Summer Severe Weather in the United States

*Blog written at 1:00pm CDT 6/26/13*

Summer is here and that means hot for most in the United States.  But along with summer heat, severe weather also poses a threat for many, especially those living in the northern Plains and northern Midwest. Obviously, severe storms can occur any day of the year, but I wanted to focus on where and in what form severe storms occur during the summer months.

You may or may not know that criteria for severe storms, as defined by the National Weather Service, are: tornadoes, and/or at least one (1) inch size hail, and/or wind gusts of at least 58 miles per hour.  As stated above, any of these three criteria can occur on any day of the the year.  During the summer months, tornado outbreaks (as defined by my definition of >10 different tornadoes in a 200 mile radius in one day) don’t usually occur that often. There are a couple of reasons why this is.  Temperatures climb into the 90s or even the lower 100s during peak heating hours of the day in most locations in June, July, and August.  While daytime heating is good for thunderstorm initiation, this usually creates a huge dewpoint depression, or the difference between air temperature and dewpoint temperature. Typically, for tornado development, dewpoint depression is often less than 20 degrees, with best opportunity at 10 degrees or less. Of course, this does not mean it is impossible for a tornado to develop with a dewpoint depression of >20 degrees.  The main threats when there is such a gap between temperature & dewpoint are wind and hail.  I will give a brief explanation why in a bit.

Now, I know what you’re thinking, what about the areas that don’t make it into the 90s and/or 100s, especially during the early summer days?  It all has to do with mid-and-upper level patterns. The best areas for tornadoes during the summer months are the central/northern Plains and northern Midwest.  Temperatures are relatively cooler in these areas during the early summer days, along with the favorable jet stream location in this area.  Typically, sometime during June, a pretty strong mid-level ridge builds into the southern/central CONUS. This not only forces the jet stream north, it acts as a “blocking pattern” in the mid-levels, which meteorologists call the “Omega Pattern”. Find more blocking patterns here.  Mid-level troughs/cutoff lows (lower isoheights) are either forced to crest the ridge or retrograde westward.  Often, during the summer, an elevated mixed layer (EML) will be present in the lower troposphere, mainly due to the upper level ridge. In simple terms, a warm layer of air is embedded just a few kilometers above ground level, which prevents surface-based storms from forming.  Tornadoes are often formed from “surface-based storms”. Storms that form above the EML are called “elevated storms” and pose a major hail and wind threat, with a very slight tornado threat.  Elevated storms that form along a boundary layer (cold front, warm front, dryline, outflow boundary, etc.) either clump together and/or form a line of storms.  This is how a Mesoscale Convective System (MCS) forms.  There are a couple of forms of MCS’s (product of severe storms) that can be recognized on Doppler Radar: bow echoes & squall lines. Bow echoes are usually formed from squall lines.

The reason why wind and hail are the main threats with any MCS is because downbursts are associated with MCS’s.  Because cold air is more dense than warm air, rain cooled air a few kilometers above the much warmer ground will crash down, at fast speeds, and spread out once it hits the surface.  This creates damaging straight-line winds, sometimes exceeding 100 mph.  Updrafts in these storms are very strong, capable of producing very large hail, as well. The stronger the updraft, the stronger the downdraft (wind & hail). Because mesoscale convective systems are usually clustered and elevated  thunderstorms, tornadoes are less likely to be produced, but not impossible to form.  In fact, tornadoes are more favorable to form at either end of bow echoes, in a area called the bookend vortex.  Rotating supercells may also be embedded in the MCS.  These systems usually last anywhere from 3-6 hours, on average and can travel hundreds of miles at times.

Derechos are also a form of MCS, but certain criteria are also needed to define a derecho:
1.) Wind damage swath has to extend more than 240 miles (400 km).
2.) Wind gusts of 58 mph (93 km/h) must be along most of its length.

Find more about derechos here.

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Severe Weather 6/11/13 & 6/12/13

*Blog written at 2:15pm CDT, 6/11/13*

An enhanced severe risk of severe weather is expected to take place today, 6/11/13, and tomorrow 6/12/13.  The primary threat will be damaging straight-line winds, each day, but very large hail, flash flooding, and a few tornadoes are also possible.  A shortwave trough will be rounding the crest of a developing mid-level ridge. By this evening, the trough axis will be located from southeastern MT to northeastern CO. A surface lee low will be advancing eastward tonight, across the central High Plains.  Rich low level moisture, daytime heating and shortwave trough influence will destabilize the boundary layer, creating severe weather from south-central & southeast MT, northeast WY, much of SD and northern NE. The severe threat will continue into IA, southern MN, southern WI and northern IL into the evening hours.  Again, the main threat will be straight-line winds, but very large hail and even tornadoes will be likely with any stronger, discreet supercells that form.  The tornado threat will likely diminish into the evening hours, once an MCS (Mesoscale Convective System)/linear line/bow echo of storms develop. SPC has issued out a “MODERATE” risk of severe storms for both days. SPC’s Day 1 (6/11/13) severe wind outlook:

SPC's severe wind probabilities for today. There will also be a significant hail threat for much of this same areas.

SPC’s severe wind probabilities for today. There will also be a significant hail threat for much of this same areas.

Thunderstorms will persist into the morning hours tomorrow, 6/12, for much of the High Plains and Midwest regions.  Surface low will deepen before reaching the Ohio Valley, tomorrow afternoon.  Few supercells will be likely by afternoon, but storms will quickly cluster together and from an MCS that will produce damaging straight-line winds and large hail. This MCS will likely trek eastward through northern/central IL, southern MI, northern/central IN, northern/central OH and much of the northeast states by Thursday morning. There is a strong possibility that a derecho, or a very large and destructive bow echo, may form.  In addition to the wind/hail threat, heavy rainfall is expected. Flash flooding will also be a huge problem!  SPC’s Day 2 (6/12/13) Severe Probability Outlook:

SPC's Day 2 severe probability outlook. Significant wind and hail threat will be likely tomorrow, 6/12/13.

SPC’s Day 2 severe probability outlook. Significant wind and hail threat will be likely tomorrow, 6/12/13.

The severe threat will continue into Thursday afternoon and evening for a good portion of the east coast.

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Summer Is Approaching

Blog written at 5:00am CDT, 6/10/13

A mid-level ridge will build in this week, 6/10 – 6/14, for much of the central CONUS. This will keep things mostly dry and very warm.  Some places, such as Wichita, KS and Oklahoma City, OK will hit >=90 degrees for the first time this year on Monday, June 10. This is a summer-like pattern setting in for much of the United States.

 

Mid-level ridge will strengthen over the middle CONUS this week. Temperatures will spike up  for most areas in the Great Plains.

Mid-level ridge will strengthen over the middle CONUS this week. Temperatures will spike up for most areas in the Great Plains. upper 90s and 100s will be likely for  areas Texas, Oklahoma, Colorado, Nebraska and Kansas. 

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Severe Weather Outlook (May 19 & 20)

*Blog written at 4am CT May 19, 2013*

Sunday, May 19, has the potential to be a severe weather outbreak in the Plains and Midwest.  Water vapor imagery currently shows a negatively tilted trough, with the trough axis running from southeast ID through northwest TX. Forecast models continue to show a closed low forming and setting up over NE/SD by tomorrow afternoon. A secondary shortwave trough will round the base of the synoptic trough/low and will be the culprit of the severe weather set-up tomorrow in the Plains.  Surface cold front will sag south/southeast and will be draped across central KS and northwest OK by early evening, Sunday.  The front will likely stall in that position.  A dryline will be also be set-up in west-central Oklahoma throughout the day.  It is still relatively unclear where and when storm initiation will happen.  However, the most likely area for all modes of severe weather will be form central OK, through northern MO.  Severe weather will be possible across 11 states in the Plains and Midwest, but not all areas will have the same severe probabilities.  Overall, hail and destructive winds will likely be the main threats Sunday, but isolated and perhaps scattered tornadoes will be possible in OK, KS, and MO.  This is where the best set-up will be.  Below is the Storm Prediction Center’s (SPC) Day 1 Outlook, issued at 06z May 19 and valid Sunday 12z to Monday 12z. A “Moderate” risk of severe storms has been issued for parts of OK, KS, MO and extreme southeast NE.  A “Slight” risk of severe storms surrounds the “Moderate” risk area. This graphic will likely change throughout the day. The “Moderate” risk area has the best chance at seeing all modes of severe weather tomorrow: hail, damaging winds, and tornadoes, possibly some strong (EF2-EF5).

day1otlk_1200

SPC’s Day 1 outlook, issued at 06z May 19. This graphic will likely change throughout the day. Don’t focus on just the red area.

On Monday, May 20, yet another shortwave will be rounding the base of the main cut-off low that will be centered over the northern Plains.  The main surface low will also be set-up in the northern Plains.  It will have cold front that will be stalled over the southern Plains, in central OK through central MO.  A dryline will also exist, though it is tough to know exactly where it will be by Monday evening.  Regardless, areas in the warm sector will be vulnerable to another severe weather outbreak.  Areas in the warm sector include central OK, eastern OK, northwest AR, southern & western MO, and eastern KS.  Storm initiation will depend on where the surface boundaries set up on Monday afternoon and locations of upper level jet streaks occur. The surface boundary locations will depend on what happens on Sunday.  However, once again, SPC has issued a “Moderate” risk of severe storms for Monday afternoon/evening.  Atmospheric environment will heavily favor destructive supercell in the warm sector.  If tornadoes develop, they can be strong.  Below is SPC’s Day 2 outlook, issued at 06z May 19 and valid Monday 12z – Tuesday 12z.  This graphic will also change over the next 24-36 hours.

day2otlk_0600

SPC’s Day 2 severe weather outlook, issued at 06z Monday, May 19. It is valid from 12z Monday to 12z Tuesday. Forecast likely to change in the next 24-36 hours.

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Severe Storms Likely May 18 – 20, 2013

*Blog written Wednesday, May 15, 2013*

Because May 18 is still 3 days away, a lot of data and information can change between now and then.

As of this writing, the NAM model is just now starting to pick up on Saturday’s outlook and I will therefore not be including it in this blog. Instead I will be referencing the GFS, CMC, and ECMWF models.

SATURDAY, MAY 18

It appears that severe weather is likely on Saturday, May 18, Sunday, May 19, and Monday, May 20, and possibly on Tuesday, May 21.  All of the reliable forecast models are picking up on a trough, that is currently off the Pacific Northwest coast, and digging into the western CONUS by early Saturday morning. It looks as if a smaller short-wave trough will push a head of the main trough and will help support thunderstorms in western and northwestern KS on Saturday evening. Return flow from the Gulf of Mexico is currently in place and will continue to transport moisture for this weekend’s events.  Both the GFS and ECMWF agree that dewpoint temperatures will reach 60F+ degrees as far north as North Dakota by Saturday afternoon. Some places in TX, OK, AR, KS and MO could even reach 70F+ degrees.

On Saturday, a dryline will be set up in western OK, western-north TX, central KS, western NE, SD & ND. The focal point of severe weather will occur in the central and high Plains.  A strong mid-level ridge will likely keep thunderstorms from developing south of Nebraska, at least prevent anything severe.  However, if the cap can break, severe weather will be likely in TX, KS and OK Saturday afternoon. 00Z CMC and 00Z ECMWF have two surface lows developing by Saturday morning, one in SD/ND and one in western KS.  06Z GFS has only one very broad surface low in SD/ND, but does eventually develop the second surface low by Saturday evening.  The biggest severe weather threat will be in the warm sector, just south of the warm front and a head of the cold front.  By Saturday evening, the warm front should be draped across SD, though IA, and south IL. The greatest forcing will be from west-central SD, down through central NE, and into west-central KS. The secondary surface low in western KS will help kick up thunderstorms, perhaps quickly becoming severe late Saturday afternoon.  It is still unclear just how the thunderstorms will evolve over time. Large hail and damaging winds WILL likely be the main threats on Saturday.  If cells become isolated and discrete, then there is enough shear for isolated tornadoes, especially in western/northwestern KS, where the short-wave trough will be located.

SUNDAY, MAY 19

On Sunday morning, showers and thunderstorms will be ongoing from Saturday. Forecast models are suggesting that the severe threat will be more scattered by Sunday afternoon and evening. The short-wave trough positioned over western KS on Saturday will round the base of the main mid-level trough/low.  This will cause a messy scene for the Midwest and northern Plains. Post-frontal severe thunderstorms will be possible in SD and NE.  Hail and wind will be the main threats for these areas.

The biggest threat on Sunday for severe weather will be from central OK, through eastern KS, to northern MO and southern IA.  The tornado threat also ramps up.  Forecast soundings and hodographs support strong supercells, capable of very large hail and a few tornadoes.

MONDAY, MAY 20

Still lots of questions for Monday. Models are in disagreement with location of the surface low(s) and frontal position by Monday evening. However, each model agrees that this has the potential to be the best day for tornadoes, out of the three days.  GFS has bulk shear values of 60+ knots in northern OK and southern KS by Monday evening. However, I do believe the cold front will be through the area by that time.  The corridor between south-central OK through southern IA has the best severe potential, including tornadoes. Below is a graphic from the Storm Prediction Center for these three days, shading in the areas with the best severe potential for the respectable days. I do agree with the graphic, for the most part.

Saturday (Red), Sunday (Purple) and Monday(Green) severe weather potential.  Image Credit: Storm Prediction Center.

Best probability of severe weather potential for Saturday (Red), Sunday (Purple) and Monday(Green).
Image Credit: Storm Prediction Center. *Subject to change in later updates*

 

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Severe Weather Outlook (May 12 – May 18, 2013)

*Blog written May 12, 2013*

Thus far in 2013, it has been a pretty inactive severe storm season.  Blocking patterns over the Pacific and western CONUS are the primary reasons why it has been relatively quiet.  It also didn’t help that unseasonably strong cold fronts swept through the Plains as recent as the first week of May.  I believe those types of cold fronts are over, until the fall of 2013.    However, the atmosphere is already showing signs of transitioning into a summer-like pattern.  So much for spring!  All hope isn’t lost, though, at least not yet.

May 12 – 14

Sunday looks to be fairly quiet around the lower 48.  An mid-level low, located over the Great Lakes region will be progressing eastward along the US/Canada border, and eventually northeast by the start of the week, next week.  The associated cold front, at the surface will be moving southeast across the state of Florida.  Thus, this will likely be the area (along the cold front) with the best opportunity to see severe weather on Sunday, May 12.

On Monday, May 13, a pretty significant shortwave trough will be entering the Pacific Northwest. This will create height falls downstream of the trough, in the states of Idaho and Montana.  With bulk shear values of 50-60 knots from the SW, dewpoints in the 40s and 50s, and strong dynamical lift, organized severe weather is likely for Montana and Idaho, and possibly on the eastern edges of Washington and Oregon.  As far as the rest of the country, a strong ridge will be in place over the southwest US and northwest Mexico.  This means mainly dry weather for much of the southwest and central CONUS.  The mid-level low, mentioned on Sunday, will be too cold to support severe weather in the northeast. In fact, it might be cold enough to snow in the northeast.

Severe weather on Tuesday, May 14, will depend on what the shortwave trough (from Monday) will do in the Northern Plains. GFS and ECMWF disagree with the outcome of the shortwave.  The ECMWF lifts it into Canada late Monday night, while the 00z GFS has a closed low forming and trekking along the US/Canada border. Given enough moisture in the lower levels, this could give some support for severe weather for North Dakota and Minnesota, if the trough doesn’t lift too far north.

May 15 – 19

A lot of assumptions are made with any forecast after 3 days out.  On Wednesday, May 15, a weak mid-level low/trough will be lifting northeast out of northwest Mexico and into the Southern Plains.  GFS and NAM are a little faster than the ECMWF with this feature.  All of the models agree, however, that there will be little shear to work with.  Therefore, severe storms will be isolated (if they occur) across Texas and Oklahoma, and possibly into Kansas.  Surface dewpoints will not be a problem. Lower and mid-60s dewpoints will be spread out across the Southern Plains by Wednesday afternoon.

The weak shortwave trough will be lifting into the Midwest and Mississippi Valley by Thursday, May 16.  This will likely spark showers and thunderstorms from eastern Oklahoma to southern Indiana. Some of these storms may become severe, though there are still questions about how these storms will be organized. Nonetheless, hail and wind will be the main threats  across these areas.  A severe outbreak is not likely, but things can change between now and then.

Friday (May 17) and Saturday (May 18) will have to closely monitored over the next few days.  A moderately amplified, open, trough will be digging into the central CONUS by Friday.  Forecast models are also hinting at lee-cyclogenesis, or a developing surface low on the lee side of the Rocky Mountains.  It’s hard to say exactly where this low will form and where it will track after it forms.  Strong mid and upper level lift, along with strong bulk shear, and ample low level moisture will support for organized severe weather on both days.  Models are hinting at dewpoint temperatures in the mid 60s and even lower 70s in places.  I can’t really tell you much, since it is almost a week out of range, but I can tell you that severe weather will likely somewhere in the Plains (from South Dakota to Texas) on both of these days. I hope to get another blog going before then.

 

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Start of May 2013: Winter Still Not Over for Some.

*Written April 30, 2013*

Some record cold temperatures will be possible for parts of the Southern and Central Plains in the next few days.  An unseasonably strong cold front will be sweeping through Wednesday/Thursday, dropping temperatures into the 30s, and even into the 20s for some areas by Thursday night/Friday morning.  Behind the cold front, a mid-and-upper level storm system will cause snow to fall and be a threat, especially for the panhandles of Texas and Oklahoma and areas of  central and southern Kansas.  Not only will record low temperatures be possible, but record daily snowfall amounts could also be measured and recorded.

NWS Amarillo has issued Freeze Watches and Hard Freeze Watches for many of their counties. They expect up to 3 inches of snow to fall in northeastern Panhandle of Texas Wednesday night into Thursday. Other Weather Forecasting Offices are calling for a light dusting to an inch of snow in places in Oklahoma and Kansas. Snowfall totals could rack up more, the farther north you go.

As far as record setting cold, many stations expect to be near or set record cold temperatures Thursday and Friday.  Oklahoma City, for example, may not only set the coldest daily low temperature, but also the all-time coldest high (44F) and coldest low temperature (32F) for the month of May (table below).

Little is still known about how much of a spring the Plains will have, especially after this cold blast in a few days. Though, I’m sure the cooler days will be welcomed and missed once the triple digit heat from summer rolls on in.

May records

May temperature Normals and Records for Oklahoma City (KOKC). The all-time record low and the all-time record coldest high temperature for the month of May could be broken this week.
Courtesy of: NWS Norman

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