March 1, 2016 Severe Weather Discussion for Oklahoma

This is a severe weather discussion valid for the morning of March 1, 2016 for the state of Oklahoma.

Models used: 00z NAM, 00z NAM-4km, 00z GFS, 21z SREF

Synopsis
Energy that will trigger this event is currently pushing into the Pacific Northwest / British Columbia region, moving directly into a small ridge positioned immediately due east of the system. Expectation is for the main body of energy to ride down on the lee side of a ridge that will build in across the Pacific Northwest as the energy continues moving eastward. Immediately south of this main piece of energy, at approximately 18z 2/29, emergence of higher mid-level vorticity values will commence across the Southwest, immediately ejecting southeastward into the Southern Plains. It will be this new piece of energy that will help to trigger the potential severe weather event.

Oklahoma…
Concentrated lower-level warm air advection will commence in the morning hours on Monday, with 850-millibar winds out of the west by 12z. Winds shift to southwesterly in the late morning, marking the warm air advection as high pressure shifts to the east, away from the Plains. Warm air advection aloft intensifies by 21z, and continues to build in magnitude through the late evening on Monday, up until the early morning hours on Tuesday when the potential severe weather event begins to unfold. Overall wind field for this event is messy. 500-millibar jet streak will round the base of the main trough as the event unfolds, reaching speeds AOA 80 knots in Colorado, as per latest model guidance.

Despite muddying of synoptic environment from previous discussion to this one, overall severe weather set-up has not deteriorated, and has actually strengthened in a handful of regards. Thunderstorms in this event will need to be elevated, due to strong temperature inversion consistent on forecasted soundings around the 900-millibar level. Surface CINH depicted at approximately -265 j/kg on latest NAM guidance, certainly conducive to a stable environment. However, 850-millibar to 500-millibar lapse rates of 7.8 degrees Celsius/km show a much more favorable environment above the PBL. As such, MLCAPE of just below 1400 j/kg with 87 j/kg MLCINH displayed at 06z Tuesday over KOUN. Wind profile shows substantial veering, perhaps with slight backing near 500-millibars, but overall much improved from previous model runs a few days ago. Surface to 3km storm-relative helicity values over KOUN progged at 435 m2/s2 from latest guidance, in addition to 0-6km shear around 50kts exemplify this improved wind field from previous model runs. LCLs outlooked at around 700 meters, certainly supportive of tornado formation, though perhaps slightly high in my opinion for any real tornado potential. SARS product over KOUN at 06z from 00z NAM depicts 65% chance of tornado, utilizing favorable MLCAPE and improved wind fields. Still skeptical of PBL temperature inversion, as well as depicted isolated storm coverage on 00z NAM-4km.

Most concerning point about this event, in addition to PBL temperature inversion, is projected coverage and evolution of convection. 00z NAM-4km projects evolution of thundershowers in extreme northern Texas, intensifying into either a group of strong cells or a single severe thunderstorm by 04z Tuesday, crossing into Oklahoma by 05z. Complex appears to strengthen and begin to bow out by 07z Tuesday, and begins to impact OKC/OUN at 08z, albeit in a slightly degraded severity. Complex continues to bow out and degrade through 12z, turning to the right and bowing out in the process as it moves into southeast Oklahoma. Scattered strong thunderstorm cells begin to fire at approximately 10z in eastern Oklahoma out ahead of the weakened segment of thunderstorms, and these could also pose a risk for marginal severe weather, in line with the latest Storm Prediction Center outlook (Slight Risk across most of central Oklahoma). In terms of probabilities, latest SREF suite places a 50% to 70% likelihood of thunderstorms occurring in an area with a supercell composite at or above 1. SREF members notably less enthusiastic with MLCAPE values over Oklahoma for this potential event, on the order of AOA 500 j/kg from about KOKC south. This uncertainty calls into question the bullish appearance of the 00z NAM, but will wait for the 03z SREF suite to see if uncertainty holds or is reduced using new 00z data. Reduction in uncertainty is certainly a possibility, as SREF suite will likely be influenced by more aggressive 00z NAM solution.

Overall, situation is a difficult one to assess. Not entirely convinced mixed layer will be conducive to severe weather event, especially given unfavorable temperature inversion around 900-millibars. In addition, very scant coverage by 00z NAM-4km only adds to concerns about any convection actually forming to begin with. Do believe that if convection is able to form, strong thunderstorms will become a good probability given favorable wind fields and potentially-favorable mixed layer environment. Severe thunderstorm formation also possible in the event convection can form and be sustained. Tornado risk somewhat low but not entirely zero in my opinion, given slightly high LCLs despite favorable wind profile, including veering winds, rather high 0-3km SRH values, and good surface to 6km shear values. Best threats will include hail and strong winds, with perhaps a low threat for a tornado if mixed layer environment can be fully realized.

Andrew

March 1, 2016 Oklahoma Severe Weather Discussion

This is a severe weather discussion for March 1, 2016 for Oklahoma.

Models Used: 2/26 00z GFS

Synopsis
Energy currently located out over the North Pacific, west of Alaska, will ride in on strong Pacific jet stream (over 190 knots as of 06z 2/26) into the Gulf of Alaska by 12z 2/28. Storm system will then drop to the southeast on lee side of a ridge just offshore of the West Coast of North America by 2/29. Slight interaction with energy immediately north of Montana/Canada border will aid to wrap system to a negative tilt by the day of the event. Vorticity max will be located over western Kansas by morning of 3/01 as system pushes east and severe weather event commences.

Oklahoma…
Similar to severe weather event on 2/23 and 2/24, a negatively-tilted trough will move into Central/Southern Plains on the morning of 3/01. Mid-level jet streak in excess of 110 knots will wrap around base and lee side of trough in the morning of March 1, providing anomalously supportive dynamics for a severe weather event. Unimpressive jet streak at 250-millibar level (90 knots) should be negated by the aforementioned mid-level dynamics. Lower-level wind fields also impressive, with nocturnal lower-level jet streak at 700-millibars nosing into southern Kansas by 09z 3/01 AOA 80 knots. 925-millibar wind field generally broad-brush 40 to 50 knots across Oklahoma out of the south, with temperatures up to +20º C in this warm sector. Surface temperatures in excess of 60 degrees F being advected northward by sub-990 millibar surface low centered in northwest Kansas combines for southerly surface winds AOA 25 knots.

Severe weather set-up not enticing from current model guidance, despite anomalous mid and lower level features. Primary factor looks to be the mere factor that it will be the morning when this event may occur (06z - 15z 3/01), and as such an inversion will be in place. Current guidance suggests favorable mid-upper level lapse rates but anemic lower-level lapse rates with CINH progged to exceed 200 j/kg in the morning hours, particularly in western and central Oklahoma. Forecasted soundings from KOUN indicate excessive CINH, as well as somewhat dry lower levels. Narrow corridor of uncapped instability forms in central Oklahoma by 12z 3/01, with just below 1000 j/kg MUCAPE available in the midst of CIN AOB 20 j/kg. LCLs around 700 meters, combined with 0-3km SRH exceeding 200 m2/s2 and surface-6km shear nearing 50 knots, do foretell a possible supercell threat with any storms that do manage to form amidst this rather stable environment. Wind profile generally veering on forecasted soundings, save for a slightly backed look around 700-millibars.

All in all, expect that the environment will not be conducive to anything more than scattered severe thunderstorms on the morning of March 1st. Any storms that do manage to form will have the potential to be severe, thanks to impressive lower and mid-level wind fields and sufficient helicity and shear. Current thinking is while there is a marginal tornado threat with this environment (SARS-indicated, in addition to favorable LCL and sufficient wind profile), isolated nature of storms in a more or less unfavorable instability-related environment should suppress any threat of a larger-magnitude severe weather event.

January 22-25 Potentially Historic Winter Storm

A potentially historic winter storm looks to be unfolding for portions of the Eastern U.S.

Tropical Tidbits

A low pressure system is forecasted to develop in the South US and push northeastward to the area just offshore the Mid-Atlantic, with this coastal storm fully unfolding around January 23rd-24th. The above image shows the GFS 500mb geopotential height contours and mean sea level pressure values for the morning of January 24th. We see the snowstorm located fairly out to sea, at approximately 985 millibars. This track would produce the snow map below, taking this solution verbatim.

Tropical Tidbits

This solution would bring about 6-12" of snow to the Kentucky and Tennessee region, extending into extreme southern Indiana and a good chunk of southern Ohio. We also see similar amounts into northern Arkansas and southern Missouri, as well as North Carolina. Amounts on the order of 12-24" are outlooked for West Virginia, particularly the southern part of the state, into the majority of central Virginia, New Jersey, and eastern Pennsylvania into southern New York, with some slightly lower amounts further east into Massachusetts. Long Island is also forecasted to receive upwards of 12" if this solution were to verify.

Tropical Tidbits

To emphasize how critical the placement of this system will be, take a look at the ECMWF model's 500mb geopotential heights and mean sea level pressure values for the morning of January 24th, the same timeframe as the GFS graphic. We see the system at 984 millibars, pretty similar to the GFS, but the system is clearly displaced further west. Given how anomalously warm the Atlantic currently is, this shift could easily push those two to three feet amounts inland, and leave the coastline with rainy conditions. This run of the ECMWF has snowfall amounts of over 30" in central Virginia, but lesser accumulations the further north and east you go.

Tropical Tidbits

The GEM model shows amounts in the 6-12 inch range for most of Missouri into the southern parts of Illinois, Indiana and Ohio, as well as much of Kentucky and northern Tennessee into western North Carolina. Amounts in excess of 30" are forecasted from Maryland into Pennsylvania, certainly an incredible amount of snow, but not entirely off the table given how strong and persistent this system may end up being. Again, the placement of this storm will be critical to who gets the heaviest accumulations, and we likely won't know who will receive this jackpot until the storm is actually happening.

Tropical Tidbits

One last bit of advice, do not bite on any particular model run for this storm. The strength of this storm, as well as how long-lasting it will be, among other factors, makes me way too uneasy to trust any single solution, hence why I presented a plethora of models here. The above graphic shows GFS ensemble spread in MSLP values for this storm. The warmer colors to the left and right of the system indicate increased uncertainty with where the storm will track. Again, this emphasizes how no single solution is set in stone, and thus should not be taken at face value. A lot of change will happen in the next several model cycles, and the heaviest amounts could plausibly shift a substantial bit as these changes unfold.

To summarize:

- A potentially historic snowstorm is forecasted to occur along the East Coast over the January 22-25 period.
- Currently, the Maryland/Virginia/Pennsylvania region is on track to receive the heaviest amounts.
- The heaviest accumulations could reach or exceed 30".
- Enough uncertainty still exists to change the storm track substantially, as well as shift the heavy snow axis and amounts.

Andrew

January 9-11 Potentially Significant Winter Storm

A potentially-significant winter storm appears to be unfolding in the January 9-11 timeframe.

Before we even dive into this storm, it's crucial to acknowledge that there are two model guidance 'camps' for this storm. As a result, there is unusually high uncertainty, so keep that in mind while reading this post.

Tropical Tidbits

The first solution we'll look at is the east-and-weaker idea. This is the latest GFS model's projected snowfall accumulation through Sunday morning. In this forecast, we see a swath of about 4-8" from southwest Missouri into extreme southwest Illinois, with a stripe of 2-4" on a northeast path until about Gary, Indiana. From that point, amounts on the order of 4-8" are outlooked for much of Michigan, particularly from the extreme southwest part of the state into the northeast part of the state.

This solution has been gaining ground in the last few model cycles, especially against the second model solution, which we'll analyze next.

Tropical Tidbits

The other solution being analyzed is a further northwest and stronger storm track. This is the 12z RGEM model, and it shows a solid 4-8" across much of the southern half of Missouri, with a small jackpot zone of 12-16" from extreme southwest Illinois into central Illinois. A zone of about 5-10" is outlooked for the Chicagoland area into the western half of Michigan.

This solution would appease many in the Midwest and Great Lakes regions looking for snowfall, but this particular solution is been losing traction to the weaker/southeast track in the last few model cycles.

Despite high model uncertainty, and the fact that I've seen stranger things happen, I'm expecting that weaker & southeast track to win out.

To summarize:

- A potentially significant winter storm is expected in the January 9-11 timeframe.
- High model uncertainty exists.
- A 2-6" snowfall swath could impact a southwest-to-northeast line from southwest Missouri to northeast Michigan.

Andrew

Stratospheric Vortex Under Attack, But Holding Strong

The stratospheric polar vortex looks to come under attack in the next several days, and possibly beyond, but all indications are that this attack will be weak, and the polar vortex aloft will remain strong.

We'll begin with a current look at the stratosphere.

CPC
(Refresh page if animation stops looping)

This animation above shows temperature anomalies at the 30-hPa level of the stratosphere, just above the middle part of the stratosphere. Note how over the past month we've seen a few instances of minor warming occurring at 'lower' latitudes, but never reaching a strength adequate enough to be an actual stratospheric warming event. Consequentially, the stratospheric polar vortex has remained pretty much undisturbed. There is some warming commencing over the Himalayan mountain region, and this could incite another minor warming event to spread over the north Pacific, as has happened before in the last 30 days on this animation. Whether it builds into anything substantial remains to be seen, but as the analysis below shows, I'm not optimistic.

FU-Berlin

The first image we'll analyze is a longitude-by-height chart, forecasted by the ECMWF model for five days' time. This chart is forecasting the presence of a geopotential Wave-1 event in the stratosphere. As the phrase 'Wave-1' indicates, the forecast here is for the risk of a single body of high pressure (hence Wave-1) forming aloft to try and displace the polar vortex. The legend on the left indicates this risk is highest from the 1-millibar to 10-millibar region, where geopotential values are highest and the warm colors are most prominent. This is a pretty strong Wave-1 event, but because it is centered above the 10-millibar level, and it will weaken shortly after this time period five days out, I'm not expecting any long-term damage to the stratospheric polar vortex.

FU-Berlin

Another tool we can use are time-by-height charts for different levels of the atmosphere to see how much, if any pressure is being applied to the polar vortex. It looks a little intimidating at first, but we'll break it down below.

The top panel shows us the forecast for Wave-1 (explained above) and Wave-2 temperature attacks at the 10-millibar level. The Wave-2 event is where one body or more of high pressure / warm temperatures form aloft (in this case, at the 10-millibar level) and try to split the polar vortex into two vortexes, hence the '2' in Wave-2. Notice the forecast for a Wave-1 event peaking at that 5-day forecast period, which we analyzed above, but then that Wave-2 line starting to rise up towards the end of the forecast period, ten days away. That could be something to monitor, but when we consider Wave-2 events are weaker than Wave-1 events, I'm not particularly encouraged by it.
This description of that top panel also applies to the third panel from the top, except now valid at the 30-millibar level. In this case, however, notice a sustained elevated Wave-1 attack throughout the forecast period, but also a strengthening Wave-2 parameter towards the end of the forecast period. Again, perhaps something to watch, but I'm not exactly keen on it impacting the vortex significantly right now.

Panels 2 and 4 from the top are nearly identical to the first and third panels described above, but these new panels show us Wave-1 and Wave-2 attacks from a geopotential (ridges instead of warm temperatures, although in essence they tend to occur together) standpoint. Note in Panel 2, showing us the forecast at the 10-millibar level, a sustained Wave-1 attack, and a pretty strong one at that, throughout the forecast period. We also see a strengthening Wave-2 event at the end of the forecast period.
The situation in the fourth panel, showing geopotential values at the 30-millibar level, is different than the 10-millibar level. We see a pretty strong Wave-1 event, but a weakening Wave-2 episode. This piques my interest, just because I want to see how that evolves more than anything, and may be something to watch down the road.

To summarize:

- Minor warming has occurred in the stratosphere over the last month, but has had very little impact on the polar vortex.
- Additional minor warming is forecast to occur over the next several days, again with little impact in the lower stratosphere.
- Long-range guidance indicates some items of interest beyond Day 10 forecasts, but I'm personally pessimistic as to the chance of a stratospheric warming event.
- This may keep the chances for a large-scale, significant cold weather event relatively low, speaking strictly from a stratospheric viewpoint.

Andrew