gulf of Mexico (Fig. 3).

90L will have to pass through an area of 20-knot wind shear (Fig. 3, Fig. 4), which is moderate, but will inhibit strengthening for as long as the system is located under that wind regime.  The GFS model does not indicate a sharp spike in wind shear over the course of 90L’s trek toward the Gulf of Mexico.

Unless the expected path drastically changes, 90L should enter the Gulf in about four or five days.  Models are unreliable for storms like this in the long range, and it should be noted that some of the models, like the GFS, are not particularly impressed with this system in the first place.  However, the cyclone-specific model HWRF does develop 90L into a 60 mph tropical storm, keeping it south of Cuba by the end of its run (126 hours out).  For my part, I am disinclined to accept a forecast of zero land interaction at this point.  However, the salient point is that any interaction with Cuba or Hispaniola will have a profoundly negative effect on 90L’s short-term intensity even if it becomes a tropical storm before reaching those areas, and avoiding those landmasses will result in a stronger cyclone that has not been delayed by a reorganization after being disrupted.

My gut forecast for a week or more out (in other words, break out the salt!) is that this system will become a tropical cyclone of moderate intensity (I’ll say Category 1, max, because of mild levels of shear in the Gulf even though the temperatures are well over 90 degrees in many areas) and that it will make landfall somewhere west of Pensacola.  I will have updates about this system if it continues to be a concern.


Figure 1: Shortwave infrared satellite of 90L, late Friday night


Figure 2: Upper-level divergence for 90L, late Friday night


Figure 3: Google Earth overlay of model tracks and shear for 90L, late Friday night


Figure 4: Wind shear tendency, late Friday night

Forecasting, Mid-latitude cyclones, Severe

Dangerous Severe Weather Situation for Tennessee Valley

So far the severe weather outbreak of late April 2011 has brought devastation to many different areas in the southern Plains and Southeast. A tornado that was probably of a violent intensity hit a small town in Arkansas last night, reportedly removing pavement from the ground, and tornadoes have touched down from Texas through Arkansas and Louisiana into Mississippi, Tennessee, and Kentucky today. The severe weather is going to continue through Wednesday as the second of two low pressure centers intensifies over the mid-South.

A powerful mid-level jet has formed, with winds screaming at 70 knots or more:


Winds and pressure at 700 mb

This will provide powerful upward forcing. Additionally, strong thermal advection feeding into the developing low pressure center (which has become quite strong today, checking in at 989 mb as of this writing) will promote instability as well as additional vertical uplift:


Temperature and wind at 850 mb

For specific tornado threat regions, the NAM model has been fixing on an area centered over the Tennessee Valley. Yesterday the bull’s eye appeared to be right over the state of Tennessee, but today, it has shifted somewhat south.

The 18 UTC run of the NAM had CAPE over 4000 J/kg in central Mississippi at peak (mid-afternoon Wednesday), with a local maximum roughly over the Golden Triangle region and Tupelo, MS:

This run also had 3 km energy helicity values literally off the charts over the same area:

The 00 UTC run has CAPE peaking around the same value as before.

It has, however, moved, the helicity indices to extreme northwest Alabama:

Bottom line? I think that at this stage, such deviations are just model noise. Observations will be needed to determine exactly what area will have the highest risk of severe weather tomorrow. However, it is important to note that the model has been very consistent with an elevated tornado threat Wednesday afternoon in the Tennessee Valley area. With that in mind, I offer up my own subjective severe weather threat map for this event. I am not assigning probabilities to any of the color regions of this map; they are present only to indicate the areas that I think are in increasingly higher danger of seeing powerful severe weather, including long-tracked tornadoes.

Forecasting, Mid-latitude cyclones, Severe

Another Multi-Day Tornado Outbreak Expected

The year is off to a record start in tornadic activity, and more is on the way. Another three-day tornado outbreak is currently underway, with day 1 having brought close to 40 tornado reports and over 250 hail and wind reports.

The system is expected to bring more severe weather to the eastern part of the U.S. through Tuesday and Wednesday. At the time of this writing, a squall line has developed in the Mississippi River Delta that is expected to push east overnight, bringing strong winds, rain, lightning, and the risk of embedded tornadoes.

This scenario is more complicated than the setup for the last big outbreak, which had a single powerful system to generate the intense weather. A closed low located over Arkansas is responsible for the day 1 activity. This low formed today from a shortwave kink in an upper trough. This low is expected to be blocked by a strong high pressure system off the Atlantic coast, causing it to move north and eventually northeast to die out over the Great Lakes area. However, a second shortwave kink is expected to enter the mid-South and undergo cyclogenesis on Tuesday afternoon or evening.


Surface low at 36 hours

The combination of the current cyclone’s development, the upper-atmospheric jet that is causing all this shortwave activity (see below), and the next cyclone’s appearance on the scene will result in there being significant sources of uplift.


250 mb jet stream


700 mb upward vertical velocities

The highest values of instability in the event are currently prognosticated by the models to occur around midday tomorrow. The NAM and GFS generally agree on the areas of high CAPE, with each model forecasting at least 3000 J/kg (and it should be noted that models do not do well with CAPE and have a tendency to underforecast. Keep an eye on observations such as soundings).


Surface-based CAPE at 18 hours, NAM


Surface-based CAPE at 18 hours, GFS

For Wednesday’s event, the highest CAPE values are expected to be over Mississippi. The NAM and GFS agree on the maximum values but have the location and orientation of the high CAPE axis different.


Surface-based CAPE at 45 hours, NAM


Surface-based CAPE at 45 hours, GFS

The Energy Helicity Index (EHI) values for 18 hours and 45 hours (midday Tuesday and afternoon Wednesday) indicate the areas that the models forecast are most conducive for tornadic supercell development. Here is what the NAM indicates for the two times:


EHI at 18 hours, NAM


EHI at 45 hours, NAM

I am especially concerned about the middle part of Tennessee on Wednesday if that is accurate. Those EHI values are almost off the scale, and they coincide with an area of at least 3000 J/kg CAPE. That area has a history of tornadoes, and between the strong instability, powerful uplift, and helical pattern to the winds, I think it is quite likely that the Tennessee valley may be in the worst part of this outbreak.

The Storm Prediction Center has already put up a Moderate risk for Tuesday and Wednesday. It is thought highly likely that at least one, if not both, of these days will see that risk upgraded to High.

Forecasting, Mid-latitude cyclones, Severe

Tornado and Severe Weather Outbreak in OK/AR/MO Thursday

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I am expecting a tornado and severe weather outbreak to occur tomorrow in what I would call the heart of Tornado Alley.

However, that’s a worthless statement from any perspective—forecasting, meteorological, geographical—without a qualification of the term “Tornado Alley.” I have long thought that the true “heart” of Tornado Alley is not over Oklahoma City at all, but is instead farther east. In truth, where that heart is depends on what one chooses to look at. (For long-tracked violent tornadoes, the bull’s eye is, beyond a shadow of a doubt, over Mississippi.) Some meteorologists developed the idea of three distinct tornado alleys: Plains Alley, the traditional storm-chasing zone in the Plains states that is often referred to with the general appellative of “Tornado Alley”; Dixie Alley, a broad region encompassing much of the Southeast but not including parts of Texas that are climatologically and geologically more similar to Plains Alley; and Hoosier Alley, which is essentially the Midwest. However, if you really take a good look at it, I think you’ll find that the entire eastern region of the country between the two mountain ranges is Tornado Alley. There is no distinct border where you can say, “There are not nearly as many tornadoes in this small region as there are in either ‘Alley’ on each side of it,” and in the absence of such a thing, the only reasonable thing to do is to say it’s all one Alley. (Besides, “Hoosier Alley”? Really?) With the whole of the U.S. between the mountain ranges as Tornado Alley, it seems reasonable to expect the core somewhere east of Oklahoma City, and the meteorological data bear this fact out.

Now that that’s out of the way, let’s take a look at the science behind this and see why there will be severe weather, as well as where it should occur.

The weather-maker will be a low pressure center forming east of the Rocky Mountains. A low that is currently analyzed at 1009 mb (which is to say, not much of a low) is located over Kansas right now. This cyclone is forecast by the NAM to open up as it weakens and become more of a broad trough. This should be occurring right now, in fact. After that low dies out, a strong upper-level low (700 mb) is set to close off its circulation over the TX/OK panhandles on Thursday. Upper-level cyclogenesis should have occurred by 9:00 to 10:00 tomorrow morning (CDT). The usual vertical tilt of these kinds of cyclones will result in the surface low being over central Oklahoma at this time, and it should be at about 1000 mb by midday tomorrow.

Thermal advections will be strong, with the greatest warm-air advection (WAA) occurring in an area just west of Tulsa, OK to north TX to the AR/LA border. Temperature advection drops off in northeast Arkansas.

Surface dewpoint temperatures will be quite high, approaching 65-70 from Tulsa south along the OK/AR border in a narrow swath. A larger area of 60-65 F dewpoints will cover the area from Oklahoma City through a diagonal line bisecting Arkansas NW-SE.

However, despite all this moisture around the surface, a dry socket should be present at the critical 700 mb level in the atmosphere. This shows up on a synoptic model forecast map as a medium brownish-gray blotch of low relative humidity.

All this will lead to mixed-layer CAPE values approaching 2500 J/kg.

The instability will definitely be present, and between the thermal advection and the diffluence-based forcing from the low, lift should be present over the target area. Decent upward vertical velocities are forecast by the models to be present over the area of interest.

A cap of CIN will be in place early in the day, but as the surface heats up, the surface temperature should remove the temperature inversion, making it easier for moist parcels of air to rise.

The disappearance of the capping inversion also shows up in forecast soundings for the area bounded by Tulsa, Fort Smith, and Fayetteville. They indicate that a substantial warm surface layer will have developed by late afternoon, and with CAPE values as high as they are expected to be, this will set off convective growth.

Helicity values are also supposed to be quite high, with 0-3 km at 300-500 over the target area, at a time coinciding with the high CAPE values and the eroded cap, which will support supercellular development instead of linear. This increases the risk of tornadoes and large hail.

The Storm Prediction Center has already put up a Moderate risk for this general area, which I agree with 100%. I wouldn’t be overly surprised to see them go with a High by tomorrow morning, though this is a small enough area that they may not do it on the basis of its size. In any case, I am growing quite confident of a risk of supercells tomorrow for eastern Oklahoma, western Arkansas, and southwest Missouri. If I were to pick one city that I think has the highest chance of a significant impact, I’d go with Tulsa. However, a slight shift in any direction of the low’s movement would result in a shifting of the severe weather risk, and these events are never points, but areas. It is looking like a potentially dangerous day tomorrow for this region of the country, so people living there should make sure they keep aware of what is going on in the afternoon and be ready to act.

Forecasting, Mid-latitude cyclones

A Big Storm, Cold, Thunder, and Perhaps Winter Precip?

Think it was pretty cold this past week? Well, wait till Sunday and Monday. But before we get to that point, we’ll have had quite a system to pass through the area, featuring thunderstorms, strong winds (and the accompanying frigid wind chills), and—though the National Weather Service isn’t officially forecasting it—I think an outside chance of frozen precipitation. Areas outside the Southeast are in line for much more frozen stuff.

Yup, it’s meteorological winter (December through February), all right.

The low that will quickly become a powerful cyclone is analyzed right now as 1002 mb. In a short period of time, it should be located in the Plains states and closing on the Midwest. By the time it reaches Iowa (approximately early tomorrow morning), it will be generating rain and snow to its north as the warm sector (that’s where we are) forms.

Look at this image from the North American Model (NAM). Classic cyclonic shape.

This is a forecast for late Saturday night or the wee hours of Sunday morning, which may be a bit slow. Our storms—generated by uplift along the cold front—may arrive earlier than this. The timing is going to be critical, especially in consideration of the outside chance of wintry precipitation. If the moist sector of the cyclone pushes through faster (forced along by cold, dry air on the other side of the cold front), then we’re not going to get anything of that kind. Jackson NWS doesn’t want to forecast any such precipitation, either; I freely acknowledge that the odds aren’t great and it makes sense that they would not want to go out on a relatively unsupported limb.

However, there is some model support for my thinking on this. Here’s the NAM forecast for 6 hours later.

Observe that precipitation is occurring north of the 0-degree line. Observe where the 0-degree line is.

Looking at imagery for the lower levels of the atmosphere, we can see that it will be a very, very close call for northeast Mississippi, and the type of frozen precipitation (if any) cannot be guessed at with any accuracy. This is because when a cold front passes, there is not a straight vertical line dividing the area of retreating warm air from the advancing cold air. Instead, it is an angle. Since cold air sinks, we are far more likely to have cold air at the surface and some warm air still lingering aloft in the first couple of hours after the “edge of the front” (a very fuzzy demarcation, but you get my point) moves through.

If we do get any winter weather as a parting gift from this cold front, it’s likely to be mixed with rain, and the ground will already be wet from the rain we already would have had (and warm from having been in the warm sector prior to the frontal passage). Accumulation is not even on the table.

What about those thunderstorms? Are we in line for another severe weather outbreak like the one from the end of last month? Probably not. That event had CAPE values that would raise eyebrows in the spring, let alone the fall. These potential energies just will not be present for this front. However, thunderstorms are expected to occur Saturday evening, likely bringing a lot of rain and lightning, and an outside chance of small hail.

After the front has passed, we are going to get a glancing blow from the arctic air mass that is behind the system. Highs are not likely to reach 40 in too many areas north of I-20 on Sunday and Monday. The wind chill on Monday morning is going to be dangerously cold, approaching 10 degrees and getting close to zero around highway 82 and points north. Take note of this if you have to go to work or school.

In the wake of the previous cold spell and the one that is coming up soon, I’m hearing some grumbling about the forecast that NOAA made for a “warmer and drier winter than average” for the Southeast. That forecast is still on tap. However, “warmer and drier than average” typically does not mean that it will be so much warmer and drier that we will be able to notice it every day! We are talking about a couple or three degrees on average for the entire winter season. That allows for plenty of below-average events, such as this upcoming one. And indeed, if you look at the long-term GFS, it shows a rather significant warmup to temperatures approaching 70 degrees until the next system pushes through just before Christmas and drives temperatures down to the 40s again. Such long-term forecasts cannot be trusted in the winter season, because the weather is so volatile, but it’s definitely food for thought!

Forecasting

Snow in Dixie Watch: Tuesday-Wednesday, Dec. 7-8?

I’ve been keeping an eye on this for several days, but the models have been cagey about it, and I have been reluctant to make a blog post about it. For the past two years, predicting snow for central Mississippi in December has been, let’s just say, a fiasco, even for the National Weather Service office, and I’m not overly inclined to get burned a third time running. As a general rule, snow in the South tends to occur in late winter. That said, this is looking promising enough to warrant a blog post.

It’s not a big system. In fact, it’s not easy to identify just what is going on with a surface chart, because the snow (if any) will be driven primarily by upper-atmospheric troughs that do not extend to the surface. What we have got here is a low pressure system off Atlantic Canada and a very strong jet streak that has dived as far south as the Gulf of Mexico.

Here is one level of the troposphere, 500 mb. The upper-level trough is evident enough here, and would you look at that—its base is right over us:

However, there is no obvious trough over the South on a surface map:

What you see in the surface map that appears to match up with the trough at 500 mb is actually a temperature gradient. The thin black lines are the pressure lines, and the thicker lines that correspond with the thick lines at 500 mb are actually temperature lines. There is a reason why this temperature gradient shows up as a trough at the upper levels. When the air is cold, it contracts. When it is warm, it expands. When you consider that the temperature change occurs over time, it can be seen as cold or warm air moving in at a given layer. This process is called thermal (or thickness) advection. Here are simple diagrams indicating how thermal advection happens at the mid levels:

  

These two diagrams are by no means representative of every advection scenario that can occur. Thermal advection can happen at any level of the atmosphere. Whether the advection results in forced ascent from the surface or forced descent to the surface depends on exactly at what level of the atmosphere the strongest contraction/expansion is occurring and what is happening below and above it.

We are going to be in a forced ascent situation, as shown in the second diagram. This results in pressure levels that are expected at a given altitude in the atmosphere (in this example, 18,000 feet) actually occurring at a lower altitude, thus the “trough.” But look: Relative to the lower levels of the atmosphere, there is a high. This is shown on the surface map. What’s important is that there will be forced ascent, not that there will be relatively high pressure at the surface.

This feature is what is going to generate precipitation. By creating a relative low aloft, it causes air at the surface to rise (nature abhors a vacuum), generating cloudiness. Precipitation is expected to fall as snow because there is no warm air getting between the surface and the layer of clouds, owing to that extremely southerly and extremely strong jet.

Will there be snowfall? Probably. The big question seems to be whether any of it will stick. It has been cold enough lately to freeze the ground, but a big question mark is a layer of dry air at the middle to lower levels of the atmosphere. The forced uplift may take a while to fill this layer of air with moisture. We’ll have to see. In any case, keep an eye to the sky tomorrow night!

Forecasting, Mid-latitude cyclones, Tropical

Well, This Is Certainly Interesting

Since I posted last night, the range of possibilities for our coming weather in the Deep South has expanded quite a bit, and the “interesting” scenario that I hinted at towards the end is suddenly looking a lot more likely to actually unfold. I am talking about the possibility that Tropical Depression 19, now upgraded to Richard, gets into the central/east Gulf of Mexico and interacts with one of the coming troughs rather than dissipating in Mexico or extreme south Texas.

The HWRF model, which was alone last night among the well-known models in showing Richard going to the northeast, has been joined now by the GFDL, ECMWF, and the majority of other models, as this Google Earth screencap shows.

The National Hurricane Center has made note of this trend as well and is expecting to move its forecast path to the east if the trends hold up. They tend to err on the side of consistency, avoiding what has been dubbed “the windshield wiper effect” when models make dramatic shifts in their forecasts. But it seems likely right now that they are already leaning in favor of an eastern track for this tropical system. The two storm-centric models, the HWRF and GFDL, turn Richard into a major hurricane and slam it into Florida, but the Google graphic indicates that there is actually quite a wide range of possible landfall locations. If the shear in the Gulf of Mexico drops off as forecast, there is really no reason why a major storm couldn’t happen (though I think there are some limits on just what is possible). Ida last year almost became a major in the Gulf in November during an El Nino autumn, after all.


(No, I do not believe Richard will actually approach Category 5.)

What about our cold-core cyclones, then? And that possible early freeze?

The GFS has (and this should surprise exactly no one) backed off its screwball idea of winter precipitation for Mississippi in the first week of November. As of 06Z’s run, it was not on board with the eastern path for Richard, which throws a major wrench into matters, but let’s look at the evolution of the trough before Richard might enter the picture. If he does get into the east-central Gulf, it’s going to be about a week from now before we can consider a landfall.

The first trough, the one expected to bring rain and thunderstorms to Mississippi on Sunday and Monday, is still on schedule. This model has increased the amount of rainfall that we are apparently going to get out of this, showing up to 2 inches in a small area and widespread totals over 1 inch. This is the trough that would pull Richard up, up, and away, sending it somewhere into the Gulf Coast and adding even more rain. (Remember, the GFS rainfall totals as of this run assume that Richard does not get into this part of the Gulf and is not picked up by this trough.) I’m having a hard time accepting a hurricane of major-plus intensity (let’s say Category 4) in the Gulf in a strong trough situation in late October, though stranger things have happened. It seems likely to me that if this trough gets it, Richard will begin to transition into post-tropical and lose some of its intensity. Cold-core cyclones do not get as strong as tropical cyclones, either in minimum pressure or in wind speeds. (There have been some non-tropical lows that went down to the 920 mb range, but these were at extremely northern latitudes. It’s much less unusual to see sub-980 mb cold lows close to the poles.)

When the energy of a tropical cyclone is entrained into a trough, the trough benefits from it. These types of systems have spawned infamous nor’easters, such as the storm created by the transitioning Hurricane Noel in 2007. The GFS already turns the trough into a 990 mb low and possible nor’easter, and as I said, that run doesn’t even take into account the possibility of tropical-origin vorticity being advected into the dominant low. The South could very easily be in for a major flooding rain event (it is unbelievable that I could realistically use the word “flood” when we are technically in a drought), but if we have the situation of a former major hurricane being pulled into a strong autumn trough, the Northeast could get a monster storm. AccuWeather.com is well-known for hyping weather events, but I think they may be onto something with their current news story about this possible storm. Let’s just say that, while nothing is definite, the potential definitely exists.

The GFS still predicts the first freeze to occur just before Halloween. This freeze would follow the second trough, which would dump even more rain on us. The freeze would be a dry freeze (in other words, a believable one) and the day following it, Halloween, would possibly not lift out of the 50s for highs. This has happened before; I distinctly recall a Halloween in the mid-90s (I am thinking 1993 or 1994) in which it did freeze overnight. After that, the cold air lifts out. As I said before, this model run has dropped the “early November Southern winter storm” foolishness of the third trough it shows.

I am going to be very mindful of the evolution of Richard and the strength of the trough that the NHC and models are increasingly convinced will get it. Things are getting interesting here, folks.