Another Late-Week Severe Weather Threat

Originally posted on ZoomRadar

As the end to another week approaches, so will the threat for severe weather. Another advancing cold front late this week will bring a slight risk of severe weather from portions of Missouri and Illinois east to New York and parts of New England on Thursday and Thursday night before affecting areas from the Carolinas north to the New York City area on Friday and Friday night. Below is the predicted surface maps from the Hydrometeorological Prediction Center starting on Thursday morning and continuing into Friday morning and Saturday morning.

The threat of severe weather has prompted the Storm Prediction Center to issue a slight risk for both Thursday and Friday which can be seen below.

Besides the infiltration of colder air behind the cold front, many factors will help lead to this severe threat. Taking a look at the latest 12Z NAM model run to view these, the first thing to notice is the upper air support that will be in place as the jet stream sinks into the United States from Canada. Below is what that may look like come Thursday evening and Friday evening.

As this trough digs and becomes more neutral, the storms will likely increase in intensity for Friday. While winds in the jet stream will only be about 50 knots above the Mid-Atlantic come Friday evening, this will still be more upper air support than this area has seen in awhile and therefore will help any thunderstorms that form that day.

The next thing to consider is the moist and unstable air mass that this front will be pushing into. Below are the images showing the dewpoint temperatures and CAPE values on Thursday evening and then below that on Friday evening.

What is being depicted here is a drastic change in the amount of moisture in the air before the front and behind the front. In front of this cold front, we may expect to see dewpoint temperatures on both Thursday and Friday in the 60s and perhaps lower 70s in a few places. CAPE values will likely remain between 1000 and 2000 J/kg for most but a few areas could see values reach at or above 3000 J/kg. Behind the front, CAPE values fall drastically to values closer to 0 as expected with a sharp cold front but more dramatically, dewpoint temperatures will fall to below 60 and in some cases to near 40! This drastic change will result in numerous thunderstorms initiating on both days as shown by the composite reflectivity from Thursday evening and Friday evening below.

Due to the fairly low shear environment across much of the threat area on most days, the threat of tornadoes should remain pretty low. However, a tornado or two cannot be ruled out, especially on Thursday across the Midwest. That being said, the main threat should be damaging winds with some large hail possible. Keep an eye on this developing situation as we head into the late work week.

Update to Weekend Severe Weather

Originally posted on ZoomRadar

As mentioned in my previous blog post, a strong cold front has been predicted to trigger strong to severe thunderstorms as it tracks to the east from the Plains to the Atlantic Coast this weekend. For this reason, the Storm Prediction Center has issued a slight risk for areas in front of this cold front today straight through Sunday. Below are the outlooks issued by the SPC.

Day 1 Outlook - Wind (Today and Tonight)

Day 1 Outlook - Hail (Today and Tonight)

Day 1 Outlook - Tornado (Today and Tonight)

Day 2 Outlook (Saturday and Saturday Night)

Day 3 Outlook (Sunday and Sunday Night)

Now let’s take a look at the specific details of why this threat exists starting with today. Below are images from today’s 12Z NAM model run showing what the scenario may look like come this evening.

Precipitation

Dewpoint Temperatures and Surface Winds

850 mb (Low-Level Jet) Winds

250 mb (Jet Stream) Winds

500 mb Relative Vorticity

CAPE

EHI

Today and tonight thunderstorms are expected to break out in a fairly unstable environment across mainly Nebraska, South Dakota, North Dakota, and Minnesota. Dewpoint temperatures will be generally greater than 55 but as high as the low 70s in some areas which will really aid in creating a moist and unstable environment. 850 mb winds will generally be pretty weak but the storms will get some help from faster winds in the jet stream reaching up to 90 knots in some spots. The trough associated with the surface cold front will still be positively tilted and positive vorticity advection will be somewhat on the low end. Taking a look at CAPE values, some areas may see values reaching above and perhaps beyond 4000 J/kg but most areas will remain between 1000 and 3000 J/kg. The last image shows us that EHI values may become pretty high with values above 6 in some areas. If this turns out to be the case we may see a small tornado threat this afternoon and evening. In fact, today may pose the best chance for tornadoes as this trough pushes across the northern portions of the country. Overall, most storms will pose a damaging wind and perhaps some large hail threat. Today will likely pose the least risk of severe weather this weekend. That being said though, there will definitely still be severe weather today and tonight across the Northen Plains, as depicted by the SPC’s outlook percentages.

Below are images from today’s 12Z NAM model depicting what conditions could be like come Saturday evening.

Precipitation

Dewpoint Temperatures and Surface Winds

850 mb (Low-Level Jet) Winds

250 mb (Jet Stream) Winds

500 mb Relative Vorticity

CAPE

EHI

Saturday I expect the severe weather to start firing up even more. The reasons behind this can be seen in the graphics above. First off, dewpoint temperatures will top out generally higher than the day before with values at and above 65 and perhaps as high as the upper 70s. Surface winds will be about the same magnitude out of the south but winds at the low-level jet level will be stronger with some areas potentially seeing these winds hit 40 knots. Winds in the jet stream will also be stronger, peaking above 100 knots across northwest Wisconsin on Saturday evening. The jet stream will also be aligned in a way now that the right rear quadrant should provide some extra lift for thunderstorms to form and become stronger. The trough will also slowly start becoming more neutral in nature and positive vorticity advection will become stronger, especially across the more northern portion of the nation. We also see that CAPE values will be generally the same as the day before with some areas topping out at and above 4000 J/kg. The last measure, EHI, will once again present some fairly high values mainly across Illinois and Indiana so we may see a slight tornado threat across this area as well as somewhat but to a lesser extent to the north of these regions. Once again, the main threat on Saturday and Saturday night will be damaging winds and some large hail.

Below are images from today’s 12Z NAM model depicting what conditions could be like come Sunday evening.

Precipitation

Dewpoint Temperatures and Surface Winds

850 mb (Low-Level Jet) Winds

250 mb (Jet Stream) Winds

500 mb Relative Vorticity

CAPE

EHI

Sunday and Sunday night will likely present the largest threat for severe weather out of the three days. Once again we will see high dewpoint temperatures from the mid 60s to as high as the mid 70s with nice southerly surface winds, helping to increase the moisture content of the air even more. Winds at 850 mb will be the strongest out of any of the three days with some areas just north of New York in Canada, seeing these winds peak at above 60 knots. The jet stream winds will once again be strong, topping out at above 100 knots, and placing that right rear quadrant right across a portion of the threat region. Our trough by now will likely have tilted negative which will help enhance that threat of severe weather as it makes this tilt towards negative on Sunday. 500 mb positive vorticity advection will likely remain fairly strong, especially across the northern portions of the threat area. CAPE values may be lower on Sunday but it is the current thought that the strengthening of upper air processes will outweigh this. Still, CAPE values will generally be above 1000 J/kg which will be enough instability for these storms to maintain themselves. EHI values will generally remain below 2 with some pockets being a little higher so the tornado threat should be pretty low. However, this does not mean that an isolated tornado risk cannot be ruled out. Once again though, the main threats will be damaging winds and some large hail.

This weekend will surely lead to multiple severe weather reports, including some large hail, perhaps a couple tornadoes, and what will likely be some widespread damaging wind reports. Please remain aware this weekend across the northern portions of the Plains, Midwest and Northeast.

Another Round of Storms for the Midwest and Northeast

Originally posted on ZoomRadar

Last week at this time, parts of the Plains and Midwest straight into the Northeast were gearing up for what would be two days of back to back severe weather with the main event occurring last Thursday. Below are the storm reports from those two days, showing the progression of the cold front which allowed storms to fire up in advance of this feature. The threat started across the Midwest and Plains and progressed eastward, affecting areas of the Northeast before pushing out to sea. While most of the damage occurred due to severe straight-line winds, Thursday did result in a handful of tornadoes, mainly along the New York and Pennsylvania state border.

Another round of strong to severe storms appears to be in the making for many of these same areas as we head into this weekend. A cold front extending from a Low centered just north of Minnesota will drape back across the Plains on Saturday morning, shown below. The advancement of this front will result in the triggering for strong to severe storms as it enters unstable and moist air ahead of this system.

The image above shows where this front will be on about Sunday morning and the below image then shows about where it will be come Monday morning.

The 12Z NAM model shows what the precipitation may look like come Friday evening as this front begins its push across the northern Plains.

CAPE values in advance of the front will be above 1000 J/kg with pockets above 3000 J/kg.

EHI values will also be quite nice with many areas in Minnesota recording values at or above 3. This is the result of good low level turning of the winds with height which means if supercells can form, there may be some tornado threat on Friday.

For these reasons, the SPC has highlighted a region across the eastern Dakotas and most of Minnesota in their Day 3 Outlook as being at a slight risk for severe storms. The main threat at this time appears to be some large hail but mainly damaging wind gusts. If supercells can get going out in front of the cold front however, we may see a few tornadoes.

On Saturday, the front will continue pushing east and even though the SPC has not highlighted an area for severe weather risk at this time, they do mention that a risk will likely be present across portion of the Midwest and Great Lakes. Below the 12Z NAM model shows what the precipitation may look like come Saturday evening with storms stretching from Michigan into Illinois and back across the Plains.

CAPE values and EHI values will once again be fairly high, with CAPE values looking much like Friday’s CAPE values but EHI values being somewhat less spread out but potentially packing areas of higher values above 6. This once again would result in a mainly damaging wind threat but a few tornadoes may not be out of the question in the areas with stronger EHI values.

Check out what hodographs may look like across central Indiana come Saturday evening. If this ends up being the case, a few tornadoes would definitely not be out of the question.

The risk of severe weather in general will also probably be worse on Saturday than it was on Friday due to the strengthening of winds aloft and better moisture across these areas. Below are images from the 12Z NAM model showing the winds at 250 mb, 850 mb, and dewpoints with surface winds. A 100 knot plus jet streak will likely be centered across the Wisconsin and Minnesota border with an 850 mb low-level jet with winds approaching 50 knots from Illinois into Michigan.

Dewpoints will also be high from the mid 60s into the low 70s across these areas. Surface winds will be mainly out of the south in front of the cold front which will aid in bringing this moist air in and help the inflow into these thunderstorms as winds could be as strong as 20 knots at the surface Saturday evening.

Storms will continue throughout Saturday night before we get to what will likely be the main event from this cold front on Sunday across the Northeast. While this event is still five days out, the SPC has highlighted an area on their Day 4 to 8 Outlook meaning that they expect a 30% chance or greater of severe weather within 25 miles of a point on Sunday and Sunday night.

The 12Z GFS model depicts this event probably a bit differently than the NAM will depict it come this time tomorrow when the NAM begins to send predictions for an event this far out. This is based on the fact that the GFS depicts the Friday and Saturday events slightly different, mainly with not as much moisture being pushed north as the NAM thinks it will. Despite this, here is what the CAPE may look like come Sunday evening and below that are images depicting 250 mb winds, 850 mb winds, and surface winds and dewpoints.

CAPE values will be above 1000 J/kg and a 100 knot jet streak will be in place north of the threat area with the right rear quadrant being placed over the threat area. Additionally, an 850 mb low-level jet will become quite strong with areas of 50 knot plus wind and widespread dewpoints at and above 70. All of these conditions along with what should be some good positive vorticity advection, will result in what should be a pretty substantial severe weather threat come Sunday. The main threat will likely be damaging winds.

This cold front will definitely be something we should all keep an eye on this weekend as it sparks strong and severe thunderstorms. This is still an evolving situation so the exact locations which may come under a threat due to these storms is still somewhat unknown. Below is my threat map depicting where I think the threats may be come Friday (shaded in brown), Saturday (shaded in yellow), and Sunday (shaded in red).