By:
Bob Henson
, 6:59PM,GMT on March 31,2015
The first ten days of April could produce more severe weather than the
modest amount racked up so far across the U.S. in 2015. The same
upper-level pattern that kept the West warm and the East cold through
most of the winter has also kept severe weather to a minimum, as
northwest flow and a series of cold fronts pushed warm, unstable air off
the U.S. mainland. This year through March 30, we’ve seen a
preliminary total of a mere 38 tornadoes,
compared to a January-March average over the preceding three years of
163. As of Tuesday morning, NOAA’s Storm Prediction Center had issued
just four tornado watches and four severe thunderstorm watches for the
year thus far. This compares to a long-term Jan.-Mar. average (1970 –
2014) of around 39 tornado and 24 severe thunderstorm watches, according
to SPC’s Greg Carbin.
Figure 1.
Severe-weather outlooks issued by NOAA’s Storm Prediction Center on
Tuesday morning, March 31, valid for Tuesday (top), Wednesday (center),
and Thursday (bottom). Image credit:
NOAA/SPC.The
2015 numbers look destined to rise over the next few days, as a major
circulation change takes place over North America. The long-prevailing
northwest flow is being replaced this week by more zonal (west-to-east)
flow, with an embedded upper-level impulse reaching the Midwest on
Wednesday and another targeting the Southern Plains on Thursday. A moist
low-level air mass is already in place, with dew points close to 60°F
from Tulsa, OK, to Birmingham, AL. Surface low pressure generated by the
upper-level impulses should continue to pull the moisture pool
northward and generate unstable conditions. SPC placed a broad belt from
Oklahoma to Georgia under a slight risk of severe weather for
Tuesday afternoon and evening, with large hail possible as the moisture continue to flow north.
A large swath of the Great Plains is under an slight risk for
Wednesday, from northwest Oklahoma to southern Minnesota, with an slight risk in place for
Thursday
across part of the Southern Plains and mid-Mississippi Valley. The
predominantly west-to-east upper flow combined with southerly low-level
flow will enhance vertical wind shear, a key ingredient in the formation
of supercell thunderstorms. The main threats appear to be high winds
and large hail, although tornadoes can’t be ruled out. As was the case
last Wednesday, when
F2 tornadoes
struck the Tulsa suburb of Sand Springs and the Oklahoma City suburb of
Moore, the corridor from Interstate 44 into east central Oklahoma could
be a particular focal point for supercell formation by late Thursday.
Severe weather may continue into the lower Mississippi Valley on Friday
before the associated cold front moves into the Gulf of Mexico.
Forecast
models indicate a strong upper-level trough will settle across the
Great Basin by early next week, setting the stage for what could be a
more extensive multiday round of severe weather beginning as soon as
Sunday. A strong surface low should develop over the High Plains by late
in the weekend, pulling unstable air back northward across a large area
beneath west-southwest upper flow. Strong thunderstorms could spread
across the Midwest and South by early next week, with several days of
focused severe weather possible.
Is El Niño about to make its presence known?Surface
waters have warmed dramatically over the far eastern tropical Pacific
over the last several weeks, and the water temperatures are now assuming
a more classic El Niño configuration that’s been absent for the last
few months, with
prominent warming
just off the coast of South America. Computer models are remarkably
consistent on projecting a strengthening of El Niño conditions over the
next few months. All eight international models
surveyed by the Australian Bureau of Meteorology on March 16
indicated that at least moderate El Niño conditions should be in place
by August (i.e., sea-surface temperatures at least 1.0°C above average
over the Niño3.4 region). However, in a
March 31 update,
the BOM cautions: “Model outlooks spanning February to May . . . have
lower confidence than forecasts made at other times of year. Some models
currently show some spread in their outlooks for tropical Pacific Ocean
temperatures, indicating that while further warming is indeed very
likely, there remains some ambiguity about the amount of warming
expected.”
One hallmark of El Niño across the U.S. is split flow,
with the polar jet stream retreating to the north and the subtropical
jet stream intensifying across the southern tier of states. This pattern
tends to keep unstable air shunted toward the Gulf Coast, hiking the
chance of severe weather there (especially in Florida) but reducing the
odds over the nation’s heartland.
Long-range models for
mid-April are suggesting an El Niño-flavored pattern may emerge, with
prominent split flow (see Figure 2, below). This is partly related to an
intense Madden-Julian Oscillation event, the strongest on record (see
our
March 18 post),
whose impacts are now reaching the eastern tropical Pacific and
reinforcing the trend toward El Niño conditions. On its heels, a strong
downwelling (or warming-phase) oceanic Kelvin wave will be pushing
slowly eastward within the eastern equatorial Pacific over the next
month, according to WSI’s Michael Ventrice. “This should favor increased
organized thunderstorm activity over the eastern tropical Pacific
basin, which will act to accelerate the subtropical jet stream over the
U.S. through the end of spring,” says Ventrice. “This spells heavy
precipitation threats (including some severe weather) across the
southeastern tier of the nation over the upcoming months under the
developing split-flow type pattern.”
Unfortunately, next week’s
strong western trough may bypass California, further reducing hope of an
“Awesome April” that might take a dent out of the severe drought and
heat now plaguing the state. If a strong El Niño were to develop and
persist, it could increase the likelihood of substantial rain in
California during the 2015-16 wet season.
Figure 2.
The GFS ensemble forecast issued at 1200 GMT on Monday, 30 March, and
valid at 0000 GMT on Monday, 13 April, shows a pronounced split-flow
pattern at the 200-millibar height (roughly 40,000 feet), with much of
the United States lying between the polar and subtropical jet streams.
Image credit: Michael Ventrice, WSI.
New insight on how El Niño, La Niña shape severe weather riskA paper
published this month
in Nature Geoscience elaborates on how the odds of U.S. severe weather
in late winter and spring tend to be boosted by La Niña and diminished
by El Niño. The authors, led by John Allen (International Research
Institute for Climate and Society, or IRI), acknowledge that it’s
difficult to examine the connection between the El Niño/Southern
Oscillation (ENSO) and severe weather. The datasets are imperfect (not
all tornadoes or severe hailstorms get reported), and there’s a great
deal of variability from year to year. “Trying to tease out an ENSO
signal from both the natural noise and the human noise becomes quite
complicated,” said coauthor Michael Tippett (Columbia University) in an
IRI news release.
“You can’t get a robust correlation using the observational record
alone.” In this new study, the state of ENSO from 1979 to 2012 is
compared not only with actual severe reports but also with the
environmental factors associated with severe weather, such as
instability and vertical wind shear, thus enabling the results to be
analyzed more comprehensively. The study is the first to examine ENSO’s
relationship to severe hail.
Figure 3.
When ENSO is in a warm, or El Niño, phase (top), the frequency of
springtime tornadoes goes down. When it is in a cool, or La Niña phase
(bottom), tornadoes increase (indicated by red areas). The effect is
strongest in the boxed area. Image credit:
IRI, from Allen et al., Nature Geoscience, 2015.
In
line with previous work by others, the largest influence found by the
IRI team in winter (December-February) is across southern Texas and
Florida, where the risk of tornadoes is roughly doubled during El Niño
events. Prior studies had been inconclusive for springtime, but the IRI
group found a significant ENSO influence focused across parts of
northern Texas, Oklahoma, and Arkansas (see Figure 3), where the risk of
tornadoes and severe hail rises dramatically during La Niña and drops
during El Niño. There’s some asymmetry in this response: while not every
El Niño event puts a dent in the region’s severe weather, La Niña
events almost always push the likehood of tornadoes or severe hail above
the climatological norm. “Naturally, this is only a simple model for
the influence of ENSO on hail/tornadoes, and there needs to be more
complexity added moving forward,” Allen told me in an email. He and
colleagues are now looking into how variations in the strength of ENSO
across a severe weather season might influence the outcome.
Given
the weak El Niño event now under way, the IRI team is calling for
slightly enhanced odds of a less-active severe season than usual. Allen
explains the forecast in a
video clip on IRI’s website.
Maysak maintains Category 5 strengthJeff Masters posted a
full report earlier today
on Super Typhoon Maysak, now plowing across the Northwest Pacific east
of the Caroline Islands. As of 8 am EDT Tuesday, Maysak’s top sustained
winds were at 160 mph, according to the Joint Typhoon Warning Center.
This makes the system one of only three Category 5 typhoons ever
observed in the Northwest Pacific prior to April (the other two are
Super Typhoon Ophelia of January 1958 and
Super Typhoon Mitag of March 2002, both with 160-mph winds). The JTWC predicts
some further strengthening of Maysak,
with a projected top wind speed of close to 180 mph by Tuesday night.
Any eyewall replacement cycle, if one occurs, could keep Maysak from
getting stronger. Fortunately, Maysak is tracking north of Yap, the most
populated of Micronesia’s Caroline Islands, and cooler water
temperatures should lead to a fair amount of weakening by the time
Maysak approaches the Philippines this weekend.
At least 5 deaths and extensive damage have been reported
on Chuuk State (Micronesia). The storm also passed just north of the
sparsely populated islands of Fais and Ulithi while close to its top
strength.
According to
intensity estimates from the Joint Typhoon Warning Center,
2015 is the first year on record to have three Category 5 storms form
in the Pacific Ocean during the first three months of the year. The
other two Category 5 storms in 2015 were
Tropical Cyclone Pam (165 mph winds), which devastated Vanuatu in mid-March, and
Tropical Cyclone Bansi
(160 mph winds), which affected ocean areas a few hundred miles east of
Madagascar. Reliable satellite records of Southern Hemisphere tropical
cyclones extend back to the early 1990s, so we only have about a 25-year
period of good records for global tropical cyclones.
We’ll have a new post by Wednesday morning.
Bob Henson
Figure 4.
An infrared image of Super Typhoon Maysak from 0444 GMT on Tuesday,
March 31. Image credit: NOAA/NASA and RAMBB/CIRA, courtesy Stu Ostro
(The Weather Channel).
Forecast
path and peak sustained winds of Maysak over the next five days.
Circles denote uncertainty in the position of the center at each
forecast point.
