I have to run off to an advanced storm spotter training and conference, so I’ll make this quick.

Both the NAM and RUC are extremely alarming for the threat of strong-violent tornadoes today. Both have an unstable atmosphere in place with INCREDIBLE low-level shear. They also depict a possible mesolow moving across the region, which is a classic signal for violent tornado outbreaks of this sort. The MCS moving through this morning is rapidly clearing out from west to east and should not impede instability.

I fully expect several long-tracked, strong-violent tornadoes across the SPC moderate risk area. Be very, very careful if you are in the vicinity.

I don’t have enough time for a full outlook, but I will post my thoughts here.

The NAM seems hell-bent on creating a very interesting day today along the warm front.  It brings a plume of 60°F dew points up to central Arkansas, northern Mississippi, and northern Alabama.  Coupled with warm temperatures and very cold air aloft, this would create a very unstable atmosphere, with SBCAPE of 1500-2500J/kg.

There will also be a large amount of wind shear in the atmosphere, especially the low-levels.  40-50kt of 0-6km shear will be in place across the warm sector, with 0-1km SRH of 200-300m^2/s^2 across the most weakly-capped areas of far eastern Arkansas and northern Mississippi by 06z.  Any surface-based storm in that environment will have strong-violent tornado potential, with new STP values of 4-6 across the region.

Everything is dependent upon the cap and whether or not there will be a sufficient lifting mechanism to break the cap across eastern AR or northern MS.  For now, I tend to agree with the SPC outlook, though I do think a 10% hatched tornado region is necessary.  If it becomes more clear that the cap will break, then a moderate risk will be necessary.

Stay safe.

Hurricane Warning
Hurricane Warning Seasonal Forecast
The 2008 North Atlantic Hurricane Season is rapidly approaching now, with less than 3 months until the season starts on June 1st. This means it is time for the second annual Hurricane Warning Hurricane Season Forecast. Last year’s forecast was fairly successful, with a call for above average activity with 11-14 named storms, 6-9 hurricanes, and 3-5 major hurricanes. There were 15 named storms storms in actuality, with only 14 fully tropical storms, 6 hurricanes, and 2 major hurricanes.

El-Niño Southern Oscillation (ENSO)

The first factor that will be considered for the 2008 hurricane season forecast is ENSO. ENSO is a large scale oscillation of ocean temperatures in the Pacific between warm and cold that has effects on the weather across the globe. In recent months, a strong La Niña has formed, which is the cold half of the ENSO cycle. The strong La Niña can be seen on the chart below; it is the area of cold water anomalies stretching from South America to the western equatorial Pacific.

A La Niña usually results in lower wind shear across the North Atlantic, providing an environment more favorable for tropical cyclone formation and propagation. The question then arises, will the La Niña continue through the hurricane season? My forecast is for the La Niña to continue into the hurricane season since it is very strong at this point but, it should start to weaken towards ENSO neutral (a relative lack of ocean temperature anomalies) towards the latter part of the season. The models that are used to predict ENSO (below) are in agreement with this assessment, although it should be noted they are subject to large errors.

Sahelian Rainfall Anomalies

The Sahel of Africa is considered a birth place of tropical cyclones. Every year, dozens of tropical waves depart Africa for the Atlantic from this region, sometimes becoming monstrous Cape Verde hurricanes. This has spawned research into the rainfall patterns of the region, in order to determine if they have any effect on tropical cyclone formation in the Atlantic. Sahelian rainfall has been below average during the last year over the southern Sahel, with the northern Sahel experiencing near normal to even above normal rainfall. In the figure below, I have put a black circle around the coastal region of the Sahel because that is the part of it which I believe is the most relevant to tropical cyclone formation. My theory is as follows. Drier than normal conditions over the coastal Sahel cause easterly waves to become drier and hotter just before they enter the Atlantic. This results in a greater temperature difference between the wave and the water, which can then help the wave become more significant and perhaps provide the extra nudge that will result in it becoming a tropical cyclone down the road. Of course, a dry Sahel is a double edged sword, and it will likely contribute to Saharan Air Layer (SAL) outbreaks. However, since the northern Sahel is near normal to wetter than normal, the more sandy part, maybe that will inhibit SAL outbreaks at the same time. It is up in the air, as always, on how much of an affect the SAL will have on tropical cyclones in the Atlantic this year. This is because the SAL likely depends on other factors than rainfall in the Sahel, such as the strength and position of the Azores high.

Normals and Adjustments Based On Current Factors

There are normally 10 named tropical cyclones in the Atlantic, 6 hurricanes, and 2.5 major hurricanes (according to the National Hurricane Center). The first adjustment for this season that I will make is +2 named storms since in recent years we have been finding tropical cyclones that we wouldn’t have normally found in the past, due to their size and sometimes remoteness. The next adjustment will be another +2 named storms due to drier than normal Sahel conditions. I will finally add another +3 for La Niña. Normal ratios will then be used to determine the amount of hurricanes and major hurricanes.

The Forecast

Named Tropical Cyclones: 16-18
Hurricanes: 8-10
Major Hurricanes: 3-4

Essentially I’m calling for a well above average season, probably more active than last year.

I don’t have time to post an outlook, but it would have a strongly-worded moderate risk for the southeastern halves of NC and SC.

This appears like a fairly classic intense tornado setup for the Carolinas, with a strong low-level jet bringing up warm, moist air into the region, a strong trough creating intense deep-layer shear, and the surface low track right along the edge of the Appalachians acting to keep the winds backed over land.

This could end up being the worst part of this severe weather event.  I strongly urge anyone in the Carolinas to pay exceptionally close attention to the weather today.

Stay safe.

A polar low which developed off of California and was mentioned in a post yesterday, has made landfall in southern Oregon after brushing the northern California coastline. There were many reports of winds in excess of tropical storm force in association with the system in California and Oregon. A short term forecast released a few hours prior to landfall by the NWS office in Eureka, CA, outlined the expected impacts from the low:

————————————————————-

SHORT TERM FORECAST
NATIONAL WEATHER SERVICE EUREKA CA
703 AM PST SUN FEB 24 2008

.NOW..
THROUGH 900 AM…A STRONG…BUT A SLOWLY WEAKENING LOW PRESSURE
SYSTEM LOCATED 60 MILES OFFSHORE OF CAPE MENDOCINO…WILL TRACK
NORTHWEST AT 25 MPH. THIS MOTION WILL BRING THE CENTER OF THE LOW
ONSHORE NEAR CRESCENT CITY BY NOON. ROTATING BANDS OF MODERATE TO
OCCASIONALLY HEAVY SHOWERS AND GUSTY SOUTHEAST WINDS OF 20 TO 30
MPH WILL SPREAD ACROSS THE REDWOOD COAST THIS MORNING.

LIGHTER SHOWERS…BUT VERY WINDY CONDITIONS WILL PERSIST ALONG THE
MENDOCINO COUNTY COAST THROUGH LATE MORNING. SOUTH AND SOUTHEAST
WINDS OF 25 TO 40 MPH SHOULD BE EXPECTED…WITH ADDITIONAL POWER
OUTAGES AND DOWNED TREES LIKELY THROUGH NOON TIME.

LARGE AND DANGEROUS SURF WILL CONTINUE ALONG NORTHWEST CALIFORNIA
COAST ALL DAY. WALKS ON BEACHES OR JETTIES IS HIGHLY DISCOURAGED.

OVER INLAND AREAS…A SNOW ADVISORY REMAINS POSTED THROUGH MID
AFTERNOON FOR ELEVATIONS ABOVE 3500 FEET. IN THE COLDER VALLEYS
OF TRINITY COUNTY SNOW LEVELS WILL HOLD AROUND 2000 FEET THIS
MORNING BEFORE RISING BY MIDDAY. MORNING COMMUTERS ALONG HIGHWAY
299 EAST OF BURNT RANCH SHOULD BE PREPARED FOR SLICK ROADWAYS.

———————————————————————————————–

One would think this was a tropical storm based on its impacts if it wasn’t for the snow that occurred with this at higher elevations!

Below are two satellite shots and one radar shot of the polar low prior to landfall, when it still had an impressive structure. An eye like feature is apparent in all imagery, and the low bears a striking resemblance to a tropical cyclone.

Click to enlarge radar image.

Relating back to my first post on this, both the NAM and the GFS have reached a consensus of painting lower 60’s dewpoints in the warm sector on Monday, in essence the middle point between extremities presented in my original post.

It seems that the main issue for this one will be lack of directional shear, and the continued possibility of moisture mixing out. Models have trended slightly weaker with the 500mb disturbance. SPC seems to have the right idea with a SEE TEXT on the Day 2 outlook, and I do not see anything higher than a low end SLGT risk to verify on Monday over the Southeast.

The next possible event looks to be around 9-10 days away, but there is plenty of time to watch that. As of right now it’s still in “possible threat” mode. More updates to come on this over the coming days as the models get a better hold over the system.

EF2 tornadoes are widely considered “significant tornadoes” in the meteorological community. But I’ve recently left myself to ponder whether or not EF2s are “significant.”

In order to answer this question, I must ask myself what is really meant by “significant.” Is significant determined by historical precident or by effect on human life? Statistically, EF2 tornadoes are fairly common, with well over one hundred occurring each year. So statistically, in the historical record, EF2 tornadoes are not overly significant. The question then boils down to effects on human life. A low-end EF2 can be rated as such by simply destroying a barn, whereas a high-end EF2 can do tens of millions of dollars in damage to a city. A low-end EF2 will generally not destroy a frame house, whereas a high-end EF2 generally will.

So the answer to this question lies in rather gray area. In my opinion, not all EF2s are significant. Some, such as the Springfield, IL, tornadoes of March 12th, 2006, which produced one hundred million dollars of damage and rendered 1000 homes uninhabitable, are. Others, such as the Macon County, IL, tornado of April 2nd, 2006, which destroyed a large barn, are not. Well-constructed arguments can be made in both directions. My opinion is that, in general, a low-end EF2 is not significant, a high-end EF2 is, and the line is drawn about in the middle.

And that’s Tony’s Tantrum of the week for this week.

A baroclinic system active off of the United States west coast has occluded to the point that it has developed a warm air seclusion near its center. Essentially, this means that there is a pocket of warm air at the center of this system surrounded by colder environmental air. This has provided a conducive environment for the development of a polar low, and one has developed. A polar low is comparable to a tropical cyclone, in that it is partially powered off of latent heat of condensation from low-level convection that develops in the warm pocket. However, cold air remains present aloft in situations like this so the cyclone can’t be considered tropical but, it can be considered sub-tropical.

A polar low formed 2 years ago around this time and hit Oregon, bringing wind and rain. This low appears to have its sights on California, so I will be updating on it for the next few days. Below is a visible loop of the polar low (which is at the center of the overall cyclone), an image of low topped convection that is developing around the polar low, and a phase diagram showing the thermodynamic nature of the polar low. The polar low is currently showing up as a shallow warm core on the phase diagram, which as explained before means that it has warm air at the core near the surface with a cold core aloft.

A major severe weather event was expected. There had been severe weather during the day. But no one quite expected what was to come. The night of February 22nd-23rd, 1998, would be the incredible herald to what would be the deadliest tornado season in twenty-four years. At its end, forty-two people would lose their lives across central Florida as seven tornadoes, three of them at F3 intensity, would rip through under cover of darkness.

We here at tornadotony.com dedicate this blog post to the forty-two people that passed away that night, as well as the eighty-eight additional people that would be killed during the horrific 1998 tornado season. Below are links to the NWS service assessment, the NWS damage survey, and the Melbourne, FL, NWS page on the outbreak.

Service Assessment

Damage Survey

NWS Melbourne, FL, page

A rare tornado outbreak occurred in South Florida on 2/12/2008, prompting 13 tornado warnings with 2 confirmed tornadoes. There were undoubtedly more tornadoes but, due to the rural nature of the Everglades there was no one there to confirm many of them. Some structural and tree damage was reported with each tornado. Below are links to the detailed summary on this event just released by the NWS.

Main Page:

http://www.srh.noaa.gov/mfl/events/?id=2008feb12

Warning Summary:

http://www.srh.noaa.gov/mfl/events/?id=2008feb12_pg2