Thoughts on the January 29/30 Severe Weather Outbreak

A strong cold front will push its way across the nation over the next few days, sparking severe weather for many areas.  The Storm Prediction Center has issued areas of concern for both Tuesday, January 29th as well as Wednesday, January 30th, which are presented below.  Based on what I have been looking at, I will give my best insight into where I believe the threat will be during the hours of January 29th into the morning of the 30th.

The 00Z NAM model is in fairly good agreement now with the past few runs of this model and the 00Z GFS model has begun to come around to a more similar solution to what the NAM has been advertising over the past day.  Therefore, I will only show the 00Z NAM model figures below, keeping in mind that the 00Z GFS model is similar but running a few hours faster than the 00Z NAM model.

The first image below is dewpoints from 06Z Wednesday, as this is when they will be maxed out for most areas that have not had the front pass them yet.  Dewpoints in the mid to upper 60s will exist across most of Louisiana and Mississippi during this time frame with 60 degree dewpoints extending as far north as Illinois.  It is also important to note the strong southerly flow in the warm sector.



CAPE values will be limited across the severe weather region but a large swath of 500 J/kg or more should exist throughout the day and overnight along the lead edge of the front.  While this is not a large amount of CAPE by any means, there will be enough forcing from other means to trigger storms.  The biggest factor that the limited CAPE could play into is limiting the threat for supercells in front of the main squall line.  However, as can be seen in the image below, which is once again for 06Z Wednesday, CAPE values may surpass 750 J/kg and near 1000 J/kg during the overnight hours, which is not bad considering that it is night and that values struggled to reach this during the day.



Couple these CAPE values with an intense low level jet, which will strengthen overnight, and the threat for tornadoes will certainly exist across portions of Arkansas, Louisiana, Mississippi, and Tennessee.  The tornado threat should also be greater at night as the trough begins to rotate from a positive tilt into a more neutral to negative tilt by morning, helping to aid in more cold air aloft, which is likely helping to raise the CAPE values too.

In my forecast I have taken into account where dewpoints of at least 55 will exist, taking into account where precipitation will be occurring during the time frame as well.  This I have outlined in the green color.  Next in orange I outlined an area which should see CAPE values in excess of 500 J/kg.  Next I looked at where low-level winds will be strongest by viewing surface winds up to 850 mb and I outlined this area in purple.  I viewed EHI values as well as helicity values to determine where these values were the best and outlined that area in red.



This information, coupled with my general meteorology knowledge and having started following this system since it was over 300 hours away, I have come up with the following map depicting the areas most at risk for damaging thunderstorms during what the SPC will consider the Day 2 time period when they issue their new outlook in less than one hour.  The yellow area represents an area where I would expect to see isolated to scattered wind damage and the red area represents an area where I would expect to see more scattered to widespread wind damage reports.  The red area receives this heightened threat due to the fact that lower level winds will be stronger at night and this is when the storms will impact this area.  Additionally, this area is also at risk for a few tornadoes as I outlined my best tornado threat area in pink.



Overall I would expect this to be mainly a damaging wind event given how strong the low-level winds are and that the main storm type should be linear.  However, even with the limited CAPE values, especially in the nighttime hours I would not rule out a few tornadoes.  The threat of tornadoes could be higher if supercells can form out in front of the main line of storms but this will likely not be known for sure until the event draws closer.  Definitely stay tuned to wherever you get your weather information and be on the lookout for another blog from me tomorrow night to update my forecast.

A New Meaning to Nuclear Winter!

With a sharp shot of cold air having pushed its way into the United States, many in the eastern half of the nation are currently experiencing the coldest air of the season.  Below is the 00Z NAM model sounding for Pittsburgh at 10PM this evening.

The modeled surface temperature at this hour was around -14 degrees Celsius.  This means that the snow growth region actually occurs right at and just above the surface.  Knowing this information, what could possibly be showing up on the Pittsburgh radar?



This precipitation is located just north of Pittsburgh and was remaining fairly stationary over time.  Give up?  Let me give you a little more information.  As you may have noticed in the model sounding from above, the forecasted wind direction near the surface was from the west northwest.  Notice how this band of precipitation is also located in this orientation.  Now I know many of you may be thinking, oh well it must be some kind of lake effect then.  Well it kind of is, but what is the source of the water?  Need help?



The red dot located just to the west northwest of the band is the Beaver Valley Nuclear Generating Station.  It is so cold that the steam being output from the nuclear power plant is forming into dendritic ice crystals and falling as snow across the northern suburbs of Pittsburgh!  In fact, the National Weather Service in Pittsburgh has actually received reports of accumulating snowfall up to one inch from this band of snow.  Try forecasting that one!

Snow Potential for Starkville!

With little to no winter weather precipitation last year, parts of the Deep South received an icy blast from old man winter over the past few days.  Ice accumulations from freezing rain piled up across parts of Louisiana, Arkansas, Tennessee, Mississippi, and Alabama while other areas of these states received flooding rainfall.  Now snow could become an issue for some areas tonight and tomorrow morning.  Below are my thoughts on how this event could evolve tonight and tomorrow for Starkville, MS.  Current radar imagery reveals lingering showers over the Golden Triangle Region with temperatures hanging out mainly in the mid 30s.

Rain showers will continue overnight but as a cold upper-level low continues to move closer to the area, the above surface air will begin to cool off.  This will result in the elevated melting layer disappearing throughout the overnight as evidenced by the 18Z NAM run for 3am local time below.  During the couple hours leading up to this time period, the elevated melting layer would be shallow and cold enough to support sleet but other issues will likely prevent this.  The issue is that the best snow growth region does not occur until around 17,000 feet at 3am.  Snow could form at lower levels possibly down near 12,000 feet but even at that height, the cloud height is not reaching these levels.  Therefore it is my concern that while the atmosphere could support snow, dendritic crystal growth will not start to occur and thus precipitation will fall as either rain or freezing rain depending on the surface temperature.



Just a couple hours later though, the 18Z NAM does show the cloud heights growing to a point where dendritic crystal growth could occur, though not optimally.  At this stage an hour or two before sunrise, I would expect very wet snow to begin falling with low snowfall ratios as most of the layer is near freezing, nearly maximizing riming potential.



As the sunrise nears, the clouds will finally reach an area for optimal dendritic crystal growth and as the layer continues to cool, snowfall ratios will increase.  Snow will likely continue to fall all morning before mixing with rain around noon and ending as rain showers in the early afternoon.  The 18Z GFS also supports a solution similar to this.

This specific event does have a few things going for it.  One, the NAM specifically has really struggled with understanding just how cold this air mass is.  Today’s high should have been in the low 40s according to the NAM in the Starkville area but the actual high was only 37 at KGTR.  Additionally, upper air temperatures upstream have been confirming a couple degrees cooler than the model soundings have been showing.  These two model errors could mean cooler temperatures at the surface and above the surface overnight resulting in a higher chance of freezing rain and some potential at a quicker turnover to snow.

Taking a look at precipitation amounts now, strong PVA should effect the region tomorrow morning, enhancing uplift.  Diffluent flow aloft will also help uplift during the early morning and mid-morning hours however, the left rear quadrant of the jet may hinder some of this uplift later in the morning.  Additionally, warm air advection from the north may help with uplift.  The below image from the 18Z NAM depicts this uplift occurring around sunrise tomorrow.



So what am I expecting for snow totals?  The 18Z GFS would support a couple inches whereas the 18Z NAM supports anywhere from 2 to 4 inches.  I expect these snowfall totals will be about correct given the upper air support but given that this is the first snow of the year for this area, the wet ground conditions, and near freezing surface temperatures, I expect ground accumulations to be limited.  Snow will stick first to colder surfaces such as grass and trees where up to an inch may be possible but on roadways I would expect all snow to melt except on bridges where some slush could develop.



If some heavier snow can develop, as sometimes occurs with these kinds of systems in the Deep South, it would not be out of the question that minor accumulations could occur on area roadways but at this time I would not expect this to occur.  Nevertheless, be sure to allow extra time to travel tomorrow and to follow winter weather driving tips such as limiting the use of your brakes and allowing extra distance between you and the vehicle in front of you.

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.